EP0347351A1 - Process for treating molten steels by the injection of gas through the bottom of the ladle - Google Patents

Abstract

The process relates to the treatment of liquid steels in a ladle by the injection of gas through the bottom of the ladle. <??>It consists in injecting at least one inert gas from locations distributed in the bottom of the ladle (3) so as to form on the surface of the liquid steel and annular swelling (14) whose outer edge (18) is close to the inner edge (16) of the ladle (1) wall lining. <??>An oxidising gas may be mixed with the inert gas for decarbonising. The process is applied in particular to the treatment of carbon steels. <IMAGE>

Description

The method and the device which are the subject of the invention relate to the treatment of the liquid steel in the ladle by means of gas injected from particular insufflation elements judiciously placed in the bottom of the ladle to ensure in particular dehydrogenation, decarburization, renitriding of steels, in particular stainless steels and decantation of the soaking inclusions.

The pocket treatment of liquid steels by injection of gas through the pocket bottom or by means of submerged lances has been used industrially for about twenty years with the aim of homogenizing the temperature of the metal, of desulfurizing it on contact slag and rid it of soothing inclusions.

The article by Grabner and Hoffgen "Einsatz und Verschleiss von Spülsteinen in der Sekundärmetallurgie" in RADEX RUNDSCHAU, Heft 3, 1983, pages 179-209 reviews the conditions for the usual implementation of this technique and its applications, flow rates argon or nitrogen gas used as well as the model and number of porous plugs used. These are generally conical in number of 1 or 2 and are generally located at 2/3 of the radius of the pocket from its center. The total surface of these porous plugs in contact with liquid steel is between 25 and 190 cm² depending on the size of the bag and the flow rates used are between 3 and 10 liters per minute and per cm² of surface of porous plug in contact with liquid metal.

We will also find in Electric Furnace Steel Making volume II Theory and Fundamentals by DC Hilty RW, FARLEY and DJ GARDE edition 1967 by E. SIMS the elements of thermodynamics necessary for understanding the mechanism of decarburization under partial pressure of CO - pages 124, 125 and 171 to 175 - for steels with a high chromium content.

We will find in the Revue de Métallurgie of January 1986 pages 25 to 41 by C. Gatelier and H. Gaye of thermodynamic considerations concerning more particularly the exchanges between the liquid steel and the hydrogen and nitrogen gases, the dimensioning of the bubbles gases emitted as well as the method of calculating their rate of climb.

However, despite the efforts made by increasing the gas flow rates conventionally used with known porous plugs and the duration of the treatment at the limits allowed by the temperature losses in the bag, neither the complete decantation of the inclusions nor sufficient dehydrogenation or renitriding can be ensured. stainless steels with nitrogen at the desired value or decarburization under low partial pressure of CO to obtain very low carbon contents of less than 0.025% by mass for stainless steels or carbon, the volumes of the argon oxygen mixture being such that conventional porous plugs do not provide sufficient flow, the temperature rise at these plugs resulting from combustion being also too high.

This is how to dehydrogenate steels the current technique consists in its general principle of subjecting the metal to a vacuum - 1 torr in general - by constantly renewing the layers of steel so that the partial pressure of dissolved hydrogen in the metal is always higher than that of hydrogen at the vacuum level considered and that therefore hydrogen can diffuse.

The renitriding of stainless steels with nitrogen (0.1 to 0.4% by mass of nitrogen) although already effective with the AOD converter (Argon oxygen decarburising Process) by the replacement of argon by nitrogen as gas of dilution of the CO formed is not sufficient and must be supplemented by final additions, in general, of very expensive nitrided ferrochrome.

The manufacture of very low carbon steels less than 0.025% by mass or steels with a high chromium content for which the combustion of carbon must be done under a partial pressure of CO lower than the unit, pressure depending on the carbon content and temperature in order to limit the degree of oxidation of the metal, is carried out either in AOD converters in which the partial pressure of CO is obtained by dilution or in vacuum degassing installations in which oxygen is blown to lance and at the desired pressure to obtain the desired CO pressure. These devices are frequently called in the specialized literature RH-OB or VOD.

Currently, rolling steels after a first settling treatment of the inclusions in the bag undergo an operation generally complementary to the vacuum elevator (R.H) in which the metal circulation rate is very high. This circulation takes place under a turbulent flow regime which increases the probability that the elementary inclusions of alumina, the size of which is close to one micron, to agglomerate and to be of sufficient size to settle in the liquid steel by density difference or clinging to the refractory walls.

The utility certificate FR 2223467 discloses a process for putting into circulation, no longer in a vacuum device but in a pocket, the whole of a cast iron bath pneumatically. The aim of this process is to introduce desulphurizing agents, such as calcium carbide or inoculating graphitizing agents, into the melt itself. These agents having a density 2 to 3 times lower than that of cast iron impose on the current of central cast iron directed from top to bottom a speed greater than that of the rise of said agents, which imposes a porous ring whose width can reach a quarter of the inside diameter of the pocket, ie three-quarters of the surface of the bottom of the pocket.

As will be seen below, the object of the present invention is quite different from that sought in FR 2223467 since it is sought in the present invention to multiply the metal gas exchange surfaces while keeping the individuality of each of the small bubbles emitted and their low rate of rise and that one also seeks to concentrate the slag in the center of the upper free surface of the liquid metal so that this slag cannot be entrained within the liquid steel itself. This requires mixing surfaces much smaller than the surfaces indicated above, very specific gas flow rates, precise positioning of the mixing elements and a suitable porosity.

The peculiarity of the industrial plants mentioned above is to require extremely large investments causing large thermal losses often requiring the heating of the metal either by alumino or silicothermy (RH-OB and AOD) or by pocket electric arcs. Processing costs are therefore high.

In the bag treatment process which is the subject of the present invention, it has been sought to multiply in the treatment bag the contact surfaces between the liquid metal and the treatment gas as well as to increase the residence time of this gas in the metal, avoiding coalescing of the bubbles emitted, the volume of which is close to 0.5 cm³, which allows them to have a low rate of rise and a very large gas-metal exchange surface.

It has also been sought to use the high gas flow rates required by the large volume of gas necessary for dehydrogenation in a short period of time to minimize the heat losses due to the duration of the treatment.

We also sought to obtain the calmest and best distributed intumescence in the free surface of the liquid metal, taking into account the high flow rate of gas used so as to avoid splashes of liquid steel and fragmentation and then the slag entrainments even metal.

We have also sought to circulate in turbulent flow regime the majority of the liquid steel contained in the ladle to increase the probability of encountering solid elementary inclusions at the temperature of liquid steel such as alumina or titanium nitrides so that they can agglomerate much faster and obtain the size sufficient to settle or cling to the refractory walls of the ladle.

Attempts have also been made to concentrate the liquid slag in the center of the surface of the ladle, where the speed of circulation of the steel is the lowest.

In the case of insufflation of an oxygen-based mixture, it has also been sought to distribute the heat resulting from its combustion in order to avoid heating at too high temperatures endangering the life of the insufflation refractories. and that the calories released by the combustion of oxygen at the nose of the blowing holes are quickly swept away by the liquid steel less hot from the top of the pocket.

The process for treating a liquid steel according to the invention applies to a steel which has been in a first phase produced in the liquid state and then transferred to a ladle. This steel is then treated in this pocket with a very large number of fine bubbles of gas or gas mixture which is injected through the bottom of the pocket from injection elements located at a distance from the center of the latter. at least equal to its half-radius and distant from the wall corresponding to the edge of the bottom of the pocket by a distance at least equal to one tenth of said radius. This results in an annular swelling (intumescence) on the surface of the liquid steel, the outer edge of which is close to the inner edge of the lining of the pocket wall.

The nature of the gas injected depends on the treatment to be provided: this gas can consist of a rare gas from the air (most commonly argon) or nitrogen or carbon dioxide or a mixture of the above gases.

Preferably the injection elements have a surface of between S / 10 and S / 30, S being the surface of the bottom of the bag and the gas injection pressure is adjusted so that the average unit flow rate per cm² of surface injection element is between 0.1 and 0.8 liters / minute in the case of dehydrogenation treatments, settling of inclusions and nitriding of stainless steels with high nitrogen content.

More preferably still in the case of decarburization treatment, the calorie intake resulting from the combustion of the gas mixture such as argon oxygen or nitrogen oxygen cannot be greater than that resulting from the combustion of 0.1 to 0.6 liters d pure oxygen per minute and per cm² of surface area of injection elements. The proportion of argon or nitrogen, which varies during the operation, is a function of the carbon content and the temperature and will be adjusted in such a way that the partial pressure PCO obtained makes it possible to have the dissolved oxygen content in equilibrium with slag containing iron oxide in the case of carbon steels or with slag containing chromium oxide in the case of steels with high chromium content.

Preferably the injection of gas by the injection elements is carried out either through oriented channels or pores whose cross section perpendicular to the gas flow is not more than 0.8 mm², the total surface of these channels being between 15 and 40 mm² per dm² of surface injection elements either through straight or non-rectilinear slits of thickness less than or equal to 0.4 mm separated from each other by a distance preferably between 1 and 3 cm, the total surface of the slits being between 45 and 105 mm² per dm² of injection element surface.

As an indication, the injection pressures used corresponding to the above values will be for the channels or pores of the order of at most 4 bars above the ferrostatic pressure at the bottom of the pocket and for the slots of around 1 bar or even 0.5 bar above the ferrostatic pressure at the bottom of the pocket.

The injection sites can be distributed generally in an annular or pseudo-annular zone, whether continuous or not. Preferably the greatest angular distance which separates these locations seen from the center of the pocket is not more than 30 °.

The permeable injection elements are refractory and can be of any shape: cylindrical, conical, pyramidal, parallelipiped or other. Advantageously, they are coated on all their surfaces with the exception of that in contact with the liquid steel with a steel sheet which is connected to the gas inlet tube on the side which, after being placed in the bottom of pocket, look out of it. These permeable elements can be arranged in a removable or fixed manner. Once mounted, they are part of the pocket bottom.

The distribution of the treatment gas to all the injection elements can be done for example from a central inlet which may or may not be linked to one of the injection elements.

It is possible to place in the pocket bottom a single annular porous refractory piece of continuous annular shape or comprising a cutout or several porous refractory pieces positioned generally annular, the angular width of each of the zones devoid of permeable elements, seen from the center of the pocket, not being greater than 30 ° and preferably 25 °.

Advantageously, the treatment operations will be carried out in a pocket under a cover allowing the capture of the fumes and the protection of the atmospheric environment and reducing the heat losses.

Also advantageously, the liquid steel and the slag in the ladle can be reheated during the injection of at least one gas, for example by one or more electric arcs crossing the layer of the liquid slag formed inside the ring d intumescence.

The process according to the invention applies in particular to the dehydrogenation of steels thanks to the multiplication of bubble-liquid metal contact surfaces allowing the partial pressures to be balanced between the hydrogen contained in the liquid steel and that admitted in the bubbles.

Satisfactory results are obtained by passing a volume of neutral gas through the liquid steel of the order of 0.5 to 1.5 Nm³ per tonne of liquid metal.

The process according to the invention also allows an acceleration of the agglomeration of solid inclusions, in the case of steels whose deoxidation or denitriding products are solid (alumina or titanium nitride for example), which are very quickly entrained in the slag layer. It is thus possible to lower in a record time the total oxygen content on semi-finished product to around 1.5 times the dissolved oxygen content of the corresponding liquid steel by the sole injection of neutral gas from the bottom.

The process according to the invention also applies to the decarburization of steels with very low carbon content, microalloyed or not and in particular those containing less than 0.05% carbon. It also applies to the decarburization of steels with a high content of martensitic chromium or not or austenitic or austenoferritic chromium nickel.

The process according to the invention also applies to the nitriding of stainless steels with a high nitrogen content (for example 0.2 to 0.4% by mass) in which the decarburization under low partial pressure of CO is obtained by a mixture oxygen nitrogen and the final content of nitrogen is adjusted after calming and desulfurization of the metal by injection of pure nitrogen.

The invention also relates to a pocket allowing the treatment of a liquid steel according to the method of the invention equipped with a pocket bottom comprising injection elements connected to gas supply means whose surface total is between S / 10 and S / 30, S being the surface of the bottom of the pocket, these injection elements being located at a distance from the center of the pocket bottom at least equal to the half-radius and distant from the wall internal pocket of a distance at least equal to a tenth of said radius, these injection elements comprising either pores or channels whose unit cross section is less than 0.8 mm², the total surface being between 15 and 40 mm² per dm² of injection element, ie slots whose thickness is less than 0.4 mm, the total surface being between 45 and 105 mm² per dm² of injection elements.

The preferred embodiments of this pocket and in particular the structure of the injection elements of the bottom of the pocket have already been described previously in connection with the description of the process and are not repeated here.

• Figure 1 : vue en élévation et en coupe suivant X-X (figure 2) d'une poche pour le traitement de l'acier liquide suivant l'invention.
• Figure 2 : vue de dessus du fond de la poche de la figure 1.
• Figure 3 : vue en perspective d'une pièce réfractaire poreuse équipant la poche des figures 1 et 2.

The figures and the example below describe in a nonlimiting manner embodiments of the device according to the invention as well as the implementation of the method according to the invention in the case of the decarburization of a steel at 13% of Cr and in that of dehydrogenation and decantation of inclusions of a bearing steel of type 100 C6 (AFNOR standard).

• Figure 1: elevational view in section along XX (Figure 2) of a bag for the treatment of liquid steel according to the invention.
• Figure 2: top view of the bottom of the pocket of Figure 1.
• Figure 3: perspective view of a porous refractory piece fitted to the bag of Figures 1 and 2.

FIG. 1 shows a pocket 1 according to the invention which allows the treatment by the method also according to the invention of a volume of liquid steel 2.

The bottom 3 of this pocket is provided, as shown in FIGS. 2 and 3, with 4 porous refractory pieces 4, 5, 6, 7 of frustoconical pyramidal shape. The side walls of these parts, such as 8, 9 are coated with a sheet of steel which is connected in leaktight manner at the level of the large base to the gas inlet tube such as 10. These refractory parts are crossed by oriented pores or channels, such as 11 which bring the surface of the large base into communication with that of the small base 12. As shown in FIGS. 1 and 2, the small base 12 is at the level of the upper face 13 of the bottom of pocket. The oriented pores 11 have an average diameter of 0.8 mm. Each of the porous refractory pieces is crossed by 500 pores distributed over the surface of the small base which is in the case of the figure 1050 cm² (100 cm long and 10.5 cm wide). The total surface "SP" of the small bases of the 4 porous parts is therefore 4200 cm² for a total number "of nt" of pores equal to 2000. The surface "S" of the pocket bottom is 4.9 m2 which corresponds to a radius (R1) of 1.25 m.

It can be seen that the SP / S ratio is equal to 0.085 value which lies within the preferential range. Just as the surface of the pores per dm² is 24 mm² value also located within the preferential range. Finally, it can be seen in FIG. 2 that the porous refractory pieces are entirely outside the circle of radius R2 corresponding to half of R1, that is 0.625 m and distant by more than R / 10, that is 0.125 meters from the edges of the bottom of the pocket. It is also noted that the largest angular width of an area devoid of pores, seen from the center of the pocket bottom, corresponds to the angle "α" which is equal to 25 °. This angle "α" is less than the maximum angular width of 30 ° of an area devoid of pores of the device according to the invention. The height "H" of liquid steel at rest in the pocket is approximately 2.5 m for a mass of steel "t" of 80 tonnes.

Is injected through the pocket bottom, according to F1, a total volume of gas of about 1600 Nl / min which corresponds to 20 Nl / t / min or even about 0.380 Nl / min per cm² of surface of the porous parts. We see that, thanks to the structure of the pocket bottom according to the invention, it is possible to have per cm2 of porous surface a very low gas flow of between 0.1 and 0.8 liters / min / cm² which allows the creation of a large number of bubbles of very small dimensions which are very unlikely to meet and merge during their ascent through the liquid steel. All of these bubbles cause a swirling displacement of the liquid steel over a large annular zone which involves a very large volume.

Thus is formed the annular bulge 14, of height "h" above the level of the steel at rest, which contains the slag 15 in the axial zone and creates a permanent exchange zone of large surface and of great activity with this milkman. The arrows F2 show the movement of the cooler liquid steel with return to the pocket bottom in the axial zone. The low unit flow rate of the pores associated with the large surface area that they cover on the bottom of the pocket and their geometric arrangement on said pocket bottom thus makes it possible to obtain a whole set of interesting results for the treatment in the pocket of the steel.

The device according to the invention also makes it possible to reheat the liquid steel and the slag by means, for example, of one or more arc heating electrodes, not shown, placed above the liquid steel in the zone close to the 'axis. By using relatively short arcs, at least partially embedded in the slag, efficient heating is obtained without risk of overheating of the upper zones of the refractory walls of the pocket due to the protection due to the annular bulge 14. At the end of treatment the steel is poured through tap hole 17.

The ladle thus described can be used in particular for the treatment, by the process according to the invention, of a steel with 13% chromium.

In this first application, a mother steel of the type containing 13% chromium is conventionally produced from scrap, from ferro-chrome fuel and from the usual additions. This steel is decarburized in the oven to 0.4% carbon then it is poured into a pocket with sufficient guard height, avoiding as much as possible the presence of slag from the oven. The composition of this pocket steel is then: Cr 13.3% in mass. Yes 0.010% en masse. VS 0.4% " " Mn 0.4% " "

This mother steel is covered with grain lime and a little spar. We injects from the bottom of the pocket an oxygen-argon mixture with increasing argon content and at constant oxygen flow rate. A significant intumescence is formed on the surface of the steel, permitted by the chosen guard height. The percentage of oxygen by volume is gradually reduced from 80% to approximately 52%, which makes it possible to lower the carbon content to 0.08% without scorification of the chromium thanks to a reduced oxygen activity in the metal. without exceeding the temperature of 1680 ° C. The additional treatments of deoxidation and calming of the nuance and settling of the inclusions are then carried out by continuing the injection of argon alone with a flow rate of 10 liters / Ton / min for about 35 minutes.

The volume of oxygen injected by the permeable elements is close to 140 m³.

A final analysis then gives the following composition: Cr 13.03% en masse VS 0.090% " Mn 0.8% " Yes 0.27% "

The steel is then poured.

The chromium yield is 98%.

The emission of red smoke is much lower than that of conventional processes.

Note that the pocket decarburization at this temperature would not have been possible by a flow of pure oxygen without significant scorification of the chromium which it would have been difficult and costly to reduce during the final deoxidation; moreover, the temperatures obtained would have been higher and would have led to a rapid and dangerous deterioration of the insufflation bricks. The low flow rate of oxygen injected per cm² of surface of the porous parts, associated with the cooling due to the dilution by argon and the sweeping of the surface of the injection elements by the coolest liquid steel makes it possible to limit and d '' gradually dissipate the heat resulting from the combustion of oxygen on the surface of the insufflation elements.

In the second application, a 60T casting is produced in an electric furnace with an eccentric tap hole of a 100 C 100 type steel with 1.1% carbon and 1.5% chromium from scrap. Poured into a pocket fitted with the injection system according to the invention, this steel with the usual additions and 300 kg of fresh and very dry lime and 30 kg of fluorspar are added.

The analysis of the steel is as follows: VS 1.1% en masse Cr 1.5% en masse Mn 0.5% en masse Yes 0.35% en masse S 0.030% en masse Al 0.100% en masse

The temperature is 1600 ° C.

The bag is transported to a bag processing installation with heating by 3 graphite electrodes so as to obtain a final temperature of 1630 °. During the whole operation the pocket is under cover so that the atmosphere above the liquid steel is free of oxygen and hydrogen coming from the atmosphere.

During the whole operation, the steel is stirred by a stream of pure argon at a flow rate of 1 Nm³ / min for 1 hour diffused by 4000 cm² of permeable elements comprising 60 channels of unit cross section 0.5 mm² per dm² of surface permeable elements.

After 1 hour of treatment, the analysis of the steel is as follows: VS 1.1% en masse Cr 1.5% " Mn 0.530% " Yes 0.360% " Al 0.015% " S 0.015% " the oxygen activity measured is 4 ppm.

The steel is then poured into an ingot mold entirely sheltered from the air to avoid any parasitic rehydrogenation and reoxidation reaction.

Analysis of the semi-finished product shows a total oxygen content of 6 ppm and a hydrogen content of 3 ppm.

The same steels having undergone the same ladle treatment operation with reheating for also 1 hour but with traditional stirring and whose samples are taken and analyzed after the same time after casting indicate an average total oxygen content of 11 ppm and a hydrogen content of 7 ppm.

Consequently, for the steel treated according to the process of the invention, there is an average drop of 4 ppm in hydrogen and a total oxygen value of 1.5 times the activity of oxygen in the ladle at the end of treatment.

Many modifications can be made to the method and to the bag according to the invention which do not go beyond the scope of the latter.

Likewise, the method and the ladle can be applied to the ladle treatment of a very wide variety of steels of all types and all compositions.

Claims (16)

1) Method for treating a liquid steel in a ladle with at least one gas injected through the ladle bottom, characterized in that this injection is carried out through injection elements whose total surface is between S / 10 and S / 30, that these injection elements are located at a distance from the center of the bottom of the pocket at least equal to its half-radius and distant from the internal wall of the pocket by a distance at least equal to 1/10 of this same radius and in that the average unit flow rate of gas per cm² of surface area of the injection elements is between 0.1 and 0.8 / 1 minute, this flow rate being permitted either by pores with a unit cross section is less than 0.8 mm² the total surface being between 15 and 40 mm² per dm² of injection elements, or by slots whose thickness is less than 0.4 mm, the total surface being between 45 and 105 mm² per dm² of injection element surface. 2) Method according to claim 1 characterized in that the injection elements are located in a continuous annular zone or not, the angular width of a zone without insufflation element seen from the center of the bottom of the pocket being not more than 30 °. 3) Method according to claims 1 or 2 characterized in that in the case of the slots, these are separated from each other by a distance between 1 and 3 cm. 4) Method according to one of claims 1 to 3 characterized in that the liquid steel is covered with an active slag whose analysis is suitable for the desired metallurgical treatment capable of gathering in mass at the center of the upper surface of the steel in the pocket during the injection operation. 5) Method according to one of claims 1 to 4 characterized in that the injected gas is a neutral gas like the rare gases of air or nitrogen with respect to the chemical elements of liquid steel for the operations of settling of solid inclusions such as alumina inclusions. 6) Method according to one of claims 1 to 4 characterized in that the gas injected for the dehydrogenation operations is one of rare gases from air or carbon dioxide or a mixture of these gases. 7) Method according to one of claims 1 to 4 characterized in that for the nitriding operations of stainless steels with high nitrogen content the gas is nitrogen. 8) Method according to one of claims 1 to 4 characterized in that the decarburization operations are carried out by a rare gas mixture of air or nitrogen and oxygen by the combustion of which the partial pressure Pco resulting therefrom is that desired by the thermodynamic equilibria governing the content of carbon and dissolved oxygen in the liquid steel at the temperature of the liquid steel during treatment. 9) Method according to claims 1 to 8 characterized in that the pocket is provided with a cover for protecting the outside atmosphere during the injection operation. 10) Method according to claims 1 to 9 characterized in that during the injection, the liquid steel is heated by means of at least one electric arc produced through at least one electrode placed above the center of the pocket. 11) Method according to claims 1 to 9 characterized in that the injection elements are refractory and can be cylindrical, conical, pyramidal parallelepipedal or other. 12) Method according to claims 1 to 11 characterized in that the faces of the injection elements with the exception of that in contact with the liquid steel are coated with a steel sheet which is connected to the inlet tube process gas (es). 13) A method according to claim 12 characterized in that the distribution of the treatment gas to all the injection elements can be done from a central inlet which may or may not be linked to one of the injection elements. 14) pocket for performing the steel treatment according to one of claims 1 to 13 equipped with a pocket bottom comprising injection elements connected to means of supply of charac gas characterized in that the total surface of the injection elements is between S / 10 and S / 30, S being the surface of the pocket bottom, these injection elements being located at a distance from the center of the pocket bottom at least equal to the half-radius and spaced from the internal wall of the pocket by a distance at least equal to a tenth of said radius, these injection elements comprising either channels or pores whose unit cross-section is less than 0.8 mm², the total surface being between 15 and 40 mm² per dm² of injection element, ie slots whose thickness is less than 0.4 mm, the total surface being between 45 and 105 mm² per dm² of element d 'injection. 15) Pocket according to claim 14 characterized in that in the case of the slots, these are rectilinear or not. 16) Pocket according to claims 14 or 15 characterized in that, in the case of the slots, these are separated from each other by a distance between 1 and 3 cm.

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