US2993780A - Method for treating steel in vacuo - Google Patents

Description

July 25, 1961 M. ALLARD 2,993,780

METHOD FOR TREATING STEEL IN VACUO Filed Nov. 12, 1958 2 Sheets-Sheet 1 A/IS A arm/f July 25, 1961 M. ALLARD 2,993,780

METHOD FOR TREATING STEEL IN VACUO Filed Nov. 12, 1958 2 Sheets-Sheet 2 United States Patent Ofitice Patented July 25, 1961 METHOD FOR TREATING STEEL IN VACUO Marc Allard, Saint-Germain-en-Laye, France, assignor t llnstitut de Recherches de la Siderurgie, Saint-Germainen-Laye, France, a professional institution of France Filed Nov. 12, 1958, Ser. No. 773,333 Claims priority, application France Nov. '16, 1957 Claims. (Cl. 75-49) The essential part played by elements existing in very small amounts in alloys and which are generally termed oligo-elements has been proved by the investigations made by all metallurgical laboratories. The modern tendency leads consequently to a maximum control of the contents of such oligo-elements in alloys and chiefly in alloys of iron.

The present possibilities of production on an industrial scale of steel in a very high state of purity allow obtaining, through various methods and starting from cast iron, steel with low contents of metalloids, such as carbon, sulphur, phosphorus and nitrogen, while retaining the desired temperature of production. The obtention of low and very low contents of carbon is however associated, as a consequence of the general laws of physical chemistry, with an increase of the oxygen contents of the metal. The deoxidization of metal through the presently used methods is accompanied, on the other hand, by the formation of inclusions within the mass of the metal. Their presence reduces consequently the advantages which might be obtained, on the other hand, by the high grade of purity of the metal.

On the other hand, it is possible to resort to deoxidizing and decarbonizing steps in order to obtain the whole range of structures between rimming steels and killed steels which latter are characterized by particularly low contents of oxygen.

Even in the case of certain applications, chiefly when it is desired to obtain forged members starting from ingots the weight of which may range up to 200 tons, experience shows that it is of advantage not to rise beyond hydrogen contents equal to 2 p.p.m. chiefly in the case of parts such as turbine or alternator rotors.

The idea of benefiting by the gasiform character of the products eliminating oxygen, hydrogen and carbon is not novel and the advantageous treatment in vacuo for removing gases, generally speaking, and for a marked decarbonization and deoxidization of metals in particular, is actually known since along time. However, the use of the arrangements known hitherto has been limited by the complexity of the arrangements to be foreseen, by the thermal difficulties and by the importance of the ferro-static metal pressures inherent to the treatments executed in a ladle.

In particular, when it is desired to remove the gases in the casting, the necessity of waiting for a complete solidification of the ingot before proceeding with a further treatment in vacuo leads to the drawback that a plant occupying considerable room inside the casting hall allows only a limited number of treatments. It should also be considered that when removing the gases during the casting, the spreading and dispersion of the jets at the output end of the nozzle in spite of the advantages obtained as far as the gas-withdrawal is considered, leads to serious drawbacks for the surface condition of the ingot.

The withdrawal of the gases before casting shows in contradistinction considerable advantages as to adaptability and, in particular, it is possible to cast during the same operation a plurality of ingots of medium size, to incorporate ingredients correcting total or partial killing, taking into account the behaviour of the metal inthe ingot mold, and also to allow readily a continuous casting of the steel.

As far as the removal of gases from the metal is concerned, anyone skilled in the art is aware that it is always of interest to obtain a pressure as low as possible while the execution of the treatment is all the more difficult when the pressure required is lower.

Now, the pressure exerted on a small volume of metal is the sum of the ferro-static pressure and of the pressure prevailing inside the chamber containing the metal. Hitherto only the pressure prevailing in the chamber has been controlled for reducing the total pressure, except in the case of the methods applying vacuum during the casting.

The main object of the invention consists in producing, during the working cycle, conditions allowing each elementary volume of metal to be subjected before casting to a reduced ferro-static pressure as provided by resorting to stirring movements produced by the gases, which allows the use of a less marked vacuum.

The method of producing steel in vacuo, according to my invention, consists in treating a metal, before its casting into an ingot mold, inside a container provided with perfect fluidtight means and in stirring same by means of gases so that each molecule is protected at some moment of the treatment, both against ferro-static pressure and atmospheric pressure.

The stirring is executed under conditions furthering to a maximum the removal of the own gases of the metal through the upper section of the metal bath and while another gas is substituted for the gases, the action of which may have failed as a consequence of a preliminary Withdrawal of the gases, of the absence of an element such as carbon or free oxygen, or of the ferro-static pressure.

In the case of so-called non-killed steels, there are produced through a spreading of the bath conditions which allow metal to be subjected throughout to the stirring due to its own deoxidization producing chiefly carbon monoxide. When this gas withdrawal slows down or when it is not sulficient, as in the case of steel killed under atmospheric pressure, there is associated the action of vacuum which should be obtained at a value under 100 mm. of mercury with a bubbling of a gas such as argon, helium, nitrogen, C0 or possibly CO or air, said gases being introduced into the lower section of the ladle or the like container through at least one porous element made of a refractory material or metal pervious for the gases and impervious for liquid metal.

The method is also suitable for baths having a substantial hei ht, for instance in the case of-operation with a casting ladle. An important auxiliary feature of the bubbling step consists in that it ensures a gas-releasing effect by setting the metal in presence of bubbles wherein the partial pressure of gases other than the substitute gas, such as hydrogen, is small or equal to zero. The efiect is however more marked, under conditions otherwise identical,

when the total pressure exerted on the bubbles is lower and consequently, when the metal is carried at a higher point of the ladle. The stirring plays therefore an essential part by producing a renewal of the upper layers.

The method for treating steel in vacuo in the case of steels which are not killed under atmospheric pressure consists in producing a steel which is not killed through its conversion inside a container provided with perfect fluid-tight means and designed in a manner such that the steel bath may assume a reduced height, after which a complete slag-removal is executed, the container is positioned so that the height of the bath may be reduced to a minimum and the gases are pumped out until a suflicient vacuum is obtained, in a manner such that the oxygen and the carbon may combine throughout the height of the bath and are removed in the shape of gasiform compounds under favorable heat conditions, ascribable in particular to the fact that the producing means operate in accordance with the laws governing the black body.

The complete skimming of the steel bath is essential since the stirring produced by the evolution of the gases in vacuo which sets in contact with each other a deoxidizedmetal and an oxidized slag would produce in the absence. of such a skimming a reduction of the slag and a reoxidizationof the bath, which would result in an increase of'theduration of operation. In the case of a phosphorus-containing pig-iron, the phosphorus would also return into the metal bath. The skimming may be made easier through the use of an electromagnetic stirring device of a conventional type. It is also possible to make the ladle turn so that the last traces of the slag stick to the wall and remain adherent thereto. The stirring means may also serve at the end of the gas-withdrawingstage in order to return the lower layers of the metal bath towards the surface and to reduce thus the ferro-static pressure to which the metal of said layers is subjected. I may also use in such a case, if no stirring means are available, the arrangement described hereinafter for steel killed under atmospheric pressure, i.e. a porous element which allows obtaining in a much cheaper manner an inputof gases at one or more points of the metal, with a view to furthering the production of bubbles and the stirring of the liquid bath.

.The deoxidizing .and/ or decarbonizing operation in vacuo is possible as a consequence of the reduced depth of the bath since all the inner walls of the container have been heated by the actual refining and consequently the losses of heat through conductivity and radiation are reduced to a minimum. Thus, it is possible to set the container in a manner depending on its shape so as to spread out theba-th without cooling same by walls having a too low temperature.

After treatment, the casting is executed inside an intermediate ladle or directly starting from the producing container inside ingot molds or inside an arrangement for continuous casting. As a matter of fact, if the treat ment is continued for-a sufficiently long time, I obtain a killed steel without any aluminium, which steel is particularly suitable for continuous casting both because it is killed, because it contains no aluminium and consequently shows no superficial defects due to the presence of said aluminium and because it associates low contents of foreign metal elements which are characteristic of steel obtained directly starting from virgin pig-iron with low sulphur, phosphorus. and nitrogen contents, such as those which may be obtained nowadays, and very low contents of carbon and oxygen.

It is also possible to control the formation of inclusions. It is lastly generally possible to control and adjust the contents of carbon underneath 0.05% which is a very diflicult matter with the methods known hitherto and, more particularly, if said contents are to be associated with low oxygen contents and consequently, with a very reduced amount of inclusions.

The arrangement for executing the above-disclosed method comprises a metallurgical container for a conversion treatment, means for closing fiuidtightly all the openings of the container, means for positioning said container in a substantially horizontal position, in order to give the bath a maximum depth and means for pumping the gases out of the container and obtaining vacuum therein.

As mentioned hereinabove,.my improved method is applicable inside a ladle tosteel killed at atmospheric pressure and, in such a case, there is produced, over said ladle a vacuum under 100 mm. Hg and a gas such as argon, helium, nitrogen, carbon monoxide or carbon dioxide is caused to bubble through the metal, said gas being introduced into the lower section of the ladle or the like container through at least one porous element of refractory or metal material, which is pervious for gases and impervious for liquid metals. 7

It is possible to begin by the bubbling after which vacuum is'applied while the gas continues being blown into the lower section of the ladle. It is also possible to begin by applying vacuum, after which an automatic adjustment of the throughput of inert gas is performed with a view to maintaining vacuum at a predetermined value depending on the rate of the gas withdrawal to be provided. The throughput of gas and the extent of vacuum may be adjusted so as to produce a swelling of the superficial section of the molten metal which is no longer covered by slag and which, consequently, more readily releases its hydrogen. Lastly, it is possible to substitute an acid slag for the basic slag with a. view to reducing to a minimum the amount of hydrogen incorporated with the slag and to avoiding any attack of the refractory material of the ladle or of the stopper.

The arrangement used for executing said modification includes a casting ladle or the like metallurgical container of a fluidtight structure carrying at its lower end a casting nozzleopening into a fluidtight recess, together with one or more porous elements of refractory brick or metal material, the upper section of the ladle being provided with a cover, the lower end of which cooperates with the fiuidtight means carried by said ladle.

For a readier understanding of my invention, I will now describe some embodiments given by way of a mere exemplizfication without any limiting sense being attached thereto. In the accompanying drawings illustrating diagrammatically said embodiment:

FIG. 1 is a partial view of an arrangement according to the invention during the conversion step, without the pumping means and without the closing cover being illustrated.

FIG. 2 is a diagrammatic view of the arrangement provided with its closing and pumping means.

FIG. 3 is a diagrammatic showing of the apparatus during the skimming stage.

FIG. 4 is a partial View of the closing means.

FIG. 5 is a sectional view of a modification for the execution ofthe invention in the case of killed steel.

FIG. 6 illustrates diagrammatically a phenomenon which mayoccur at the surface of the bath during the operation inside the arrangement of FIG. 5

The metallurgical container used as illustrated in FIG. 1 is given a preferably elongated shape. It is provided with trunnions 2 which allow setting it on a support 3. The opening of the container is provided with an outer flange 4 carrying a frusto-conical element 5 the part played by which will be disclosed hereinafter, A lance 6 is adapted to provide for the treatment of a metal bath 7 inside the container.

The gas withdrawing system illustrated in FIG. 2 includes all the elements of my improved arrangement. The flange 4 mentioned hereinabove cooperates with a packing 8 made for instance of rubber or plastic material and carried by a cover 9 provided with an inspection gate 10 which allows checking the withdrawal of the gas.

FIG. 4 shows diagrammatically on a much larger scale the position of the packing 8 with reference to the bearing surface on the flange 4. The packing 8 is urged against-said'bearing surface4 by the elastic means illustrated. symbolically at 11 in FIG. 2. In FIG. 4, it is apparent that the frusto-conical member 5 plays the part of a guide for the positioning of the cover 9. The latter includes at its outer end an elastic coupling member illustrateddiagrammatically at 12 and adapted to further its positioning and to make up for any lack of alignment with reference to the container 1, said elastic coupling being connected with the cooling means 13. Lastly, said cooling means 13 are connected in their turn with a vacuum pump 14 provided with dust-removing means which are not illustrated.

The container 1 rests on the electromagnetic skimming device 15 carried by the carriage 16. This electromagnetic skimming :devicemay pivot. round a spindle .17, as shown in FIG; 3, so as to allow the skimming to..be

executed. Lastly, a porous element 151: is fitted in a wall of the container. Said element provided with a connection which is not shown in the drawings for connection with a gas-feeding pipe may serve, as mentioned hereinabove, for instance in the case of the absence of any stirring means. I

I will now describe an operation executed in conformity with the method according to my invention.

First example.--About 6 metric tons of liquid phosphorous pig-iron are introduced into the container 1 and are treated with oxygen, for instance through the nozzle, inside the container. Once the operation has reached a stage of progression such that the carbon contents of the bath are of a magnitude of 0.03 to 0.09% with suitable phosphorus contents, for the applications considered, of about 0.020%, after at least one conventional skimming operation has been performed, the container 1 is brought onto the carriage 16 as shown in FIG. 3 and set in a substantially horizontal position. The tilting of said container is obtained through any known means which are not illustrated, whereby the major fraction of the slag is poured into a vat 18, after which the skimming is brought to completion through the agency of the skimming and stirring means 15, associated with the rabbles 19 until the bath is made bare.

The container I mounted on the carriage 16 is then brought into registry with the gas withdrawing arrangement described hereinabove and illustrated in a highly diagrammatic manner in FIG. 2. Said carriage is brought to the front of the gas evolving head, after removal of the parts not illustrated, providing a protection against projections for the frusto-conical member 5 and the flange 4 engaging the packing. The member 5 cooperates in the ready positioning of the packing 8 over the flat flange 4, which positioning is made easier by reason of the elastic vacuum-tight connection 12 already referred to. When the packing 8 has reached substantial contact with the flange 4, the elastic means illustrated diagrammatically at 11 urge the packing against the flange so as to obtain a fluidtightness which is sufficient for initiating vacuum.

The vacuum pumps are then started and the attendant may check through inspection gates 10, the intensity of bubbling of the bath at the beginning of operation. Vacuum is maintained under a predetermined pressure and for a predetermined duration. If it is desired to obtain a complete killing, the bath is stirred through the porous element 15a in order to save time. When the vacuum is broken, the carriage is caused to recede and the container 1 is suspended to a travelling crane and the casting is performed either directly out of the container or else, after transfer, into an intermediate ladle for continuous or discontinuous casting.

Obviously, the embodiment described hereinabove is not the only one which may be executed; it is possible for instance to provide a stationary container with a movable gas-withdrawing head and, similarly, it is possible to execute the conversion through any other suitable method. It is also possible, in the case of a rotary furnace, to use a plurality of gas-withdrawing heads or a single gas-withdrawing head and a plurality of covers similar to the cover 9 with its auxiliaries including the cooling means.

The modification illustrated in FIG. 5 includes chiefly a casting ladle 20 which is similar in shape to the ladles usually resorted to in steelworks, but the walls and bottom of which are vacuumtight. The bottom of the ladle 21 is provided with a conventional casting nozzle 22 opening into the center of a ring 23 welded to the bottom of the ladle and carrying a shoulder 24. An aluminium plate 25 provided with a suitable packing 25a closes the ring 23. The vacuum obtained above said sheet 25 through the connection 26 urges said sheet against the packing 25a.

The bottom of the ladle is equipped with means 27 pervious for gases and impervious for liquid metal. Said means may include a sintered, refractory metal or compound element connected with external means in a manner such that the gas may be guided solely towards the molten material. The sintered materieal may include a substance, the incorporation of which in the bath through wear or dissolution improves the grade of the metal, either through nucleation or through dissolution or reaction of the oligo-elements. The action of the gas ensures a homogeneous distribution of thwe oligo-elements or traces of elements.

The casting ladle is provided with a stopper 32 cooperating with the nozzle 22 and with a cover 33 which is provided with a lining 34' made of a refractory cement. The edge of said cover is provided with a groove constituted by a channel iron 35 inside which is fitted a packing 36 for instance of a material having as a base a silicone, said packing engaging in its operative position an annular angle bar 37 coaxial with the ladle so as to prevent the packing from being damaged by the comparatively high temperature of the ladle; said angle bar is carried by an annular disc 38 welded at 39 to the ladle and stayed by the stay plates 39a. The outer diameter of the annular disc is large enough for the control slider 40 of the stopper 32 to remain carried by the ladle with a view to maintaining the plug or the nozzle or stopper head over the latter throughout the gas-withdrawing operation. Said stopper-controlling slider passes at 42 through the disc 38 and a packing which is not illustrated ensures its fluidtight passage at said point 42. Lastly, the cover 33 carries a pipe 43 connected with the vacuum producing means. To obtain the desired vacuum, I resort to an ejector or to a mechanical pump provided with heat exchangers and/or suitable dust removers.

I will now disclose two further examples of operation according to my invention.

Second example.1n a ten-ton basic electric furnace there are obtained twelve tons of steel of the following composition:

C 0.07 Mn 0.40 Si 0.20 Cr v 16.80 Ni 2.40

The final slag is removed from the furnace. However,

with a view to increasing safety, an acid slag produced by another operation is positioned in the ladle which has been preheated in the usual manner, said slag being measured through suitable additions, so as to form altogether 3% of the weight of the steel and having the following composition:

The casting temperature being equal to 1,680 C., the ladle is filled with steel without any of the usual additions of deoxidizing agents and the cover is set in position. Before applying vacuum, I begin blowing through the porous wall sections argon or one of the other inert gases already mentioned. During this operation, a fraction of the dissolved hydrogen is removed. This result being obtained, vacuum is applied gradually, While the throughput of argon is reduced continuously until the desired pressure of 75 mm. of Hg is obtained. The hydrogen contents of the steel inside the furnace at the moment of the casting were equal to 8.5 ppm. while the final contents according to a sample removed from said ingot were equal to 2.7 ppm. The total volume of argon used was 250 litres per ton. It should be noted that during the vacuum treatment the deoxidization is brought to an end which deoxidization has been executed without any of the conventional deoxidizing reagents, so that less inclusions are obtained than hitherto.

Third example.ln a basic six-ton electric furnace, 7.5 tons of steel having the following composition have been obtained:

C 0.33 Ni 3.25 CT 1.25 Si 0.25 Mn 0.40 Mo 0.30 S 0.015 P 0.020

The final slag being removed, there is introduced into the preheated ladle an acid slag obtained by a further operation, the weight of which slag has been adjusted so as to form altogether 2% of the weight of steel, while it has the following composition:

Percent Si 60.0 A1 0 3.6 FeO 10.5 MnO 15.0 CaO 4.5

The temperature of the cast material being equal to 1,670 C., the ladle was filled to the extent of one-half and the argon was then introduced through the porous wall section. As soon as the ladle has been filled and the cover positioned, vacuum was applied to the ladle and the throughput of argon was produced in a continuous manner so as to obtain the final desired pressure of 60 mm. of Hg the hydrogen contents of the steel inside the furnace at the moment of the casting being equal to 6.5 p.p.m. while the final contents measured on two samples removed from the first ingot were respectively equal to 1.4 and 1.5 ppm. The total volume of argon used was equal to 350 litres per ton.

.Such an introduction shows furthermore a considerable advantage. There is produced over the free surface of themetal bath a swelling illustrated at 44 in FIG. 6. Said swelling, the location of which depends on the location. of the. porous element or elements is completely freed of slag so that the conditions are much more favorable for the withdrawal of gases and there is no layer of slag in the swollen area. This fact is important since recent experiences seem to show that a fraction of the hydrogen contents in the metal is due to the presence of the slag. When the layer of slag covers the metal, it is therefore necessary to remove the gases from the slag so as to lower the level of the hydrogen contents in the metal. In contradistinction, when no layer of slag covers the metal, the hydrogen carried along by the bubbles of argon is pumped directly out'of the liquid steel and this cuts out any pollution 'due to the simultaneous stirring of the slag with the metal.

Obviously, it is possible to introduce the gas through a plurality of porous wall elements which may be positioned along the side walls of the ladle. Similarly, the arrangement for producing vacuum may be carriedby the cover of the ladle. Lastly, it is possible tocontrol in a continuous manner the ratio between the volume of exhausted gases and the throughput of inert gas, so as to obtain the optimum conditions for exhausting gas out of the liquid metal.

It is also possible after the gas withdrawing step to protect the casting jet by nitrogen or the like inert gas pro jected by nozzles carried by a collar or the like support and spraying a protecting screen round the casting jet.

What I claim is:

1. A method of producing steel, comprising the steps of converting a liquid metal bath to steel in a reaction chamber, completely skimming the liquid steel bath, changing the depth of the bath in the chamber without loss of heat to reduce the bath depth to the lowest possible level in the chamber, blowing bubbles of a gas which is inert to the bath through the bath to stir the same, simultaneously applying a vacuum to the chamber, and then casting the liquid steel out of the chamber.

2. The method of claim 1, wherein the liquid metal bath is converted in an upright reaction chamber and the bath depth is changed by tilting the chamber on its side.

3. The method of claim 1, wherein said vacuum is less than mm. of mercury.

4. A method of producing steel, comprising the steps of converting a liquid metal bath to steel under a basic slag, completely skimming the liquid steel, then adding an acid slag to the liquid steel, blowing bubbles of a gas which is inert to the liquid steel through .the steel to stir the same and applying a vacuum below 100 mm. of mercury to the liquid steel until its hydrogen content has been reduced to a maximum of 2.7 ppm.

5. The method of claim 4, wherein the amount of the inert gas and of the vacuum as well as the location of the gas bubble source are so adjusted that a swelling of the upper level of the liquid steel is produced to clear the same of the slag and to increase the rate or" the removal of hydrogen.

References Cited in the file of this patent UNITED STATES PATENTS 1,921,060 Williams Aug. 8, 1933 2,040,566 Rohn May 12, 1936 2,054,923 Betterton et a1. Sept. 22, 1936 2,093,666 Vogt Sept. 21, 1937 2,776,204 Moore Jan. 1, 1957 2,811,346 ,Spire Oct. 29, 1957 2,837,790 Rozian June 10, 1958 2,848,317 Coupette et a1. Aug. 19, 1958 2,852,246 Janco Sept. 16, 1958 2,893,715 Harders et al. July 7, 1959 2,893,860 Lorenz July 7, 1959

Claims (1)

1. A METHOD OF PRODUCING STEEL, COMPRISING THE STEPS OF CONVERTING A LIQUID METAL BATH TO STEEL IN A REACTION CHAMBER, COMPLETELY SKIMMING THE LIQUID STEEL BATH, CHANGING THE DEPTH OF THE BATH IN THE CHAMBER WITHOUT LOSS OF HEAT TO REDUCE THE BATH DEPTH TO THE LOWEST POSSIBLE LEVEL IN THE CHAMBER, BLOWING BUBBLES OF A GAS WHICH IS INERT TO THE BATH THROUGH THE BATH TO STIR THE SAME, SIMULTANEOUSLY APPLYING A VACUUM TO THE CHAMBER, AND THEN CASTING THE LIQUID STEEL OUT OF THE CHAMBER.

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