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RH Vacuum Degassing Technology

• satyendra
• October 7, 2013
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• RH degassing, secondary steel making, snorkel, Vacuum treatment,

RH Vacuum Degassing Technology

Today, secondary metallurgical units represent the versatile usable connection between primary steelmaking process and the continuous casting process of the liquid steel. Vacuum degassing is an important secondary steel making process. This process was originally used for hydrogen removal from the liquid steel but presently it is also being used for secondary refining and has become increasingly important process of secondary steelmaking. Lower hydrogen and nitrogen content, ultra low carbon content, ultra low sulphur content, lower total oxygen content as well as steel cleanliness are the reasons for installing vacuum treatment facilities in the steel melting shop..

In the new constructed steel plants, vacuum degassing facilities are considered and intergrated in the steel production line. There is also a trend for existing plants to install vacuum treatment facility to provide an opportunity for steel plant to extend the product mix and to be more flexible in order to respond the steel market situation.

Since 1950s, several vacuum technologies have been developed for the degassing purpose. These technologies include DH (Dormund Hoerder) degassing, RH (Ruhrstahl Heraeus) degassing, vacuum tank degassing (VTD), vacuum arc degassing (VAD), and vacuum induction melting (VIM). In the present scenario, RH degassing and VTD processes are normally used for mass production of steel in order to reduce gases and carbon contents in the liquid steel. The selection of RH degassing or VTD is strictly dictated by steel grades to be produced in the steel plant. In the majority of the cases, the installation of RH degassing is more dominant, especially for big heat size, compared to VTD due to its excellent mixing performance, and short cycle time for decarburization and degassing which results into a great number of heats treated per day. Because of the short cycle time, RH degassing process can treat a large number of heats per day. Further, due to the excellent mixing behaviour achieved during the process, this short treatment time is attainable irrespective of the size of the ladle.

The RH degassing technology was first introduced in the late 1950s in Germany where the first RH degassing plant was developed and installed. The RH degassing process has been named after Ruhrstahl and Heraeus where this process was initially developed. Since then, a lot of process improvements have been done on the RH degassing plant. These improvements include the installation of oxygen lance, the enlargement of snorkel and vessel diameters, as well as the application of powder injection for desulphurization. Comprehensive model for decarburization on RH degassing plant has been introduced by Kuwabara considering the vacuum pressure, lift gas flow rate, vessel as well snorkel diameters. It has been reported that the time needed to achieve carbon content of less than 20 ppm (parts per million) can be completed in less than 15 minutes in a RH degassing plant.

When equipped with an additional top-lance, the RH degassing is called RH-TOP degassing. RH degassing and RH-TOP degassing units (Fig 1) use the principle of the vacuum recirculation process, and are in particular applied for the production of steel grades with very low contents of carbon under economically favourable conditions. The main functions of RH degassing plant is the removal of hydrogen, natural and forced decarburization, chemical heating of the liquid steel,  and for the precise adjustment of the chemical analysis and temperature of the liquid steel. These activities are carried out under vacuum conditions. Low hydrogen content is the main prerequisite for producing high strength steel grades and grades intended for use in the oil and gas industries. The RH degassing technology allows achieving very low hydrogen contents in a short vacuum time.

Fig 1 Cut sections of RH and RH-TOP degassing units

As a tool for liquid steel secondary refining, the RH degassing process has wide applications owing to its multiple metallurgical functions, such as vacuum degassing, de-carburization, inclusion removal, de-nitrogenation, and inclusion removal. It is widely used for the production of ultra-low carbon steels, bearing steels, pipe line steels, spring steels, and silicon steels etc.

RH degassing unit typically consists of a refractory lined block-type or split-type vessel, equipped with two refractory lined snorkels at the vessel bottom, which is connected to a vacuum pump. Further components are a hydraulic or mechanical vessel or ladle lifting system, in case of an RH-TOP, a multiple function top blowing lance, and a measuring and sampling system. Material addition under vacuum is executed by means of a vacuum hopper system. Refractory repair and preheating of vessels, snorkels and top part can be executed in separate stands. Characteristics of the design are the single vessel installation (vessel lifting system), fast vessel exchange (ladle lifting system) or duplex vessel installation for increased availability.

The RH circulation degassing process has proved its vast suitability in large number of steel melting shops worldwide, for operation with short tap to tap times covering heat sizes upto 400 tons. The vacuum treatment in RH degassing plants produces steel which fulfills the demand of high steel qualities. To achieve this, the liquid steel is allowed to circulate in a vacuum chamber where a considerable drop in pressure causes it to disintegrate into the smallest of the parts. The increase in the surface area allows the liquid steel to degas to the best possible extent. The process needs reliable vacuum units which is able to suck off very large flow rates under very difficult conditions of dusty atmosphere and high temperatures.

The RH degassing process depends on sucking the liquid steel from the ladle to the vacuum chamber equipped with two snorkels (up-leg and down-leg). When the inert gas is blown to the liquid steel, then the circulation flow of liquid steel between vacuum chamber and ladle is forced. The degassing process mainly occurs in liquid internal, at splashed metals in vacuum chamber and bubble surfaces, which involves complex chemical reactions and transport phenomena. Fig 2 shows principle of the process and the lining of vessel and other parts of the process.

Fig 2 RH degassing process

Process development

When the RH process was initially introduced, the primary objective was to reduce hydrogen content in the liquid steel. The first result was not as successful as expected due to the insufficient vacuum in the vessel. The application of steam ejector vacuum pumps in the early 1960s enabled sufficiently low pressure to be reached, leading to hydrogen contents of less than 1 ppm. Since then, RH degassing process is continuously being developed with respect to vacuum condition, reaction vessel design and geometry (size and shape), cross section of snorkels, and capacities of RH degassing units.

Application of the RH degassing process for decarburization was first introduced at the end of the 1970s. Today extremely low final carbon contents of less than 20 ppm can be obtained with the use of this process, as needed for the production of automotive sheets. The addition of alloying elements during degassing has the advantages of achieving higher yields for ferro-alloys and high accuracy of chemical analysis of steel due to the absence of air and the avoidance of metal slag reactions.

Further developments were the use of gaseous oxygen during RH degassing treatment in RHO, RH-OB, RH-KTB, RH-MESID, and MFB processes. In the MFB process, the RH degassing unit is equipped with a multi function burner (MFB). MFB is a device which enables fuel and oxygen to be blown from a single lance that is inserted into the vacuum chamber. It allows heat to be retained within the vacuum chamber both during vacuum processing and while on standby. This reduces the adhesion of metals within the chamber while making it possible to produce ultra low carbon steel by means of oxygen blowing during processing. The aim of these processes were to accelerate the decarburization reaction, to reheat liquid steel by alumino-thermic reaction, to remelt skulls, to keep vessel at high temperature by converting generated carbon mono-oxide gas to carbon di-oxide gas during the decarburization period, and to heat the refractory lined vessel between the treatments. Recently, some RH-TOP lances have been used for blowing powder into the liquid steel for reducing sulphur or carbon contents to the lowest levels. Today all these processes, except RH-OB, are called RH-TOP degassing process.

Basically, the development of the RH degassing and the RH-TOP degassing processes which are important are (i) faster decarburization and degassing by improving vacuum pump, snorkel design, vessel design, improved conditions for decarburization, (ii) increased speed of ferro-alloy additions, (iii) separation of activities like alloying or wire additions from the RH treatment by installing a dedicated station for these activities, and (iv) optimized plant layouts to reduce the effect of the ladle transport time and the snorkel immersion time on the cycle time.

RH degassing plant concepts

One unique feature of the RH degassing process is the wide range of plant concepts which can be reasonably be built to suit the specific lay-out of the steel melting shop, the needed cycle time, and the meeting of the availability requirements. A series of the design criteria which are available for consideration for the RH degassing plant are given in Tab 1

The cycle time, metallurgical capability and, routinely achieved high quality production from the RH degassing unit are dependent upon (i) concept of the RH degassing plant, (ii) embedding the RH degassing unit into the process flow of the steel melting shop, (iii)  design of the RH vessel, (iv) performance of the vacuum system and other components of the RH degassing unit, (v) regular maintenance of the refractories, (vi) slag conditioning and slag metallurgy, (vii) overall stable production conditions, and (viii) the automation system. Fig 3 shows typical basic concept and the main components of a RH degassing plant.

Fig 3 Typical basic concept and the main components of a RH degassing plant

The RH degassing plant is normally equipped with the Level-2 automation system. Level-2 automation system which includes hardware, system software, and application software is realized based upon the metallurgical models. Level-2 application software and model software is designed as independently executable programmes. The applicable software supplies the model with the data from various sources and receives calculated model data. The communication between the application software and the model software is realized by means of database tables which provide the input data to the model and receive the output of the model. On the other hand, Level-2 collects all treatment data for transmittal and heat report generation. Level-2 automation is mainly operated by a single dialogue which is normally designed for accompanying process observation and providing set point data to be executed on Level-1automation. Level-2 automation needs only a small input from the operator.

RH degassing process characteristics

The process mainly consists of a refractory lined cylindrical reaction vessel with two steel pipes attached to the bottom of this vessel. The reaction vessel is lined with the fire-clay /alumina bricks in the upper portion and alumina /magnesite bricks in the lower portion. The two steel pipes are the inlet and the outlet snorkels. Both of them are completely refractory lined with alumina refractories on the inside but only the lower part is refractory coated on the outside. The inlet snorkel is equipped with a number of gas injection pipes arranged in the lower section in one or two levels and equally distributed around the circumference. The reaction vessel is designed such that the liquid steel is raised through the inlet snorkel and falls back into the steel ladle after degassing through the outlet snorkel. Top side of the reaction vessel is provided with exhaust, facilities for ferro-alloy additions along with observation and control windows.

The RH degassing unit is normally employed for vacuum treatment and decarburization of long sequences of low-carbon steel grades. The metallurgical and operational features of the RH degassing process include (i) fast decarburization down to less than 20 ppm, (ii) hydrogen and nitrogen removal, (iii) use of less expensive high carbon ferro-alloys, (iv) chemical heating of killed and un-killed heats, (v) improved steel cleanliness in terms of non-metallic inclusions, and (vi) good composition control.

The top blowing lance system is installed above the RH degassing vessel and combines several functions. Oxygen blowing rates of 2,000 N cum/hour to 4,000 N cum/hour and installed burner capacities of 2 MW to 4 MW are typical design features of the process. For process supervision, the lance can be equipped with a TV camera. In addition, the top blowing lance can be equipped with a powder blowing function in order to conduct a deep desulphurization of the liquid steel. RH-TOP degassing process functions include (i) oxygen blowing for forced decarburization and chemical heating, (ii) heating of RH vessel refractory material by gas / oxygen combustion, (iii) powder blowing for desulphurization, (iv) fast skull removal by usage of the oxygen jet, and (v) advanced ignition by external ignition facility.

There is a wide spectrum of mass steel qualities which can be produced most economically or even uniquely by the RH degassing processes. Extremely low carbon and hydrogen contents are achieved in short treatment times. There is only a minimum loss of temperature. No special slag measures, ladle free-board, or porous plugs are needed. The chemical composition can be precisely adjusted. An extended product mix, high quality products, increased productivity, and minimized ladle maintenance are further benefits.

RH vacuum degassing process normally does not reach equilibrium and the amount of hydrogen, carbon, and nitrogen removal are governed by kinetic considerations. The decarburization mechanism is fairly complex as the reaction kinetics depends both on the circulation rate and the rate of decarburization. The bath mixing has also effect on the decarburization.

Since the RH degassing process is based on the exchange of liquid steel between the steel ladle and the RH vessel, the rate of steel recirculation determines the velocity of metallurgical reactions and the duration of the process assuming a defined metallurgical target. Liquid steel circulation depends on the geometry of the equipment such as snorkel diameter, the radius of the equipment, and the position and number of lift gas tuyeres. The liquid steel density for the design assumed at 1,600 deg C is 6.94 tons per cubic meter. The atmospheric pressure exerted on the ladle surface causes the steel in the snorkels to rise to a barometric height of around 1.45 m under deep vacuum conditions. The mechanism of the vacuum treatment of liquid steel in the RH degassing process is shown in Fig 3.

Fig 3 Mechanism of the vacuum treatment of liquid steel in the RH process

Automotive and other exposed sheet as well as sheet for the electrical industry (e.g. for transformers) are typical final products produced from the liquid steel processed in RH / RH-TOP degassing units.

Operational steps

Various steps in the operation of RH degassing process are described below. Reaction vessel is first preheated to the desired temperature which normally varies in the range of 900 deg C to 1,500 deg C as per the plant requirements.

The RH degassing process starts with the movement of the steel ladle containing the liquid steel into the treatment position by a ladle car and either the reaction vessel is lowered or the ladle is lifted to the desired level so that the snorkels are submerged in the steel. The degassing process is started after both snorkels are sufficiently immersed into the liquid steel. Before snorkel immersion the injection of inert gas, normally argon, is started in the gas pipe of the inlet snorkel. Argon acts as a lifter gas to increase the liquid steel velocity which is entering into the inlet snorkel.

After achieving the required immersion depth of the snorkel, the reaction vessel is evacuated by means of a vacuum pump system which is connected to the reaction vessel through off take duct (exhaust). A vacuum (negative pressure) is created, and the liquid steel is drawn into both snorkels. Argon gas which is injected into the mix, increases the pressure in the up-leg snorkel. This pressure creates a circulation of liquid steel through the snorkels.  Now the metallurgical treatment steps, such as degassing, oxygen blowing, and adjustment of chemical analysis and temperature can be carried out. Alloy additions can be made at the end of degassing depending on the superheat of the liquid steel.

Liquid steel in the reaction vessel is degassed and flows back through the outlet snorkel into the steel ladle. The degassed steel is slightly cooler than the liquid steel in the steel ladle. Buoyancy force created by density difference (density of cooler degassed liquid steel being greater than the hot liquid steel in the ladle) stirs the bath. Rate of circulation of liquid steel in the reaction vessel controls the degassing. Circulation rate depends upon the amount of lifter argon gas and the degree of vacuum. The cycle time is normally in the range of twenty to thirty minutes. Depending on its size, a RH degassing unit has the capability to circulate 85 tons per minute to 135 tons per minute of the liquid steel.

When the chemistry of the liquid steel is determined and found to be satisfactory, the degasser snorkels are removed from the liquid steel, the argon is shut off, and nitrogen is introduced in the up-leg snorkel to keep the injection tubes from freezing over. The degassing operation is then complete, and the steel ladle is transferred to the post-treatment or take-over position.

During production, the operators are guided by a process automation system. This system uses a number of mathematical models in order to forecast metallurgical parameters and to create set-points, for example for steel temperature cyclically calculated based on different received parameters and processing time, chemical composition by determining received steel samples and added materials through the process. Forecasts and set-points are also created for status of degassing functions like hydrogen and nitrogen removal depending on initial contents, degassing time, vacuum pressure curve, lift gas rate and others, status of decarburization by determining cyclically carbon and oxygen content of steel, and set-pointing for various functions like oxygen blowing, vacuum and lift gas patterns etc.

Furthermore, the Level-2 system is connected to the production planning and the process automation of preceding and subsequent units as well as with the laboratory, in order to provide all relevant data to the operator. The data tracking is collecting all relevant data from Level-1 system and process models for the creation of different heat and production reports. All these data are stored in a database to make the system ready for future data applications.

RH degassing unit availability

Besides a short cycle time, the availability of the RH degassing unit with a view to production planning in the steel melting shop is to be considered. Most critical is the time needed for the maintenance of the snorkel. After treating a sequence of six heats, the snorkel needs intermediate maintenance (deskulling, and refractory gunning). Depending on the specific slag and treatment conditions and the tools available, the maintenance work needs 20 minutes to 60 minutes. More frequent maintenance results in an increased snorkel life time.  After 60 heats to 300 heats, depending again on the treatment conditions as well as on the quality of the refractory and the design of the snorkels, the snorkel needs replacement. Further refractory maintenance is needed, mainly in the bottom area, every 2 to 3 snorkel campaigns. In the present day RH degassing plant, the vessel is exchanged for snorkel replacement and vessel maintenance for reducing the downtime of the plant.