US3606291A

US3606291A - Molten steel degassing apparatus and method

Info

Publication number: US3606291A
Application number: US852955*A
Authority: US
Inventors: Philip E Schneider
Original assignee: Dravo Corp
Current assignee: Dravo Corp
Priority date: 1969-05-15
Filing date: 1969-05-15
Publication date: 1971-09-20
Anticipated expiration: 1988-09-20

Abstract

THERE IS DISCLOSED A METHOD AND APPARATUS FOR THE DEGASSING OF MOLTEN METAL, PARTICULARLY STEEL, WHEREIN THERE IS A VERTICALLY ELONGATED DEGASSING CHAMBER HAVING ITS AXIS INCLINED AWAY FROM THE VERTICAL. THERE IS A HEARTH AT THE BOTTOM OF THE DEGASSING CHAMBER THAT SLOPES FROM THE HIGH SIDE TOWARD THE LOW SIDE WITH AN OUTLET PORT FOR DISCHARGING METAL AT THE LOW SIDE. THERE IS A RISER LEG THAT OPENS THROUGH THE HEARTH AT THE HIGH SIDE AND EXTENDS DOWNWARDLY FROM THE HEARTH FOR A LENGTH RANGING BETWEEN TWO AND TWO AND ONE-HALF TIMES THE LENGTH OF THE BAROMETIC COLUMN OF MOLTEN METAL THAT MAY BE DRAWN UP INTO THE LEG ABOVE THE LEVEL OF MOLTEN METAL IN THE LADLE. WHEN THE APPARATUS IS OPERATING, PROVISION IS MADE FOR INTRODUCING GAS INTO THE RISER LEG AT A LEVEL BELOW THE TOP OF THE BAROMETRIC COLUMN. THE METAL IS CARRIED BY EXPANDING GASES UP THE LENGTH OF THE TUBE ABOVE THE TOP OF THE BAROMETRIC COLUMN AS DROPLETS AND EXPEDDED INTO THE DEGASSING CHAMBER, FALLING BACK ONTO THE SLOPING HEARTH CLEAR OF THE RISER LEG. A RECEIVING VESSEL IS CONNECTED TO THE OUTLET PORT INTO WHICH ALLOYING INGREDIENTS MAY BE INTRODUCED INTO THE DEGASSED METAL.

Description

SePt- 20,' 1971 I5. E. SCHNEIDER 3,606,291

MOLTEN STEEL DEGASSING APPARATUS AND METHOD Filed May 15, 1969 5 Sheets-Sheet 1 Iz f/sooo 45 6 ATToRNEvs. l

5 Sheets-Sheet 2 MOLTEN STEEL DEGASSING APPARATUS AND METHOD 4MToRNEYs.

Sept. 20,` 197;

Filed May 15, 1969 MOLTEN STEEL. DEGAssING APPARATUS AND METHOD Filed May 15, 1969 Sept. 20, 1971 P. E. SCHNEIDER 5 Sheets-Sheet 3 lll INVENTOR.

ATTORNEYS.

Sept. 20, 1971 3,606,291

MOLTEN STEEL DEGAssTNG APPARATUS AND METHOD Filed May 15, 1969 P. E. SCHNEIDER 5 Sheets-Sheet t INVENTOR. `IIIILIF LSCHNEIDER.

BY Ml PMF? ATTORNEYS.

SePt- 20, 11971 P. E. SCHNEIDER 3,606,291

MOLTEN STEEL DEGA'SSING APPARATUS AND METHOD Filed May l5, 1969 5 Sheets-Sheet 5 Jgyls E 397215* 0: u b Q L lt Y lu -s O h. L o lr u e e u: Ln Q a I k o 3971s di a r'* u m D g x v lr E `X L E s k @KIS Q b b V) k 4 l l n l n s s a s a s s a a a h h N '/l/n03 'HIV/wd y.0 HnOH/sa/vnod. M ?SHVHO Nou Ons srl/131MB 01107 HIV NVENTOR. PHIL/P E SCHNEIDER yz. da

Alfrneys United States Patent lOce Patented Sept. 20, 1971 MOLTEN STEEL DEGASSING APPARATUS AND METHOD Philip E. Schneider, Mount Lebanon, Pa., assignor to Dravo Corporation, Pittsburgh, Pa. Continuation-impart of application Ser. No. 522,547, Jan. 24, 1966. This application May 15, 1969, Ser.

Int. Cl. C21c 7/10 U.S. Cl. 266-34V 5 Claims ABSTRACT OF THE DISCLOSURE There is disclosed a method and apparatus for the degassing of molten metal, particularly steel, wherein there is a vertically elongated degassing chamber having its axis inclined away from the vertical. There is a hearth at the bottom of the degassing chamber that slopes from the high side toward the low side with an outlet port for discharging metal at the low side. There is a riser leg that opens through the hearth at the high side and extends downwardly from the hearth for a length ranging between two and two and one-half times the length of the barometic column of molten metal that may be drawn up into the leg above the level of molten metal in the ladle. When the apparatus is operating, provision is made for introducing gas into the riser leg at a level below the top of the barometric column. The metal is carried by expanding gases up the length of the tube above the top of the barometric column as droplets and expelled into the degassing chamber, falling back onto the sloping hearth clear of the riser leg. A receiving vessel is connected to the outlet port into which alloying ingredients may be introduced into the degassed metal.

CROSSeREFERENCE TO RELATED APPLICATION This application is a continuation-in-part of my copending application Ser. No. 522,547 filed Ian. 24, 1966, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to the vacuum degassing of molten metal, particularly steel, and is for an improved apparatus for and method of continuous vacuum degassing and transferring of metal from one vessel, as for example a ladle, into a receiving element, such as a tundish, a continuous casting mold, a succession of standard ingot molds, or another ladle.

The degassing of molten steel to remove oxygen, hydrogen and some nitrogen is becoming increasingly irnportant with the demand for cleaner steel and tighter control of quality, and this is especially true with continuous casting.

Until recently removal of oxygen has been effected by the addition of aluminum and silicon to metal before it is teemed. This produced what the trade termed killed steel. Some of the aluminum and silicon combined with the oxygen, and the presence of the oxide destroyed the cleanliness of the steel. The unoxidized metal remaining is desirable as an alloying ingredient. By vacuum degassing, the silicon and aluminum are required only as alloying ingredients, and the formation of substantial oxides of these elements is avoided.

Continuous degassing is a term used in the art where the metal flows continuously through the degassing chamber, as distinguished from a discontinuous or intermittent method wherein the metal is exposed as a dormant pool in a degassing chamber and then discharged. Stream degassing is a term used for a method wherein metal falls by gravity from one receptacle through an evacuated atmosphere into another vessel or receiver. These methods are in use at the present time, but they all havev certain limitations.

In continuous casting where the metal is transferred through the degassing unit from a ladle to a mold, the alloying ingredients must be added in the ladle before degassing, with the result that the steel is killed and degassing is largely effective only for removal of hydrogen.

ln continuous degassing where the metal moves up one leg of a degassing vessel into a degassing chamber and out the other leg with a vacuum being maintained in the degassing chamber, whether the metal flows back into the ladle from whence it is removed, or transferred to another ladle, mold, or continuous casting unit, the length of the leg through which the metal is raised to the degassing chamber has been limited by the height of the column of metal that can be supported by barometric pressure acting against the reduced pressure in the degassing vessel, i.e., a leg length equal to the barometric height less the depth of metal in the degassing chamber. This imposes a limitation on the depth of the ladle that may be used in transferring metal from the ladle to a receiving element other than the ladle. It also limits the control that may be exercised in the rate of flow and retention time in degassing systems. In a once-throug degassing transfer system this lack of control and flexibility imposes limitations on the overall utility of the apparatus, especially since the liquid level of the metal in the ladle is continuously lowering in once through degassing as metal is removed from the ladle, giving rise to certain diculties not encountered in a degassing apparatus where the metal flows or recirculates back into the ladle. This disadvantage cannot be satisfactorily overcome by transferring metal from the ladle to a tun-dish or similar shallow vessel then to a degasser, because the heat loss is too great and the metal is subjected to another source of refractory contamination.

The present invention has for its object to provide a novel degassing apparatus and method designed to give greater flexibility and better control, particularly in a once-through operation where the metal is transferred from a vessel, such as a ladle, t0 another receiving device during degassing, although not necessarily restricted thereto.

A further important object of the present invention is to provide greater flexibility or better control of the degassing operation and to make more effective use of the gas contained in the metal for lifting the metal into the degassing chamber.

A further object of the invention is to provide a degasser and method capable of using certain refractories that remove some impurities from the molten metal during degassing and wherein provision is made for compensating for loss of heat in the metal during degassing.

A further object is to provide for the introduction Of alloying or addition materials into the metal inthe final stages of degassing, and thoroughly mixing these materials into the molten metal.

A further object is to provide a degassing apparatus having certain mechanical advantages.

A further object is to provide an improved method and apparatus for controlling the flow of metal through the degasser. l

These and other objects and advantages are secured by this invention as will hereinafter more fully appear.

SUMMARY OF THE INVENTION The invention embodies the use of a degassing chamber whose vertical height preferably exceeds its diameter, and from the lower end of which projects a tubular leg, the terminal of which is designed to be immersed in the ladle of steel to be degassed. This leg is substantially longer than is customarily provided, being in excess of 60 inches. Sixty inches is approximately the height to which a column of steel under a vacuum of 500 microns will be raised by barometric pressure, and it is here contemplated that the continuous phase liquid level of the metal will not be raised to the top of the tube where it opens into the degassing chamber. The expansion of the gas present in the metal with or without additional gas injected into it, in this conned shape, along with a vacuum in the chamber, projects the metal in drops, mist and globules of irregular size into the degassing chamber to a height of several `feet. This action may be controlled by selective injection of a gas into the leg at one or more locations. Preferably means are provided for selectively injecting a lifting gas into the leg below the barometric height to increase the rate of metal ow into the degassing chamber, and a second means is provided for selectively injecting what is termed a suppressor gas into the leg above the barometric height to suppress the rate of metal flow therein. The lifting and suppressor gases may be the same or diderent gases depending on several considerations later discussed. The term barometric height or the like, unless otherwise indicated, refers to the height to which a column of metal will be raised by atmospheric pressure when the column is subjected to the sub-atmospheric pressure then prevailing in the degassing chamber.

The bottom or hearth of the degassing chamber slopes to an outlet port, and the dispersed drops and globules fall to the hearth and flow as a stream out said port so that there is no pool of metal on the hearth as is customary, especially above the lifting leg, and the degree of slope and the diameter of the hearth will, to some extent, determine retention time of the metal in the degassing chamber.

While the degassed metal could flow into a discharge leg also exceeding 60 inches in length, I prefer to discharge it into a auxiliary vessel of unique construction in which a vacuum is also maintained. Desirably this vessel is arranged to supply heat to the molten metal, if necessary to secure uniform and controlled casting temperature, and for circulating the metal to better diuse alloying ingredients therethrough.

BRIEF DESCRIPTION OF TI-LE DRAWINGS FIG. l is a transverse vertical section through the apparatus preparatory to being put into use;

FIG. 2 is a similar view showing the apparatus in conjunction with a ladle;

FIG. 3 is a more or less schematic view similar to FIG. 2, but showing the relation of the degasser to the ladle when the ladle is nearly empty. Also in this view the auxiliary chamber has been removed and a second discharge leg has been substituted; and

FIG. 4 is a section showing more or less schematically another form of the auxiliary vessel, the view showing only a fragment of the degasser and being a transverse vertical section,

FIG. 5 shows the performance curve of a typical steam ejector suitable for degasser use.

In the accompanying drawings, corresponding reference numerals indicate corresponding parts.

DETAILED DESCRIPTION The degassing apparatus as shown in FIGS. l and 2 comprises a vertically-elongated structure. For use with a ZOO-ton ladle, for example, the overall length of the apparatus would be of the order of 30 feet, but this dimension may vary and is mentioned only as an illustration of the approximate size of an installation. It comprises an upper chamber portion designated generally as :2 in the form of a vertically-elongated hollow cylinder which is tilted from a vertical position. It may be circular in section, or it may be elliptical with the long axis in the plane of the view shown in FIG. l, or it may be more or less rectangular with the long axis extending in the plane of the section shown in FIG. 1. It is several feet in diameter, as for example, on the order of ve feet, or more. It is generally in the form of a metal shell that is lined with a refractory. It has a separable dome 3 and in this dome there may be an opening 4 to Which a closure is attached, as hereinafter explained. There is a bottom or hearth 5 which slopes from the high side of the chamber enclosed within the structure toward an outlet port 6 in the side wall on the low side of the tilted structure.

Extending downwardly from the high side ofthe hearth so that it is eccentric to the long axis of the structure is a refractory-lined leg 7 in which is a passage l8, the internal diameter of which will ordinarily not be less than 6 inches. This leg may be in sections bolted together as indicated in the drawing. The leg may comprise a cylindrical metal shell lined with refractory. At the lower end of the main portion of the leg there is a replaceable terminal section 9, and between the terminal 9 and the main portion of the leg there is a flange plate 10 for the selective introduction of gas from an external supply indicated by the tube 12. The end of the terminal 9 is on an angle to the longitudinal axis of the leg so that it terminates in a generally horizontal plane.

Spaced above the plate 10 there is a second anged plate 10u also having gas inlet ports 11a for the selective introduction of gas from an external supply indicated by the tube 12a. The two spaced gas inlet ports 11 and 11a in the leg 7 are located so as to be respectively below and above the surface of liquid metal normally maintained in the leg 7, which level is indicated at A-A and this normally is about 50 to 60 inches above the level of metal in the container in which the leg is immersed. Gas inlet ports 11 are preferably near the lower end of the column of liquid metal in the leg.

There is an opening in the upper wall of the chamber indicated at 13 that communicates with a duct (not shown) leading to a vacuum pump, such as a steam jet pump of the type commonly used in the steel industry in connection with degassing equipment of this general nature. The height of the chamber in the upper portion 2 from the hearth to the dome is desirably of the order of 10 feet or more while the length of the leg 8 from the inner surface of the hearth to the flange plate 10` is desirably considerably more than 60 inches and the vertical distance from the opening in the end of the terminal 9 to the level of the hearth must be in excess of 60 inches, and preferably is about 2 to 2% times the barometric height. It may be explained that the height of a column of molten steel which may be maintained in the interior of the leg by atmospheric pressure when a vacuum of .500 microns is drawn in the chamber will be about 60 inches, and it is desirable that the leg 8 be long enough so that the level of the barometric column of metal will be some distance below the hearth. The overall length of the leg 8 from the vessel hearth to its lower end must be such that when the leg is projected into the metal to the full depth of the ladle the degassing vessel will clear the top of the ladle, and for the purposes of this invention, should exceed the height of the barometric colurnn.

According to the preferred embodiment of the invention as shown in FIGS. 1 and 2, the outlet port '6 opens into the interior of an enclosed auxiliary vessel designated generally as 15. This vessel is detachably secured and sealed to the main body of the degasser. It has a discharge port 16 through the bottom and a water-cooled metal cover schematically indicated at 17. Positioned axially of the port 1-6 within the vessel which is refractorylined is a stopper rod 18 which passes through a gland in the cover 17 and is provided'with means for raising and lowering it as schematically indicated at 19. Embedded in the refractory walls of the vessel 15 is an aircooled or Water-cooled induction coil 20= which surrounds the open chamber 21 within the vessel.

The stopper rod 18 may be formed of stainless steel tubing with means for circulating water axially therethrough to cool it. This tube is designated 18a and it is surrounded by an insulating sleeve and terminal portion 18b. The top 17 of the auxiliary vessel is provided with a port 22.

Since one purpose of the apparatus is to degas metal while it is being transferred from one vessel such as a ladle to another vessel or receiving element, for example, a continuous casting mold, there is schematically indicated in FIGS. 1 and 2 the upper end of a continuous casting unit, this being indicated as A, and the discharge port 16 is centered over the continuous casting mold A.

In starting up the apparatus, it is desirable to preheat it. During preheating, heating means such as an electric heater or a uid fuel burner (either oil or gas) is inserted in the opening 4 in the dome of the degassing unit, and another heating means or burner is inserted in the opening 22 in the auxiliary vessel. These burners in FIG. l are designated 23 and 24 respectively. When the unit has been brought up to operating temperature, these burners are removed and provision is made for observing the operation within the respective vessels. In FIG. v2 a television camera has been schematically indicated at 25 over the port 4 for scanning the interior of the vacuum chamber, and an observation glass 26 is placed over the port 22.

In operation a vessel containing the molten metal to be degassed and transferred to a mold or other receiving vessel is brought to position under the degassing unit and raised to a level where the terminal 9 is immersed only a few inches into the molten metal. A vacuum of the order of about 500 microns is maintained in the main degassing chamber and the metal rises in the leg 8 approximately 60 inches due to the external atmospheric pressure. This column of metal inside the leg terminates below the level of the gas ports 11a and below the hearth as previously explained. Under these conditions the gases in the metal are released from a considerable depth in the metal and as they rise toward the surface they increase in size and in velocity due to expansion. As they leave the surface of the metal they propel droplets or globules of the molten metal with considerable velocity out the top of the leg into the expansion chamber. These droplets may range in size from practically a vapor dispersion of the metal to globules rarely as large as an inch in diameter. There are droplets of metal dispersed throughout a gaseous medium that are injected into the chamber contrary to prior art devices Where gaseous bubbles lifted a metal stream into a chamber. They are propelled upwardly into the vacuum chamber varying distances, averaging perhaps 4 orv5 feet above the hearth. Preferably they are not lifted to the height of the exhaust port 13. Afterv reaching their maximum altitude they descend to the hearth where they collect and ow through the discharge port. 6. Because of the inclination of the degasser from a vertical position, the metal injected in this Way from the leg into the vacuum chamber does not fall back into the leg 8 and its trajectory is away from the succeeding drops or globules that are spraying continuously from the upper end of the leg.

To increase the violence and rapidity with which the metal is expelled from the leg into the vacuum chamber, gas is introduced-from the supply pipe 12 into the flange 10, discharging into the metal near the lower end of the column. Argon gas being inert and not readily absorbed into the molten metal, is preferably used for this purpose, but other gases as known inthe industry may be used.

For example if 150 liters per minute of gas are used in the bubble pump, i.e., injected through the flanged grate 10 as described, it is the equivalent to .0145 poundmol of argon per minute. The .05% carbon steel could lose 500 p.p.m. of O2 in the form of CO at a working pressure of 500 microns in the degassing vessel. This is equivalent to about .100 pound-mol per minute of CO, if the steel is flowing through the degasser at a rate of labout tons per minute. The gas in the steel in this example supplies ten times as much expansion energy as the lifter gas.

It is known that finely-divided particles of steel need reside for 0.25 to 7.0 seconds at the working pressure in the vacuum chamber to reach a practical equilibrium of oxygen content for a given carbon content and working pressure. The precise residence time depends upon the type of steel to be degassed. In the described apparatus, the path of the droplet is up into the vessel and down onto the hearth or lower side wall, and finally into the receiving vessel. The average height and trajectory of the droplet to achieve the proper residence time is determined in part by the slope and length of the vessel and hearth area of the degassing vessel and by the crosssectional area of the auxiliary chamber. Therefore ample exposure of the metal to the vacuum can be obtained, even though the retention time of the metal on the hearth is short as compared to those methods where a pool of metal is retained in the degassing vessels.

The lifting gas injected through port 10 below the barometric height may only be necessary if the metal is killed or semikilled or like cases in which the gas in the steel possess insufficient energy to lift the -metal the remaining distance into the degasser. In some cases the lifting gas may only be necessary to initially start the atomization and flow of metal globules or particles into the degasser. Because of the expansion of the lifting gas and the entrapped gases in the steel as the metal rises in the leg 7, the flow of metal into the chamber may tend to accelerate. To control this acceleration a second gas inlet port 11a is provided at a level above the barometric height and preferably below the juncture of leg 7 with the hearth 5, for injection of a flow suppressor gas into the rising column of gas and steel particles. The gas injected may be argon or the like, and acts to create a pressure in the leg 7 against which the expanding gases rising through the leg must operate, whereby expansion and therefore acceleration of the lifting and entrapped gases is suppressed and the flow of metal controlled. Thus by selectively regulating the lifting and suppressor gases the metal flow or throughput may be controlled. Usually the volume of suppressor gas is not suicient to destroy the barometric column, although it may vary the overall height of the column. The suppressor gas may be used alone or in combination with an external lifting gas, although ordinarily its use will not be required except where a lifting gas is employed. Other means for regulating the flow of metal are possible, such as controlling the vessel vacuum through the ejector system, but the described method is preferred because of its relative simplicity and economy. The control of the How of metal through the degasser is important both to keep a reasonably constant level in the auxiliary chamber and prevent overtilling and also because the ow must, in many cases, by co-related to a continuous casting unit.

FIG. 5 shows a typical performance curve of a four stage steam ejector suitable for use with the present invention. The effect of the suppressor gas on the vessel vacuum is illustrated 011 the curve of the first stage ejector. Both abcissa and ordinate are on logarithmic scale. Point A on the curve illustrates a typical operating condition wherein the working pressure in the vessel is at l.750 microns (0.75 mm. Hg absolute) While withdrawing gas from the steel plus the lifter gas at a combined rate of 200 lbs./hr. (70 F. dry air equivalent). If a suppressor gas is introduced into the upleg at the rate of lbs/hr. (70 F. dry air equivalent), the pressure over the metal in the contined space of the upleg Will increase considerably, but vessel suction pressure will increase only slightly because of the greater volume in the degassing chamber. Suction pressure may increase rapidly to about 850 microns, as indicated at point B, however, the vessel pressure and rate of gas removal will tend to stabilize at some lower point on the curve because less gas is being evolved from the metal since metal flow is retarded by the addition of the suppressor gas. With the 'use of the suppressor gas it is possible to control the flow of metal while maintaining a substantially constant vessel pressure, whereby all the metal is substantially uniformly degassed.

In the arrangement shown in FIGS. 1 and 2 the molten metal flowing out the port 6 enters the auxiliary vessel 15. At the beginning of the operation the outlet opening 16 is closed. Several methods of closing the outlet may be employed, as for example the arrangement here shown where the stopper rod 18 is lowered against the interior of the discharge port while an aluminum plate is sealed across the outside of the discharge port. The stopper rod is incapable of holding vacuum, but is capable of preventing the hot metal from running down into the discharge port against the aluminum plate. With the discharge port 16 sealed against the ingress of air and the outflow of metal, the vacuum generated in the degassing vessel is also generated in the auxiliary vessel and as the metal flows into this vessel it collects. When the depth of metal in the auxiliary chamber reaches the barometric height or approximately 60 inches, the stopper rod may be lifted from its seat and the molten metal, contacting the aluminum plate, will burn through the aluminum plate and ow into the mold. The size of the outlet orifice 16 should be adjusted and regulated so that after the casting starts, the level in the auxiliary chamber will be maintained nearly constant. As long as the metal in the auxiliary chamber is kept above 60 inches from the discharge end, it will flow through the outlet port, but the molten metal will block the atmospheric air from coming up through the opening 16. If the depth of metal in the auxiliary vessel were not suflicient to maintain a barometric column over the bottom port 16, air would of course be drawn up through the molten metal.

The flow from the auxiliary chamber is commenced when the height of metal therein exceeds the barometric height; preferably it is started when the level of metal in the auxiliary chamber is but a few inches above the barometric height. 'I'hus the effective velocity head through the orifice 16 is only that resulting from a few inches of metal above 60` inches and therefore is of low velocity through the orifice and into the casting mold which is located below the orice. This low velocity is desirable because experience has shown that as a general rule the lower the velocity, the better the quality of the casting, and conversely, the higher the velocity the lower the quality of the casting. The flow control method and apparatus previously described enables the effective head of metal in the auxiliary chamber to be maintained at the desired level of a relatively few inches to achieve a fairly uniform low velocity flow through the orice 16.

It is therefore important to the operation of the present invention that the column of metal sustained in the inlet leg be less than the length of the leg, but over 60 inches, and it is important that some provision be made to provide for the continuous ow of molten metal without permitting the ingress of air, and this is accomplished through some means capable of maintaining a depth of metal above the discharge port of at least about 60 inches.

In the usual degassing practice the molten metal in the inlet leg is lifted by the vacuum in the vacuum chamber to a level where a pool of metal lies on the hearth of a degassing vessel over the top of the inlet leg, whereas the slope of the hearth in the present invention is such that metal does not collect in a pool over the hearth to a level where it can block the upwardly-traveling droplets rising out of the inlet leg of the vessel.

As the metal in the ladle lowers with the removal of metal from it, relative movement is effected between the ladle and the terminal of the inlet leg of the degasser to keep the terminal immersed to approximately the same level at all times. This can be done as indicated by raising the ladle with a crane hook as the level of metal falls, or by supporting the ladle in some other manner for vertical movement. The use of the present apparatus wherein the riser leg is of a length two to two and one-half the barometric height enables the use of much larger ladles than previously thought possible. Previously, the ladle depth was limited by the barometric height since metal containing gas bubbles owed to the chamber rather than metal, as droplets, being propelled in a gaseous medium as in the present invention. Various means for detecting the change in metal may be employed, as for example visual observation or loss of weight measuring device.

The heat balance in the melt is affected by radiation and convection losses and various exothermic and endothermic reactions taking place in the ladle, degasser and auxiliary chamber. In some heats a net heat loss may occur, consequently it is desirable to provide a means for supplying heat to the metal to maintain a temperature suitable for casting. In the embodiment described this is accomplished by induction heating of the metal in the auxiliary chamber 21. Through the use of a stainless steel water-cooled tube inside with its refractory covering as previously described, little of the induced coil may be used to keep the metal in the auxiliary vessel agitated. This is desirable particularly where alloying materials or addition substances are being continuously introduced into the degassed metal. In FIGS. 1 and 2 I have indicated a port at 27 in the degassing chamber through which alloying materials may be discharged onto the hearth. Such materials would be fed at a uniform rate from a vacuumtight feeder of the kind presently available, and forming no part of the present invention.

If desired, there may be an induction coil at 28 around the lower end of the inlet leg of the degassing apparatus above the terminal 9 which is a Water-cooled coil, and which may function as a split phase induction electromagnetic pump to be used in conjunction with the argon, or possibly in lieu of it.

In FIG. 3 instead of showing an auxiliary vessel for receiving the degassed metal from the port 6, I have shown a tubular leg of a length suflicient to retain by barometric pressure a column of metal, this tube having a discharge opening at its lower end designed to regulate the outow of metal to the rate of now, and because of the barometrically supported column of metal above the discharge orice, prevent the influx 0f air to the degassing chamber. In this figure, 30 designates the discharge leg, and 31 is the discharge orice at the end. It is of a length fairly comparable to the length of the inlet leg, and at least long enough to retain a column of metal slightly in excess of 60 inches below the discharge port 6. It will of course not ordinarily 'be longer than is required, as otherwise provision must be made for clearance for the continuous casting mold or other mold or vessel into which the outlet leg discharges.

In FIG. 4 there is more or less schematically illustrated a slightly different form of auxiliary vessel. In this view, 35 designates the auxiliary vessel having a water-cooled metal cover 36. The vessel is refractory lined. Intermediate the ends of the vessel there is a partition or wier 37 forming a separate well 38 into which the molten metal from the degasser is rst discharged. An air-cooled induction coil 39 surrounds this well and the metal overflows from this well into a second compartment 40 which may have one or more discharge openings in the bottom as indicated at 41. The outow of metal through these ports may be controlled by stopper rods 42, one for each port. The same method of sealing may be used as previously described in connection with FIGS. 1 and 2, or some other temporary closure can be provided for these ports. The compartment 40 is of a depth to hold in excess of 60 inches of molten metal. In this figure, each port discharges into molds A such as might be used for multiple strand continuous casting. Instead of introducing the alloying material on to the hearth of the degassing chamber as previously described, alloying material may be introduced into the auxiliary vessel with any of the several forms of auxiliary vessels here used. This is indicated in FIG. 4 where 43 indicates an opening for discharging alloying material into the metal in the well 38. Here again the induction coil will serve to agitate the metal to disperse the alloying material through the metal, as well as to supply heat to replace in whole or in part some of the heat lost.during degassing. In FIG. 1 I have indicated a nozzle in the bottom of the auxiliary vessel through which argon gas -may be introduced for the purpose of agitating the metal to Secure more uniform distribution of alloying ingredients through the metal. This nozzle is indicated at 45, and a similar arrangement may be provided in FIG, 4 although itis notshown. v

v YAny of the modifications or arrangements Vshown in any one figure may be, to the extent desired or applicable, interchangeably used with modifications shown in another figure..

It will be seen that since only the lowermost end of the part 9 on the inlet tube is immersed below the surface of the molten metal in the supply ladle, the main length of the inlet leg may be surrounded by. a metal casing,

and this is important to the maintenance of a barometric column of molten metal in the inlet leg since the refractory itself is porous. The volume of metal carried by expanding gases in the suction leg above the barometric height ofthe metal may be changed byuincreasing or'decreasing the'depth to which the riser' leg 7 is immersed in the molten metal, since this will change the relation between the gas inlet 11 and the toppof the barometric column of molten metal and thereby alter the bubble characteristics and the effect of the bursting of the bubbles and the formation of drops and fragments which are vcarried up the remaining height of the leg and expelled into the degassing chamber by the rapidly expanding gases. Also, lmolten metal can be removed from the ladle until the very bottom is approached by raising the ladle in the manner described w-hile keeping the height of metal in the leg uniform.

With the present invention it is also possible to use in the auxiliary chamber special refractory such, for example, as compacted calcium oxide refractory. Calcium oxide will react with the sulphur in the steel and when oxygen and sulphur are low, nitrogen can be removed. Because of the low rate of dissociation of calcium oxide (2caO-2Ca-l-O2) in low oxygen steels and the consequent low release of gas therefrom into the melt, more effective degassing conditions can be maintained in the vessel, with the result that more nitrogen can be removed than where more common refractories are employed. At the same time the consumption of the calcium oxide by reaction with sulphur is so small that its life is not seriously impaired over that of any other refractory. However, because of the use of the calcium oxide on the inside of this vessel it can at all times be kept at a temperature such that the lime will not absorb atmospheric moisture and therefore will not swell and disintegrate as will calcium oxide used as refractory in most environments.

The auxiliary vessel permits alloying elements to be introduced into the metal while the metal is still under vacuum but in an enviroment where most of the gases in the steel have been removed, so that these alloying elements are added and mixed during the once-through transfer of metal from a ladle to a mold, continuous casting unit or other receiver with no appreciable oxidation of the alloy.

The numerical data contained herein refers to situations where steel is the metal being processed; but it 'will be apparent to those skilled in the art that other metals such as copper and aluminum, for example, will involve different figures than those presented herein. Denser metals, for example, will have a barometric height proportionally less than steel, and less dense metals will have proportionately greater barometric height under the same vacuum conditions.

While I have shown and described certain preferred ing the same, it will be understood that this is by way of illustration and that various changes and modifications may be made within the contemplation of my invention and under the scope of the following claims.

I claim:

1. A molten metal degassing apparatus comprising a vertically-elongated chamber having its longitudinal axis sloped from a vertical plane and having a hearth at the bottom thereof sloping .continuously from a high side to an outlet port at the low side whereby molten metal may not be retained as a pool thereon, a tubular hot metal inlet leg opening through the sloping hearth into the chamber at the high side of the hearth, and means near the upper end of the chamber for connection to a vacuum pump, the degassing chamber being many times larger in diameter than the internal diameter of the leg, the leg being of a length of about two to two and one-half times the length of the barometric column of molten metal drawn up into said leg by the vacuum whereby there is, under any operating condition, a length of leg above the barometric column of metal drawn up into said leg by the vacuum in which the metal is propelled into the vacuum chamber "by expanding gases.

2. A molten metal degassing apparatus arranged for use wherein molten metal is transferred during degassing from a ladle to other molten metal receiving means and the inside depth of the ladle exceeds the barometric height to which molten metal'may be raised by vacuum comprising a vertically-elongated chamber having its longitudinal axis sloped from a vertical plane and having a hearth at the bottom thereof sloping continuously from a high side to an outlet port at the low side whereby molten metal may not be retained as a pool thereon, a tubular hot metal inlet leg opening through the sloping hearth into the chamber at the high side of the hearth, means near the upper end of the chamber for connection to a vacuum pump, an outlet port at the low side of the bottom, and a molten metal receiving means into which said port opens, said receiving means having a restricted outlet at the bottom and being of a depth to retain by lbarometric pressure ya column of molten metal therein to form a seal against the entrance of air when the outlet is open and a vacuum is maintained in the degassing chamber, the degassing chamber being many times larger in diameter than the internal diameter of the leg, the leg being substantially longer than 60 inches and of a length beyond which the continuous phase liquid level can rise under any operating conditions to thereby provide a length of leg above the liquid level where gases are confined until they reach the degassing chamber and thereby acquire a velocity suicient to project entrained fragments of molten metal up through the leg above said liquid level into the degassing chamber, the molten fragments then falling to the hearth, the inclination of the vertically elongated chamber preventing most of any such fragments from falling back into the inlet leg.

3. The apparatus as dened in claim 1 including means at a fixed position on the leg for selectively introducing a gas into the metal in the leg at a level below the barometric height to which metal will be lifted in the leg.

4. A molten metal degassing apparatus comprising a vertically-elongated chamber having its longitudinal axis sloped from a vertical plane and having a hearth at the bottom thereof sloping continuously from a high side to an outlet port at the low side whereby molten metal may not be retained as a pool thereon, a tubular hot metal inlet leg opening through the sloping hearth into the chamber at the high side of the hearth, and means near the upper end of the chamber for connection to a vacuum pump, the degassing chamber being many times larger in diameter than the internal diameter of the leg, the leg being substantially longer than sixty inches whereby there is, under any operating condition, a length of leg above the barometric column of metal drawn up into said leg by the vacuum in which the metal is propelled into the vacuum chamber by expanding gases, said leg having at a fixed position thereon means for selectively introducing a gas into the metal in the leg at a level below the barometric height to which metal will be lifted in the leg, and a further means, at a ixed position thereon for selectively introducing a gas into the leg at a level above said barometric height.

5. Apparatus for the continuous degassing of molten metal of the type wherein there is a degassing chamber connected to a vacuum pump, the chamber being vertically elongated with a hearth at the bottom and having a tubular hot metal inlet leg of substantially smaller internal diameter than the degassing chamber depending from the hearth and opening into the degassing chamber through the hearth so arranged that when the lower end of said metal inlet leg is submerged into a ladle of molten metal, the metal will be sucked into the tube by the vacuum above the level of the metal in the ladle to a barometric height, there being an outlet from the degassing chamber at the level of the hearth at the opposite side of the degassing chamber from the inlet leg characterized by:

(a) the inlet leg having a length in the range of two to two and one-half times the maximum barometric height to which the metal may rise in said tube,

(b) the leg having means below the barometric height of the metal for introducing lifting gas into the molten metal in the leg whereby the ow of metal above the barometric height -upwardly through the leg into the degassing chamber is effected by the upwardly-moving gases liberated from the metal and aided as necessary by lifting gas supplied through said gas inlet,

(c) the vertical axis of the degassing chamber is inclined from a vertical plane in such manner that the hearth slopes from the high side where the inlet tube opens through the hearth to the lower side where the outlet port is located and so arranged that the molten metal projected as a discontinuous stream by the gas from said inlet tube into the degassing chamber will not fall back into the inlet tube or accumulate as a pool on the hearth area,

(d) and means into which the outlet port opens with a discharge opening at the bottom thereof for retaining a depth of metal greater than the barometric height of the metal between said outlet port of the degassing chamber and the discharge opening at the bottom of said means.

References Cite'd UNITED STATES PATENTS 1,921,060 8/ 1933 Williams 266-34X 3,042,510 7/ 1962 Armbruster 75-93X 3,310,850 3/ 1967 Armbruster 164-64 3,321,300 5/1967 Worner 266-34X 3,367,396 2/ 1968 Sickbert et al 75-49X FOREIGN PATENTS 954,214 4/ 1964 Great Britain 264-33 JAMES M. MEISTER, Primary Examiner U.S. C1. X.R. --49