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Steel DegassingStronger, higher quality steel results when the molten steel is treated under vacuum

Steel DegassingVacuum degassing of steel takes place after the molten steel has left the furnace and before the steel is poured into ingots or processed through a caster. The main objectives of steel degassing are:

• Reduction/elimination of dissolved gases, especially hydrogen and nitrogen

• Reduction of dissolved carbon for more ductile steel

• Preferential oxidation of dissolved carbon over chromium when refining stainless steel grades

After leaving the furnace, molten steel is moved in a ladle to the degassing area and positioned inside the degasser. The ladle is covered with a layer of slag that is penetrated approximately 18” deep by the snorkels. As the snorkels are inserted, the Nash steam ejectors create a vacuum of 0.5 mm HgA in the vacuum chamber to draw the steel into the chamber. The lower partial pressure within the vacuum chamber removes both hydrogen and nitrogen gases from the liquid steel, which are both vented as the steel is continuously circulated. The evacuation time is usually five minutes or less.

The steel degassing process requires:

rapid evacuation of the vacuum tank maintenance of vacuum while at the same time sucking out a heavy flow of inert gas immediate availability

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Hybrid System for Steel Degassing   - another ejector job for Nash

Gardner Denver Nash is busy creating an ejector/liquid ring pump hybrid system, destined for a Twin Tank Car Ladle Degasser, for a major U.S. steel manufacturer. Nash has many systems installed for steel degassing, both all ejector and hybrid. The main reason for our success is our technical expertise, total system responsibility, customer support and superior products. The systems are designed to meet end point capacity and vacuum level, evacuation time and multiple other design points based on the steel that will be produced.

Years ago, steel making involved the direct transfer of liquid steel, via a ladle from a basic steel making vessel or an Electric Arc Furnace, into ingots, producing a crude composition of steel.

In the 1950’s, attempts were made to reduce dissolved gases, like carbon, oxygen and hydrogen, by vacuum degassing in order to produce ultra low carbon steel and prevent hydrogen induced defects.

Vacuum degassing became a standard part of the secondary steel making process; taking place after the ladle has left the furnace and before the steel is poured into the ingots. There are two main types of degassers: recirculating and non-recirculating. The first type, RH Degassers (invented by Heraeus-Rheinstahl in Germany), involves inserting two legs, or snorkels, of a vacuum chamber into a ladle of liquid steel. The metal is drawn into the chamber via one snorkel which also injects Argon to promote turbulence; it is then exposed to the vacuum to remove gases and recirculated back through the second snorkel. The other system, a tank degasser, is a vessel into which the ladle is set and stirred by the injection of argon. The chamber is depressurized to remove gases and, finally, the ladle is removed. Both operations are batch processes.

The multi-stage ejector system can have as many as five stages, or four stages with a liquid ring vacuum pump replacing the fifth stage (as shown here). A hogger is used to meet evacuation time requirements.

This system is only one possible solution.

Gardner Denver Nash has sold vacuum systems, which have been operating successfully for decades, for all types of degassers. Nash-designed systems meet the evacuation requirements to maintain the production rate as well as meets the capacities at different pressures to maintain product quality.

Typical systems sold in this application are four or five stage all ejector systems or hybrid systems. System design and cooling water temperatures determine the number of stages required to meet customer’s requirements.

The four stage hybrid system we are currently working on saves 13,500 PPH of steam and uses 540 BHP. Using a steam cost of $7.5/ 1,000 lbs power at $ 0.065/ KWH, operating savings using a hybrid system is over $500,000. The payback time for this hybrid system is less than 7 months! 99% of the hybrid systems for steel degassing application will give a payback of less than 1 year because of the large volume of NCG gas that has to be handled.

The customer’s decision to go with hybrid system, in spite of higher initial cost, was due to:

Steam savings that reduced the boiler size Performance of GDN systems supplied Stability of a hybrid System

Steam jets work on a constant mass flow basis, while liquid ring vacuum pumps work on a constant volume basis.  Nash can help you determine the optimal economic break-even point of a system that uses both. taking advantage of the best characteristics of each. 

Experience with GDN vacuum pumps

For more information on steel degassing, click here

  The vacuum circulation or RH process is widely used to produce ultra-low carbon steels. Besides decarburisation, it also comprises hydrogen and nitrogen removal under vacuum. The course of the decarburisation treatment can be observed, in principle, on the basis of off-gas measurement data, while this is not possible for the removal of nitrogen and hydrogen. 

This has led to the development of a dynamic process model based on thermodynamics and reaction kinetics. It includes the calculation of the C, O, H, and N contents and of the steel temperature. With the aid of the model it is possible to simulate and analyse the process behaviour as a means of optimising the design and operation of RH degassers. 

The model is utilised for the on-line observation of the

 

current process status and for predicting the course of the treatment. This prediction serves as the basis for calculating set-points for the supply of oxygen via a lance or tuyeres for the purpose of forced decarburisation and chemical heating. 

Accuracy of the process model(standard deviation of the modelling error) 

Carbon contentOxygen content prior todeoxidationNitrogen contentHydrogen content

6 ppm

30 ppm

8 ppm0.2pp

m

Temperature

5 K

Structure of the process model and required input variables

 

Example: Observation of a decarburisation treatment

Within a dynamic control concept the model is used for on-line calculation of the current process status and for prediction of the evolution of decarburisation and temperature behaviour. From a comparison with the target values from the continuous casting plant it is decided if an additional oxygen supply for forced decarburisation or for chemical heating is required, or if carbon for pre-deoxidation or cooling scrap have to be added. By this process control system the accuracy in meeting the aim temperature can be improved while reducing on the average the consumption of oxygen and aluminium for deoxidation.

    

 Structure of a model-based control concept for a RH plant with oxygen lance

   On-line applications of the process model

                                                                       

LTV Steel Indiana Harbor, USA                                                                   Bethlehem Steel, USA LTV Steel Cleveland, USA voestalpine Linz, Austria (RH/1 with oxygen lance, RH/2)

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The degasser operation imposes severe stress into the refractory lining system owing to rapid changes in temperature.

 In the recirculatory degassing operation, liquid steel is forced from the ladle

into an evacuated refractory chamber by atmospheric pressure. The low pressure in the chamber then allows the entrapped gases to expand and rise to the surface, resulting in the denser degassed steel returning to the ladle.

The action of the degassing process also results in high turbulence within the steel volume giving rise to homogenisation. Gases from the chamber are

removed through off takes and coolers.  

RH AND DH DEGASSING VESSELS  

Vacuum degassing is carried out in two distinct recirculatory units, the DH -Dortmund-Hoerde utilising a single snorkel leg whereby steel is drawn into the chamber and after

degassing leaves through the same leg. RH - Ruhrstaal - Heraeus using an upleg snorkel leg through which steel is drawn into the chamber and the denser degassed

steel leaves through the down leg.  

The DH degassing system is typically used in the production of high alloy and speciality steels from low tonnage electric arc furnace shops, whereas the RH

degassing system is associated with high tonnage BOS shops producing low-carbon aluminium-killed steel.

 RH degassing is generally preferred owing to the metallurgical advantage of

downstream refining processes to produce large tonnage's of high quality lower cost continuously cast steel.

 Development of the RH degassing system has led to

 RH-OB - oxygen blown to produce low carbon (<0.015%) steel.

 RH-PB - powder injection of synthetic slag desulphurisers to remove or modify

sulphur inclusions.  

RH-OB - aluminium heating by the addition of aluminium metal.  

 REFRACTORY WEAR PROCESSES

Refractory selection for the lining of vacuum degassers is invariably determined by consideration of the various features of the process.

 TEMPERATURE

Temperatures in the degassing vessel vary from 1480 to 1760oC, with the temperature often being sustained at 1480oC between heats, and increased up to 1760oC during

treatment.

 EROSION

The action of molten steel and slag entering the vacuum chamber places a highly erosive action upon the refractory lining.

 ABRASION

Abrasive forces exists in the gas off takes owing to the action of fine steel particles entrapped in the exiting gases.

 THERMAL CYCLING

Owing to the intermittent nature of the process there is inevitably temperature cycling in the vessel, this coupled with invasion into brick matrices leads to disruptive spalling

of the refractory lining.  

REQUIREMENTS OF THE REFRACTORY LINING The performance of the working lining is totally governed by the presence of basic

slags and iron oxide, demanding a basic refractory lining. The greatest wear occurs in the snorkel legs and bottom of the chamber.

Refractory requirements are high strength, good slag resistance, and high thermal shock resistance.

 SNORKEL LEG

Materials based upon direct bonded sintered and fused co-clinker have been shown to give optimum performance.

Snorkel leg materials are supplied with all mating faces diamond ground to tight tolerance to allow construction without the use of mortar joints.

 SNORKEL LEG OUTER LINING

Many plants provide the snorkel leg with an outer cast using a high quality refractory concrete. Dyson Refractories prefer to enhance the refractory concrete with metal fibres to increases the resistance to material loss by Thermo-mechanical damage.

 WORKING BOTTOM AND LOWER SIDEWALL

The working bottom of the degasser is normally constructed using a soldier course design, with materials based upon fused magnesia chrome clinker. In this area of the

vessel, high demand is placed upon the materials ability to resist both erosion and slag attack.

 ALLOY CHUTE

The alloy chute demands refractory materials with high resistance to thermal shock and abrasion, and here again fused grain magnesia chrome clinker are preferred.

 UPPER VESSEL WORKING LINING

The upper vessel working is less prone to attack by either erosion or slag, but places special demands upon the refractory material. The upper working lining is primarily

affected by temperature variation, and to a lesser extent by metal and or slag contact. As such a refractory lining offering high resistance to thermal shock is required.

 SAFETY LININGS

All areas of the vessel require a high quality economical safety lining capable of resisting metal at high temperature.

Materials based upon Andalusia have proven to be the most sensible choice   

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Melting/Forming/Joining: Vacuum Degassing for the Steel Industry by Kyle Shoop November 15, 2006

ARTICLE TOOLS

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Enlarge this picture

Fig. 1. Cut-away view of a ladle degasser

Vacuum degassing, which involves exposing the liquid steel to a low absolute vacuum, serves several purposes from reduction of dissolved gases to carbon removal from high-chrome stainless-steel melts. Several different types of vacuum degassers are available. In addition to reviewing the primary objectives of vacuum degassing, the various types of vacuum degassers will also be reviewed.

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

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Industrial Heating

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 Gases

Enlarge this picture

Fig. 2. Solubility of hydrogen and nitrogen in iron at 1-atmosphere

The 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 Removal

Enlarge this picture

Fig. 3. Solubility of hydrogen and nitrogen in iron at 1600˚C (2912˚F)

Vacuum 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 Oxidation

Enlarge this picture

Fig. 4. Relationship of carbon and oxygen at

1600˚C (2912˚F)

Another 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 Degassing

There 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 Degassers

Enlarge this picture

Fig. 5. Effect of temperature and pressure on carbon-chromium equilibrium

The 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 Degassers

The 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 Degassers

Enlarge this picture

The 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 System

Enlarge this picture

Fig. 6. Plan view of a twin-tank vacuum degasser arrangement

Regardless 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.

References

1. 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

Ladle refiningDr. Dmitri Kopeliovich

Vacuum ladle degassing o Recirculation Degassing (RH)o Recirculation Degassing with oxygen top lance (RH-OB)o Ladle Degassing (VD, Tank Degassing)o Vacuum Oxygen Decarburization (VOD)

Ladle Furnace (LF) Ladle desulfurization by injection of active agents

o Powder injectiono Cored wire injection

Ladle-to-mold degassing

Vacuum ladle degassing

Methods of vacuum ladle degassing utilize the reaction of deoxidation by carbon dissolved in steel according to the equation:

[C] + [O] = {CO}

where:[C] and [O] - carbon and oxygen dissolved in liquid steel; {CO} - gaseous carbon monoxide.

Vacuum treatment of molten steel decreases the partial pressure of CO, which results in shifting equilibrium of the reaction of carbon oxidation. Bubbles of carbon monoxide form in the liquid steel, float up and then they are removed by the vacuum system.

In addition to deoxidation vacuum treatment helps to remove Hydrogen dissolved in liquid steel.

Hydrogen diffuses into the CO bubbles and the gas is then evacuated by the vacuum pump.

Movement of the molten steel caused by CO bubbles also results in refining the steel from non-metallic inclusions, which agglomerate, float up and are absorbed by the slag. CO bubbles also favor the process of floating and removal of nitride inclusions and gaseous Nitrogen.

Steels refined in vacuum are characterized by homogeneous structure, low content of non-metallic inclusions and low gas porosity.

Vacuum degassing methods are used for manufacturing large steel ingots, rails, ball bearings and other high quality steels.

Vacuum ladle degassing methods:

Recirculation Degassing (RH)

Recirculation degassing unit uses a vacuum chamber having two snorkels connected to the chamber bottom. One of the snorkels is equipped with pipes supplying Argon through its refractory lining.

The snorkels of the vacuum chamber are immersed into the ladle with molten steel. Liquid metal fills the chamber to a level determined by the atmospheric pressure (4.2ft/1.3m). Argon bubbles floating up in one of the snorkels (up-leg) force the melt to rise in the snorkel. Through the

second snorkel (down-leg) the molten steel flows down back to the ladle producing circulation. The circulation rate may reach 150-200 t/min.The recirculation degassing vacuum chambers are usually equipped with addition hoppers, through which alloying elements or/and desulfurization slag may be added.

Benefits of Recirculation Degassing (RH):

-Hydrogen removal (degassing);-Oxygen removal (deoxidation);-Carbon removal (decarburization);-Sulfur removal (desulfurization);-Precise alloying;-Non-metallic inclusions removal;-Temperature and chemical homogenizing.

Recirculation Degassing with oxygen top lance (RH-OB)

In this method a conventional Recirculation degassing (RH) vessel (chamber) is equipped with a vertical water cooled lance for blowing oxygen on the molten steel surface.Oxygen intensifies the reaction [C] + [O] = {CO} resulting in fast and effective decarburization. Oxygen also oxidizes phosphorus like in Basic Oxygen Process (BOP) or in oxidizing slag stage in Electric-arc furnace.

Oxidation reactions have also heating effect therefore the treated metal may be heated to a required temperature without any additional energy source.

Benefits of Recirculation degassing with oxygen top lance (RH-OB):

-Hydrogen removal (degassing);-Fast carbon removal (decarburization);-Phosphorus removal (dephosporization);-Sulfur removal (desulfurization);-Reheating; -Precise alloying;-Non-metallic inclusions rem

Steel-making processes

Abstract: Steel is made by the Bessemer, Siemens Open Hearth, basic oxygen furnace, electric arc, electric high-frequency and crucible processes. In both the Acid Bessemer and Basic Bessemer (or Thomas) processes molten pig iron is refined by blowing air through it in an egg-shaped vessel, known as a converter, of 15-25 tonnes capacity. In the Siemens process, both acid and basic, the necessary heat for melting and working the charge is supplied by oil or gas.

(Both the gas and air are preheated by regenerators, two on each side of the furnace, alternatively heated by the waste gases. The regenerators are chambers filled with checker brickwork, brick and space alternating. The high nitrogen content of Bessemer steel is a disadvantage for certain cold forming applications and continental works have, in recent years, developed modified processes in which oxygen replaces air.

Steel is made by the Bessemer, Siemens Open Hearth, basic oxygen furnace, electric arc, electric high-frequency and crucible processes.

Crucible and high-frequency methods

The Huntsman crucible process has been superseded by the high frequency induction furnace in which the heat is generated in the metal itself by eddy currents induced by a magnetic field set up by an alternating current, which passes round water-cooled coils surrounding the crucible. The eddy currents increase with the square of the frequency, and an input current which alternates from 500 to 2000 hertz is necessary. As the frequency increases, the eddy currents tend to travel nearer and nearer the surface of a charge (i.e. shallow penetration). The heat developed in the charge depends on the cross-sectional area which carries current, and large furnaces use frequencies low enough to get adequate current penetration.

Automatic circulation of the melt in a vertical direction, due to eddy currents, promotes uniformity of analysis. Contamination by furnace gases is obviated and charges from 1 to 5 tonnes can be melted with resultant economy. Consequently, these electric furnaces are being used to produce high quality steels, such as ball bearing, stainless, magnet, die and tool steels.

Figure 1. Furnaces used for making pig iron and steels. RH side of open hearth furnace shows use of oil instead of gas

Acid and basic steels

The remaining methods for making steel do so by removing impurities from pig iron or a mixture of pig iron and steel scrap. The impurities removed, however, depend on whether an acid (siliceous) or basic (limey) slag is used. An acid slag necessitates the use of an acid furnace lining (silica); a basic slag, a basic lining of magnesite or dolomite, with line in the charge. With an acid slag silicon, manganese and carbon only are removed by oxidation, consequently the raw material must not contain phosphorus and sulphur in amounts exceeding those permissible in the finished steel.

In the basic processes, silicon, manganese, carbon, phosphorus and sulphur can be removed from the charge, but normally the raw material contains low silicon and high phosphorus contents. To remove the phosphorus the bath of metal must be oxidised to a greater extent than in the corresponding acid process, and the final quality of the steel depends very largely on the degree of this oxidation, before deoxidisers-ferro-manganese, ferro-silicon, aluminium-remove the soluble iron oxide and form other insoluble oxides, which produce non-metallic inclusions if they are not removed from the melt:

2Al + 3FeO (soluble) 3Fe + Al2O3 (solid)

In the acid processes, deoxidation can take place in the furnaces, leaving a reasonable time for the inclusions to rise into the slag and so be removed before casting. Whereas in the basic furnaces, deoxidation is rarely carried out in the presence of the slag, otherwise phosphorus would return to the metal. Deoxidation of the metal frequently takes place in the ladle, leaving only a short time for the deoxidation products to be removed. For these reasons acid steel is considered better than basic for certain purposes, such as large forging ingots and ball bearing steel. The introduction of vacuum degassing hastened the decline of the acid processes.

Bessemer steel

In both the Acid Bessemer and Basic Bessemer (or Thomas) processes molten pig iron is refined by blowing air through it in an egg-shaped vessel, known as a converter, of 15-25 tonnes capacity (Fig. 1). The oxidation of the impurities raises the charge to a suitable temperature; which is therefore dependent on the composition of the raw material for its heat: 2% silicon in the acid and 1,5-2% phosphorus in the basic process is normally necessary to supply the heat. The "blowing" of the charge, which causes an intense flame at the mouth of the converter, takes about 25 minutes and such a short interval makes exact control of the process a little difficult.

The Acid Bessemer suffered a decline in favour of the Acid Open Hearth steel process, mainly due to economic factors which in turn has been ousted by the basic electric arc furnace coupled with vacuum degassing.

The Basic Bessemer process is used a great deal on the Continent for making, from a very suitable pig iron, a cheap class of steel, e.g. ship plates, structural sections. For making steel castings a modification known as a Tropenas converter is used, in which the air impinges on the surface of the metal from side tuyeres instead of from the bottom. The raw material is usually melted in a cupola and weighed amounts charged into the converter.

Open-hearth processes

In the Siemens process, both acid and basic, the necessary heat for melting and working the charge is supplied by oil or gas. But the gas and air are preheated by regenerators, two on each side of the furnace, alternatively heated by the waste gases. The regenerators are chambers filled with checker brickwork, brick and space alternating.

The furnaces have a saucer-like hearth, with a capacity which varies from 600 tonnes for fixed, to 200 tonnes for tilting furnaces (Fig. 1). The raw materials consist essentially of pig iron (cold or molten) and scrap, together with lime in the basic process. To promote the oxidation of the impurities iron ore is charged into the melt although increasing use is being made of oxygen lancing. The time for working a charge varies from about 6 to 14 hours, and control is therefore much easier than in the case of the Bessemer process.

The Basic Open Hearth process was used for the bulk of the cheaper grades of steel, but there is a growing tendency to replace the OH furnace by large arc furnaces using a single slag process especially for melting scrap and coupled with vacuum degassing in some cases.

Electric arc process

The heat required in this process is generated by electric arcs struck between carbon electrodes and the metal bath (Fig. 1). Usually, a charge of graded steel scrap is melted under an oxidising basic slag to remove the phosphorus. The impure slag is removed by tilting the furnace. A second limey slag is used to remove sulphur and to deoxidise the metal in the furnace. This results in a high degree of purification and high quality steel can be made, so long as gas absorption due to excessively high temperatures is avoided. This process is used extensively for making highly alloyed steel such as stainless, heat-resisting and high-speed steels.

Oxygen lancing is often used for removing carbon in the presence of chromium and enables scrap stainless steel to be used. The nitrogen content of steels made by the Bessemer and electric arc processes is about 0,01-0,25% compared with about 0,002-0,008% in open hearth steels.

Oxygen processes

The high nitrogen content of Bessemer steel is a disadvantage for certain cold forming applications and continental works have, in recent years, developed modified processes in which oxygen replaces air. In Austria the LID process (Linz-Donawitz) converts low phosphorus pig iron into steel by top blowing with an oxygen lance using a basic lined vessel (Fig. 2b). To avoid excessive heat scrap or ore is added. High quality steel is produced with low hydrogen and nitrogen (0,002%). A further modification of the process is to add lime powder to the oxygen jet (OLP process) when higher phosphorus pig is used.

Figure 2.

The Kaldo (Swedish) process uses top blowing with oxygen together with a basic lined rotating (30 rev/min) furnace to get efficient mixing (Fig. 2a). The use of oxygen allows the simultaneous removal of carbon and phosphorus from the (P, 1,85%) pig iron. Lime and ore are added. The German Rotor process uses a rotary furnace with two oxygen nozzles, one in the metal and one above it (Fig. 2c). The use of oxygen with steam (to reduce the temperature) in the traditional basic Bessemer process is also now widely used to produce low nitrogen steel. These new techniques produce steel with low percentages of N, S, P, which are quite competitive with open hearth quality.

Other processes which are developing are the Fuel-oxygen-scrap, FOS process, and spray steelmaking which consists in pouring iron through a ring, the periphery of which is provided with jets through which oxygen and fluxes are blown in such a way as to "atomise" the iron, the large surface to mass ratio provided in this way giving extremely rapid chemical refining and conversion to steel.

Vacuum degassing is also gaining ground for special alloys. Some 14 processes can be grouped as stream, ladle, mould and circulation (e.g. DH and RH) degassing methods, Fig. 3. The vacuum largely removes hydrogen, atmospheric and volatile impurities (Sn, Cu, Pb, Sb), reduces metal oxides by the C – O reaction and eliminates the oxides from normal deoxidisers and allows control of alloy composition to close limits. The clean metal produced is of a consistent high quality, with good properties in the transverse direction of rolled products. Bearing steels have greatly improved fatigue life and stainless steels can be made to lower carbon contents.

Figure 3. Methods of degassing molten steel

Vacuum melting and ESR. The aircraft designer has continually called for new alloy steels of greater uniformity and reproducibility of properties with lower oxygen and sulphur contents. Complex alloy steels have a greater tendency to macro-segregation, and considerable difficulty exists in minimising the non-metallic inclusions and in accurately controlling the analysis of reactive elements such as Ti, Al, B. This problem led to the use of three processes of melting.

(a) Vacuum induction melting within a tank for producing super alloys (Ni and Co base), in some cases for further remelting for investment casting. Pure materials are used and volatile tramp elements can be removed. (b) Consumable electrode vacuum arc re-melting process (Fig. 4) originally used for titanium, was found to eliminate hydrogen, the A and V segregates and also the large silicate inclusions. This is due to the mode of solidification. The moving parts in aircraft engines are made by this process, due to the need for high strength cleanness, uniformity of properties, toughness and freedom from hydrogen and tramp elements. (c) Electroslag refining (ESR) This process, which is a larger form of the original welding process, re-melts a preformed electrode of alloy into a water-cooled crucible, utilising the electrical resistance heating in a molten slag pool for the heat source (Fig. 5). The layer of slag around the ingot maintains vertical unidirectional freezing from the base. Tramp elements are not removed and lead may be picked up from the slag.

Figure 4. Typical vacuum arc remelting furnace

Figure 5.Electroslag remelting furnace

DEGASSER® is built around two major components. A degas chamber (vacuum

container) enclosing a degas membrane module that is fabricated with gas/liquid

separation membrane such as hollow fiber non-porous membrane. A vacuum control

device controls a vacuum outside or inside of the membrane module.

For the sake of discussion here, we use a most fundamental schematic as below though a

way of membrane fabrication and a way of vacuum application are varied by purposes

and ambient environment differences.

 

In this case, a liquid is pumped (suctioned) through a hollow fiber membrane module and

is expelled out of outlet port. Though the hollow fiber membrane is non-porous so called,

small and highly mobile gaseous compounds (molecules) infuse into and permeate

through the membrane wall.

Dissolved gaseous compounds near by membrane surface infuse and permeate the

membrane wall and diffuse out it to the exterior where regulated vacuum is applied. At

first, those dissolved gaseous compounds (molecules) migrate to the region near by

membrane surface by mainly diffusing action that is explained by the Fick's Law.

And, Henry's Law, which describes an equilibrium action between different solute

concentrations, governs a mechanism of infusion into the membrane wall. This same

principle keeps fueling the migration of gaseous compounds inside of wall and to the

outside of membrane.

The migration speed differs at inlet port and outlet port because concentrations of

dissolved gaseous compounds are different there. Within the flow path, diffusion speed is

influenced by rheological state and linear velocity. Also, the migration speed inside of

membrane wall differs by characteristics, molecular structure and a crystal structure of

membrane.

Furthermore, degassing efficiency is influenced by an existence and state of compounds

inside of membrane wall that may affect a migration of gaseous compounds.

Although Fick's Law and Henry's Law could explain the basis of migration theory, many

factors influence each other. Thus the mechanism of degassing is fairly complex and

cannot be described like a rather simple model of gas/gas separation or equations.

ERC optimizes a degassing mechanism based on those principles and many years'

experiments and keeps its uncompromising effort to improve the efficiency further.