Vacuum degassing of liquid steel is one of many secondary-steelmaking practices that can be used by today's steelmakers. Secondary steelmaking is the processing of liquid steel after it has left the Basic Oxygen Furnace (BOF) or the Electric Arc Furnace (EAF) and before the steel is poured into ingots or processed through a continuous caster. Vacuum degassing involves exposing liquid steel to a low absolute vacuum. The actual process vacuum needed depends on the goals of the steelmaker, but absolute vacuum levels below 1 Torr are common. The primary objectives of vacuum degassing are as follows:
- Reduction of dissolved gases (hydrogen, nitrogen and oxygen) in the molten steel
- Reduction of dissolved carbon from the molten steel
- Preferential oxidation of dissolved carbon over chromium in the application of refining stainless-steel grades
Vacuum degassing also provides the following benefits:
- Homogenize the liquid-steel composition and bath temperature
- Removal of oxide-inclusion materials from the liquid steel
- Provide the means and technical conditions that are favorable for final desulfurization
- Reheating of the steel melt through the oxidation of reagent elements in solution in the steel or through arc reheating
- Provide the delivery of bulk alloys through vacuum lock or micro-alloying elements via cored wire
Reduction of Dissolved GasesThe effect of exposing liquid steel to a low-pressure atmosphere (i.e. vacuum) has several benefits to the steelmaker. The first is the reduction of dissolved gases. The reduction of hydrogen is the main objective for most vacuum degassers. Nitrogen can also be reduced during vacuum degassing. It should be noted, however, that the reduction of nitrogen is limited and not as easy as hydrogen. In addition to final vacuum level and purge gas rate, nitrogen removal is also dependent on the quantity of oxygen and sulfur in the liquid steel. These surface-active elements severely limit the nitrogen removal rate. The solubility of dissolved gases in steel decreases as the steel solidifies and cools (Fig. 2). This results in the formation of internal stresses, flakes, cracks and pinholes in the steel. The exposure of liquid steel to a vacuum can reduce the dissolved-gas content such that internal stresses are avoided. The quantity of dissolved gases in liquid steel is proportional to the square root of the partial pressure of the dissolved gas. This relationship was determined by Sievert and is expressed by the following equation
[% X] = K (Px2)1/2
where [%X] is the percent dissolved gas in the molten steel, K is the equilibrium constant and PX2 is the partial pressure of the dissolved gas expressed in terms of atmosphere. This relationship for hydrogen and nitrogen is as follows:
[Hppm] = 25.6 * (PH2)1/2 at 1600°C
[Nppm] = 456.7 * (PN2)1/2 at 1600°C
Equilibrium solubility of hydrogen and nitrogen in molten iron at various partial pressures is shown in Fig. 3.
In any vacuum-degassing system, the following conditions affect the amount of dissolved gas removal: surface area of the molten steel exposed to the vacuum; mean free path for diffusion from the steel; the amount and type of deoxidizers used before vacuum degassing; vacuum degassing pressure; exposure time at vacuum; initial dissolved gas content; use of purging gases; and dissolved gas pickup from contamination environments (refractories, slags, moisture, air) during vacuum degassing.
Carbon - Oxygen RemovalVacuum degassing can also be used to produce ultra-low-carbon steels. Exposing liquid steel to an oxidizing environment (e.g. unkilled steel, slag or oxygen injection) will reduce carbon content of the steel. Likewise, the oxygen content of the steel will also be reduced as per the following reaction:
[C] + [O] = CO(g),
[C] and [O] are the dissolved carbon and oxygen levels in the steel. The equilibrium relationship for this reaction is as follows:
[%C] * [%O] = 0.002 * PCO at 1600°C
Under proper vacuum conditions, the steel can be decarburized to levels less than 0.005%. Figure 4 shows the carbon-oxygen relationship at two partial pressures of CO in liquid steel at 1600°C (2912°F).
Preferential OxidationAnother application for vacuum degassing is the production of stainless steels. The preferential oxidation of carbon over chromium in a high-chromium melt at various temperatures and pressures is illustrated in Fig. 5. The thermodynamics of carbon reduction of stainless-steel melts indicates that a high operating temperature or a low partial pressure of CO is required if excessive amounts of chromium are not to be oxidized. The production of stainless steel at high operating temperatures results in high operating cost, excess refractory wear and low productivity. A reduced partial pressure can be used to produce low-carbon-grade stainless steels at lower operating temperatures. The reduction of CO partial pressure can be accomplished by dilution of CO by argon or exposing the liquid melt to a vacuum. The former is known as Argon Oxygen Decarburization (AOD), and the latter is Vacuum Oxygen Decarburization (VOD).
Types of Vacuum DegassingThere are three basic types of vacuum degassers (stream, recirculation and ladle). All three methods are batch-type operations. The choice of a vacuum-degassing system is determined by many factors. These include primary objective of vacuum degassing, capital investment, operating costs, temperature losses, tonnage throughput, space limitations and turnaround time.
Stream DegassersThe stream degassing process occurs as the tapping ladle is being emptied and a receiving ladle/ingot mold is being filled. The low-pressure atmosphere (i.e. vacuum) for stream degassing can occur in a vacuum chamber, or the teeming/receiving ladle can be used as the vacuum enclosure. For either system, the steel is transferred into another ladle while the pouring steel stream is broken up into myriads of droplets as it is exposed to the vacuum. The possible arrangements of stream degassing include ladle to mold (LMD), ladle to ladle (LLD) and tap degassing (TD).
Recirculation DegassersThe recirculation degassing process occurs as the liquid steel in a ladle is forced by atmospheric pressure into an evacuated chamber where it is exposed to a low absolute pressure and then returned back to the ladle. The steel is circulated through the evacuation chamber until the desired level of degassing has occurred. This is repeated for 30 to 60 cycles. The possible arrangements of recirculation degassers include Dortmond Horder (DH) - single snorkel and Ruhrstahl Heraeus (RH) - dual snorkel.
Ladle DegassersThe ladle degassing or tank degassing process occurs when a full ladle of steel is placed into a vacuum tank or a vacuum cover is placed directly onto the ladle of steel (Fig. 1). The steel is circulated to the top of the ladle and exposed to the vacuum by either gas stirring (i.e. porous plugs) or induction stirring. The possible arrangements of ladle degassing include Vacuum Tank Degasser (VTD), Vacuum Arc Degassing (VAD), Vacuum Oxygen Decarburization (VOD) and Lid Degasser (LD). The fundamental requirements for the ladle vacuum-treatment process include:
Freeboard (i.e. the distance between the slag/metal interface and the ladle rim) must be sufficient in order to contain the slag and steel boiling intensity during pump-down and vacuum treating. Too little freeboard will require a slower, more rigorously controlled pump-down and hence a longer overall treatment time at the ladle degassing station. Table 1 provides a guideline for freeboard and pump-down of ladle degassing.
Ladle Stirring: There are two methods for ladle stirring: gas stirring through the use of a porous plug and/or induction stirring. The liquid steel must be stirred at variable intensities that are appropriate for the metallurgical and process work during the degassing process. Typically for gas stirring, the flow rate is minimized during pump-down but is increased during deep degassing. This promotes the interaction of the steel with the vacuum and allows the dissolved gases in the steel to dissolve into the argon.
Temperature Loss: The temperature of the steel at the beginning of the process should be sufficient, allowing for the temperature losses during degassing, subsequent feeding of cored wire and quiet rinse stirring following exposure to vacuum.
- Refining Slag: The steel must be covered with a refining slag whose weight (hence thickness), composition and fluidity are suitable for the process objectives. For example, for typical gas removal and desulphurization to the lowest sulfur level, the slag should be fully deoxidized.
The Vacuum Tank Degasser (VTD) can be configured in many different arrangements depending on the shop layout and flow of steel through the facility. A stationary, foundation-mounted twin-tank arrangement is shown in Fig. 6. This allows one tank to be processing a heat while the second tank is available for processing the next heat. The arrangement of the facility provides for ladles to be transported to and from the VTD by the existing overhead ladle crane. At the tank degasser, ladles are supported during the vacuum-treatment process on structural members integral to the vacuum-tank assembly.
Vacuum Pumping SystemRegardless of the type of vacuum degasser used, the vacuum pumping system has to be designed to meet the process goals of the steelmaker. Parameters required for designing the vacuum pumping system include the following:
- Quantity of dissolved gases to be removed from the steel and slag, including hydrogen, nitrogen and oxygen. These gases will be removed at different rates depending on absolute pressure, steel chemistry and argon flow rate.
- The system volume, including the tank, drop-out chamber and ducting, and the process time requirement to reduce the system from atmosphere to deep vacuum degassing operation (e.g. 1 Torr).
- The final absolute pressure of the system. This is also known as the system blank-off point and determines the quantity of stages needed to reach the desired vacuum level.
- The quantity of argon required during deep degassing, as this will determine the stirring energy and the rate of removal of the dissolved gas.
- The in-leakage rate, which is the rate at which air is leaking into the system.
References1. Ahindra Ghosh,Secondary Steelmaking, Principles and Applications,CRC Press LLC., New York, NY, 2001.
2. E. T. Turkdogan,Fundamentals of Steelmaking,The University Press, Cambridge, UK, 1996.
3. M. A. Orehoski and R. D. Gray, "Ladle Refining Processes," AISE Fall Meeting, Pittsburgh, PA, 1985
4. K.J. Shoop, R.W. Arnold and K. Perala, "Start up and Commissioning of a Twin Tank Vacuum Degasser for SDI's Structural and Rail Mill", AISTech 2004, Indianapolis, IN