The terms secondary metallurgy or ladle metallurgy are used in particular in steel production and refer to the aftertreatment of steel after refining . Secondary metallurgy is therefore not to be understood as an independent metallurgy , but rather describes various, alternatively or sequentially applicable, metallurgical measures that improve the melting.
The introduction of secondary metallurgy has fundamentally changed the steel industry. Whereas in the past the steel was ready after freshening, nowadays it is usually followed by a series of subsequent treatments in order to achieve the required quality of the steel.
The sulfur and phosphorus content may:
- for stainless steel 0.025%
- for quality steel 0.035%
- for base steel 0.045%
There are the following methods of secondary metallurgy:
Treatment in the ladle oven
- The exact casting temperature for the continuous caster is set by means of an electric arc . ± 3 ° C
- Setting the analysis by alloying (FeMn, FeCr, FeSi, FeMo, ...)
- Homogenize by flushing the pan with argon or nitrogen
- Deoxidation using aluminum or calcium
Many gases are dissolved in the melt, which negatively influence the properties of the steel during further processing. To safely remove these, the post-treatment is carried out under vacuum . So z. B. the hydrogen content of the steel can be reduced. The hydrogen would make the steel brittle ( hydrogen embrittlement ). Furthermore, the carbon can be further reduced during the vacuum treatment of the steel in order to make the steel suitable for deep drawing. Alloying agents which are sensitive to oxidation, such as e.g. B. titanium can be introduced.
In the vacuum system z. B. generated a low vacuum via steam jet (with a residual pressure of approx. 5 mbar).
Plant designs are:
- Ruhrstahl-Heraeus process (RH process): the only continuous degassing process in which a portion of the metal is conveyed by the flushing gas into a vacuum vessel and from there continuously flows back into the melting pan.
- Dortmund-Hörde process (DH process): With a DH system, the vessel is also immersed in the melt. The vessel is then cyclically raised and lowered and treated under vacuum, which is why it is called the vacuum lifting process. The pan is rinsed intensively with argon to keep the melt moving. This is also a partial degassing system.
- Ladle degassing ( vacuum degassing (VD) or vacuum oxygen decarburization (VOD)) and related processes: The entire melting ladle is exposed to a vacuum in a hermetically sealed vessel and flushed with inert gas via one or more purging stones. The first discontinuous degassing process capable of industrialization was introduced at the Bochumer Verein in the mid-1950s . The improved RH process followed in the late 1950s.
- Pouring stream process: The melt flows from a container through a small opening into an evacuated vessel. The gases can easily emerge from the thin pouring stream, which is why this process is very effective, but also very complex.
- Vacuum arc furnace (VLBO, Eng .: Vacuum arc remelting , VAR): The alloy metals are in an evacuated arc furnace melted down.
- Wolfgang Bergmann: Material production - material processing - material application (= material technology . No. 2 ). 4th edition. Hanser Verlag, Munich 2009, ISBN 978-3-446-41711-3 , chap. 1.2 Iron and Steel, p. 43 f . ( limited preview in Google Book search).
- Patent DE1116870 : Method and device for casting in vacuum. Published June 22, 1955 .