US3179512A - Method for transporting and degasifying a melt - Google Patents

Description

2 Sheets-Sheet 1 mvENToR ER\K ALLAN OLSSON Bv Qua/@41 THU E. A. OLSSON METHOD FOR TRANSPORTING AND DEGASIFYING A MELT April zo, 1965 Filed Aug. 8, 1962 ATTORNEYS April 20, 1965 E. A. oLssON 3,179,512

METHOD FOR TRANSPORTING AND DEGASIFYING A MELT Filed Aug. 8, 1962 2 Sheets-Sheet 2 Fig.2

|NvENToR ERM ALLAN OLSSON BY www ATTORNEYS United States Patent O z claims. (ci. rs-io) It has been known for some time that by degasifying molten metal with vacuum treatment appreciable improvements in quality can be obtained. Vacuum treatment of iron and steel is rapidly gaining importance, and several methods are known and are in practical use. A. normal procedure involves placing the melting furnace or casting ladle and the mould, wherein the contents of the furnace or ladle are to be cast, within an airtight container which is evacuated. Then the vacuum chamber containing the complete furnace and die is tilted so that the melt runs from the furnace down into the mould. This method is suitable mainly for a relatively small charge quantities, and then only for casting a single ingot or a small number of ingots. For larger charges to be cast into a number of ingots it is preferable to place the ladies intended for casting within an evacuated container or chamber which has been provided at the top with arrangements for introducing the melt from a furnace or another ladle. The main part of the charge is degasied when the melt enters the airless space from above as a jet. The jet breaks up under the action of the overpressure oi the dissolved gases. The degasitied melt collected in this way in the ladle in the vacuum chamber is later cast into casting moulds or ingot moulds outside of the chamber. To prevent the melt from absorbing oxygen and/or hydrogen during casting, the casting jet is enveloped by inert gas which is also employed to lill the mould.

Another process is characterized by the melt in the casting ladle being drawn up one or more times into an evacuated receptacle so that the contents of the casting ladle are degasitied as completely as possible.

A third process is as follows: the melt is caused to circulate from the casting ladle or corresponding container through an evacuated chamber connected by two pipes or channels with the ladle or container which is at a lower level. The melt is thus drawn by vacuum into the upper container, and circulation of the melt is effected by blowing inert gas into one of the pipes. After being treated according to requirements the entire contents o the ladle are cast into ingot moulds or casting moulds.

Vacuum treatment according to the above-mentioned methods takes a relatively long time during which the temperature of the melt drops owing to heat losses which normally occur through radiation and convection. Other reasons for a temperature drop in the melt are the endothermic reaction which takes place with the release of gases from the melt, and the heat transfer to the often only slightly preheated linings in the vacuum treatment installations. it is therefore desirable to carry out degasiiying as quickly as possible. This requires large containers and large vacuum pumps.

This invention provides a method for degasifying the y li, l W l Z Patented Apr. 20, i965 ICC melt during the' casting process and permits degasifying of even larve charges with relatively small degasifying equipment. The method according to the invention is characterized by passing the melt as it is cast, through an evacuated container, wherein it is supplied with heat energy whereby temperature drop of the melt is prevented or held within permissible limits.

The invention is described more in detail in connection with installation for continuous casting.

FlG. l shows a rst embodiment in cross section in which only the components of the installation necessary to clarity the invention are included.

FIG. 2 shows in a similar manner a modilied embodiment.

'he installation as shown in FIG. l comprises an airtight container 3 lined with refractory material and tted with two downward pipes l and 5. The container is connected through a pipe lil with a vacuum pump (not shown) for evacuation. When the melt lills the receptacle 2. and the mould 6 so that air is prevented from being drawn in through pipes 4 and 5, the container 3 is iilled under vacuum with melt. The level of melt is such that the prevailing internal pressure in the container 3 plus the weight of the liquid column over the surface of the melt in the receptacle Z and in the mould d is balanced with the prevailing external atmospheric pressure. When this balance is obtained and melt is supplied to the receptacle 2 from a casting ladle l with bottom emptying and the level in the receptacle 2. tends: to rise, the melt runs over into the mould 6 via pipe 4, container 3 and pipe :'i. The same thing happens when the level in the mould 6 sinks for certain reasons, eg., extraction of the partially solidilied cast 7 in the die. A means of transport for the melt is obtained as a result of both free surfaces of the melt seeking to take up the same level. The supply of melt to the mould can be determined by the extraction speed of the cast '7 out of the mould, assuming that the surface of the melt in the receptacle 2 is maintained at an unchanged level by the supply of new melt regulated automatically in a simple manner. As the continuously moving melt to the container 3 releases dissolved gasses, owing to the lower partial pressure of respective gases within the container 3, the vacuum pumps must be kept in continual operation. The pumps must be designed to maintain the required reduced pressure in the chamber 3 notwithstanding the release of gases described above.

An improvement in degasifying can be brought about by connecting up an inletfor inert gases under the Surface of the melt so that inert gas can bubble up through the melt and generate bubbles within the melt. These bubbles provide a relatively large free surface, and the dissolved gases can pass into the bubble space which initially has a zero partial pressure in relation to the dissolved gases. The enlargement of the free surface of the melt resulting from the permeation of inert gas through the melt (boiling) also promotes release of gases in the melt to the space above which is under considerably reduced pressure.

The process in question incurs relatively large heat losses due to heat being conducted away through container 3 and the walls of pipes d and 5 and due to the endothermic reaction in connection with gas release.

3 When sms` l quantities of melt per unit time flow through the degasiying and transnorting arrangement shown, there is a risk or the melt solidifying especially in the relatively slender pipe 5 to the mould. rThis disadvantage can, however, be completely eliminated by the use of the wellknown inductive heating method. Inserted in a space extending through the container 3 is a yoke 9 made from transformer plates forming the primary winding il that induces a secondary voltage in the vmelt in container 31. The melt acts as the secondary winding. ln order to close the secondary circuit an electrically conducting rod or plate lll is'inserted to connect the inlet and outlet sides of the melt. Through a connection between the die 6 or cast 'l' and the melt in receptacle 2, another close electrical circuit can be obtained. lt is diilicult to maintain a high electrical eticiency with this system and theretore a relatively low power factor must be ticipated. Instead of the above method it can occasionally be simpler to supply heat energy through direct resistance heating of the melt in which case an electric current may be passed directly through the melt in pipes or 5. A vertical hole may be made in the lining or" the container 3 between the two pipes and 5 and a water-cooled electrode inserted. The design of the electrode should be such that melt on entering the space above the electrode is caused to solidify close to the electrode owing to the cooling effect ot the latter, while on the other hand the metal above remains iolten. When voltage is applied between the electrode in question and the cast, an electric current will pass through and heat the melt between these terminals. 'l he strength of this current is determined by the electrical resistance and the applied voltage. Similarly, an electric current can be passed through to heat the melt in thc other pipe, if, for instance, the melt in container 3 is connected to the other terminal in the electrical circuit by an immersed electrode. Overheating of the vacuum chamber and pipes by secondary induction currents may be avoided 'oy employing suitable design and material.

The installation described here is well suited as a conveying arrangement for multiple casting when it is provided with a number of'pipe 5 correspoi rig to the num- Special regulation of the flow ber of casts and moulds. of melt to each separate mould is completely unnecessary, as each mould is automatically filled with melt to a level corresponding to the level of molten metal in reeptacle 2. This arrangement has advantages over other methods when casting from a relatively large ladle into a large number of moulds. The treatment time for the contents of a casting ladle can be extended over the entire casting period, consequently reducing the requirements regarding size pumping capacity of the installation. The arrangement is provided with a iiow outlet (nozzle) corresponding to the desired casting speed (rising speed of melt in the mould). When one mould is filled, filling is transferred to the next mould. Either the moulds are placed on cars and the degasifying installation is stationary or vice versa. To prevent the melt from rcabsorbing undesirable gases, eg., oX gen and/ or hydrogen, during the passage into the mould, it is advantageous to fill the moulds with inert gas and to envelop the freefalling jet of melt in inert gas supplied through nozzles arranged around the outlet opening.

irrespective of whether the electric current is supplied directly to the melt via contact devices or is induced in the melt, inwardly directed forces arise perpendicular to the direction of the current, i.e., the pinch effect. This effect serves to constrict the melt in the channels and thus aiects the how-through of melt according to the current density. Since the reduced pressure prevailing in the evacuated container has considerably reduced the static pressure of the melt that acts against these forces, constriction is more noticeable at relatively low current strength than would be the case if there were full static pressure. When the current is increased to provide more heat at lower through-flow in order to compensate for heat losses and temperature drop ofthe melt, the constricting effect is also increased and can be used for regulating the rate of flow of the melt through the installation. This type of regulation has an especially important application in the continuous casting of ingots so small that the discharge pipe, even with the thinnest permissible wall thickness, cannot enter the mould opening. It is a well-lrnown fact that discharge of small quantities or metals that are difficult to melt is combined with great dihiculties due to the risk of solidication in the nozzle.

PEG. 2 presents an arrangement that is specially suited for this case. The current is supplied to the melt directly via two electrodes la and l5, eg., graphite electrodes, which form pipes similar to l and 5 of FIG. 1. The current provides'heat both to the melt in the evacuated container 3 `and especially to the melt in the discharge pipe where the conducting area is relatively small. Furthermore, the current tends to have a constricting effect on the melt especially in channel il?. owing to the abovementioned pinch etect. increased current strength produces both the constricting effect, Le., the tendency to diminish the through-flow of the melt, and the generation of heat in the melt. A nozzle i3 of heat resistive material is inserted in the bottoni or the pipe l5 which forms one of the electrodes. The diameter of this nozzle is suited to pass a smooth iet of melt and prevents the iet from spreading out and deviating on the cessation of the influerce of the above-mentioned pinch eliect. it is of course possible to supply the current through an electrode in thereceptacle 2 instead ot through pipe 14 as shown. It is also possible to supply the current via the cast, eg., via the mould 6 or via the rollers beneath the die which press against the cast 7. However, local overheating can easily occur at the place of contact due to Contact resistance and damage the cast.

It the above-mentioned pinch effect is not required, it may be reduced or eliminated by means of special arrangements, e.g., by arranging parallel conductors with opposite current directions. Both conductors may consist of the melt, or only one may consist or the melt, while the other comprises another conductor with a larger area and/ or a larger conducting capacity than the column o melt.

I claim:

l. A method vfor continuously transporting and degasitying molten metal during continuous casting, comprising the steps of continuously feeding molten metal t0 a container subject to atmospheric pressure, continuously evacuating and drawing orf gases in a degassing chamber isposed above said container, continuously siphoning molten metal from said container through a first conduit into and through said degassing chamber and discharging the molten metal to a continuous casting mold through a second conduit connected to the bottom of the degassing chamber, heating the molten metal while passing through the degassing chamber by passing electric current directly through the moving molten metal in said tirst and second conduits and said degassing chamber, and increasing and decreasing the flow of molten metal siphoned from said container through the degassing chamber to said mold by respectively decreasing Yand increasing the electric current supplied for heating thereby decreasing and increasing, respectively, the constricting influence of the pinch elect in said conduits while preventing soliditication of the molten metal flowing in said conduits.

2. A method for continuously transporting and degasitying molten metal during continuous casting, comprising the steps of feeding molten metal to a container subject to atmospheric pressure, evacuating and drawing off gases in a degassing chamber disposed above said container, siphoning molten metal from said container through a first conduit into and through said degassing chamber and discharging the molten metal to a continuous casting mold through a second conduit connected to the bottom of the degassing chamber, inductively heating the molten metal while passing through the degassing chamber by passing electric current directly through the moving molten metal in said first and second conduits and said degassing chamber, and increasing and decreasing the ow of molten metal sphoned from said container through the degassing chamber to said mold by respectively decreasing and increasing the electric current supplied for heating thereby `decreasing and increasing, respectively, the constricting iniluence of the pinch effect in said conduits while pre- References Cited by the Examiner UNlTED STATES PATENTS Rozian 75-49 Harders et al 2654-34 Lorenz 75-93 X Lorenz 75-93 Chambers 22-73 LorenzV 75-49 venting solidication of the molten metal flowing in said 10 DAVID L RECK Primary Examiner conduits.

Claims (1)

1. A METHOD FOR CONTINUOUSLY TRANSPORTING AND DEGASIFYING MOLTEN METAL DURING CONTINUOUS CASTING, COMPRISING THE STEPS OF CONTINUOUSLY FEEDING MOLTEN METAL TO A CONTAINER SUBJECT TO ATMOSPHERIC PRESSURE, CONTINUOUSLY EVACUATING AND DRAWING OFF GASES IN A DEGASSING CHAMBER DISPOSED ABOVE SAID CONTAINER, CONTINUOUSLY SIPHONING MOLTEN METAL FROM SAID CONTAINER THROUGH A FIRST CONDIUT INTO AND THROUGH SAID DEGASSING CHAMBER AND DISCHARGING THE MOLTEN METAL TO A CONTINUOUS CASTING MOLD THROUGH A SECOND CONDIUT CONNECTED TO THE BOTTOM OF THE DEGASSING CHAMBER, HEATING THE MOLTEN METAL WHILE PASSING THROUGH THE DEGASSING CHAMBER BY PASSING ELECTRIC CURRENT DIRECTLY THROUGH THE MOVING MOLTEN METAL IN SAID FIRST AND SECOND CONDIUTS AND SAID DEGASSING CHAMBER, AND INCREASING AND DECREASING THE FLOW OF MOLTEN METAL SIPHONED FROM SAID CONTAINER THROUGH THE DEGASSING CHAMBER TO SAID MOLD BY RESPECTIVELY DECREASING AND INCREASING THE ELECTRIC CURRENT SUPPLIED FOR HEATING THEREBY DECREASING AND INCREASING, RESPECTIVELY, THE CONSTRICTING INFLUENCE OF THE PINCH EFFECT IN SAID CONDIUTS WHILE PREVENTING SOLIDIFICATION OF THE MOLTEN METAL FLOWING IN SAID CONDIUTS.

Images