The strict requirements for modern steels make it necessary for molten crude steel to undergo subsequent treatment. This applies to both the blast furnace-to-converter and electric arc furnace methods. In the steel life cycle, the secondary metallurgy stage takes place between the converter or electric arc furnace and the casting system. In this phase, the objective is to put the molten steel through certain chemical and/or metallurgical processes to adjust its chemical composition, its purity level, and last but not least its casting temperature to the required parameters. This includes the following steps:
Homogenisation of the melt
Removal or conditioning of non-metallic inclusions
Like the range of materials involved, the metallurgical activities to be carried out at both basic oxygen and electric steelworks are also highly varied.
Secondary metallurgy treatment is trending towards improved steel quality levels with increasingly flexible metallurgical systems. For this reason, improving steelworks’ logistics has become an area of focus. As the systems and equipment in steel plants increase in number, their complexity is growing. The systems’ functionality is also on the increase.
A Primetals Technologies AOD converter (photo: Primetals Technologies)
Gas mixtures in the AOD converter
Treating the material in the argon oxygen decarburisation (AOD) converter is a stage of the process between the electric arc furnace and ladle treatment. This technology has been in use for about 60 years but only has a limited range of applications.
It is used to help make stainless steel – the input material is melted stainless steel scrap. Stainless steels contain a large proportion of alloying elements, primarily chromium and nickel, to meet the strict demands in terms of corrosion resistance, thermal stability and mechanical strength. The AOD converter makes it possible to reduce the melt’s carbon or silicon content by blowing in a mixture of argon (inert gas) and oxygen without burning or slagging other elements such as chromium.
The sequence in the converter is as follows: A common practice in the industry is to perform two or three blowing stages with different ratios of oxygen to inert gas. If the bath’s carbon content decreases, the ratio of oxygen to inert gas also drops. The point at which the planned carbon concentration changes is at the end of each intermediate stage – namely at 0.40% or 0.25% – until the final concentration (above 0.1%) is reached. The entire mixture injected by the blower reacts with the bath.
The oxidation reaction is exothermic and results in a significant temperature increase from an initial 1,400–1,500ºC to more than 1,700ºC by the end of the oxygen blowing phase. However, this high temperature can damage the AOD vessel’s refractory inner lining, which is why scrap is added for cooling purposes. This heats up and absorbs additional thermal energy during melting. As part of this process, the necessary materials are also added to attain the exact composition of chemical elements desired within the stainless steel. These materials are iron (Fe), silicon (Si), chromium (Cr), nickel (Ni), manganese (Mn), molybdenum (Mo) and copper (Cu). Lime is also added to control the viscosity of the slag.
All these practices make it difficult to build a theoretical analysis model. In the past, attempts were made to develop models to explain the reactions and processes taking place. However, their practical applications for the AOD converter are limited. The parameters the model depends on are difficult to know or estimate. In addition, there is no mathematical model that fully describes the coupled thermometallurgical/fluid dynamic processes.
The key oxidative reactions that take place in the bath are:
½ O2+ C → CO + Q (1)
Si + O2 → Si O2 + Q (2)
2 Cr + ⅔ O2 → Cr2 O3+ Q (3)
Mn + ½ O2 → Mn O2 + Q (4)
Oxygen’s chemical affinity to silicon is highest, followed by carbon. As a result, the aim is to oxidise all the silicon at the beginning of the first blowing process. In this case, the resulting silicic acid and other oxides with a lower specific weight will blend into the slag.
Current efforts to develop AOD converters for plant engineering are aimed at creating damping systems that can reduce vibrations during the blowing process as well as individual control of the bottom tuyeres (blowing nozzles) to maximise their lifespan. In addition, developers are focusing on suspended converter systems for extra-fast vessel changes and sub-lances for temperature measurement and sampling.
A ladle furnace at ThyssenKrupp Steel (picture: ThyssenKrupp Steel)
Ladle treatment: heating the melt
Stainless steel production is not the only context in which the available crude steel undergoes further treatment. In secondary metallurgy, the steel is chemically refined in a ladle vacuum treatment process. In systems used for this purpose, unwanted accompanying elements are separated out from the molten crude steel. Once special materials are added, these inclusions are bound up in the form of slag.
A ladle furnace is often used for secondary metallurgical treatment of molten steel. Its main role is to heat the melt. The use of a ladle furnace at this stage is key for delivering quality and productivity improvements in steelmaking.
Different ladle furnace designs are used in steel plants. One of the options is called a single station, which can either have a fixed or swivelling gantry; alternatively, the oven can be equipped with a ladle transfer car and a rotating turret.
If a single station does not provide sufficient capacity, double or twin stations may be considered. A twin station consists of two treatment stations with two covers, which are operated by a swivelling gantry. Designs aim to keep the systems compact overall so that their footprints remain small and their covers are easy to access. This makes it simple to replace electrodes.
A VOD/VD system (picture: Primetals Technologies)
Vacuum treatment: different system types in use
Vacuum treatment is based on the observation that when steel solidifies, dissolved gases only have limited ability to escape. As a result, the steel’s technological properties worsen. When the external pressure is lowered, the gases dissolved in the metal escape more easily from the melt. All vacuum processes rely on this reality. Cost-effective generation of very low pressure levels (0.1–0.5 mbar) made vacuum processes possible on an industrial scale.
Further metallurgical reactions are carried out in vacuum conditions, such as fine decarburisation, alloying, deoxidation and purity optimisation. The advantages of vacuum-treating molten steel are the high purity, low gas content levels and narrower alloy tolerances. During vacuum treatment, the vessels can undergo additional heating to minimise heat losses or set specific temperatures. When vacuum treatment is combined with use of a lift gas, this accelerates and supports the metallurgical reactions.
VD/VOD systems – vacuum degassing for maximum metallurgical flexibility
During vacuum treatment, the carbon, oxygen, nitrogen, hydrogen and sulphur content are reduced in different stages. A vacuum alloying additive system allows for specific compositional adjustments to the steel. Good homogenisation and high alloy yields are characteristic features of this process.
To increase capacity and productivity, the vacuum degassing (VD) system can also be designed as a twin-vessel system. The vacuum oxygen decarburisation (VOD) system is a tank degassing unit that is additionally equipped with an oxygen lance.
This additional oxygen supply can be used for the production of extra-low-carbon stainless steel grades, also known as forced decarburisation. Alternatively, it can be used to chemically heat the melt in conjunction with the addition of aluminium-silicon, also known as a vacuum degassing with oxygen blowing (VD-OB) process. The vacuum pump must be designed with a higher capacity to support this.
An RH system in Taiwan (photo: SMS Group)
RH systems – the vacuum recirculation process
The RH (Ruhrstahl Heraeus) process is carried out in a refractory-lined vessel equipped with two snorkels which are immersed in the steel bath. By reducing the system pressure, the melt rises into the vacuum vessel where decarburisation, degassing and other degassing reactions take place.
The injection of lift gas into one of the snorkels causes the steel bath to circulate. This makes it possible to treat the whole melt quickly. The snorkels are immersed either by lowering the vacuum vessel or by lifting the ladle. In principle, replacing a vessel is a challenging process. To keep the time expended short here, quick-change systems are used.
The vacuum ladle degassing (VLD) process represents the ideal solution for secondary metallurgical operations for small melt sizes ranging from 5 to 20 tonnes. As do the other system set-ups, the VLD process spans all required steps. Internationally, these systems are used in smaller steelworks, forges and foundries.
The ArcelorMittal plant in Eisenhüttenstadt, Germany (photo: ArcelorMittal)
A look behind the scenes: ArcelorMittal Eisenhüttenstadt company case study
The ArcelorMittal plant in Eisenhüttenstadt has an RH system, a ladle furnace and a vacuum system with the following performance figures:
Year of manufacture: 1995
Transformer power: 30 MVA
Energy consumption: 86 kWh/t
Heating rate: 3–4 degrees per minute
Electrode diameter: 450 mm
Electrode consumption: 09 kg/t
Ladle purgers: 2
Lift gas: Argon/nitrogen
Year of manufacture: 1984/Messo
Upgrade: 2000/SMS Mevac
Vacuum generators: Three-stage steam jet pumps and two liquid ring pumps
Vacuum pressure: Around 1 mbar
Lance system: Multi-functional lance for heating the vessel, melting off deposits, heating the melt, supporting decarburisation