EP0255450B1 - Device for gas-shielding a molten metal jet - Google Patents

Abstract

Molten steel, normally exposed to an atmosphere of air, is protected against pick-up of impurities by placing carbon dioxide gas in such quantities and in such proximity to the surface to cause dissociation of the carbon dioxide at a rate which furnishes an atmosphere of carbon monoxide and gives off a negligible amount of oxygen to the steel, thus providing a gas barrier or shroud isolating the steel from the surrounding atmosphere and preventing pick-up therefrom of oxygen, nitrogen or hydrogen. This method may be applied to protecting steel being transferred from a ladle to a mold, or from a ladle to a tundish and from the tundish to a mold in continuous casting. In a method where a number of shrouding operations are carried out in series, carbon dioxide vapor, under pressure, is bled, in increments, from a storage vessel containing a body of liquid carbon dioxide in overlying ullage space containing vapor. Each increment is superheated, after it leaves the vessel, and is ultimately expanded and dispersed at ambient temperature to form the shroud. As each increment of vapor is removed from the vessel, it is replaced by withdrawing liquid carbon dioxide, vaporizing it and returning it to the ullage space.

Description

The invention relates to a method of forming a protective gas shield around the steel to prevent oxidation, when this steel is poured from a container in the form of a liquid stream until the moment when it solidifies.

In normal practice, the liquid steel produced by any of the well known methods usually contains a high oxygen content. This is detrimental to its quality. To avoid this drawback, the steel is calmed by introducing into the liquid steel deoxidizing agents, for example, silicon, in the form of ferrosilicon, or aluminum or these two substances at the same time. This is usually done in a transfer pocket, on casting.

Following the deoxidation treatment, the quenched liquid steel has a strong affinity for oxygen, which it absorbs when it is exposed to the atmosphere, when it is poured into ingot molds, to form billets. or slabs. This results in defects in the resulting steel.

To avoid or reduce this absorption of oxygen, various protection procedures have already been used. One method consists in protecting the streams of liquid steel poured in the open air by passing them through ceramic pipes between the casting basin and the ingot mold. This technique was an established practice adopted to maintain high quality in the continuous casting of large section blooms and slabs. Unfortunately, it cannot be applied to smaller blooms and billets due to space limitations. An example of this type of process is described in Canadian Patent No. ₀ 1097881.

In another method, liquid argon is poured into the molds. The argon evaporates when it comes into contact with the liquid steel and isolates the latter from the atmosphere during the rest of the casting in the ingot mold. The main drawbacks of this process are that the storage and transport of the equipment is difficult to adapt to the severe working conditions of the casting platform and, moreover, the cost of argon is high, relatively at the price of normal steel grades.

The inert gas protection of continuously cast steel has also been described in the article "Gas Shrouding of Strand Cast Steel at Jones & Laughlin Steel Corporation" by Samways, Pollard & Fedenco, Journals of Metals, October 1974, as well as in U.S. patents 3,908,734, 3,963,224 and 4,023,614.

Another method uses liquid nitrogen to form a protective shield for the stream of liquid steel as it is cast in a continuous casting machine. This process is described in the brochure entitled "Conspal Surface Protection" published by Concast AG, Zurich, Switzerland, March 1977, as well as in US Patent No. 4,178,980, issued in the name of L'Air Liquide. In general, liquid nitrogen gives a degree of protection which provides a good improvement compared to other processes. However, handling this substance under the harsh conditions of the casting platform makes it difficult in some cases to obtain continuity of the casting during the operation.

The Applicant has found that, against all expectations, carbon dioxide (C0 ₂ ) can be used effectively as a protective gas to protect the liquid steel from oxidation by the atmosphere in the case of continuous casting, or of casting in ingots, when this steel is poured from a container in the form of a liquid stream until it solidifies.

An apparatus in accordance with the preamble of claim 1 is described in DE-A 1 920 421. In this continuous casting apparatus, a gas is injected into the closed volume delimited by the lower external part of the upper container (A) and the upper part of the lower container (B). A constant pressure is thus maintained above the metal entering the mold.

Patent DE-A 2 040 504 describes the use of carbon monoxide CO so as to maintain a reducing atmosphere above the molten metal, on the one hand, and the reduction of the iron oxide FeO contained in the metal bath by injecting CO into said metal bath, on the other hand.

It is known to use carbon dioxide to protect liquid metals such as lead, zinc, copper, metals which have a melting point below the decomposition temperature of carbon dioxide. Based on thermodynamic considerations, it should be expected that such protection will not be achieved by the casting of metals having a melting point above the decomposition temperature of carbon dioxide. In particular, it should be expected that, under the effect of contact between carbon dioxide and liquid steel, the latter will be oxidized by the decomposition of the gas, since its decomposition temperature is much lower than that of liquid steel. Unexpectedly, the Applicant has found that the kinetics of the reactions are such that, on coming into contact with liquid steel, and although the carbon dioxide decomposes at the gas-metal interface, a negligible quantity of oxygen dissolves in the metal and the carbon monoxide formed behaves like a screen layer at the gas-metal interface. Not only is oxidation considerably reduced compared to the level it would reach in the absence of a shielding layer between the metal and the atmosphere, but also absorption of nitrogen and hydrogen (from moisture from air) by liquid steel. The absorption of oxygen from decomposition is less than about 60 parts per million and can be reduced to 40 parts per million. Carbon dioxide is therefore capable of forming an effective screen between liquid steel and the surrounding atmosphere when this steel is poured from a container in the form of a liquid stream until it solidifies, which considerably reduces the rate of oxidation.

This process is the subject of European application 154 585, of which the present application is a division.

In the implementation of the process, a protective carbon dioxide screen is formed around the stream of liquid steel, close to its source, and this screen is kept in contact with the steel until the latter is solidifies. The general criteria to be observed for the use of carbon dioxide as a protective screen are generally the same as in the case of the use of argon or other inert gases. For example, in the case of pouring an ingot from above into an ingot mold, the ingot mold is purged in advance using carbon dioxide to remove the oxygen and to form an anhydride atmosphere in the ingot mold. carbon dioxide into which and through which the steel is poured.

In this way, the oxygen content of the mold, before casting, can be reduced to less than 3% by volume, and preferably, to 1% at most.

The protective screen can be formed by means of an annular ramp pierced with outlet orifices which are arranged around the stream of liquid steel, close to its source, for delivering carbon dioxide in the form of jets. which come together to form a cover that follows the surface of the steel stream. In the case of casting in an ingot mold, an annular distribution ramp can surround the outlet nozzle of the ladle.

The steel forming the liquid stream is usually at a temperature between 1,625 ° C and 1,650 _° C.

The invention also relates to the use of mixtures of argon and carbon dioxide in steel casting to prevent oxidation.

• la figure 1 est une vue en perspective qui montre les positions relatives de la poche de coulée et d'une rangée de lingotières, pendant la mise en oeuvre du procédé selon l'invention;
• la figure 2 est une coupe verticale, en partie en élévation, d'une lingotière pendant l'opération de purge à l'anhydride carbonique qui sert à préparer cette lingotière pour la réception de l'acier liquide;
• la figure 3 est une vue partielle à échelle agrandie qui montre un intercalair e en acier ondulé qui supporte la base de la lingotière ;
• la figure 4 est une coupe verticale, en partie en élévation,qui montre une opération de coulée en lingots; et
• la figure 5 est un schéma montrant la disposition des éléments d'un équipement approprié pour la mise en oeuvre d'un procédé selon l'invention, et les liaisons de transmission des fluides qui relient ces éléments.

The invention will be better understood with the aid of the following embodiments, given without limitation, together with the figures which represent:

• Figure 1 is a perspective view showing the relative positions of the ladle and a row of molds, during the implementation of the method according to the invention;
• Figure 2 is a vertical section, partly in elevation, of an ingot mold during the purging operation with carbon dioxide which is used to prepare this ingot mold for the reception of liquid steel;
• Figure 3 is a partial view on an enlarged scale which shows a corrugated steel spacer which supports the base of the mold;
• Figure 4 is a vertical section, partly in elevation, which shows an ingot casting operation; and
• FIG. 5 is a diagram showing the arrangement of the elements of equipment suitable for implementing a method according to the invention, and the transmission links of the fluids which connect these elements.

FIG. 1 represents a pocket A containing liquid steel which is poured into an ingot mold B. A protective gas, composed of carbon dioxide, is conveyed through an annular distribution ramp (represented in FIG. 4), supplied by a supply line 15.

An ingot mold 8 ₁ , which waits its turn to receive the liquid steel, is shown while receiving carbon dioxide purge gas via a line 17 and the following ingot molds Bi and B ₂ wait their turn to be processed.

Before entering the treatment phase, each of the molds is provided with a cap 19 formed of an aluminum film. The cap 19 has been torn locally to form an opening for the introduction of the gas pipe.

FIG. 2 shows, in a more detailed manner, the ingot mold B ₁ during purging with carbon dioxide. Line 17 is passed through an opening 20 in the aluminum foil cap and it ends in a nozzle 18 through which carbon dioxide is introduced into the bottom of the mold to move the air and replace it with a carbon dioxide atmosphere.

The mold 8 ₁ has a wall 22 which encloses a molding cavity 23 of decreasing section. The base of the wall 22 is supported on a corrugated metal interlayer 24, which is itself supported by the plate of a carriage C and is intended to form a seal between the base of the wall 22 and the plate of the carriage, by letting a certain quantity of carbon dioxide gas escape laterally.

Carbon dioxide is injected into the ingot mold Bi until this ingot mold has an oxygen content of not more than 3%, and preferably not more than 1%. The mold is now ready for casting. It is then brought to the position of the mold B and the casting operation is carried out as described with reference to FIG. 4. A valve arranged in the pocket A is opened, by means of a remote control, for allow the liquid steel to flow through an outlet passage 25 formed in the pocket A and pass in the form of a vertical current at the level of a protective gas diffuser 27. The diffuser 27 is supplied with gaseous carbon dioxide by a pipe 15, which has the effect that a gas screen surrounds the stream of liquid metal and accompanies the latter when it enters the carbon dioxide atmosphere contained in the ingot mold B. From the moment it leaves the pocket until it reaches its destination in the mold, the liquid steel is isolated from the atmosphere by a continuous curtain of carbon dioxide. When the ingot mold is cast, the valve of the ladle is closed to stop the flow of liquid metal and the next ingot mold and the ladle are brought into positions of mutual alignment so that this ingot mold receives its liquid steel content.

To ensure a supply of carbon dioxide in due time, on order, use is made of feeding equipment such as that shown in FIG. 5.

EXAMPLE

For the purposes of this example, equipment is used which is substantially as shown in FIG. 4. A bag is used having a capacity of 50 tonnes and ingot molds each having a capacity of 8 to 9 tonnes. The pocket has a circular opening or nozzle with a diameter of 5 to 6.5 cm. Each mold has a depth of 240 to 260 cm. The distance from the bottom of the nozzle to the upper surface of the mold is 75 cm. Each ingot mold rests on a trolley-mounted interlayer, of the type used to evacuate the solidified ingots from the casting station.

The bag is equipped with a perforated circular ramp, located just below the nozzle and capable of forming a protective screen for carbon dioxide gas. This ramp is connected to a continuous source of carbon dioxide gas supply. In addition, the installation includes conventional equipment for purging the ingot mold by means of carbon dioxide gas.

Just before casting, each mold is purged using carbon dioxide gas, at a rate of 2.8 cubic meters per minute, to expel the air from inside the mold. The air is expelled from the interior of the ingot mold by the carbon dioxide purge at a flow rate of 2265 to 2832 liters / minute (80 to 100 sefm standard cubic foot minute), for approximately 3 minutes before the casting of each ingot . A rubber hose with a protective asbestos coating is introduced into the mold, through the aluminum film, so that the diffuser plunges as low as possible, as shown in Figure 2. The gas flow is extended to the air has been expelled from the mold, to such an extent that the concentration of oxygen in the mold does not exceed 1% by volume. The gas injection is extended to an instant immediately preceding the pouring into this ingot mold, this to take account of the gas leak between the ingot mold and its interlayer.

While the steel is in the oven, the molds are prepared for casting according to the following procedure. A strong jet of compressed air is projected onto the interlayer to remove any free particles from it. A coating composed of a dispersion of cement in dilute phosphoric acid is then applied to the interlayer. Four strips of corrugated steel sheet of approximately 150 mm x 750 mm 1.6 mm are placed on the interlayer, in a square or a rectangle. When the mold is placed in position, its weight has crushed the corrugations to reduce the risk of leakage of the liquid steel (see detail in Figure 2).

On the insert, inside the mold, an oblong chimney, made of thin sheet steel, measuring approximately 500 mm x 1,000 mm x 1,250 mm, to reduce the intensity of projections to the time of the start of the pouring of liquid metal into the mold. Exothermic "boards" (hot elevators or "hot tops") are fixed on the upper end 12 ′ of the internal surface of the ingot mold, these boards generating heat by coming into contact with the liquid steel, this for to slow the cooling at the part of the ingot, and to thereby reduce the depth of the recess formed in the upper part of this ingot and which must be cut before the subsequent rolling. A thin aluminum foil cap is placed over the top of the mold to limit exposure to the atmosphere before the mold has been purged with carbon dioxide.

At the start of casting, the liquid steel punctures a small hole in the thin aluminum sheet, thereby reducing the amount of ambient air that is drawn into the mold. The temperature of the current steel is from 1,625 _° C to 1,650 _° C. During the filling of each ingot mold, a screen of carbon dioxide is formed near the source of the current, that is to say say just below the bottom of the pocket, under the nozzle. The screen formed around the current of liquid steel is entrained with the steel and forms a protective screen insulating from the atmosphere from the moment the steel leaves the nozzle until its impact in the mold. The carbon dioxide flow rate sent to the screen is 2.8 cubic meters per minute.

The pocket containing the 50 tonnes of steel is positioned above the first ingot mold, already purged and the flow of screen gas is put into action. The purge pipe was previously transferred to the second ingot mold without interrupting the flow of gas.

The valve is open to start casting (see Figure 4). At times, the nozzle is blocked by solidified metal or by slag. In each case, it is necessary to inject oxygen to the lance to clear the nozzle (see Figure 4).

Although carbon dioxide is supplied in liquid form, C0 ₂ gas is used at the two injection points (purging and forming a screen). A device is therefore used which has a vaporization capacity to provide a flow rate comparable to that of an inert gas, for example argon. The composition of the CO ₂ feed is shown in Figure 5.

It is the first ingot that requires the least time for casting, since the static pressure of the metal gradually decreases during casting. In approximately 3 minutes, the ingot mold is filled and the valve is closed (for approximately 20 to 30 seconds) while the crane operator positions the pocket above the second ingot mold. During this time, the purge gas pipe is transferred to the next ingot mold and the valve is opened again to fill the mold that has just been purged. The sequence is continued until the pocket is emptied of its metal charge.

The charge contained in each mold is allowed to cool, in the conventional manner, with a neck che of protective flux on its surface, so as to form a solid ingot. The ingot molds are then emptied of their ingots.

Each ingot is hot rolled into a strip, according to standard practice, then subjected to ultrasonic control for the detection of surface defects. The acceptable strip was then rolled into a sheet and the sheet was then made into a helically welded tube. The tube was then subjected to ultrasonic testing for fault detection.

Control heaters were then performed in an identical fashion.

During the entire mold casting operation, the gas flow rate was 2.8 cubic meters per minute in the case of carbon dioxide and 2.8 cubic meters per minute in the case of l 'argon. Each mold was purged for approximately 2 minutes and the stream of liquid metal was protected for the duration of the casting operation, approximately 25 minutes.

A comparison of the results is given below in terms of surface defects on the strips formed by rolling from billets produced:

Faults detected by the ultrasonic effect on a helically welded tube:

Thanks to the relatively low cost of carbon dioxide and the ease with which it can be obtained, compared, for example, to argon, and thanks to the fact that it can be produced and supplied continuously, this gas constitutes an extremely useful gas when used according to the invention. Carbon dioxide is heavier than air (1.5: 1) unlike argon (1.5: 2) and therefore it maintains an effective protective screen longer than lighter gases since it does not does not disperse into the atmosphere as easily.

Carbon dioxide can be used in the form of carbon dioxide snow to provide a concentrated form of C0 ₂ gas for the use of the ingot mold in ingot casting or in the ingot mold of a continuous casting installation.

It is also noted from the above comparative results that the use of argon at the level of the screen and carbon dioxide for purging the ingot mold unexpectedly gives further improved results. It is also clear from these examples, that a mixture of carbon dioxide and argon, at the screen, whatever the proportions, associated with a purging of the mold with carbon dioxide, will give results greater than or equal to those obtained with carbon dioxide alone.

Claims (1)

1. Apparatus for producing a gaseous protection for a jet of molten metal cast from a upper container (A) provided with an opening and closing system situated in the lower part of the container, the opening of the system allowing the jet of metal to be cast into a lower container (Bi) provided with an opening for receiving the metal cast whilst the opening and closing system is open, a protective gaseous sheath being formed around the molten metal jet to protect it against the impurities of the ambient atmosphere during the casting of the metal jet into the lower container, characterised in that it comprises:

- means (15) of injecting gaseous carbon dioxide around the casting jet, which are situated in the first container and surround the tap-hole,

- means (17) of injecting gaseous carbon dioxide into the lower container to ensure its purging,

- a vessel containing carbon dioxide in liquid form in its lower portion and in gaseous form in its upper portion,

- means of drawing liquid carbon dioxide from the lower portion of the vessel, vapourising it and injecting it into the upper portion of the vessel to replace the gas drawn off,

- a conduit situated in the upper portion of the vessel for conveying the gaseous carbon dioxide towards the means of injecting carbon dioxide gas around the casting jet and the means of injecting carbon dioxide gas into the lower container,

- means of controlling the injection of carbon dioxide into the second container to ensure its purging before the casting of the metal,

- means of controlling the injection of carbon dioxide around the metal casting jet issuing from the first container.

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