Steel Degassing

During the steelmaking process, while the molten steel is still in the ladle and before it is poured, the steel must be degassed in order to: (1) reduce/eliminate dissolved gases, especially hydrogen and nitrogen; (2) reduce dissolved carbon (to improve ductility); and (3) to promote preferential oxidation of dissolved carbon (over chromium) when refining stainless steel grades.

In the steel smelting process, unwanted gases are dissolved in the liquid, which could produce any number of imperfections and defects. A common method used to remove these undesired gases is vacuum degassing. The process is done after the molten steel has left the furnace and before being poured into ingots or introduced into a continuous caster.

History of Steel Degassing

Throughout the early part of the 1900s, the steelmaking industry was plagued by poor quality steel This was due to the fact that during the production process, hydrogen and nitrogen gases were present in the molten steel. During cooling and subsequent solidification, the dissolved hydrogen caused pinholes and porosity in the final product since hydrogen has low solubility in steel at ambient temperature. The result was the release of hydrogen during solidification. Even the presence of a few parts per million of hydrogen gas causes defects and loss of yield strength.

The idea of using a vacuum to remove dissolved gases from molten steel was initially proposed in 1940 by Soviet scientists Dr. Alexi M. Samarin and Dr. L.M. Novik. It was first tried industrially in 1952 in the Soviet Union and in 1954 in Germany. Soon after, this technology became an essential part of the steelmaking process and is now used throughout the world.

Steam Ejectors

Steam jet ejectors use the venturi principle to create a vacuum by forcing steam through a nozzle. The nozzle provides controlled expansion of the steam, which effectively converts pressure into velocity, producing a vacuum. This draws in and entrains any gases present which enter at the bottom. The steam is mixed with the pumped gases and passes out of the ejector.

Steam ejectors are commonly used in multiple stages for vacuum degassing by piping them in series, sometimes with condensing sections between them. While a single stage ejector can provide a 40 mbar vacuum, multiple stage Ejectors can attain an ultimate pressure of 4 x 10-3 mbar. Steam ejectors have the advantage of no moving parts and correspondingly, high reliability and low maintenance. A major criterion when considering steam ejectors is the availability and cost of steam.

Water Ring Pumps

One of the vacuum pump systems used for vacuum degassing combines both steam Ejectors with liquid ring pumps and involves putting several ejectors in series, followed by the liquid ring pump. The liquid ring pump is considered a positive displacement, wet style pump.

Summary

Vacuum degassing plays a critical role in the manufacture of steel. The vacuum degassing process allows the production of high quality, low carbon, and ultra-low carbon steels and stainless steels, with minimal oxygen, hydrogen, and nitrogen contamination. The main benefits include:

  • Reducing hydrogen content to avoid embrittlement of steel.
  • Reducing oxygen content to enhance microcleanliness.
  • Improving the distribution of alloying elements and other additives.
  • Controlling the composition of the steel to tighter chemistry specifications.
  • Improving mechanical properties such as uniform transverse ductility, fatigue resistance, and high-temperature performance.
  • Achieving exceptionally low carbon content steel heats beyond those obtainable by conventional means.