AU463587B2 - Controll of vacuum decarburization of molten steel - Google Patents

Description

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This invention relates to the process control of molten

metal vacuum decarburization systems where the low pressure

(subatmospheric) and a mixture of oxidizing and/or inert

gases are used to minimize metallic oxidation. The term

"vacuum decarburization" as used herein refers to decarbu-

rization systems using subatmospheric pressure. The use of

low pressure to promote decarburization of steel and its

alloys is ;ell known (see U.S. Patent No. 3,249 issued

December 29, 1868). Additionally, U.S. Patent No. 2,040,566

suggests that alloy steel may be decarburlzed under vacuum

without the oxidation of metallics. However, in spite of

these teachings, vacuum decarburization has not become a

commercial process for the decarburlzation of steels.

It is suggested that lack of a responsive method of control-

ling the reaction has prevented commerical exploitation of

the vacuum decarburization process.

In recent years the advent of high capacity steam ejector

systems have made available economic vacuum pumping systems

suited for use in steel making areas. Full utilization

of this equipment and realization of a commercial vacuum

decarburization process awaits the development of a sound

control system which minimizes metallic oxidation in the pro-

cess. A previous invention of mine, disclosing a method

for dynamic controlling the decarburization of steel and

disclosed in U.S. Patent 3,594,155, teaches a method of

control of the decarburization process while minimizing

mntalljc oxidat'.on. My present invention hereinafter describ-

ed teaches improvements in the dynamic control of decarbu-

rization of steel wherein this decarburization is accomplished

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in a vacuum environment.

'UrIARY OF THE INVENTION~ A method of decarburizing miolten steel within a closed vessel under a vacuum, wherein thr- c:,idj.zing material is introduced into the vessel and wherein it is desired to minimize the oxidation of certain metal values contained within the steel bath according to the follow- ing operative steps: 1. Establishing a predetermined ambient pressure within said vessel. 2. Measuring the composition and flow rate of input gases to the closed vessel. 3. Measuring the composition and off gases from the closed vessel, and determining the expected rate of carbon removal from the steel. 4. Determining the expected ambient pressure within,-the closed vessel. Measuring the actual pressure within the closed vessel and ajusting the composition and flow rates of the input and off gases to maintain the actual pressure within the vessel at least as great as the expected pressure within the vessel.

DESCRIPTICN OF THE DRAWINGS The figure illustrates by block diagram, apparatus for performing the method of my Invention.

DESCRIPTION OF THE PREFERREDEMBODIMENT A basic problem in all decarburization systems is to avoid oxidation of the metallic values such as chromium and iron. There are two consequences of over oxidation of

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metallics. The first and perhaps most obvious one is

the economic loss of the metallic values which must be recovered. The second problem has to do with the overall

control of the process. During vacuum decarburization, early over oxidation of the melt leads to the formation

of scum or oxide slag which may later combine in an un-

controlled manner with carbon in the melt, leading to

uncontrolled eruptions. A close monitoring of the react- ions as they

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22.415no occur is necessary to minimize the aforementioned difficulties und this close monitoring is achieved by the method of my inven- tion. The basic objective of the invention is to control the o::idation of carbon from the melt during vacuum processes. The reaction is controlled in a manner such that excessive build up of easily reducable oxides such as iron and chromium does not occur during the decarburization process. In U.S. Patent No.

3,420,657, it is taught that oxygen or other oxidizers may be added-to a melt at a pressure calculated from thermodynamic con- siderations. It is also taught that the addition of oxygen or o-xidizers is terminated when the pressure within the vessel suddenly increases. This teaching presumes that once the thermodynamic considerations are met, the desired reactions will proceed and no kinetic considerations need be applied. I

have found that the teaching of the above patent is in error and that a continuing monitoring of the procedure must be affected and controls on the reactions impressed thereon, otherwise undes- irable okidation of certain metallics within the bath will occur. My invention which involves the monitoring of the rate of oxygen blow (or oxidizer input), the output gas composition and the pressure of the system allows more straightforward and complete control of the decarburization process and may be used to avoid oxidation of the metallics within the bath. As is known to those familiar with the art,-when oxygen, gaseous oxidizers or even solid oxidizers are added to a melt containing carbon, the oxidizers can either react with the car- bon in the melt to form gaseous products or reaction, such as carbon monoxide and carbon dioxide, or they can react with other elements within the melt, such as manganese, iron phosphorus,

silicon, chromium, titanium, etc., to form solid oxides. For

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the purpose of the description of this embodiment, it is

assumed that oxygen or oxygen and inert gas mixtures are

used as the oxidizers. Where oxygen is the basic oxidizer

used, the following reactions may occur:

02 2C 2CO

02 C CO2

02 02 (Oxygen by passes the melt) (Fe) x 02 Y (Cr) MyOx 2 (Si) (etc.)

(where M is a metallic element and MyOx are oxides such as

FeO, FeOCr20 3 Cr2 03 Si0 2 etc., which can form).

For the inert or diluent gases

A(ir) A (out)

N2 (in) 2 (out) MN

or N 2 2N

or. N N2 (Small amounts of nitrogen will either dissolve in or evolve

from the bath).

H2 in H2 out

H2 211

(11) 2H H2

(Small amounts of hydrogen will either dissolve in or evolve

from the bath).

If carbon monoxide is used as one of the input gases,

the following reactions are possible:

(12) CO (in) CO (out)

(13) yCO+xM MxOy yC

(where M is a metallic element such as silicon, manganese,

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chromium, etc.).

if carbon dioxide is used as one of the Similarly, may occur. input gases, the following reaction

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22.415/70 (14) 02 C 2CO

y CO2 y y C y 2

From the above reactions, it may be seen that if metallic

oxidation is to be avoided, reactions which produce an increase or at least do not produce an alteration in the total gas volume within the vessel are preferred. The reactions to be avoided (which are equations 4, 13 and 15) are those which cause a decrease in the total gas volume. It will be noted that these are the equations which describe the production of metallic oxides, being those of silicon, manganese, chromium iron, etc. Ideally, all of the gaseous oxygen introduced into the system will be converted into carbon monoxide. Actually some of the oxygen passes through the vessel unreacted and some of it combines to form carbon dioxide. The uncontrolled oxid- ation will allow some of the oxygen to go into the form of a solid oxide. It vd11 be noted at this point that even though some of the metallic oxides which are formed may be volatile, such as silicon monoxide, these oxides condense on the cooler parts of the vessel.. The net effect upon the total system is that all metallic oxides of concern may be considered as solid oxides. Thus, whenever a predetermined addition of oxygen (oxidizer) is made to the bath and a determination is made of the average conmosition of the off gases, it is possible to estim .e an over- all output of gases by known procedures. This estimated output of gases constitutes either a part or all of the output load that the vacuum pumping system must evacuate. Empirically, it is determined that a capacity of the pumping system increases as the pressure increases. Further, knowledge of the expected load from the decarburization reactions (the estimated output of gases)

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22,4 15 n and the load from any ballast input to the system (supplied to establish a particular desirable pressure for the reaction) enable an estimate of the pressure which should be reached within the system, to be made. This expected pressure may then be com- pared with the actual measured pressure within the system. If the observed measured pressure within the system is equal or higher than that calculated or expected, it may be concluded that metallic oxidation is not occurring. It will be evident to those skilled in the art that this conclusion is valid when there are no substantial leaks in the system. If the pressure observed or measured is lower than the calculated or expected pressure, metallic oxidation is occurring. The discussion thus far assumes that the decarburization process is to be conducted such that no metallic oxidation is desired or expectedto occur. In practice, small metallic losses can be tolerated and allow- ances may be made for these by setting minimum efficiency limits for the conversion of oxygen, carbon monoxide, carbon dioxide and oxygen. Efficiency is defined as follows: (16) S100 [OX/2 (Volume CO)+Volume CO, Conversion efficiency in pet. Volume 02 as off gas

Volume of 02 injected Whenever the actual or observed pressure is less than the expected pressure, corrective action must be taken to avoid or minimize the undesirable oxidation of those metallic values with- in the bath sought to be retained. The particular corrective action necessary depends upon the status of the system. The fact that the actual pressure is less than the expected pressure sug- gests that the system pressure is higher than that desirable to support efficient decarburization. It is necessary to decrease the total overall system pressure to one which will support

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22,415/70 effective decarburization one in which loss of metallic

values is not experienced). In those instances where both a ballast load and an oxidizer input are being used, the system pressure may be decreased by decreasing the ballast load. Re-

ducing pressure in the system accelerates the decarburization reaction and thus, may reduce the metallic oxidation reactions, thereby reducing the metallic losses. The supply of oxidizer might also be reduced (oxygen), thereby reducing the amount of oxidizer available for the decarburization reaction to a point in which the oxidizer is essentially totally consumed by that reaction, leaving none available for the oxidation of the metal- lic values sought to be minimized. Likewise, both the ballast supply and the oxidizer supply may be.reduced together to accomlish a combination of the above described effects. .Upon the observation of an actual pressure less than the expected pressure indicating metallic oxidation, I prefer to reduce the ballast load gradually, comparing the difference between new actual and expected pressures (by computing eqpected pressured for each new ballast load supplied to the system) until reaching a pressure which the system indicates effective decarburization is co-curring. This would be where the system pressure was re- duced sufficiently to a point where the actual pressure existing within the vessel equaled or exceeded the expected pressure by calculation of the thermodynamic considerations of the decarbur- ization reaction. Whenever the system pressure is greater than or equal to the expected pressure from calculation of the thermodynamic con- siderations, it indicates that the decarburization processes are progressing satisfactorily, no metallic oxidation is taking place). It is, however, possible that the decarburization

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22,415/70 system is not being utilized in its most optimum fashion. Thus,

for control of the vacuum decarburization process according to

system pressure, so long as the actual pressure within the system

is greater than or equal to the epected pressure of the calcul- ations, oxidation of the metallic values has been minimized to the

degree desired or totally eliminated.

The method of control suggested herein is a dynamic one and may be utilized Loth in an automatic control system or in a

manual manner. he accompanying figure illustrates a vacuum do- carburization system where control of the decarburization may be

affected by my method. Reference numeral 2 indicates a vessel

such as a basic oxygen furnace or a processing ladle containing

a bath of molten metal 3 and having a sealed covering 4 attached

thereto such that a vacuum may be drawn within the vessel 2. A supply tube 6 is provided for feeding an oxidizer such as oxygen into the vessel and supply tube 8 is provided for feeding a gaseous ballast load such as nitrogen or argon to the interior

of the vessel 2. Connected to the vessel 2 is a vacuum pump such as a steam ejector system 10, capable of providing a vacuum

to the vessel 2 to support efficient decarburization of the bath

3 within vessel 2. I prefer touse a double connected steam ejector system having a main line vacuum 12 and a seccndary line

vacuum line 14 connected to the vessel 2 such as at cover 4. I use the double connection to permit a throttled evacuation of the

reaction vessel 2. The steam ejector system 10 I emrloy is a multiple ejector system having a single first stage 16, a double

stage 18 and 18' coupled directly to the first stage 16 and each of which are connected to a steam condensor 20. A ballast supply 22 is provided to first condensor 20. Third stage ejectors 24, 24' and 24" are connected to condensor 20 and

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22,4!,5 no difectly to ejectors 18 and 18'. Third stage ejectors 24 and

-are individually connected to second stage condensors 26 and

26' which are in turn connected to fourth stage ejectors 28 and

28'. An additional. fourth stage ejector 28" is connected

directly to third stage ejector All three of the

fourth stage ejectors 28, 28' and 28" are connected to a final

stage condensor 30. Ballast is supplied to this system at 22

to assist the establishing of a particular desired pressure

w-ithin the vessel 2 in conjunction with oxidizer supply at 6.

The ballast supply at 8 is conventionally an emergency nitrogen

ballast which may be fed directly into the vessel. This same

nitrogen ballast system may be used for back-filling the

reaction system when the decarburization and other gasing

cycles are completed. It is possible to supply the ballast

load for regulating the ambient pressure of the system through

supply 8 though practical advantage is gained at 22. The de-

carburization apparatus would also include flow measurement

and control devices 32 inthe oxidizer supply system being capable

of measuring the flow of the oxidizer and control therefor, as well as any inert gases which may be also injected therein.

Vacuum pressure gauges will be provided as at 34 to measure

the vacuum within the vessel, as well as desirable control

points within the steam ejector system such as preceding each ejector stage. off gas analysis system is provided as is known

in the art for measuring the particular off gases and the com-

position of the off gas according to its components. Likewise.

as with the flow control and measuring devices at 32, similar

flow control and measuring devices are necessary in line 22 as

at 36 for the measure and Control of the ballast supplied through

line 22. Suitable control and measuring means are comercially

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22.41 5/70 available and known in the art. It is also preferable to have a computor available to perform the calculation

of the decarburization reaction to rapidly provide the

calculation of the expected pressure according to the

various parameters established within the system.

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Claims (4)

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   THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:- 1. In the method of vacuum decarburizing molten steel within a closed vessel wherein oxidizing material is in- troduced into the vessel and wherein it is desired to mini- mize the oxidation of certain metal values contained within said steel by establishing a predetermined ambient pres- sure within said vessel and measuring the composition and flow rate of input gases, measuring the composition of the off gases to determine the expected rate of carbon removed from said steel, the improvement comprising: deter- mining the expected pressure within the vessel according to the expected rate of carbon removal from said steel; measuring the actual pressure within said vessel and ad- justing said composition and flow rates of said gases and maintaining said actual vessel pressure at least as great as said expected pressure.
2. 2. The improvement according to claim 1 wherein maintain- ing said actual vessel pressure at least as great as said expected pressure includes reducing the quantity of oxidiz- ing material introduced into the vessel while maintaining the flow rate of input gases constant, thereby maintaining a constant ambient pressure within said vessel.
3. 3. The improvement according to claim 1 wherein maintain- ing said actual vessel pressure at least as great as said expected pressure includes decreasing the ambient pressure within said vessel accelerating the decarburization reac- tion and thereby reducing oxidation of said metallic values.
4. 4. The improvement according to claim 3 including reduc- ing the quantity of oxidizing material introduced into the vessel. 14 463.587

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   A method according to claim 1 substantially as herein-

   >.-efore described.

   DATED this 18th day off NOVEMBER, 1970

   ALLEGHENY LUDLUM INDUSTRIES, INC.

   By its Patenit Attornt--ys: CLE14ENT HACK CO Fellows Institute of Patent Attorneys of Australia

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