US3226224A

US3226224A - Process for vacuum degasification of metal

Info

Publication number: US3226224A
Application number: US200599A
Authority: US
Inventors: Sickbert Adolf
Original assignee: Bochumer Verein fuer Gussstahlfabrikation AG
Current assignee: Bochumer Verein fuer Gussstahlfabrikation AG
Priority date: 1961-06-09
Filing date: 1962-05-29
Publication date: 1965-12-28
Anticipated expiration: 1982-12-28
Also published as: GB1014102A; CH422025A; AT265347B

Description

Dec. 28, 1965 A. slcKBERT PROCESS FOR VACUUM DEGASIFICATION OF METAL 3 Sheets-Sheet l INVENTOR /I//ff/ Filed May 29, 1962 ADOLF SIC/(BERT @www Dufw mais Dec. 28, 1965 A. slcKBERT 3,226,224

PROCESS FOR VACUUM DEGASIFICATION OF METAL Filed May 29, 1962 3 Sheets-Sheet 2 INV ENTOR ADOLF S/C/(BERT l f f f ,V /f fw uf//f f/ Dec. 28, 1965 A. slcKBr-:RT

PROCESS FOR VACUUM DEGASIFIGATION OF METAL 3 Sheets-Sheet 3 Filed May 29, 1962 INVENTOR ADOLF S/C/CEERT United States Patent O 3,226,224 PROCESS FOR VACUUM DEGASIFICATION F METAL Adolf Sickbert, Wattenscheid-Eppendorf, Germany, as-

signor to Bochumer Verein fr Gusstahlfabrikation AAG., Bochum, Germany, a corporation of Germany Filed May 29, 1962, Ser. No. 200,599 Claims priority, application Germany, .lune 9, 1961, B 62,840 7 Claims. (Cl. 75-49) This is a continuation-in-part of co-pending applications Serial No. 152,372, led September 27, 1961 (now U.S. Patent No. 3,146,503, granted September l, 1964), and Serial No. 153,842, filed November 21, 1961, and now abandoned.

This invention relates to new and useful improvements in the vacuum degasifcation of metal. In order to remove undesirable gas constituents from various metals, including iron and steel, it is known to subject the metal to low pressures, as for example in vacuum chambers so that the partial pressures of the undesirable constituents will cause their migration out of the molten metal.

In accordance with one process for the vacuum degasification of metal which is particularly applicable for iron and steel but which is also utilizable in connection with other metals, the molten metal to be degased is caused to liow into a vacuum chamber in the form of a spray of droplets, Due to this particular form the action of the low pressure has been found to be optimum in removing the undesirable gas constituents. By `gas constituents is not only meant materials which are normally considered as gases, but also other impurities which may be converted to gas or vapor form under the pressures and temperatures in question. The above mentioned mode of vacuum degasication is sometimes referred to in the art as stream degasication in that the major portion of the degasication occurs while the metal is flowing in the form of a stream.

Stream degasiiication was conventionally effected by tapping the molten metal from the furnace or converter (generically referred to as furnace herein) into a transport or storage ladle which may be of the tapping variety, and thereafter transferring the molten metal from this intermediate ladle into the vacuum chamber generally by means of a further intermediate tapping ladle positioned directly above the vacuum chamber and hermetically sealed thereto. The use of the various intermediate ladies however, resulted in a great loss of heat due to the high sensible heat content of the molten metal, and in order that the molten metal would remain sufficiently uid in the vacuum chamber and retain sufficient heat quantity for further handling and treatment it was necessary to superheat the metal prior to leaving the furnace or to provide additional heat during the intermediate handling, as for example, by electrically heating one or more of the intermediate ladles.

Aside from the obvious expense involved in providing this additional heat either by way of super-heating or through the intermediate ladles, the use of the higher temperatures required would adversely affect the equipment, as for example, the refractory linings thereof.

While it has been proposed in the literature of the art to effect a degasing of steel which has not been completely killed, the same was not satisfactory from a commercial or practical standpoint due to the tendency of unkilled steel to violently boil in a transport ladle and the danger which would inherently result therefrom.

In spite of the obvious disadvantages of the use of the intermediate transport or storage ladles or containers, and the obvious economical disadvantages thereof, their use was universally believed necessary in the art and no satisfactory solution had been proposed to avoid the same.

One object of this invention is an improved mode of effecting the above idcntied stream degasing process without these disadvantages.

A further object of this invention is a greatly improved and more efcient mode of effecting stream degasing of metal, such as iron and steel. This and still further objects will become apparent from the following description read in conjunction with the drawings in which:

FIG. 1 is a diagrammatic vertical section of an embodiment of equipment for effecting the process in accordance with the invention,

FIG. 2 is a diagrammatic vertical section of a further embodiment of equipment for effecting the process in accordance with the invention, and

FIG. 3 is a vertical section of a still further embodiment of equipment in accordance with the invention.

In accordance with the invention it has been suprisingly discovered that stream degasing may be effected in a more efficient, economical and improved manner if the molten metal is passed from the furnace in the form of a contiguous stream directly into the vacuum chamber wherein the degasing occurs.

In accordance with the process of the invention the metal to be degased is initially melted in the conventional manner in a furnace. The term furnace is being generically used herein to designate any of the devices con ventionally used for melting metal, as for example, open hearth furnaces, electrical furnaces or converters in the steel art, or blast or shaft furnaces in the iron-making art. The molten metal formed in the furnace after the melting and treatment therein is then directly passed in the form of a substantially contiguous stream from the furnace through an inlet opening into a vacuum chamber maintained at low pressure while a pool of molten metal from said stream is maintained above the opening in the vacuum chamber in order to seal the same. As the stream of molten metal passes through the inlet opening which may be in the form of a nozzle into the vacuum chamber, the same is divided into a spray of droplets in accordance with the stream degasication process. The `body of liquid metal which forms from the stream in the vacuum chamber is furthermore, if necessary or desired, subjected to the action of the vacuum for further degasication.

The process in accordance with the invention is applicable for the degasing of any metals, as for example, aluminum or other non-ferrous metals, but is particularly adaptable for the degasiflcation of ferrous metals, such as iron and steel, and partially ferrous metals such as ferrochrome.

The invention will be described herein with further reference to the degasiiication of steel, it being understood however, that the general principles as described are also applicable to other metals. In connection with the degasiication of killed steel the pressure in the vacuum chamber should be maintained below about 20 mrn. Hg, whereas in connection with unkilled steel the pressure should be maintained below about 30 mm. Hg. In connection with other metals pressures even as high as 50 or mm. Hg may be effective.

Referring to FIG. 1 of the drawing, 1 represents a tapping ladle constructed from materials conventionally used for this purposes in the steel-making art including, for example, a refractory lining. The ladle 1 is provided with a vacuum-tight cover 2 which is provided with a connecting tubular elbow 3 joined to the suction of evacuation pipe 4 and provided with the closure valve 6. The suction pipe 4 and if desirable, the elbow 3, may be made of flexible material, or may be constructed in a flexible manner by providing the ball and socket joints 15. The suction pipe 4 is preferably also provided with cooling means in the form of the pipes 17 surrounding the same and provided with the sprays or jets 19 through which a cooling agent such as compressed air is blown. The pipes 17 are also provided with the flexible links 20 so that the same follow the movement of the suction pipe 4. Positioned on the cover 2 is a relatively small feed hopper 5 which is funnel-shaped and may, if desired be provided with a stopper rod 13 constructed in the manner conventional for Stoppers in tapping ladles. The feed hopper 5 is heremtically sealed i.e. sealed in a vacuum-tight manner by the cylindrical sleeve 23 which surrounds the opening 7 leading through the cover of the chamber. The opening of the feed hopper 5 leading into the opening 7 may, if desired, be sealed by a removable sealing device such as the plug 9. Surrounding the opening 7 is the splash shield or sleeve 7a which protects the stopper device 8 of the ladle 1 from damage from the melted steel introduced. Additionally a feeding device for additives r treating agents in the form of a lock-sluice or feed-worm 11 is provided through which, for example, alloying deoxidizing agents or other treating agents may be fed. A television camera 12 may also be provided to permit observation and control of the degasing operation. The entire arrangement as above described is moveable, as for example, on a conventional crane and hanger which engages the trunnions 4 so that the arrangement may be positioned directly below the discharge chute and tap 16a of a furnace such as the electric arc furnace 16. Due to the mobility of the arrangement and the flexible connection of the suction pipe 4 it is possible for the apparatus to follow the movements of the tapping chute 16a of the furnace. An overflow chute 14 for slag or the like from the hopper is also provided.

In operation the steel is melted in the electric arc furnace, as for example from scrap and pig iron, in the conventional manner, at a temperature between about 1600 and 1650 C. After the melt is formed, the same may be killed in the conventional manner by adding treating treating agents, such as silicon, aluminum and calciumsilicon, which will combine with the oxygen in the molten metal, removing the same and forming a slag. The tapping ladle 1 in the form of the vacuum chamber is then positioned by a crane beneath the tapping spout 16a of the furnace. If the stopper rod 13 is provided, the same is closed, and the ladle 1 may be evacuated through the suction pipe 14, with the valve 6 open, down to the low pressure desired, as for example down to 0.5 milliameter Hg or below, depending on the efliciency of the pump or evacuating device. The furnace 16 is then tapped in the conventional manner through the tapping chute 16a, so that a stream of steel from the tapping chute flows into the hopper 5. When the level of the steel in the hopper 5 is sufciently high to seal the opening 7, the stopper rod 13 is lifted, so that the steel will ow through the 4opening 7 into the interior of the ladle 1. The pressure ,in the interior of the ladle 1 is maintained through continuous evacuation, at as loW as possible a value, which in connection with killed steel may remain at the values given above, but in connection with unkilled steel, due to the volume of gas generated, the pressure may somewhat increase. If a plug 9 is not provided, the steel simply runs through the opening '7, and as the same enters the interior of the ladle 1, is converted into a stream of droplets, the individual particle size of which may range from about as tine as possible to up to about ten millimeters. In this form the steel is very effectively degased by the low pressure in the chamber. The pouring rate vfrom the tap 16 is so controlled that a sufficient pool of molten steel remains in the hopper 5, so that the opening 7 will remain sealed. It should be emphasized that basically this is the sole purpose of retaining the pool of steel in the hopper 5, and in connection with the efciency of the process, it is desirable that the smallest quantity of the steel be retained in this form and that the hopper be constructed as small as possible for this purpose. The hopper 5 should, of course, be large enough so that variations in the tlow rate from the tap 16a will not cause an overow and spillage. Generally, the pool of molten steel should be maintained at a height sufficient to prevent the down-ilowing steel from completely cavitating through the opening which would allow the sucking in of the ambient atmosphere. For practical considerations a height of about 10 inches of steel in the -pool is generally suicient for this purpose.

The pouring rate and rate of entry of the stream into the vacuum chamber should be so controlled that the stream will continue to divide into a spray of droplets, and generally for this purpose the opening 7 should have a diameter between about 1 and 4 inches, depending upon the size of the furnace and rate of pouring, etc. The optimum size hole and rate of pouring, for a particular operation however, may be very easily empirically determined, as for example by observing the process through the television camera 12.

lf the tapping is effected so that slag runs olf with the molten metal, the ow rate may be so controlled that the slag, or a portion thereof, is continuously overowed from the top of the hopper 5, as for example into the overflow chute 14. In certain instances it may be desirable to allow the slag vor a portion thereof to also flow into the vacuum chamber, to act on the steel or as a heat-seal therefor. Thus, for example, the flowing of the slag with the steel into the Vacuum chamber, will result in an intimate contacting which may be effective for desulfurization of the steel. It is also possible at the end of the operation to allow a layer of the slag to build up in the hopper 5 to maintain the seal and prevent excessive loss of temperature, particularly if the body of steel forming in the ladle 1 is to be subjected to further treatment under the action of the vacuum, as for example stirred or admixed with treating agents, alloying materials or the like. During the pouring operation it is also possible to admix treating agents with the stream of steel, as for example by pouring the same into the hopper 5 or adding the same to the hopper 5 during the pouring operation. Thus, for example, separate slag of various oxides, such as calcium oxide, aluminum oxide, silicon oxide, calcium fluoride, may be added for desulfurization or alloying elements, or ennobling agents may be added, the term treating agent being used herein to generically define any additive conventionally used, whether the same serves for removal of a component or remains in the steel itself.

After the desired quantity of steel has been tapped from the furnace 16 through the tapping spout 16a into the chamber 1, the chamber may be sealed by closing the stopper rod 13 and allowing a body of metal or slag to remain in the hopper. The steel in the chamber 1 may then be further subjected to the action of the vacuum until the same becomes quiescent, or partially quiescent, and further treating agents may be added, if desired, as for example through the vacuum-tight sluice or lock 11. lf it is not necessary to maintain the vacuum at the low level, the Valve 6 may simply be shut, so that the pressure in the chamber will simply increase in proportion to the gas generated.

If desired `or necessary, during the evacuation the suction pipe 4 may be cooled, as for example by means of passing compressed air through the pipe 17, which is sprayed from the jets 19 in the form lof the sprays 18 against the pipe 4. This is particularly desirable when treating unkilled steel where a large quantity of hot gases are generated and must be withdrawn through the pipe 4. The body of treated molten steel in the bottom of the vacuum ladle 1 may then be removed and treated in any known or conventional manner. Withdrawal of the steel may be effected by simply lifting the stopper rod 8, so that the liquid steel will run through the tap hole, melt the fusible membrane 10, as for example of lead or the like, which serves the purpose of preventing gas from being initially sucked into the chamber through the tapping hole. If the ladle is to be transported prior to the removal of the steel, the valve 6 may be shut and the ladle disconnected from the flexible pipe 14. Alternatively, if it is not necessary to maintain the steel under low pressure, the valve 6 may remain open. The steel from the ladle 1 may, for example, be passed directly into a mold for casting an ingot or foundry form, may be passed into a further vacuum chamber for further vacuum treatment, and if desired cast in this chamber, under vacuum, or may be subjected to continuous casting, directly or by way of intermediate further vacuum treatment.

It should be emphasized that the stopper 13 does not function during the normal operation and is merely utilized before and/ or after the pouring operation. Within the broadest aspects of the invention it is not necessary to provide such a stopper. If stopper 13 is not provided and the apparatus is not sealed with plug 19, operation may still be effected provided that the vacuum pump is of suticient high capacity so that as the pouring ybegins and the sealing pool of molten steel builds up in the hopper 5, evacuation of the chamber 1 may be effected in a sufficiently short time. Alternatively, a plug 9, of fusible material, as for example lead, may be provided to allow initial evacuation of the chamber. When the pouring of steel is commenced and the sealing pool builds up in the hopper 5, the plug will automatically be melted away, allowing the steel to run into the chamber 1 as previously described. It should be furthermore emphasized that during the major portion of the operation, the stream of steel from the furnace 16 into the chamber 1 is contiguous, i.e., a continuous, uninterrupted stream of steel extends from the furnace into the pool in the hopper 5 which continuously runs into the chamber. Thus, in effect, it may be considered that the steel is poured directly from the furnace into the vacuum chamber, the pool of steel maintained above the vacuum chamber, merely serving the practical function of sealing the inlet opening from the surrounding atmosphere.

In accordance with the invention it has been found that decided advantages may be achieved by directly pouring the unkilled steel from the furnace 16 into the chamber 1. The use of the unkilled steel enhances the degasification process and allows the killing of the steel in a more effective and efficient manner, with a reduced quantity, or in some cases without the special addition of a deoxidizing agent. Thus, as the unkilled steel flows into the vacuum chamber, the carbon present therein will react with the oxygen in the steel, so that this carbon acts as a deoxidizing or killing agent, and the oxygen in turn acts as a decarbonizing agent, simultaneously reducing the oxygen and carbon content. The reacted carbon and oxygen combine, forming gaseous carbon monoxide, which is withdrawn with the gas. By a suitable initial oxygen content in the steel or by the specific addition of oxygen, the carbon content of the steel may be reduced to almost any desired value in a very simple and economical manner. This offers the possibility of very simply producing extremely low carbon steel which is useful for many purposes, as for example, producing sheets which are to be coated with vitreous enamel or as electrical laminates, such as for transformers. The carbon content thus, for example, may be reduced to 0.1% and lower. The formation of this low carbon steel furthermore allows the production of a high silicon steel by the addition of silicon in further treating operations, which additionally adds to the heat economy of the process due to its heat of dissolution.

The low carbon content steel which may be produced as described above is also useful in the production of stainless steel, as for example, by the addition of chromium.

Furthermore, the degasiiication of the unkilled or partially deoxidized steel allows a very accurate and convenient adjustment of the degree of the deoxidation through the vacuum and thus allows an accurate control of the thickness of the non-segregated surface zone, as for example, when the steel is continuously cast from the vacuum chamber. The invention also allows the economical utilization of the evacuation equipment as it is possible to use a single evacuation pump or device alternately or intermittently for various separate chambers. Thus for example, the valve 6 may be shut and the suction pipe 4 connected to a different vacuum chamber after the pouring operation has terminated. It is also often preferable to maintain the suction pipe 4 under the vacuum while it is being connected and disconnected to various devices in which case a separate valve may be provided in this line.

FIG. 2 shows a further embodiment of the invention in which a conventional tapping ladle 46 is positioned within the vacuum chamber 41 provided with the hinged cover 2 connected to the main body of the chamber by means of the hinge 60. In this embodiment the hopper 47 is in the form of a small cylindrical container provided with the tapping hole 54 and positioned on the cover 2 in sealing engagement about the opening 4S provided with the anti-splash shield 7a. The device as shown is used in conjunction with the converter 52 and the molten steel from the converter is poured through the guide chute 50 into the hopper 47. The chute 50 being adjustable by means of the link 51. The vacuum tight seal of the hopper onto the cover 2 is effected by means of the sealing ring 48. In all other respects operation is identical with the embodiment described in connection with FIG. l, except after the pouring and vacuum treatment the ladle 46 must be removed from the vacuum chamber by opening the cover 2.

In connection with tapping ladles, such as the tapping ladle 46, a problem has been encountered in the prior art in that an accidental unseating of the stopper rod would sometimes occur and result in spillage and loss of the molten steel from the ladle.

In accordance with a further embodiment of the invention as shown in FIG. 2, this is prevented by packing the tapping hole of the ladle with particled refractory material such as the sand 52, and holding the sand in place by means of the plug 60 of wood, carbon refractory material or any other suitable material. The plug may be held in place by the hinged metal plug 62 and spring latch 63. The accidental unseating of the stopper rod would simply cause the steel to flow in the sand and solidify preventing accidental spillage and discharge. When it is desired to discharge the steel through the tapping hole the latch 63 is released, the cover 62 to which the plug 60 may be attached swung out of place and the sand runs out of the hole allowing discharge in the conventional manner.

In the embodiment shown in FIG. 3 the device is used in conjunction with an open hearth furnace 64 provided with the discharge spout 65. In this case a conventional tapping ladle 46 is positioned in the vacuum chamber 41 provided with the movable cover 2. The feed hopper 47 provided with the opening 54 leading into the opening 66 in the cover 2 is sealed in a vacuum-tight manner on the cover by means of the seal 67. An anti-splash device 7a is also provided. The tapping ladle 46 is however, sealed to the bottom of the vacuum chamber 41 by means of the sealing ring 68 in a vacuum-tight manner surrounding the discharge hole 69. This allows tapping of the ladle 46 without removal of the same from the vacuum chamber 41. If the stopper rod in the ladle 46 Will not provide a hermetic seal, or as a precautionary measure it is possible to seal the tap hole of the ladle 46 and/or the opening 69 with, for example, a fusible plug or plate. In all other respects operation is as pre- "7 viously described and the chamber 41 is evacuated through a conventional hose connection (not shown).

The initial melting of the metal such as thesteel may as indicated, be effected in the well known or conventional manner. In connection with the melting of steel the melting operation prior to the tapping of the furnace generally involves a refining and decarbonization treatment. Usually the initial charge for the furnace is made up with a higher carbon content than that ultimately desired in the steel to be formed and this carbon content is reduced during the decarbonization treatment. This decarbonization period generally takes about 1-2 hours, except in the case of a blowing process. Where special quality or special application steels are to be produced these rening and decarbonization periods often last for a considerably longer period of time. During the decarbonization the carbon is converted to carbon monoxide by oxidation and the formed carbon monoxide causes an agitation of the melt and promotes the purification, partial degasication and homogenization of the steel. The refining serves primarily to remove undesirable components, especially non-metallics by reducing them with slags.

In accordance with a further embodiment of the invention it has been surprisingly found that when utilizing the vacuum degasing treatment as described above, the furnace may be initially charged with a charge having a carbon content not substantially in excess of that desired in the ultimate steel to be produced and that the decarbonization treatment may be eliminated, thus substantially shortening the processing time without any detrimental effect on the quality of the steel. In accordance with this embodiment of the invention the `carbon content of the material when charged into the melting furnace, which is operated at atmospheric pressure, is kept within the range of the c-arbon content required in the steel or blow. The charge is then melted down at the normal melting temperature, as for example, at a temperature between about 1500 and 1550 C., and after the charge is melted and without a decarbonization tre-atment the temperature is raised to the pouring temperature, as for example between about 1600 and 1700 C. and the charge is then immediately poured directly into the vacuum chamber for the stream degasification, as described above. The reduction of melting time in this method may be about to more than 50% of the conventional melting time.

Prior to tapping the charge into the vacuum chamber, and preferably prior to raising the temperature lto the pouring temperature, the melt should be dephosphorized -if necessary, -as the lower temperature at this point promotes dephosphorization. Furthermore, it is desirable to lower or oxidize out the silicon content, including any ferrosilicon that may have been added. The slag is drawn off and a new slag -or new slag-forming agents may be added. Furthermore, any necessary correction and alloy element contents may be made before tapping, or if desired, during tapping. Thus, for example, if the initial charge is selected with a carbon content below that desired in the steel, carbon preferably in the form of graphite powder may be added at this point. While the heating of the charge may be effected in any of the known furnaces, the same is preferably effected in an electric arc furnace as the same allows a very rapid heating-up and thus a very substantial saving in the processing time. The vacuum stream degasing is effected in the manner described above and pressures below 30 mm. Hg, and preferably below 20 mm. Hg are used with the upper limit being contemplated in connection with completely unkilled steel. For higher grades of steel it is preferable to use pressures of less than 3 to 0.5 mm. Hg, and preferably 0.11 mm. Hg. After tapping the steel may be treated and processed in the identical manner as described above and the other expedients .described are also applicable. The treatment in accordance with this preferred embodiment of the invention results in a substan-r tial saving in time in which the melt is heated in the furnace and thus in a substantial economy without any detrimental eifect on the steel quality whatsoever.

The invention will be described in further detail with reference to the following examples which are given solely by way of illustration and not limitation.

Example 1 A 50-ton charge consisting of 90% scrap and 10% pig iron was melted, by heating to a temperature of about 1500" C., in an electric arc furnace of the type shown in FIG. 1. The melt formed had a composition of .40% C, .35% Mn, .06% Si, .035% P, and .032% S.

2000 lbs. of burnt lime, lbs. of CaF2 and 600 lbs. of iron ore are successively added into -the furnace. The temperature of the melt is then raised to about 1600 C. over a period of about 45 minutes. During this period and after the additives had formed a liquid slag, normal cubic meters of oxygen were blown into the liquid melt. As a result `of this treatment the melt had a composition of .10% C, .25% Mn, .00% Si, .012% P and .032% S. The slag was partially removed and the melt which contained about 6 p.p.m. H2, 0.006% oxygen and about 0.005% nitrogen, was treated -by the addition of manganese to bring the content to .35%.

The steel was then tapped from the furnace into an apparatus as shown in FIG. 1 and having a 70ton capacity. The size of the opening 7 was 1% inches in diameter and the splash guard 7a had a length of 20 inches. The capacity of the hopper .5 was five tons. The ladle 1 was evacuated through the suction pipe 4 to a vacuum of about 0.2 millimeter Hg, utilizing a vacuum pump. When the steel reached the level of about 12-14 inches in the hopper l5, the stop rod 13 was raised and the fusible plug 9 of lead melted, causing the steel to flow downward through the opening 7 and into the chamber in the form of a spray of particles having a size of below about 10 millimeters. Due to the fact that the steel was unkilled and relatively large quantities of gas generated, the pressure in the ladle 1 rose to about 2 millimeters Hg. The pipe 4 became heated due to the hot gases drawn off and were cooled with lthe compressed air jets 13. The rate of pouring from the furnace 16 was maintained at about 6 tons per minute, which maintained a substantially uniform height of the liquid pool in the hopper -5 at between about 10-20 inches. After the complete charge had been emptied into the ladle 1, `the stopper 13 was closed and the hopper 5 substantially filled `with the slag from the furnace. The steel was maintained in the ladle 1 under the influence of the vacuum until it was substantially quiescent, which took about 6-10 minutes.

The liquid steel charge contained in the ladle 1 was thus partially killed by the degasication action and the reaction of oxygen with the carbon reducing the carbon content to .08%. The valve 6 was then closed, the suction pipe 4 disconnected, the valve 6 opened to allow entry of the ambient atmosphere. The ladle was then transported by a crane and the steel tapped from the ladle by lifting the tapping rod 8. This caused a melting of the fusible plate 10, and the stream of steel was poured into an ingot mold for the formation of a plate ingot. The gas content of the treated steel was reduced to 11/2 parts per million hydrogen and .002% oxygen and .003% nitrogen.

Example 2 Example 1 was repeated except that 250 normal cubic meters of oxygen were blown into the melt to reduce its carbon content to .03%. The steel obtained after the degasing operation as described in Example 1 had a carbon content of .008% and was excellently suited for applications requiring low carbon steel, as for example pro- 9 ducing plates for enameling, transformer laminates, and the like.

Example 3 Example l was repeated except that the steel was initially killed in the furnace by first removing the liquid oxidizing slag and adding 2% burnt lime, 1/2 CaF2, and 100 kg. ferro-silicon having a 75% silicon content. After these additives had formed a surface slag, 30 kg. of carbon powder were distributed over the surface of the slag. 175 kg. of ferro-silicon were then added to the melt and the killed melt obtained was tapped into the vacuum device as described in Example 1. As the stream passed into the vacuum device, 250 kg. of a desulfurizing slag containing calcium oxide, aluminum oxide, silicon dioxide, CaFZ and FeSi, were added over the period of the pouring through the sluice 11. This resulted in a reduction of the sulfur content to below .010%. The melt in the ladle was then magnetically stirred unde the action of the vacuum for minutes and ingots were cast from the steel, as for example for the production of boiler plate steel.

Example 4 Example 1 may be exactly repeated except the steel produced in a converter, rather than the electric arc furnace.

Example 5 A 60-ton charge of 60% scrap and 40% pig iron was melted in an open-hearth furnace to obtain an initial temperature of the melt of about l500 C. The melt contained 1% C, .35% Mn, .06% Si, .035% P and .032% S.

The steel was heated and treated in the conventional manner for about 2 hours, resulting in a melt which contained .35% C, .30% Mn, .020% P and .022% S. The steel was then tapped into the vacuum ladle as described in Example l, and a treating agent in the form of .2% Mn was added to the stream of the steel as it flowed into the hopper 5. After the pouring had been completed .25% silicon, in the form of ferro-silicon, was added through the sluice 11 and the melt magnetically stirred for 5 minutes while the melt was maintained under vacuum.

The magnetic stirring is effected by constructing the ladle with its bottom portion to the height of about 1 meter of non-magnetic steel and surrounding this bottom with a rotating magnetic field provided through a coil. The ladle was then disconnected from the suction line and transported and sealed in place on the top of a second vacuum chamber provided with a forging mold. The steel was then tapped from the ladle 1 into the second vacuum chamber directly into the forging mold. The second vacuum chamber was maintained under a pressure at least as low as the pressure in the first vacuum chamber during the initial degasification. As molten metal poured from the ladle 1 into the second vacuum chamber, it was subjected to a second stream degasication. In the same manner, instead of being directed into the second vacuum chamber, the steel may be continuously cast as it leaves the ladle 1.

Furthermore the molten steel may be directly tapped from the ladle 1 into a foundry mold.

Example 6 In a 50-ton electric arc furnace with a transformer output of 20,000 kw., the following material was charged:

Kg. Scrap 48,000 Pig iron 2,000 Calcined lime 1,200 Iron ore 800 With this charge the following analysis was theoretically expected when the meltdown became complete:

Percent C 0.45 Mn 0.55

Cr, Ni, and Cu were expected to be present only in slight amounts as tolerable impurities.

The first test should produce the following approximate analysis in view of the changes known by experience to occur during the melt-down:

Percent C 0.35 Si Less than 0.02 P 0.018 S 0.031

The standard analysis for CK 35 carbon steel for crankshafts is as follows:

C 0.32 to 0.37. Mn 0.50 to 0.70. Si 0.15 to 0.35. P 0.020 max. S 0.015 max.

The scrap was fully melted down after 2 hours. In the last 20 minutes before sampling, 250 kg. of lump ore and 50 kg. fluorspar were shoveled by hand into the slag, and then the melt was tested.

Test 1 gave the following analysis:

Percent C 0.30 Nin 0.41 S1 0.01 P 0.017 S 0.033 Cr 0.04 N1 0.07 Cu 0.11

Therefore, the addition of lime and ore to the charge and the later injection of a small amount of ore resulted in a dephosphorization during the melt-down to 0.017% P, while the theoretically expected content was 0.031%.

In the case of sulfur, the corresponding figures were 0.042% S and 0.033% S in the sample.

The first slag was largely drawn off and a new slag was added having the following composition:

Kg. Ground Calcined lime 1,400 Fluorspar (ground) 450 Coal dust 50 Silicon dust The sample of the melt had a temperature of 1530" C. Then the heat input was increased with a higher power output. In 45 minutes a temperature of l670 C. was reached. In the meantime, an electric agitator was run several times for several minutes each time to stir up the steel bath and the slag.

20 minutes before tapping 110 kg. FeMn (75% Mn) were thrown into the furnace to bring the manganese content up to within the prescribed limits (0.50-0.70% Mn); the addition was calculated to make it 0.56% Mn.

During the heat-up period of 45 minutes provision was made by the addition of 60 kg. silicon dust (75 and 32 kg. coal dust to produce a reducing slag and to keep its reducing power constant or improve it.

minutes before tapping, Sample 2 was taken, which showed the following analysis:

C .Percent 0.34

Mn do 0.56

Si do 0.06

P do 0.018

S do 0.023

H2 6.1 cc./100 g. steel. O2 -Percent 0.007

N2 do 0.007

Shortly before tapping another sample was taken from the furnace (Sample 3) and showed the following analysis:

C .Percent 0.33 Mn do 0.56 Si do 0.23 P do 0.017 S do 0.018 H2 6.4 cc. per 100 g. O2 .Percent 0.008 N2 do 0.007

Immediately after Sample 3 120 kg. FeSi (75% Si) were thrown into the furnace to bring the silicon content of the heat up to about 0.30% according to the standard.

2 hours and 45 minutes after the current was turned on, the charge was tapped and at the same time subjected to a vacuum treatment in a degasing ladle as shown in FIG. 1. The pressure in the vacuum equipment was 0.1 mm. Hg at the beginning and did not exceed 2.0 mm. Hg during the treatment.

After tapping the fourth sample analysis was as follows:

C Percent 0.36 Mn do 0.58 Si do 0.27 P do 0.018 S do 0.014 H2 1.2 cc./100 g. O2 .Percent 0.003 N2 do 0.005

The tapping temperature was 1680o C. and the pouring r temperature 1570 C. The current consumption amounted to 578 kwh. per metric ton for a melting time from tap to tap of 3 hours 10 minutes, whereas the melting time in this furnace under the operation methods used hitherto would have amounted to about 5 to 51/2 hours.

5ton ingots were top-poured with hot tops. The pouring rate was about 21/2 metric-tons per minute. The steel corresponded at least to steel made in the normal manner, in all its chemical and physical characteristics namely in cross samples from top and bottom, Baumann print, etch test, blue shortness tests, purity in core and skin, analyses of skin and core for C, S, O and N, in specimens from the center of the ingot, longitudinal tear specimen from skin and core as delivered and after cooking out at about 160 C. notch impact strength transition temperature in skin and core on longitudinal specimens, microstructure in skin and core, fracture structure in skin and core, hydrogen analysis from the core, conversion behaviour, annealing characteristic, Baumann print (transverse), etch test (transverse), blue shortness, purity in skin and core, analysis for C, S, O and N from skin and core.

Example 7 A steel was to be made for dies with the following specified analysis:

Percent C 0.53-0.58 Si O20-0.35 Mn 0.50-0.70 P 0.020 S 0.020 Cr 0.60-0 80 Mo 0.30-0 35 Ni 1.50-1.80 Va 0.07-0.12

The charge melted down in the arc furnace showed the following analysis on the first test:

Percent C 0.45 Si 0.12 Mn 0.55 P 0.025 S 0.016 Cr 0.73 Mo 0.33 Ni 1.70 Va 0 The silicon content has been lowered extensively by the addition of ore. The melting down temperature amounted to 1510 C. 15 minutes after the first test the melt was slagged off and a light carbidic slag was added. Shortly before the expiration of an hours heat-up time ferrovanadium, ferromolybdenum and some aluminum were added to the melt. After an hour the temperature reached 1695 C. and the melt was directly poured into the tapping ladle shown in FIG. 1 at a temperature of about 1690 C. During the entire tapping time the pressure in the Vacuum ladle was less than 5 torr. The duration of this vacuum treatment was about 7 minutes. The charge was poured from the tap ladle into an ingot mold that had been set up in a vacuum chamber. The vacuum there was about 1 torr. The pouring of the 10- ton ingot took about 10 minutes. 0.10% graphite was added to correct the carbon content. The steel of this ingot gave the following final analysis:

Percent C 0.57 Si 0.21 Mn 0.59 P 0.015 S 0.007 Cr f 0.77 Mo 0.35 Ni 1.57 Va 0.10 A1 0.01

This steel had strength qualities entirely in accordance with those found in steels of this type when they are made by the former methods.

While the invention has been described in detail with reference to certain specic embodiments, Various changes and modifications which fall within the spirit of the invention and scope of the appended claims will become apparent to the skilled artisan. The invention is therefore only intended to be limited by the appended claims or their equivalents, wherein I have endeavored to claim all inherent novelty.

What I claim is:

1. Process for the vacuum degassing of metal which comprises melting the metal in a furnace under substantially atmospheric pressure; directly passing a contiguous stream of the molten meltal downwardly from the furnace into a container open to the atmosphere and surrounding the inlet opening of a vacuum chamber maintained at an absolute pressure below mm, Hg and through said inlet opening into the vacuum chamber, maintaining the flow rates of molten metal from the furnace to the container, and through the inlet opening into the vacuum chamber in relation to each other to maintain a pool of molten metal with an upper surface open to the atmosphere in said container to seal the inlet opening, thereby preventing loss of vacuum in the chamber; separating slag entrained in the molten metal from the surface from said molten metal in the container by flotation upward to said surface open to the atmosphere, passing a molten metal stream through said inlet opening, dividing said stream into a spray of droplets as it passes into the vacuum chamber; and removing said separated slag from the surface of the molten metal pool so that the metal introduced into the vacuum chamber is substantially slag-free.

2. Process according to claim 1 in which the metal is steel.

3. Process according to claim 1 in which said metal is a ferrous metal and wherein the pressure in the vacuum chamber is maintained below 50 mm. Hg.

4. Process according to claim 3 in which said metal is steel and wherein the pressure in the vacuum chamber is maintained below 30 mm. Hg.

5. Process according to claim 4 in which the stream of molten steel is passed from the furnace in a substantially unkilled condition and is at least partially killed as it passes into the vacuum chamber.

6. Process according to claim 1 in which said metal is steel which is melted in the furnace from a charge having carbon content approximately equal to the carbon content of the steel to be produced and in which the temperature of the melt is raised to the pouring temperature immediately prior to passing said stream into the container.

7. Process according to claim 6 which includes dephosphorizing the melt prior to raising the temperature thereof to the pouring temperature.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES AIMME Transactions (Iron and Steel Division), published by the Institute, New York, 1929, pp. 428-445, article by Ziegler,

Metals and Alloys, vol. 1, No. 15, September 1930, pp. 712-713.

The Making, Shaping and Treating of Steel, 7th ed., 1957, United States Steel Corp., Pittsburgh, Pa., p. 314.

Transactions of the American Electrochemical Society, vol. 32, 1917, pp. 165-182, articles by Yensen.

DAVID L. RECK, Primary Examiner.

WINSTON A. DOUGLAS, Examiner.

Claims

1. PROCESS FOR THE VACUUM DEGASSING OF METAL WHICH COMPRISES MELTING THE METAL IN A FURNACE UNDER SUBSTANTIALLY ATMOSPHERIC PRESSURE: DIRECTLY PASSING A CONTIGUOUS STREAM OF THE MOLTEN METAL DOWNWARDLY FROM THE FURNACE INTO A CONTAINER OPEN TO THE ATMOSPHERE AND SURROUNDING THE INLET OPEING OF A VACUUM CHAMBER MAINTAINED AT AN ABSOLUTE PRESSURE BELOW 100 MM. GH AND THROUGH SAID INLET OPENING INTO THE VACUUM CHAMBER, MAINTAINING THE FLOW RATE OF MOLTEN METAL FROM THE FURNACE TO THE CONTAINER, AND THROUGH THE INLET OPENING INTO THE VACUUM CHAMBER IN RELATION TO EACH OTHER TO MAINTAIN A POOL OF MOLTEN METAL WITH AN UPPER SURFACE OPEN TO THE ATMOSPHERE IN SAID CONTAINER TO SEAL THE INLET OPENING, THEREBY PREVENTING LOSS OF VACUUM IN THE CHAMBER; SEPARATING SLAG ENTRAINED IN THE MOLTEN METAL FROM THE SURFACE FROM SAID MOLTEN METAL IN THE CONTAINER BY FLOTATION UPWARD TO SAID SURFACE OPEN TO THE ATMOSPHERE, PASSING A MOLTEN METAL STREAM THROUGH SAID INLET OPENING, DIVIDING SAIS STREAM INTO A SPRAY OF FROPLETS AS IT PASSES INTO THE VACUUM CHAMBER; AND REMOVING SAID SEPARATED SLAG FROM THE SURFACE OF THE MOLTEN METAL POOL SO THAT THE METAL INTRODUCED INTO THE VACUUM CHAMBER IS SUBSTANTIALLY SLAG-FREE.