US3540515A

US3540515A - Casting method with molten metal degassing during teeming

Info

Publication number: US3540515A
Application number: US750168A
Authority: US
Inventors: Richard E Lyman
Original assignee: United States Steel Corp
Current assignee: United States Steel Corp
Priority date: 1968-08-05
Filing date: 1968-08-05
Publication date: 1970-11-17
Anticipated expiration: 1987-11-17

Description

United States Patent [72] Inventor Richard E. Lyman llomewood Village, Illinois [21] Appl. No. 750,168

[22] Filed Aug. 5, 1968 Continuation of application Ser. No. 511,513, Dec. 3, 1965, abandoned.

[45] Patented Nov. 17 19 70 73] Assignee United States Steel Corporation a corporation of Delaware [54] CASTING METHOD WITH MOLTEN METAL FOREIGN PATENTS 609,687 1 1/1960 Canada 266/34V 1,336,389 12/1964 France 164/254 Primary Examiner-Robert D. Baldwin Attorney- Donald S. F erito ABSTRACT: A method of degassing molten metal during teeming thereof is described in which molten metal is discharged from a ladle into an evacuated degassing chamber so that it impinges effervescently on the interior of the degassing chamber and passes therethrough, without accumulating therein, to an evacuated vertical duct. A vacuum is continuously maintained in the degassing chamber as the molten metal is passed therethrough to the vertical duct. The molten metal flows from the vertical duct into an evacuated tundish. After a sufi'lcient amount of molten metal has accumulated in the tundish to establish a barometriccolumn of molten metal in the vertical duct having a ferrostatic pressure exceeding one atmosphere, the tundish is opened to atmospheric pressure and the molten metal is discharged therefrom into a mold.

Patented Nov. 17, 1970 nwhr i v m C/Q m (w V {Pt l i Rn w f INVEN TOR. RICHARD E. L YMAN Arforney CASTING METHOD WITH MOLTEN METAL DEGASSING DURING TEEMING This is a continuation-in-part of my U.S. Pat. application Ser. No. 511,513, filed Dec. 3, 1965, now abandoned.

The present invention relates generally to teeming molten metal and, more particularly, to an improved and unique method for degassing molten metal during teeming thereof.

Vacuum degassing, which has become an important step in many modern steel making processes, basically involves exposing molten steel to a vacuum so that dissolved oxygen, hydrogen, and nitrogen may be partially removed from the steel prior to teeming. It is well established that effectively degassed molten steel yields improved product with respect to internal soundness and freedom from included nonmetallic matter.

It is the primary object of my invention to provide an improved method for teeming molten metal in which the molten metal is degassed during the teeming process.

It is a more specialized object of my invention to provide a method as set forth in the statement of object above wherein molten metal is discharged from a ladle in a stream of relative- 1y small cross section into an evacuated degassing chamber. The molten metal is not accumulated in the degassing chamber but passes effervescently therethrough into a vertically disposed duct of relatively large cross section without being exposed to the atmosphere. The outlet opening of the degassing chamber is offset from its inlet opening and a vacuum is maintained in the degassing chamber as the molten metal passes therethrough from its inlet to its outlet. From the vertical duct, the molten metal flows into an evacuated tundish. After a sufficient amount of molten metal has accumulated in the tundish to establish a barometric column of molten metal in the vertical duct having a ferrostatic pressure exceeding one atmosphere, the tundish is opened to atmospheric pressure and the molten metal is discharged therefrominto a can be degassed as it is teemed from a ladle L into a mold M in accordance with the method of the invention. The degassing and teeming unit includes a horizontally extending degassing chamber 22 having a discharge opening 23 that is connected by a relatively large diameter vertical duct 24 and a short horizontally disposed connectingduct 26 to a tundish 28. The entire unit is refractory lined and the elongated horizontally extending chamber 22, vertical duct 24, connecting duct 26 and tundish 28 are well insulated. The end of the horizontal chamber 22'adjacent the vertical duct 24 is provided with an upturned-flange exhaust opening 30 for connection with an articulated vacuum line 32. At the opposite end of the degassing chamber 22, an upturned-flange inlet opening 34 is provided for making a vacuum-tight connection with the bottom discharge nozzle 36 of the ladle L. Opening 34 may be provided with a diaphragm (not shown) which is perforable by liquid steel, to permit evacuation of the degassing unit before the ladle is connected thereto. It will be noted that the discharge nozzle of the ladle is of such inside diameter that the molten metal flows therefrom in a stream of relatively small cross section. The greater diameter of the duct 24 in relation to the smaller diameter ladle discharge nozzle ensures that the molten metal will flow continuously through the degassing chamber 22 and not accumulate therein.

The vertical duct 24 projects downwardly from the bottom of .the degassing chamber 22 for a distance of approximately 2 to 3 feet greater than the height of a column of molten steel which can be supported by atmospheric pressure (approximately 57 inches). From the lower end of the vertical duct 24,

the connecting duct 26 leads into the tundish 28. The length of the vertical duct 24 may vary somewhat depending on the depth of the tundish. A deeper tundish would require a greater length of vertical duct. The tundish 28 may be provided with a discharge opening 38 closable by a rotary gate valve 40.

In operation, the ladle L, containing undegassed molten steel, is lowered to a vacuum-sealed connection with the vacuum chamber 22 wherein the ladle discharge nozzle 36 is enclosed in the inlet opening 34 of the vacuum chamber. The interior of the entire degassing and pouring apparatus 20, including the degassing chamber 22, vertical duct 24, connecting duct 26 and tundish 28, is then evacuated by means of vacuum pump apparatus 42 connected with the line 32.

After the degassing and pouring unit 20 has been evacuated, the discharge nozzle 36 of the ladle L is opened to permit a stream of molten steel of relatively small cross section to enter the degassing chamber 22 with a velocity greater than 16 feet per second as determined by the sum of the ferrostatic and atmospheric pressures. The degassing chamber 22 is operated with no contained accumulation of molten steel so that the high-velocity pouring stream 44 from'the ladle L is shattered by its impact on the exposed bottom of the degassing chamber 22 with the result that the liquid steel splashes in an effervescent state over the entire interior surface of the chamber. This phenomenon results from the fact that the bottom of the refractory lined degassing chamber is exposed to the impact of the incoming stream of molten metal from the ladle and the kinetic energy of the metal stream is utilized to produce splashing and cffervescenceof the molten metal as it passes through the degassing chamber. In this manner, prolonged exposure to the vacuum of a large surface area of the molten steel is obtained with no ferrostatic pressure present to sup press gas evolution.

The effervescing molten steel flows toward the discharge end of the degassing chamber 22 where the gas released from the degassed molten metal flows into the vacuum line through the exhaust opening 30, and the degass d steel drains through discharge opening 23 into the vertical duct 24 in a stream of relatively large cross-sectional area. The molten metal discharged from the degassing chamber 22 passes through the vertical duct 24 and the connecting duct 26 into the tundish 28. When sufficient steel has accumulated in the tundish 28 to provide a barometric column above the opening of connecting duct 26 into vertical duct 24 so that a ferrostatic head with pressure exceeding one atmosphere is established in the vertical duct, the vacuum in tundish 28 is released by allowing air to bubble up through the discharge opening 38 or by introducing an inert gas such as argon into the head space above the liquid steel in the tundish 28. After these conditions are established, the column of molten steel in the duct 24 will maintain a vacuum seal between the degassing chamber 22 and the tundish 28 and the rate of steel flow into the tundish will be equivalent to the rate of steel flow into the degassing chamber. With discharge opening 38 open over the mold M,

the rate of steel flow into the mold M will depend on the size of the discharge opening 38 and the height of molten steel in the tundish 28.

In the event the vacuum is accidentally lost, the steel remaining in the vertical duct 24 may be discharged into the tundish 28. Because the tundish 28 is much greater in cross sectional areas than the duct 24, the steel from the duct 24, in the event that the vacuum is accidentally lost from the ap 'paratus, will cause only a small rise in the liquid steel level in the tundish.

It will be noted that in the practice of the method of the invention only a moderate vacuum pump capacity is required because the degassing rate is limited to the rate of pouring the degassing steel. Further, added steel-handling timeis nominal metal is discharged from the ladle into the horizontally exthan conventional stream degassing with a minimum of head tion while it splashes about and coats the interior surface of 5 the degassing chamber 22 in flowing from the point of impact beneath discharge nozzle 36 of the ladle L to the offset discharge opening 23 of the degassing chamber 22.

While I have shown but one embodiment of my invention, other adaptations and modifications may be made without departing from the scope of the following claims.

Iclaim:

1. A method of casting which includes degassing molten metal during teeming thereof from a ladle having a bottom discharge opening to an evacuated teeming unit which opens to a mold, which comprises the steps of discharging said molten metal in a stream of relatively small cross section in to an evacuated degassing chamber portion of said unit through an inlet in the top of said chamber so that said molten metal impinges effervescently on the interior thereof, discharging said molten metal from said chamber at a point offset from its inlet into an evacuated duct portion of said unit of relatively large cross section without exposing the molten metal to the atmosphere and in such a manner that said molten metal does not accumulate in said chamber, maintaining a vacuum in said chamber as said molten metal passes therethrough, then directing said molten metal from said duct into an evacuated tundish portion of said unit, accumulating sufficient molten metal in said tundish to establish a barometric column of molten metal in said duct having a ferrostatic pressure exceeding one atmosphere, then opening said tundish to atmospheric pressure, and then discharging molten metal from said tundish into said mold.