EP0196952A2 - Process for obtaining a killed steel with a low nitrogen content - Google Patents

Abstract

The process for producing killed steel having a low content of nitrogen comprises pouring an effervescent steel from a converter into a ladle in which there are added to the molten steel contained in the ladle, in the course of the pouring operation, additives for killing the steel, such as aluminium, silicon, etc. . . . The invention comprises pouring the effervescent steel into the ladle before introducing the killing additives, and prior to the introduction of the killing additives carbon dioxide in the form of carbon dioxide snow in the vicinity of the lower part of the pouring jet and on the surface of the bath of steel in the ladle in a quantity sufficient to protect the surface of the molten metal from the surrounding air upon the introduction of the killing additives into the ladle.

Description

The present invention relates to a process for obtaining a calm steel with low nitrogen content by pouring an effervescent steel from a converter into a ladle, in which molten steel located in the ladle is added during of the casting operation, in particular additives for calming this steel such as aluminum, and / or silicon.

During the production of metals, various constituents of the ore are eliminated, while new bodies are inserted into the molten metal, in particular in contact with air.

Certain metals or alloys of metals can in particular, during their development, see their quantity of nitrogen increase if no special precautions are taken. This is the case, for example, of steel when it is poured from a converter into a pocket or more generally a transmitting container into a receiving container. It has been found that the presence of nitrogen in a steel, in the form of interstitial impurities promotes hardening by aging and reduces its resistance. In particular, it is found that a sheet having a too high nitrogen content is poorly resistant to aging and corrosion after deep drawing.

A first solution to this problem has been proposed in Japanese patent application No. 51-77.519 in the name of NIPPON STEEL CORP. To reduce the nitrogen content in the steel, it is proposed in this Japanese application to pour the steel from a converter where it has been refined, in a ladle, previously filled with a non-nitriding gas, said ladle having been closed by means of a cover made of consumable material.

As specified in this patent application, the essential principle of this process consists, before any metal casting, in completely replacing the air in the bag, a non-nitriding gas by injecting this gas into said bag and covering this bag of a cover, the two measures, taken separately, not making it possible to achieve the desired goal, that is to say the reduction in the amount of nitrogen absorbed by the steel during this ladle pouring. According to one embodiment, carbon dioxide is used in the form of blocks of carbon dioxide ice ("dry ice" according to English terminology) deposited in the bottom of the pocket before closing it by the cover.

More recently, the authors of this patent application published an article entitled: "Conditions for preventing nitrogen absorption during steel making" - Y. ABE - Y. KATAYAMA - M. NISHIMURA - T. TAKAHASHI - NIPPON STEEL CORP. J.S.C. (Science Council of Japan) - 21.05.1981 - in which they specify the use of these blocks of "dry ice" or dry ice and its limits. Thus, it is specified that the ice blocks must have a maximum size of the order of 600 mm per side to avoid projections of molten metal. In addition, the minimum size is of the order of 40 mm, to avoid total sublimation before casting, and air infiltration into the pocket. In practice, it is recommended to use blocks 100 to 200 mm side. Furthermore, the time between the deposit of the blocks in the receiving container and the metal casting is of the order of one hour.

From these various publications, it thus appears that this process consists in completely expelling the air from the pocket, while maintaining a cover above the latter. The use of dry ice blocks suggests that, due to their relatively slow sublimation, gaseous carbon dioxide gradually expels the air from the pocket (density of CO ₂ higher than that of air ), which would probably not have been the case if the sublimation of carbon dioxide had been rapid, generating gas streams in the pocket and thus maintaining a mixture of carbon dioxide and air in it.

In addition, it is recommended in these various publications to maintain dry ice blocks on the surface of the liquid metal in the pocket, throughout the casting thereof.

Following various experiments, the Applicant has found that the process described above has a certain number of drawbacks.

First of all, the necessary presence of a consumable cover, without which the announced results cannot be obtained increases the manufacturing costs (material and additional labor). In addition, the operator must adjust the jet of liquid metal on the zone of least resistance of the cover.

In addition, the use of ice blocks requires many manipulations (cutting blocks, packaging, supply, handling, etc ...) going against a steel simplicity.

It was also found that the protection of the casting jet was generally ineffective due to the presence of an air mixture and carbon dioxide gas, a phenomenon aggravated by the temperature gradients in the ladle. Finally, if we follow the teaching of the patent, that is to say the maintenance of dry ice blocks on the surface of the liquid metal in the pocket, the process is particularly dangerous. Indeed, there are explosions in the molten metal, due to the sublimation of carbon dioxide under the blocks, thus forming pockets of gas bursting on the surface, causing splashes of molten metal. The cover made of consumable material is not sufficient to avoid splashes of molten metal creating more air intakes in the pocket, which goes against the desired objective.

It is also known from Belgian patent 677958 to add carbon dioxide C0 ₂ , in particular in the form of carbon dioxide snow in the bottom of the mold before the casting of effervescent steel from a ladle therein and on the surface of the liquid metal during the filling of the mold. Effervescent steel, that is to say steel containing a large amount of dissolved oxygen, has the great advantage of producing ingots whose surface in contact with the ingot mold is completely free of waste. Thus carbon dioxide, under the operating conditions described in this patent decomposes into oxygen and carbon monoxide which burns on contact with air while oxygen makes it possible to intensify the phenomenon of effervescence sought.

Contrary to the teaching of this Belgian patent, the method according to the invention makes it possible to use carbon dioxide in the form of carbon dioxide snow to protect the surface of the steel bath so as to obtain both a small amount of dissolved oxygen in the steel, after soaking, as well as a small amount of nitrogen, while avoiding the drawbacks mentioned above. The method of obtaining callus steel _- ied from effervescent steel according to the invention is characterized in that the effervescent the steel is cast into the bag sufficient to allow the introduction of additives and calmage in that a few moments before the introduction of these calming additives, carbon dioxide is injected in the form of carbon dioxide snow, near the base of the pouring jet and on the surface of the steel bath in the ladle, in sufficient quantity to protect the surface of the molten metal from the surrounding air as soon as the calming additives are introduced into the ladle.

It has in fact been observed that, if the presence of carbon dioxide snow around the base of the casting jet had no effect on the nitrogen uptake by the effervescent steel, on the other hand from the introduction of the calming additives such as aluminum, silicon, etc ..., the presence of carbon dioxide at the foot of the jet and on the surface of the steel bath, would prevent the re-nitriding of the quenched steel without it being necessary to proceed as in the Japanese patent mentioned above, which would be, moreover, impossible. In addition, it can be seen that the process as described above makes it possible, unexpectedly, to reduce the losses of dissolved aluminum in the steel by around 25%, which makes it possible to improve the profitability of the process since the the amount of aluminum required for soothing is thus reduced. In addition, this process is economical compared to the process described in the aforementioned Japanese patent, since the introduction of carbon dioxide takes place later, therefore consuming less carbon dioxide.

As a general rule, it can be seen that the density of the carbon dioxide snow used (density of the solid particles of this carbon dioxide snow) must be less than or equal to 1.1 kg / dm ³ .

In practice, the carbon dioxide snow which is suitable for implementing the invention is snow produced by an apparatus known as a cyclone. This snow comes from the sudden expansion of liquid carbon dioxide, generally stored at a temperature of - 20 ° C and a pressure of 20 bars directly in the atmosphere, that is to say at ambient temperature and pressure. The snow thus formed is used as is, generally without further treatment. In practice, this makes it possible to have the carbon dioxide snow generator near the place of pouring and to inject this snow into the pocket by a supply line connected to the cyclone. Continuous or sequential feeding can thus be easily controlled by the operator who controls the casting of metal.

Generally, the quantities of dry ice required vary from 0.2 to 5 kg per tonne of metal cast.

With regard to the introduction of dry ice into the pocket, those skilled in the art generally know that the pouring of a converter into a pocket has a duration t _l , which varies according to erosion of the converter tap hole. On the other hand, the time necessary for the introduction and dissolution of the soothing additives has a fixed value t ₂ for a given volume. Under these conditions, the skilled person will introduce the snow at the latest at time t3, from the start of the pouring, equal to t _l - ^t ₂ .

Of course, this method is preferably applied for the protection of the pouring jet between the converter and the ladle but can also be applied for the casting of a first ladle in a second ladle or in a continuous casting distributor, as well as the distributor in the molds, etc ...

• - la figure 1, une vue en coupe de la coulée d'acier effervescent d'un convertisseur dans une poche utilisant le procédé selon l'invention ;
• - la figure 2, une variante de réalisation du procédé de la figure 1, comportant une alimentation in situ en neige carbonique.
• - la figure 3, une vue schématique d'une installation de coulée utilisant le procédé selon l'invention.

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

• - Figure 1, a sectional view of the effervescent steel casting of a converter in a pocket using the method according to the invention;
• - Figure 2, an alternative embodiment of the method of Figure 1, comprising an in situ supply of dry ice.
• - Figure 3, a schematic view of a casting installation using the method according to the invention.

In FIG. 1, the effervescent steel 1 is located in the converter 2 under the orifice 3 from which the bag 4 is brought.

When the bag is partially filled, a predetermined amount of carbon dioxide snow is injected before adding the calming additives such as aluminum or silicon as well as the additives (if necessary) such as silico manganese, ferrovanadium, ferromanganese fuel, ferroneobium, carbon in the form of carburite, etc. well-known additives to give the desired properties and grades to steels. The liquid metal 5 immediately sublimates the carbon dioxide snow present in the zone of the jet foot 6, and in the zone situated above the layer of liquid metal 7, thus creating a layer 8 of carbon dioxide, surmounted by a layer 9 of air. The whole liquid metal 7 and carbon dioxide sheet (heavier than air) thus form a piston, as the liquid level rises, which expels the air from the pocket, the jet foot thus being constantly protected.

FIG. 2 shows a variant of FIG. 1, in which the carbon dioxide snow is injected into the pocket just before the addition of the calming additives (continuously or sequentially), using a supply 10, on the one hand connected to the reservoir 12 of liquid C0 ₂ and on the other hand connected to the bag 4 by the expansion valve 11.

Snow 14 spreads over all of the liquid metal. For this, a symmetrical supply can be provided at several points.

Continuous or sequential feeding using the reservoir 12 makes it possible to generate, by expansion of the liquid C0 ₂ , approximately 40% solid and 60% gas. The latter dilutes the atmosphere of the ladle and improves the protection of the pouring jet. Furthermore, this gas, heavier than air, heats up in contact with the liquid metal before being entrained towards the surface of said metal, thus avoiding excessive cooling of the metal.

FIG. 3 represents a simplified view of implementation of the method according to the invention. A receiving pocket 32 is placed under the converter 30 containing the molten steel 31, this pocket being supported by a carriage 33 moving on the rails 34, 35. At the same level as these is placed a reservoir 36 of carbon dioxide liquid 50. This reservoir is protected by a firewall 37. The liquid carbon dioxide is sent (by means not shown in the figures) via the pipe 38 to an apparatus 41 for making carbon dioxide snow called carbocyclone, such as those sold by CARDOX.

Line 38 ends with two nozzles shown diagrammatically at 39 oriented 180 ° from one another, playing the role of expansion orifices at ambient temperature and pressure of the carbon dioxide stored at approximately -20 ° C. and 20 bars in the tank 36. This expansion in the downward-facing cone 40 causes the appearance of carbon dioxide snow which is stored in the bucket 42 placed on the scale 43. When the desired quantity is stored in the bucket, the injection is stopped and the bucket is moved on the pouring floor 51 located at the level of the converter, above the ladle. Through the opening 52 in the floor 51, the snow from the bucket 42 is poured into the pocket 32, a few moments before the addition of the soothing additives.

Using such a device, knowing the hourly production of the carbocyclone, it is easy to produce the quantity of snow in due time, so as to be ready when the quantity of steel poured into the pocket is deemed sufficient. In practice, a flow carbocyclone 1200 kg / hour snow snapshot is ideal for feeding a steel mill.

EXAMPLE 1:

An effervescent steel comprising 1.5% carbon, 10% chromium, 0.09% silicon, 0.08% manganese, 0.012% sulfur and 0.011% carbon is poured from a converter into a 1 ton bag. phosphorus.

When the bag is filled to about a third of its height before the calming operation, carbon dioxide snow is injected from the sudden expansion at room temperature of liquid carbon dioxide stored at - 20 ° C and 20 bars. The amount used was approximately 1 kg. A few seconds after having finished injecting snow into the pocket, brought to around 900 ° C., additives, in particular steel calming additives, are continued pouring until the pocket is completely filled. performs in about 1 minute.

A sample of the liquid steel is taken from the converter before casting and from the ladle after casting.

The same operation is carried out, under the same conditions without using dry ice, which will be called reference casting, and the samples are taken in the same way.

The results obtained are as follows:

Compared to the reference casting, the renitriding of the casting according to the invention has decreased by 37%.

It should be noted that the initial nitrogen concentration in the bag is not the same in the two cases, since it is impossible to have the same initial nitrogen concentrations for two successive mergers carried out under the same conditions. On the other hand, the Applicant has verified that the reduction in renitriding does not depend on the initial nitrogen concentration.

EXAMPLE 2:

The operation is carried out under conditions similar to that of Example 1 but with a bag receiving 6 tonnes of effervescent steel comprising from 0.2 to 0.3% of carbon, from 0.6 to 0.7% of manganese and 0.2 to 0.7% silicon. The steel is poured into the pocket up to about a third of its height. Then injected about 5 kg of dry ice (in one or more times until the end of the casting). Then the steel calming additives are introduced, in a manner known per se.

The results obtained are as follows:

The renitrutation decreased by 40%.

• - diminution de l'absorption d'azote pendant la coulée
• - diminution de l'absorption d'azote après la coulée pendant au moins dix minutes.

From the table above, there are two effects of dry ice:

• - decrease in nitrogen absorption during casting
• - decrease in nitrogen absorption after pouring for at least ten minutes.

Example 3: Under the same conditions as in Example 2, an effervescent steel of the following composition is poured:

As before, this steel is quenched with aluminum, the carbon dioxide snow having been injected just before the introduction of aluminum. The following results show a significant improvement in renitriding and reoxidation (reduction in dissolved aluminum losses):

This table also shows that we can modulate the quantity of dry ice introduced according to the result that we aim to obtain: we will put more dry ice / per tonne if we want to avoid as much as possible re-nitriding, while avoiding reoxidation, while the addition of a small amount of C0 ₂ / tonne of metal unexpectedly decreases reoxidation, while also reducing renitriding.

Protecting the casting with dry ice reduces the loss of aluminum dissolved in the steel by 25%. This shows in particular the inerting effect of carbon dioxide used under the conditions mentioned above: if it was oxidizing with respect to air, the loss of dissolved aluminum would be very significant and in any event much higher than that observed without CO ₂ protection.

Claims (7)

1. - Process for obtaining a calm steel with low nitrogen content by pouring an effervescent steel from a converter into a ladle, in which the molten steel located in the ladle is added during the casting operation, in particular additives for calming this steel such as aluminum, silicon ... characterized in that the effervescent steel is poured into the ladle in sufficient quantity to allow the introduction of additives calming and in that a few moments before the introduction of these calming additives, carbon dioxide is injected in the form of carbon dioxide snow in the vicinity of the bottom of the casting jet and on the surface of the steel bath in the ladle, in sufficient quantity to protect the surface of the molten metal from the surrounding air as soon as the soothing additives are introduced into the mold. 2. - Method according to claim 1, characterized in that the density of the dry ice (14) is less than or equal to 1.1 kg / d _m 3. 3. - Method according to one of claims 1 or 2, characterized in that the introduction of carbon dioxide (8) into the pocket (4) takes place in the form of carbon dioxide snow injection obtained directly by sudden expansion at atmospheric pressure and at ambient temperature of liquid carbon dioxide stored under the usual conditions of temperature and pressure. 4. - Method according to one of claims 1 to 3, characterized in that the bag is filled to about a third of its height before injecting the carbon dioxide snow. 5. - Method according to one of claims 1 to 4, characterized in that the injection of carbon dioxide snow (14) continues for at least part of the duration of the casting of the effervescent steel (1) in the pocket (4). 6. - Method according to one of claims 1 to 4, characterized in that the injection of carbon dioxide snow (14) continues throughout the duration of casting of the effervescent steel (1) in the pocket (4). 7. - Method according to one of claims 1 to 6, characterized in that one injects into the pocket (4), in one or more times, from 0.2 to 5 kg of dry ice (14) by ton of cast steel.

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