The typical production route for stainless and special steel grades uses AOD converters and VOD ladle treatment facilities. The vacuum converter (CONVAC) offers significantly reduced production costs resulting from reduced consumption of argon, shorter process times and hugely lower refractory costs as well as a new option to refine ferroalloys to low carbon (LC) and ultra-low carbon (ULC) grades. Our expert Tim Kleier describes the technology and typical applications.
The AOD converter (Argon Oxygen Decarburization) is the most common type of converter for the production of stainless and special steel grades. Limitations of the conventional AOD are the achievable target values for carbon, hydrogen and nitrogen due to its exclusive reliance on inert gases to reduce the partial pressure of CO (pCO). The Vacuum Oxygen Decarburization process (VOD) lacks decarburization speed due to the limited available reaction volume in ladle metallurgy.
The vacuum converter (CONVAC) process technology combines the specific strengths of AOD and VOD in a single unit, taking a significant shortcut in stainless and special steelmaking routes and adding a new option for ferroalloy refining. The vacuum converter technology enables ultra-low carbon, nitrogen and hydrogen content in the production of stainless steel within a single facility.
Especially stainless steel grades requiring low final carbon and nitrogen contents benefit from significantly reduced production costs resulting inter alia from reduced consumption of argon, shorter process times and hugely lower refractory costs in ladle metallurgy. Cost reduction add up to 10 € per ton of steel (e.g. low carbon / low nitrogen stainless steel), as well as an increase in productivity of approx. 15% can be achieved. The concept has been proven by several tests in practice as well as laboratory conditions down to carbon contents of less than 0.1% and is now under execution in a brand new special steelmaking complex in Austria.
The main feature of the vacuum converter CONVAC is the possibility of a two-stage process in one unit: The AOD type process at ambient pressure and the decarburization and degassing under vacuum enabling the refining to low and ultra-low contents of carbon and nitrogen.
This results in a single innovative facility combining the advantages of these processes:
- High decarburization rate and
- Excellent desulphurization
- Deep decarburization and
- Excellent degassing for nitrogen as well as hydrogen.
The vacuum converter technology can be used for the production of stainless steel as well as for the refining of ferroalloys. The converter system is equipped with a movable vacuum cover in addition to the dedusting hood for primary off gas collection during atmospheric process conditions as per established AOD concept. During vacuum treatment, the vacuum cover rests on a water-cooled flange welded to the converter vessel top cone below the mouth. During atmospheric treatment, the flange on the vessel is protected against slag or steel accretions in order to maintain a smooth surface.
In order to best accommodate the vacuum converter process stages, the vessel is designed for minimized slopping and high mixing energy, thereby shortening treatment times even further. In terms of process economy and metallurgical results and productivity levels achieved, the vacuum converter is considered superior to the conventional AOD converter.
The figure below depicts an exemplary arrangement for the interchangeable vacuum cover and conventional dedusting hood on a laterally traversing car. Depending on layout constraints and process parameters, different setups to accommodate the hood and cover for the two treatment stages are possible.
Optionally, the vacuum cover can be equipped with a vacuum bin system. A top lance may be used during atmospheric treatment as well as optionally during vacuum treatment.
The submerged tuyères for injection of process gases known from conventional AOD converters are individually equipped with sealings to avoid air inlet into the vessel during vacuum treatment. The number of tuyères varies depending on the size of the converter. The tuyères may be located either in the bottom or the side wall of the lower cone of the converter vessel.
With the vacuum converter process, the decarburization of stainless steel grades and ferroalloys in the converter is split into an atmospheric treatment and a vacuum treatment stage. The typical vacuum converter treatment cycle comprises of the following steps:
- Atmospheric conditions: 1. Charging of pre-metal - sample taking and temperature measurement / 2. Desiliconization and main decarburization using top lance and submerged tuyères to inject oxygen / 3. Dynamic decarburization using submerged tuyères to inject oxygen/inert gas mixtures at a flow rate adapted to the relevant process requirements
- Reduced pressure conditions using a pump system to regulate the pressure across the entire range of approximately 30 to 1,000 mbar: 1. Final decarburization using submerged tuyères only / 2. Reduction
- Atmospheric conditions: Deslagging and tapping
As a result of the combined process, the overall treatment time of stainless steel in a vacuum converter is shorter than in a conventional AOD converter.
In melt shops that did not utilize a VOD before, the installation of a new vacuum converter or the conversion of an existing AOD to a vacuum converter ensures high reproducibility of ultra-low carbon, nitrogen and hydrogen content in the production of stainless steel. The product portfolio for high-chromium steel grades can be expanded accordingly.
Compared to the conventional AOD process, the combination of reduced pressure and a small amount of injected inert gases through the submerged tuyères leads to a lower CO partial pressure with better mixing energy at the same time.
In comparison to both, AOD and VOD processes, the chemical equilibrium as well as the reaction kinetics in the vacuum converter process are enhanced.
Resulting from the higher decarburization rate, the total treatment time can be reduced by approximately 15% (Figure below), equivalently affecting the productivity.
In particular during the production of low carbon / low nitrogen stainless steel, the consumptions of argon, oxygen, nitrogen and reduction materials are reduced considerably, leading to significant production cost savings of approximately 10 € per ton of steel.
For the production of duplex grades with high nitrogen and chromium as well as low carbon contents the vacuum converter process technology is also beneficial.
The refining process for ferroalloys to produce low and ultra-low carbon grades in a vacuum converter is characterized by achieving high added value at low conversion costs. In the vacuum converter, high carbon ferrochromium can be refined to carbon levels of less than 0.1 %.
Using a vacuum converter to refine ferroalloys is an option for dedicated ferroalloy producers (as an alternative to established but expensive processes with regards to OPEX) and stainless steelmakers (alternating with regular steel production in order to deepen in-house added value).