Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/10—Supplying or treating molten metal
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
  • C21C5/00—Manufacture of carbon-steel, e.g. plain mild steel, medium carbon steel or cast steel or stainless steel
  • C21C5/56—Manufacture of steel by other methods
  • C21C5/567—Manufacture of steel by other methods operating in a continuous way
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/10—Supplying or treating molten metal
  • B22D11/11—Treating the molten metal
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10S75/00—Specialized metallurgical processes, compositions for use therein, consolidated metal powder compositions, and loose metal particulate mixtures
  • Y10S75/957—Continuous refining of molten iron

Definitions

• the invention relates to the production of liquid steel. It is particularly applicable to the production of high purity steels, with extremely low carbon contents, and even also nitrogen and oxygen.
• the circulation of metal between the pocket and the tank causes gentle agitation of the metal in the pocket, which favors good settling of non-metallic inclusions.
• a processing time of 10 minutes may be enough to lower the carbon content in the steel from 300 ppm to 20 pp.
• the object of the invention is to propose a new type of metallurgical reactor, which gives access to the carbon contents in the liquid steel of the order of 10 ppm and less under satisfactory productivity conditions.
• This reactor should also be able to be used to produce less deeply decarburized steels but with very low content of oxidized inclusions.
• the subject of the invention is a process for adjusting the composition of a liquid metal, characterized in that, from a starting container, the said liquid metal is sucked inside a reactor which is puts under reduced pressure with respect to said first container, said metal is made to flow inside said reactor according to a regime close to a piston flow and it is subjected to a metallurgical treatment, and in that, after its passage in said reactor, said metal is discharged into an inlet container with respect to which said reactor is also placed under reduced pressure.
• said starting container is supplied with metal continuously, and said metal is made to flow from said arrival container also continuously.
• the liquid metal is steel
• the metallurgical treatment comprises decarburization, denitriding or deoxidation by carbon under vacuum
• the arrival container is a continuous casting distributor.
• the invention also relates to an installation for adjusting the composition of a liquid metal, such as steel, characterized in that it comprises a starting container containing said liquid metal, a reactor, and a container of arrival, said reactor comprising a tank provided with means allowing the maintenance of its interior at a reduced pressure compared to those prevailing in said departure container and said arrival container, of a first plunger said ascending plunger of which one end is quenched in the liquid metal contained in said starting container and the other end of which is connected to the bottom of said tank, of a second plunger, said descending plunger, one end of which dips in the liquid metal contained in said arrival container and of which the the other end is connected to the bottom of said tank, and means for ensuring a continuous circulation of said liquid metal between said starting container and the edit inlet container through said plungers and said reactor vessel, and in that said vessel is shaped so as to impose on said metal liquid a flow close to a piston type flow on its path between said plungers.
• This installation can, according to the invention, be inserted in an installation for adjusting the composition of the liquid steel comprising:
• a coarse decarburization reactor for liquid metal comprising a tank provided with means for bringing it under reduced pressure, a first plunger dipping in the liquid metal contained in said first container and a second plunger dipping in an intermediate container, said tank also comprising means for blowing oxygen into said liquid metal;
• the new proposed reactor has points in common with RH, namely the presence of a tank placed under vacuum and two plungers, through which the metal enters the tank and leaves. But the principle of continuous circulation of the liquid metal between the ladle and the reactor is abandoned: the metal leaving the reactor here pours into another container than its starting container and will no longer return to this same reactor.
• the liquid steel in the reactor according to the invention must have a flow close to a piston flow. If necessary, as will be seen, this pseudo-piston flow is obtained by fragmenting the reactor into a multiplicity of perfectly mixed cells between which the exchange of matter is reduced to a minimum.
• this reactor can advantageously be inserted in a continuous or semi-continuous production line for liquid steel.
• FIG. 2 which shows schematically the whole of an example of production line and casting of an ultra-low carbon steel, in which is inserted a reactor according to the invention.
• the principle of the continuous recirculation of the metal between the ladle and the tank means that the tank is constantly supplied with less decarburized metal than is the metal being decarburized which is already there.
• the tank behaves like a perfectly stirred reactor, we can define at any time an average carbon content Cy of the metal in the tank, as well as an average carbon content CL of the metal in the ladle.
• CL is greater than Cy, it is understood that the decrease in Cy is slowed down by the addition of less decarburized metal, of carbon content C- ⁇ . This is well reflected in the mathematical models of decarburization which involve the
• Cy c— must be as close as possible to 1. It is of the order of 0.6 on average in conventional RH, and varies according to the stage of advancement of decarburization and the rate of recirculation of the metal.
• Figure 1 shows schematically the principle of an elementary reactor according to the invention, and an example of its insertion in a plant for the production of steel continuously.
• This installation comprises a channel 1 for supplying liquid steel 2 which flows continuously with a controllable flow rate from a metallurgical container not shown, such as a steelworks pocket or a machine.
• This primary production machine also works continuously.
• This channel 1 is connected to a cover 3 which covers an intermediate container 4 exposed to atmospheric pressure, in which the steel flows
• this reactor 5 internally coated with refractory, comprises:
• a tank 6 intended to contain at a given time 25 a certain quantity of liquid steel 2 in circulation; like the RH tanks, this tank 6 must be high enough so that its upper part is not likely to be too damaged by projections of liquid metal;
• a gas suction installation 7 intended to establish a reduced pressure in the tank 6;
• this descending plunger 10 does not open into the same container as the ascending plunger 8, but into an inlet container 11 which, in the example shown, is a continuous casting distributor.
• this distributor 11 is equipped with at least one outlet nozzle 12 thanks to which the liquid metal 2 flows continuously, with a flow controllable by a stopper rod or a drawer not shown, in at least one ingot mold 13 bottomless, with walls energetically cooled by an internal circulation of water. It is in this ingot mold 13 that the solidification of a crust 14 of steel begins which gives rise to a steel product 15, slab, bloom or billet according to the format of the ingot mold 13.
• the RH tanks have a substantially cylindrical shape, with an inside diameter of a few meters at most. This configuration gives RH the properties of a perfectly stirred reactor.
• the tank 6 must confer on the reactor 5 properties as close as possible to those of a piston flow reactor, where the steel which has undergone degassing and / or decarburization to given levels cannot then be mixed with less purified steel.
• one solution consists in giving the inside of the tank 6 the shape of a corridor, that is to say a channel of rectangular or approximately rectangular section, long and narrow, at the ends of which are connected the divers 8, 10.
• the ratio between the distance separating the divers 8, 10 and the width of the channel is at least equal to 6.
• the tank 6, for example, a width of 1 m to 1.50 m and a length separating the divers 8, 10 from 8 to 10 m.
• the efficiency of decarburization and degassing is largely a function of the ratio between the surface of liquid metal 2 offered under vacuum and the volume of this same liquid metal 2 present in the tank 5. This ratio must be as high as possible, which implies that, for a given volume of metal, the depth "e" of the metal present in the tank 5 must not be too great (0.40 to 0.80 m for example).
• This depth is governed by the geometry of the set of the installation, (in particular the differences in level between the tank 6, and the intermediate container 4 and the distributor 11) and also by the pressure difference ⁇ P between the inside of the tank 6 and the atmosphere to which the surfaces are exposed 16 and 17 of the liquid steel 2, respectively in the intermediate container 4 and in the distributor 11.
• ⁇ h is called the average difference in level between the surface of the liquid steel 2 in the tank 6 of the reactor and said surfaces 16 , 17 exposed to the atmosphere, we have the relation ⁇ h ⁇ ⁇ pg where p is the density of the liquid steel (approximately
• the depth e of the liquid metal 2 in the tank 5 is therefore relatively little dependent, in the usual pressure ranges, on the level of vacuum obtained in the tank 5.
• the circulation of the metal between the containers 4, 11 through the reactor 5 is governed in part by the injection of gas into the ascending plunger 8 if it is practiced there. But in all cases, the existence of this circulation and the flow of metal which it brings into play depend on the difference in level between the surface 16 of the liquid metal 2 in the intermediate container 4 and the surface 17 of the liquid metal 2 in the distributor 11. This difference in level is linked in particular to the difference between the supply flow rate of the intermediate container 4 and the flow rate of metal 2 leaving the distribution valve 11.
• the corridor shape for the tank 6 of the reactor 5 is the most suitable for establishing a piston-type flow for the liquid metal 2.
• the arrival of the metal in origin of the ascending plunger 8 and the gassing due to argon possibly injected into this plunger 8 and the decarburization of the metal cause strong agitation which can significantly degrade and in an uncontrollable manner the conditions of this flow. This is why it is advisable to carry out a flow approaching a piston flow by dividing the tank 6 of the reactor 5 into a series of perfectly mixed cells and between which the exchanges of liquid metal 2 are as limited as possible.
• dams 24-28 are arranged transversely to the general orientation of the tank 6 and delimit cells 29-34, each equipped with a fluid injection device 19-23 (except possibly the first cell 29, if an injection of fluid is carried out in the ascending plunger 8; it is then this injection which causes the agitation of metal in this first cell 29).
• This configuration contributes to good control of the flow rate of the metal 2 in circulation.
• Openings 36-39 are provided in the other dams 25-28 to allow minimal communication of the cells 30-34 between them ensuring the progression of the liquid metal 2 in the tank 6. These openings 36-39 are preferably placed in the parts lower dams 25-28 to allow complete emptying of the tank 6.
• the reactor 5 is provided with all the equipment (not shown) that can usually be encountered on RH, namely: one or more television cameras allowing operators to observe the surface of the liquid metal 2 in the tank, one or more devices for taking metal samples (it is advantageous to provide several staggered along the tank 5 to follow the evolution of the composition of the metal during vacuum treatment), one or more devices introduction of alloying elements, one or more oxygen blowing devices, one or more graphite resistors providing a preheating of the refractories of the tank 6. It is advantageous to install an oxygen insufflation device (lance or nozzle) at least in the first cell 29.
• one or more television cameras allowing operators to observe the surface of the liquid metal 2 in the tank
• one or more devices for taking metal samples it is advantageous to provide several staggered along the tank 5 to follow the evolution of the composition of the metal during vacuum treatment
• one or more devices introduction of alloying elements it is advantageous to provide several staggered along the tank 5 to follow the evolution of the composition of the metal during vacuum treatment
• the fluid injected into the bottom 18 of the tank 6 by at at least some of the devices 19-23 may not be argon, but a gas capable, initially, of partially dissolving in liquid steel, and the departure of which under the effect of vacuum tends to promote decarburization .
• This gas can be nitrogen and, above all, hydrogen. In doing so, of course, it is accepted that the content of the metal in this gas in most of the tank is higher than with the usual practice of argon insufflation.
• carbon is added to the bath in solid or gaseous form (for example in the form of CH4) at one or more places of tank 6, so that it combines with dissolved oxygen and decreases its concentration.
• a level deemed sufficiently low for example 80 ppm
• carbon is added to the bath in solid or gaseous form (for example in the form of CH4) at one or more places of tank 6, so that it combines with dissolved oxygen and decreases its concentration.
• Another advantage of the installation according to the invention is that the metal flow rate which passes through it is very moderate compared to that which circulates in an RH (10 rpm against 240 rpm). The refractories therefore wear considerably less quickly, in particular at the level of the plungers.
• this reactor 5 can also be used simply so as to decant said metal from a container initially containing a certain quantity of liquid steel (and no longer receiving it later) in another container, pocket or distributor, initially empty.
• a container initially containing a certain quantity of liquid steel (and no longer receiving it later) in another container, pocket or distributor, initially empty it is however necessary to arrange so that the surface of the metal in the arrival container is permanently at an altitude lower than that of the surface of the metal in the departure container. This may require moving these devices relative to each other during operation in a rather complex way, and requiring the departure and arrival containers to be shallow, so that their movements do not have a too large amplitude.
• the initial carbon content of the liquid steel is not very high at the inlet of reactor 5 (for example of the order of 80 ppm), a single reactor of reasonable dimensions makes it possible to reduce this content to levels of the order of 5 ppm, as will be seen in the example described below. If the initial carbon content is a few hundred ppm, an additional reactor can be added to reactor 5 located upstream of it in the continuous production line, which would have the function of bringing the carbon content to the level required at the inlet of reactor 5 (80 ppm in our example).
• This production line firstly comprises a primary production machine 40. Its function is to produce, continuously or discontinuously, a liquid steel whose composition must be adjusted in the course of operations.
• This machine 40 can, as shown in Figure 2, be a conventional LD type converter, that is to say in which the liquid iron which is introduced therein is transformed into liquid steel by decarburization. This decarburization is obtained by insufflation of oxygen by means of an emerged lance 42.
• any other known primary production machine may be suitable, for example a converter of the LWS type with oxygen blowing from the bottom, a blown converter, or an electric furnace producing steel liquid from scrap.
• this primary production machine 40 feeds liquid steel 2 to a large steelworks pocket 43 which acts as a buffer container.
• this pocket 43 pours continuously into a first container 44.
• the feed rate of this first container 44 is controlled by a drawer closure (not shown) of a type known per se (or its functional equivalent) ) located on the pocket 43.
• the liquid steel 2 is introduced into the first container 44 by a protective tube 45 in refractory designed to limit the absorption of atmospheric oxygen and nitrogen by the jet of liquid metal.
• the flow of steel leaving the pocket 43 is, for example, around 10 rpm, corresponding to the average productivity of the production machine 40.
• various operations for adjusting the composition of the liquid steel 2 can take place, in particular the addition of alloying elements, and especially desulfurization. It is indeed advisable to desulfurize the steel 2, when necessary, before the vacuum treatment. One reason for this is that this operation requires the addition of materials such as lime which may have a high moisture content. They are therefore capable of supplying hydrogen to the liquid steel 2.
• the desulfurization requires an intense mixing of the liquid steel 2 which thus risks absorbing atmospheric nitrogen. It is therefore necessary for the degassing of the liquid steel 2 to follow the desulfurization to compensate for the negative effects on the dissolved gas content. In addition, the departure of nitrogen during this degassing is all the easier when the sulfur content of the liquid steel 2 is lower.
• the first container 44 is divided into two compartments 46, 47 by a transverse partition 48, the lower part of which is provided with one or more openings 49 providing communication between the two compartments.
• the first compartment 47 is that into which the liquid steel 2 coming from the bag 43 is introduced, and where desulfurization takes place.
• conventional means for introducing solid materials make it possible to permanently maintain in this first compartment 47 metallurgical conditions favorable to the desulfurization of the metal, namely:
• a very low content of dissolved oxygen in the liquid steel 2 is obtained by a periodic or continuous addition of aluminum which combines with dissolved oxygen to form alumina; the presence on the surface of the liquid steel 2 of a layer of a slag 50 with a high lime content and a high fluidity, so as to react the lime with the sulfur of the metal which is then trapped in slag in the form of CaS; it is necessary to permanently maintain a satisfactory composition for this slag, while it gradually becomes saturated with CaS as the operation progresses, and it is gradually enriched with alumina originating from deoxidation; it is therefore necessary to periodically remove a fraction of this slag, and simultaneously add lime (and possibly other constituents ensuring good fluidity to the slag) to compensate for this removal.
• the first compartment 46 is also equipped with means for stirring the liquid steel 2, such as means 51 for blowing argon, making it possible to ensure the intense stirring between the metal 2 and the slag 50 necessary for the desulfurization, and eliminating a large part of the alumina inclusions formed during the introduction of aluminum.
• the sulfur content of the liquid steel 2 obtained as a result of this treatment also depends on the sulfur content of the raw materials from which it was produced, the quantity of slag 50 surmounting the liquid steel 2, and the average residence time of the liquid steel 2 in this first compartment 46.
• the capacity and the geometry of this first compartment 46 must therefore be calculated to guarantee the liquid steel 2 an average residence time high enough for the sulfur content to reach the desired level.
• This reactor 53 has, in the example shown, a configuration quite similar to that of a conventional RH. It comprises a cylindrical tank 54 provided with two plungers, namely said ascending plunger 52 which can be equipped with an argon injection device 55, and a descending plunger 56, the lower end of which dips in an intermediate container 57 separate from the first container 44.
• the tank 54 is equipped with a gas suction device 58 making it possible to maintain a reduced pressure in the reactor 53, for example of the order of 50 torr or less, under the effect of which liquid metal 2 is sucked inside the tank 54.
• the gas possibly blown into the ascending plunger 55 contributes to ensuring the circulation of the metal 2 between the first container 44 and the intermediate container 57, with a flow rate which, in operation permanent, is substantially equal to the feed rate of the first container 44. This maintains stable operating conditions throughout the installation if we manage to maintain a drop constant designation between the surfaces of the metal in the first container 44 and the intermediate container 57.
• the vessel 54 of the reactor 53 is equipped with a lance 59 (or an equivalent device) making it possible to inject oxygen into the liquid metal, preferably in an area situated at the level of the ascending plunger 52.
• the oxygen thus introduced on the one hand consumes the aluminum present in the bath, and on the other hand, dissolves in steel 2 where it can thus combine with carbon to effect the coarse decarburization required. This should bring the carbon content of metal 2 from 200-800 ppm to, for example, about 80 ppm. Since decarburization is very rapid in this range of contents, this perfectly stirred reactor 53 is sufficient to obtain this result.
• the liquid steel 2 therefore has a carbon content already considerably reduced compared to its initial content, and contains sufficient dissolved oxygen so that decarburization can continue until an ultra-low content if it is treated in a reactor of the type shown in FIG. 1. Such treatment constitutes the next step in the production chain.
• the liquid steel 2 is withdrawn from the intermediate container 57 by an advanced decarburization reactor 5, identical to that previously described and shown in FIG. 1. For this reason, it is not useful to describe it here in more detail.
• the intermediate container 57 here fulfills the functions of the intermediate container 4 in FIG. 1.
• the other elements common to the two installations are designated in FIG. 2 by the same reference signs as in FIG. 1.
• Means 60 have been added to it.
• the continuous addition to the liquid steel 2 of alloying elements such as aluminum, silicon, manganese in the last cell or cells of the advanced decarburization reactor 5.
• decarburization is considered as completed, and the metal can be definitively nuanced after the capture by the aluminum of the residual dissolved oxygen.
• the installation previously described and shown in FIG.
• the decarburized and nuanced liquid steel 2 enters a continuous casting distributor 11, in which the descending plunger 10 of the advanced decarburization reactor 5 is immersed. Then it begins to solidify in the ingot mold (s) 13 to form in each of them a slab, a bloom or a billet 15.
• This distributor 11 is preferably equipped with all the known improvements which make it possible to pour high-quality metallurgical products, in particular from the point of view of inclusion cleanliness: obstacles increasing the residence time of the metal, devices for mixing by gas blowing or by induction, cover covering the distributor and under which neutral gas is blown.
• the metallurgical vessels 43, 44, 57 are provided with covers and means for blowing a neutral gas under these covers (not shown) to limit the contact of the liquid metal with the ambient air.
• this reactor 5 has the following characteristics: length: 9 m; width: 1 m; average depth "e" of metal 2: 40-45 cm; working pressure: 50 torr or less; number of cells: 9, including the first cell 29 into which the ascending plunger 8 opens; the last of these cells is devoted to nuancing and does not participate in decarburization if deoxidizing elements (aluminum, silicon, etc.) and other alloying elements are added thereto; length of the first cell 29: 1.8 m; length of other cells: 0.9 m; flow of argon in each cell: approximately 15 l / sec; average residence time of the metal in each cell: 15 sec, except in the first cell 29 where it is 30 sec, hence a total residence time of 150 sec in the reactor; carbon content in the first cell: 30 ppm; carbon content in the 8th and 9th cells: 5 ppm;
• the ultra-low carbon contents obtained often go hand in hand with very low nitrogen contents (30 ppm and less), provided that the nitrogen content at the outlet of the production machine 40 is not too high, and that the following stages (in particular the transfers from one container to another and the stirring of the metal) do not bring about significant nitrogen recovery.
• the preceding procedure would not be optimal.
• the denitriding in the last cells would be very low since the deeply decarburized metal would have a high content of dissolved oxygen.
• the CO sweep produced by decarburization would therefore be too reduced to be able to offset the negative effect of dissolved oxygen on the denitriding kinetics.
• the reactor 5 can no longer be used as an advanced decarburization reactor, but as a reactor for deoxidation of the metal by carbon under vacuum.
• the advantage is to obtain a final metal with a very low content of inclusive oxygen, since a minimum quantity of aluminum is necessary for the final deoxidation of the liquid metal.
• the carbon content of 80 ppm (for example) obtained thanks to the coarse decarburization reactor 53.
• the various injections of argon are replaced by injections of a liquid or gaseous hydrocarbon such as methane CH 4 which decomposes by cracking into carbon and hydrogen.
• Carbon combines with the oxygen dissolved in metal 2 to form CO, the formation kinetics of which is accelerated by hydrogen.
• the quantity of hydrocarbon injected can optionally be modulated in each cell so that the carbon content of the bath remains constant as the deoxidation takes place. It is thus possible to obtain a dissolved oxygen content of approximately 10 ppm while retaining the initial 80 ppm of carbon.
• the invention is not limited to the examples which have been described and shown, and modifications can be made to the constitution of the metallurgical reactor according to the invention and to the process for the preparation of the liquid steel which produces it. use.
• the main thing is to keep the general principle governing the design of the reactor, namely, a flow of liquid metal within it as close as possible to the ideal case of a piston flow.
• the reactor which has been described can without problems also be used for the production of steels whose carbon content is not particularly low, and for which no research that a high inclusive cleanliness and a low dissolved gas content.
• the invention can find applications in the preparation of metals other than steel.

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• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Mechanical Engineering (AREA)
• Manufacturing & Machinery (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Treatment Of Steel In Its Molten State (AREA)
• Adhesives Or Adhesive Processes (AREA)
• Physical Or Chemical Processes And Apparatus (AREA)
• Refinement Of Pig-Iron, Manufacture Of Cast Iron, And Steel Manufacture Other Than In Revolving Furnaces (AREA)

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