EP2047926A1 - Method of manufacturing stainless steels comprising fine carbonitrides, and product obtained from this method - Google Patents

Method of manufacturing stainless steels comprising fine carbonitrides, and product obtained from this method Download PDF

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Publication number
EP2047926A1
EP2047926A1 EP07291236A EP07291236A EP2047926A1 EP 2047926 A1 EP2047926 A1 EP 2047926A1 EP 07291236 A EP07291236 A EP 07291236A EP 07291236 A EP07291236 A EP 07291236A EP 2047926 A1 EP2047926 A1 EP 2047926A1
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EP
European Patent Office
Prior art keywords
stainless steel
titanium
semi
nozzle
liquid metal
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Application number
EP07291236A
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German (de)
French (fr)
Inventor
Jean-Michel Damasse
Paul Naveau
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Aperam Stainless France SA
Original Assignee
Ugine et Alz France SA
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Ugine et Alz France SA filed Critical Ugine et Alz France SA
Priority to EP07291236A priority Critical patent/EP2047926A1/en
Priority to EP08860262.8A priority patent/EP2197608B1/en
Priority to ES08860262.8T priority patent/ES2690310T3/en
Priority to US12/682,380 priority patent/US20100278684A1/en
Priority to PCT/FR2008/001320 priority patent/WO2009074736A1/en
Priority to KR1020107010145A priority patent/KR101220791B1/en
Publication of EP2047926A1 publication Critical patent/EP2047926A1/en
Withdrawn legal-status Critical Current

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    • 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
    • B22D11/108—Feeding additives, powders, or the like
    • 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
    • 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
    • C21C5/005—Manufacture of stainless steel
    • 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
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/0006—Adding metallic additives
    • 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
    • C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
    • C21C7/0068—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00 by introducing material into a current of streaming metal
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium

Definitions

  • the invention relates to a method for manufacturing stabilized stainless steels for economically obtaining a very fine dispersion of carbonitrides after solidification, with a minimized risk of nozzle clogging during casting.
  • the invention also relates to stabilized, continuously cast stainless steels having a very fine dispersion of homogeneously distributed carbonitrides.
  • stabilizing elements are made in the bag. It is known that a possible precipitation of chromium carbides at the grain boundaries can lead to a local depletion of chromium and thus to an awareness of intergranular corrosion.
  • Elements such as titanium, zirconium, niobium and vanadium which form carbides, nitrides or carbonitrides which are more stable than chromium carbides, are therefore used as stabilizing elements for fixing carbon and nitrogen.
  • Continuous casting of steel is a well known process: it consists of pouring from a pocket, a liquid metal in a tundish for regulating the flow and then, from the latter, to make a casting in the upper part of a water-cooled bottomless copper mold with vertical reciprocating motion.
  • the semi-solidified product from the lower part of the mold is extracted by means of rollers.
  • the liquid steel is introduced into the mold by means of a tubular conduit called nozzle arranged between the tundish and the mold.
  • the liquid metal is poured on the top of a refractory material dome of a distributor body.
  • the shape of this dome causes a flow of the metal towards its periphery, the flow being deflected towards the inner wall of the nozzle or an intermediate vertical tubular member.
  • a volume without liquid metal is created in which it is possible to carry out additions via an injection channel.
  • the device thus described is called a hollow jet nozzle or "Hollow Jet Nozzle".
  • the patent BE1014063 discloses a method of adding metal powders to form oxides during solidification.
  • a steel having a dissolved oxygen level (O 2 ) given from the distributor to the mold is cast, an addition (M) of metal powder is carried out, the M / O 2 ratio is controlled and the mixture is mixed. the powder to the liquid metal so as to form metal oxides.
  • O 2 dissolved oxygen level
  • the patent WO2006096942 discloses an addition of technical ceramic nanoparticles in a hollow jet nozzle.
  • These ceramic particles may be oxides, nitrides, carbides, borides or silicides. These particles are characterized by a high thermal stability, so that no reaction occurs substantially between them and the liquid metal. However, this process is difficult to implement due to agglomeration of the nanoparticles which tend to form larger particles possibly causing the aforementioned defects. Again, the application of such a technique to stainless steels is not mentioned in the patent.
  • the object of the invention is to provide a process for the manufacture of stabilized stainless steels having a fine and regular dispersion of nitrides and / or carbonitrides.
  • it seeks to obtain a large number of fine precipitates, less than 2.5 microns in size, while limiting the number of coarse precipitates larger than 10 microns.
  • Another object of the invention is to propose a process having a better efficiency with regard to the yield of the additions of stabilizing elements, compared with the methods of addition in the bag.
  • Another object of the invention is to provide a method for minimizing the risk of plugging nozzles in continuous casting of stainless steels.
  • Another object of the invention is to provide stainless steel semi-finished products having an equiaxed solidification structure at the end of the continuous casting, even without the use of electromagnetic stirring techniques.
  • Another object of the invention is to provide stainless steel semi-finished products having a good homogeneity on a cross section relative to the direction of continuous casting.
  • the subject of the invention is thus a process for producing a stabilized stainless steel semi-finished product comprising a casting step using a hollow jet nozzle disposed between a tundish and a continuous casting mold, the nozzle comprising in its upper part a distributor member for deflecting the liquid metal arriving at the inlet of the nozzle, thus defining an interior volume without liquid metal.
  • the method is characterized by supplying, in the form of liquid metal in the tundish, an unstabilized stainless steel containing no precipitates of nitrides, carbides and carbonitrides, and then pouring the liquid metal by means of the nozzle simultaneously performing an addition of metal powder in the interior volume of the hollow jet, the metal powder containing at least one element for stabilizing the stainless steel, the addition being carried out at a temperature of the liquid steel between T liquidus + 10 ° C and T liquidus + 40 ° C, and solidified said semi-finished product.
  • the invention also relates to a process characterized in that the solidification of the liquid metal begins less than 2 seconds from the addition to obtain the semi-finished product.
  • the subject of the invention is also a process according to one of the above modes, characterized in that the stabilizing element is chosen from one or more of the following elements: titanium, niobium, zirconium, vanadium.
  • the stabilizing element is titanium, the titanium, carbon and nitrogen contents of the stainless steel, expressed in mass percentage, satisfying: Ti ⁇ 0,15 + 4 (C + N)
  • the steel is a ferritic stainless steel, or austenitic stainless, or martensitic stainless or austenitic ferritic stainless.
  • the invention also relates to a semi-finished product manufactured by a method according to one of the above modes, characterized in that its solidification structure is completely equiaxed.
  • the invention also relates to a stainless steel product made from a semi-finished product produced by a method according to one of the above modes, characterized in that the stabilizing element is titanium and that the number of precipitates of nitrides or titanium carbonitrides smaller than 2.5 microns, is greater than 15000 / cm 2 .
  • the number of titanium nitrides or carbonitrides greater than 10 microns in size is preferably less than 50 / cm 2 .
  • the average interprecipitated distance is less than 15 micrometers.
  • the invention to be described is directed to a wide range of stainless steels capable of being stabilized by additions of titanium, niobium or zirconium, vanadium or other stabilizing elements, these elements being used alone or in combination.
  • the invention is advantageously used in the manufacture of ferritic stainless steels of the X 3CrTi17 type, of composition according to NF.EN 10.088-1 and 2: C ⁇ 0.050, Si ⁇ 1.00%, Mn ⁇ 1.00%, P ⁇ 0.040%, S ⁇ 0.015%, Cr: 16.00-18.00%, N ⁇ 0.045%, 0.15 + 4 (C + N) ⁇ Ti ⁇ 0.080%, the contents being expressed as a percentage by mass.
  • a liquid metal containing a content of nitrogen N and carbon C present in the form of dissolved elements is poured from the ladle into the distribution basket: the composition and the temperature of the liquid metal are such that it does not exist precipitates of nitrides, carbides, carbonitrides, under these conditions.
  • the carbon and nitrogen contents make it possible to adjust the amounts of stabilizing elements which will be added later.
  • the ladle is poured into a tundish 1 comprising a bottom with a closure device 2 whose more or less complete closure makes it possible to regulate the flow towards a casting nozzle 3.
  • a tundish 1 comprising a bottom with a closure device 2 whose more or less complete closure makes it possible to regulate the flow towards a casting nozzle 3.
  • the temperature of the liquid steel must not be too important. It will be seen later that the additions made in the hollow jet nozzle must be carried out at a temperature having a limited distance from the liquidus temperature (referred to as T liquidus ) of the steel.
  • the process according to the invention requires the use of a hollow jet nozzle.
  • This nozzle comprises a distribution dome 4 made of refractory material pierced with one or more injection channels which open into the central lower part of the dome in the form of injection tubes 5. It is thus possible to add a metallic powder entrained by a vector gas.
  • the injected powder 6 mixes with the liquid metal which has been deflected by the upper part of the dome towards the walls of the nozzle or of an intermediate tubular member between the nozzle proper and the tundish.
  • the powder supply is carried out by one or more tubes 7 themselves connected to one or more tanks 8.
  • the upper part 9 of these powder reservoirs is pressurized with a carrier neutral gas such as argon, which helps protect the powder from oxidation.
  • a flow adapted gas forces the powder to flow to the hollow jet nozzle with a flow rate corresponding to the amount that it is desired to add.
  • the flow of the powder can also be facilitated by a mechanical device such as a worm.
  • the particle size of the powder must be chosen so as to ensure easy flow between the tanks and the nozzle and a near-immediate melting in the liquid metal. A spherical particle size of between 100 and 200 micrometers is well adapted to these requirements.
  • Powders of these metal elements can be naturally mixed so as to achieve a particular combination such as, for example, titanium-niobium bi-stabilization. It is also possible to mix the above powders with ferroalloys or iron powder in order to reduce the overheating temperature at the outlet of the hollow jet nozzle so as to increase the equiaxed zone fraction of the semi-finished product after solidification.
  • the stabilizing element Upon addition to the hollow jet nozzle, the stabilizing element is melted by contacting the liquid metal within a few tenths of a second. Since the powder is protected from oxidation by the neutral gas until it comes into contact with the liquid metal, the yield of the addition is high.
  • Sufficient stabilizing elements are added so that nitrogen and carbon are completely precipitated and the solubility product corresponding to the formation of these precipitates is reached or exceeded at the temperature at which the addition is made.
  • the nitrides and / or carbonitrides then immediately precipitate in a very fine form.
  • the solidification of the liquid metal is begun in less than 2 seconds, the latter starting on the walls of the mold 10.
  • This very limited hold time of the precipitates in the liquid metal makes it possible to avoid an increase in their size.
  • Those skilled in the art will be able to adapt the various parameters at their disposal such as: height of the injection device with respect to the mold, injection flow rate, more or less significant implementation of the heat exchangers, extraction speed of the half -product, overheating temperature, complementary injection of ferroalloy powder to accelerate the solidification, so that the delay between the addition and the beginning of the solidification is less than 2 seconds.
  • a particularity of the stainless steels obtained according to the invention lies in the great homogeneity of the dispersion of the nitrides and carbonitrides with a smaller mean inter-precipitate distance, so that possible awareness due to a depleted area locally is reduced.
  • the above parameters, and in particular the powder injection rate and the superheating temperature are adapted so as to obtain a completely equiaxed semi-finished solidification structure.
  • This last term designates for example a slab (thickness of the order of 200mm), a slab (thickness of the order of 50-80mm), a thin strip (thickness of the order of 1-3 mm), a slab billet, not yet mechanically deformed hot.
  • Such an equiaxed structure is particularly advantageous in the field of ferritic stainless steels to minimize the ragging defect. It is known that this defect is manifested by the formation of surface irregularities after stamping parallel to the rolling direction. It is due to the presence of heterogeneous structures before cold rolling and annealing, themselves resulting from columnar solidification structures.
  • the addition of powder proves to be advantageous for obtaining a totally equiaxed structure because the precipitates act as germination sites, thus preventing the formation of a less favorable columnar or basaltic type solidification.
  • the invention thus makes it possible to avoid possibly implementing electromagnetic stirring techniques which are usually used for this purpose.
  • the semi-finished product After manufacture of the semi-finished product, it can be hot-rolled or cold-rolled, pickled, annealed, according to conventional methods, to thereby obtain a product which can take various forms such as hot strip, thin sheet, or long product of various forms.
  • grade B the addition of titanium was carried out in pocket, in the form of a titanium sponge.
  • the liquid metal in the tundish does not contain titanium.
  • This element was added in a hollow jet nozzle in the form of ferro-titanium powder (titanium 70% -fer 30%) with a particle size of between 100 and 200 microns.
  • the addition temperature of the powder is T liquidus + 35 ° C.
  • the solidification of the metal begins less than two seconds after addition, on the walls of the mold.
  • Different slab-shaped castings have been made according to the invention without encountering a problem of nozzle plugging. This is a consequence of the characteristic late precipitation of the process, the low retention time of the precipitates within the liquid metal and an advantage over conventional addition processes.
  • a density of fine precipitates ( ⁇ 2.5 ⁇ m) greater than 15000 / cm 2 guarantees a very homogeneous distribution of titanium nitrides. In this way, the trapping of carbon and nitrogen is ensured in a very complete and uniform manner.
  • a density of coarse precipitates (> 10 ⁇ m) less than 50 / cm 2 makes it possible to ensure that failure initiation does not occur prematurely during mechanical stressing.
  • the invention makes it possible to multiply by a factor of about 2 the number of fine precipitates and to divide the number of coarse precipitates by a factor of about 3.
  • the yield of the addition of titanium (ratio of the titanium present in the final product to the titanium added in the form of a powder) is 95 to 100% in the process according to the invention. This yield is therefore much higher than that of the conventional process, of the order of 60%.
  • the method according to the invention thus makes it possible to economically and reliably produce stabilized stainless steel grades having a very fine dispersion of nitrides or carbonitrides.

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Abstract

The process for manufacturing a stabilized stainless steel semi-product, comprises supplying a non-stabilized stainless steel in the form of a molten metal in a pocket distributor (1), continuously flowing the molten metal using a hollow jet nozzle (3) disposed between the pocket distributor and an ingot mold, and solidifying the semi-product. The nozzle comprises a distributor body in its upper part for deflecting the molten metal arriving at an inlet of the nozzle. The molten metal is flowed by simultaneously adding a metallic powder (6) in an internal volume of the hollow jet. The process for manufacturing a stabilized stainless steel semi-product, comprises supplying a non-stabilized stainless steel in the form of a molten metal in a pocket distributor (1), continuously flowing the molten metal using a hollow jet nozzle (3) disposed between the pocket distributor and an ingot mold, and solidifying the semi-product. The nozzle comprises a distributor body in its upper part for deflecting the molten metal arriving at an inlet of the nozzle. The molten metal is flowed by simultaneously adding a metallic powder (6) in an internal volume of the hollow jet at a temperature of T l i q u i d u s+10[deg] C to T l i q u i d u s+40[deg] C. The metallic powder contains an element for the stabilization of stainless steel. The solidification is carried out after 2 seconds of addition of the metallic powder. An independent claim is included for a stainless steel semi-product.

Description

L'invention concerne un procédé de fabrication d'aciers inoxydables stabilisés permettant d'obtenir de façon économique une dispersion très fine de carbonitrures après solidification, avec un risque minimisé de bouchage de busette lors de la coulée.The invention relates to a method for manufacturing stabilized stainless steels for economically obtaining a very fine dispersion of carbonitrides after solidification, with a minimized risk of nozzle clogging during casting.

L'invention concerne également des aciers inoxydables stabilisés, coulés en continu, présentant une dispersion très fine de carbonitrures répartis de façon homogène. Pour stabiliser ces aciers inoxydables, on procède à des additions d'éléments stabilisants, en poche. On sait en effet qu'une précipitation éventuelle de carbures de chrome aux joints de grains peut conduire à un appauvrissement local en chrome et donc à une sensibilisation à la corrosion intergranulaire. Des éléments tels que le titane, le zirconium, le niobium, le vanadium qui forment des carbures, nitrures ou carbonitrures plus stables que les carbures de chrome, sont donc utilisés comme éléments stabilisants pour fixer le carbone et l'azote.The invention also relates to stabilized, continuously cast stainless steels having a very fine dispersion of homogeneously distributed carbonitrides. In order to stabilize these stainless steels, additions of stabilizing elements are made in the bag. It is known that a possible precipitation of chromium carbides at the grain boundaries can lead to a local depletion of chromium and thus to an awareness of intergranular corrosion. Elements such as titanium, zirconium, niobium and vanadium which form carbides, nitrides or carbonitrides which are more stable than chromium carbides, are therefore used as stabilizing elements for fixing carbon and nitrogen.

Les additions de titane ou de ferro-titane en poche se font par exemple sous forme de fil fourré ou d'éponge. Il existe cependant des inconvénients à ces additions précoces, c'est à dire au stade de la poche :

  • compte tenu du délai s'écoulant entre les additions et la solidification en lingotière, une partie des précipités a le temps de coalescer et de s'agglomérer au sein du métal liquide, conduisant à un accroissement de la taille moyenne des précipités et à la présence de certains précipités de taille plus importante. Ceci a une influence néfaste sur les propriétés mécaniques car l'amorçage de l'endommagement intervient en premier lieu sur les précipités de plus grande taille. De plus, certains agglomérats de précipités peuvent se retrouver sur la peau des demi-produits après coulée et entrainer des défauts de surface qui doivent être éliminés par des traitements mécaniques coûteux.
  • Par ailleurs, une oxydation partielle des éléments stabilisants peut intervenir et un certain nombre de précipités a le temps de décanter, ce qui diminue notablement le rendement des additions de ces éléments.
The additions of titanium or ferro-titanium pocket are for example in the form of cored wire or sponge. However, there are disadvantages to these early additions, that is to say at the stage of the pocket:
  • given the time between additions and solidification in the mold, part of the precipitates has the time to coalesce and agglomerate within the liquid metal, leading to an increase in the average size of the precipitates and the presence some larger precipitates. This has a detrimental influence on the mechanical properties because the initiation of the damage intervenes primarily on the larger precipitates. In addition, some agglomerates of precipitates can be found on the skin of the semi-finished products after casting and cause surface defects that must be eliminated by expensive mechanical treatments.
  • In addition, a partial oxidation of the stabilizing elements can occur and a number of precipitates have time to settle, which significantly reduces the yield of additions of these elements.

Il est envisagé de stabiliser les aciers inoxydables au stade de la coulée continue. La coulée continue d'acier est un procédé bien connu : celui-ci consiste à couler à partir d'une poche, un métal liquide dans un panier répartiteur destiné à réguler le débit puis, à partir de ce dernier, d'effectuer une coulée dans la partie supérieure d'une lingotière en cuivre sans fond refroidie à l'eau et animée d'un mouvement vertical alternatif. On extrait au moyen de rouleaux le demi-produit solidifié de la partie inférieure de la lingotière.It is envisaged to stabilize stainless steels at the stage of continuous casting. Continuous casting of steel is a well known process: it consists of pouring from a pocket, a liquid metal in a tundish for regulating the flow and then, from the latter, to make a casting in the upper part of a water-cooled bottomless copper mold with vertical reciprocating motion. The semi-solidified product from the lower part of the mold is extracted by means of rollers.

L'acier liquide est introduit dans la lingotière au moyen d'un conduit tubulaire appelé busette disposé entre le répartiteur et la lingotière.The liquid steel is introduced into the mold by means of a tubular conduit called nozzle arranged between the tundish and the mold.

On a ainsi proposé un dispositif de coulée permettant des additions au stade de la lingotière, décrit dans le brevet EP269180 du Centre de Recherches Métallurgiques : le métal liquide est coulé sur le sommet d'un dôme en matériau réfractaire d'un organe répartiteur. La forme de ce dôme provoque un écoulement du métal vers sa périphérie, l'écoulement étant dévié vers la paroi interne de la busette ou d'un organe tubulaire vertical intermédiaire. On crée ainsi, dans la partie centrale de la busette sous l'organe répartiteur, un volume sans métal liquide au sein duquel il est possible d'effectuer des additions par un canal d'injection. Le dispositif ainsi décrit porte le nom de busette à jet creux ou « Hollow Jet Nozzle ».It has thus been proposed a casting device allowing additions to the stage of the mold, described in the patent EP269180 of the Metallurgical Research Center: the liquid metal is poured on the top of a refractory material dome of a distributor body. The shape of this dome causes a flow of the metal towards its periphery, the flow being deflected towards the inner wall of the nozzle or an intermediate vertical tubular member. Thus, in the central part of the nozzle under the distributor member, a volume without liquid metal is created in which it is possible to carry out additions via an injection channel. The device thus described is called a hollow jet nozzle or "Hollow Jet Nozzle".

Utilisant ce dispositif, le brevet BE1014063 décrit un procédé d'addition de poudres métalliques pour former des oxydes lors de la solidification. Dans ce but, on coule un acier présentant un taux d'oxygène dissous (O2) donné à partir du répartiteur vers la lingotière, on effectue une addition (M) de poudre métallique, on contrôle le rapport M/O2 et on mélange la poudre au métal liquide de façon à former des oxydes métalliques.Using this device, the patent BE1014063 discloses a method of adding metal powders to form oxides during solidification. For this purpose, a steel having a dissolved oxygen level (O 2 ) given from the distributor to the mold is cast, an addition (M) of metal powder is carried out, the M / O 2 ratio is controlled and the mixture is mixed. the powder to the liquid metal so as to form metal oxides.

Même si la formation de ces oxydes peut jouer un rôle favorable en augmentant la fraction de zone équiaxe sur le demi-produit solidifié, ce procédé ne permet cependant pas d'apporter une réponse à la stabilisation des aciers inoxydables puisqu'il ne concerne pas le piégeage du carbone et de l'azote. L'application d'un tel procédé aux aciers inoxydables n'est d'ailleurs pas mentionnée dans ce brevet.Even though the formation of these oxides may play a favorable role by increasing the equiaxed zone fraction on the solidified half-product, this process does not make it possible to provide a response to stabilization. stainless steels since it does not concern the sequestration of carbon and nitrogen. The application of such a method to stainless steels is also not mentioned in this patent.

Le brevet WO2006096942 décrit une addition de nanoparticules de céramique technique au sein d'une busette à jet creux. Ces particules de céramique peuvent être des oxydes, nitrures, carbures, borures, siliciures. Ces particules sont caractérisées par une grande stabilité thermique, si bien qu'aucune réaction n'intervient pratiquement entre celles-ci et le métal liquide. Ce procédé est cependant difficile à mettre en oeuvre en raison d'une agglomération des nanoparticules qui ont tendance à former des particules de plus grande taille provoquant éventuellement les défauts mentionnés ci-dessus. Là encore, l'application d'une telle technique aux aciers inoxydables n'est pas mentionnée dans le brevet.The patent WO2006096942 discloses an addition of technical ceramic nanoparticles in a hollow jet nozzle. These ceramic particles may be oxides, nitrides, carbides, borides or silicides. These particles are characterized by a high thermal stability, so that no reaction occurs substantially between them and the liquid metal. However, this process is difficult to implement due to agglomeration of the nanoparticles which tend to form larger particles possibly causing the aforementioned defects. Again, the application of such a technique to stainless steels is not mentioned in the patent.

Le but de l'invention est de proposer un procédé pour la fabrication d'aciers inoxydables stabilisés présentant une dispersion fine et régulière de nitrures et/ou de carbonitrures. On cherche en particulier à obtenir un grand nombre de fins précipités, de taille inférieure à 2,5 micromètres, tout en limitant le nombre de précipités grossiers de taille supérieure à 10 micromètres.The object of the invention is to provide a process for the manufacture of stabilized stainless steels having a fine and regular dispersion of nitrides and / or carbonitrides. In particular, it seeks to obtain a large number of fine precipitates, less than 2.5 microns in size, while limiting the number of coarse precipitates larger than 10 microns.

Un autre but de l'invention est de proposer un procédé présentant une meilleure efficacité concernant le rendement des additions d'éléments stabilisants, comparés aux procédés d'addition en poche.Another object of the invention is to propose a process having a better efficiency with regard to the yield of the additions of stabilizing elements, compared with the methods of addition in the bag.

Un autre but de l'invention est de disposer d'un procédé permettant de minimiser le risque de bouchage de busettes en coulée continue d'aciers inoxydables.Another object of the invention is to provide a method for minimizing the risk of plugging nozzles in continuous casting of stainless steels.

Un autre but de l'invention est de disposer de demi-produits d'aciers inoxydables présentant une structure de solidification équiaxe à l'issue de la coulée continue, même sans mise en oeuvre de techniques de brassage électromagnétique.Another object of the invention is to provide stainless steel semi-finished products having an equiaxed solidification structure at the end of the continuous casting, even without the use of electromagnetic stirring techniques.

Un autre but de l'invention est de disposer de demi-produits d'aciers inoxydables présentant une bonne homogénéité sur une section transversale par rapport à la direction de coulée continue.Another object of the invention is to provide stainless steel semi-finished products having a good homogeneity on a cross section relative to the direction of continuous casting.

L'invention a ainsi pour objet un procédé de fabrication d'un demi-produit en acier inoxydable stabilisé comportant une étape de coulée au moyen d'une busette à jet creux disposée entre un panier répartiteur et une lingotière de coulée continue, la busette comprenant dans sa partie supérieure un organe répartiteur permettant de dévier le métal liquide arrivant à l'entrée de la busette, définissant ainsi un volume intérieur sans métal liquide. Le procédé est caractérisé en ce qu'on approvisionne, sous forme de métal liquide dans le panier répartiteur, un acier inoxydable non stabilisé ne contenant pas de précipités de nitrures, de carbures et de carbonitrures, puis on coule le métal liquide au moyen de la busette en effectuant simultanément une addition de poudre métallique dans le volume intérieur du jet creux, la poudre métallique contenant au moins un élément permettant la stabilisation de l'acier inoxydable, l'addition étant réalisée à une température de l'acier liquide comprise entre Tliquidus+10°C et Tliquidus+40°C, et on solidifie ledit demi-produit.The subject of the invention is thus a process for producing a stabilized stainless steel semi-finished product comprising a casting step using a hollow jet nozzle disposed between a tundish and a continuous casting mold, the nozzle comprising in its upper part a distributor member for deflecting the liquid metal arriving at the inlet of the nozzle, thus defining an interior volume without liquid metal. The method is characterized by supplying, in the form of liquid metal in the tundish, an unstabilized stainless steel containing no precipitates of nitrides, carbides and carbonitrides, and then pouring the liquid metal by means of the nozzle simultaneously performing an addition of metal powder in the interior volume of the hollow jet, the metal powder containing at least one element for stabilizing the stainless steel, the addition being carried out at a temperature of the liquid steel between T liquidus + 10 ° C and T liquidus + 40 ° C, and solidified said semi-finished product.

L'invention a également pour objet un procédé caractérisé en ce qu'on débute la solidification du métal liquide moins de 2 secondes à partir de l'addition pour obtenir le demi-produitThe invention also relates to a process characterized in that the solidification of the liquid metal begins less than 2 seconds from the addition to obtain the semi-finished product.

L'invention a également pour objet un procédé selon l'un des modes ci-dessus caractérisé en ce que l'élément permettant la stabilisation est choisi parmi un ou plusieurs des éléments suivants : titane, niobium, zirconium, vanadium.The subject of the invention is also a process according to one of the above modes, characterized in that the stabilizing element is chosen from one or more of the following elements: titanium, niobium, zirconium, vanadium.

Préférentiellement, l'élément permettant la stabilisation est le titane, les teneurs en titane, carbone et azote de l'acier inoxydable, exprimées en pourcentage massique, satisfaisant à : Ti≥0,15+ 4 (C+N)Preferably, the stabilizing element is titanium, the titanium, carbon and nitrogen contents of the stainless steel, expressed in mass percentage, satisfying: Ti≥0,15 + 4 (C + N)

Selon un mode particulier, l'acier est un acier inoxydable ferritique, ou inoxydable austénitique, ou inoxydable martensitique ou inoxydable austéno-ferritique.In a particular embodiment, the steel is a ferritic stainless steel, or austenitic stainless, or martensitic stainless or austenitic ferritic stainless.

L'invention a également pour objet un demi-produit fabriqué par un procédé selon l'un des modes ci-dessus, caractérisé en ce que sa structure de solidification est totalement équiaxe.The invention also relates to a semi-finished product manufactured by a method according to one of the above modes, characterized in that its solidification structure is completely equiaxed.

L'invention a également pour objet un produit en acier inoxydable fabriqué à partir d'un demi-produit élaboré par un procédé selon l'un des modes ci-dessus, caractérisé en ce que l'élément permettant la stabilisation est le titane et que le nombre de précipités de nitrures ou de carbonitrures de titane de taille inférieure à 2,5 micromètres, est supérieur à 15000/cm2.The invention also relates to a stainless steel product made from a semi-finished product produced by a method according to one of the above modes, characterized in that the stabilizing element is titanium and that the number of precipitates of nitrides or titanium carbonitrides smaller than 2.5 microns, is greater than 15000 / cm 2 .

Le nombre de nitrures ou de carbonitrures de titane de taille supérieure à 10 micromètres, est préférentiellement inférieur à 50/cm2.The number of titanium nitrides or carbonitrides greater than 10 microns in size is preferably less than 50 / cm 2 .

Selon un mode préféré, la distance moyenne interprécipités est inférieure à 15 micromètres.In a preferred embodiment, the average interprecipitated distance is less than 15 micrometers.

D'autres caractéristiques et avantages de l'invention apparaîtront au cours de la description ci-dessous donnée à titre d'exemple et faite en référence à la figure 1 jointe qui représente schématiquement un exemple de dispositif pour mettre en oeuvre le procédé selon l'invention.Other features and advantages of the invention will become apparent from the following description given by way of example and with reference to the figure 1 attached which shows schematically an example of a device for implementing the method according to the invention.

D'autres caractéristiques et avantages de l'invention apparaîtront au cours de la description ci-dessous donnée à titre d'exemple.Other features and advantages of the invention will become apparent from the following description given by way of example.

L'invention qui va être exposée s'adresse à une large gamme d'aciers inoxydables susceptibles d'être stabilisés par des additions de titane, de niobium ou de zirconium, de vanadium ou d'autres éléments stabilisants, ces éléments étant utilisés seuls ou en combinaison.The invention to be described is directed to a wide range of stainless steels capable of being stabilized by additions of titanium, niobium or zirconium, vanadium or other stabilizing elements, these elements being used alone or in combination.

On met en particulier avantageusement l'invention en oeuvre dans la fabrication d'aciers inoxydables ferritiques du type X 3CrTi17, de composition selon la norme NF.EN 10.088-1 et 2 : C<0,050, Si<1,00%, Mn<1,00%, P<0,040%, S<0,015%, Cr:16,00-18,00%, N<0,045%, 0,15+4(C+N) <Ti<0,080%, les teneurs étant exprimées en pourcentage massique.In particular, the invention is advantageously used in the manufacture of ferritic stainless steels of the X 3CrTi17 type, of composition according to NF.EN 10.088-1 and 2: C <0.050, Si <1.00%, Mn < 1.00%, P <0.040%, S <0.015%, Cr: 16.00-18.00%, N <0.045%, 0.15 + 4 (C + N) <Ti <0.080%, the contents being expressed as a percentage by mass.

Le procédé selon l'invention est le suivant :

  • On élabore au moyen d'un procédé connu en soi, un métal liquide destiné à la fabrication d'acier inoxydable ferritique, ou inoxydable austénitique, ou inoxydable martensitique ou inoxydable austéno-ferritique. Au stade de la poche, avant coulée, l'acier liquide peut faire l'objet de différentes opérations métallurgiques :
  • additions complémentaires pour mise à la nuance de l'acier
  • désoxydation du métal liquide
  • brassage du bain par un gaz neutre de façon à assurer l'homogénéisation thermique avant coulée
The process according to the invention is as follows:
  • A liquid metal for producing ferritic stainless steel, or austenitic stainless steel or martensitic stainless or austenitic ferritic stainless steel is produced by means of a process known per se. At the ladle stage, before casting, the liquid steel can undergo various metallurgical operations:
  • additional additions for shading of steel
  • deoxidation of the liquid metal
  • bath mixing by a neutral gas so as to ensure thermal homogenization before casting

A ce stade, même si le métal liquide peut éventuellement contenir une faible quantité d'élément permettant la stabilisation de l'acier inoxydable, aucune précipitation de cet élément n'intervient. L'addition principale d'élément stabilisant et sa précipitation interviennent ultérieurement, comme décrit ci-dessous.At this stage, even if the liquid metal may possibly contain a small amount of element allowing the stabilization of the stainless steel, no precipitation of this element does not intervene. The main addition of stabilizing element and its precipitation occur later, as described below.

On coule à partir de la poche vers le panier répartiteur un métal liquide contenant une teneur en azote N et en carbone C, présents sous forme d'éléments dissous : la composition et la température du métal liquide sont telles qu'il n'existe pas de précipités de nitrures, carbures, de carbonitrures, dans ces conditions. Les teneurs en carbone et en azote permettent d'ajuster les quantités d'éléments stabilisants qui seront ajoutés ultérieurement.A liquid metal containing a content of nitrogen N and carbon C present in the form of dissolved elements is poured from the ladle into the distribution basket: the composition and the temperature of the liquid metal are such that it does not exist precipitates of nitrides, carbides, carbonitrides, under these conditions. The carbon and nitrogen contents make it possible to adjust the amounts of stabilizing elements which will be added later.

La poche est coulée dans un panier répartiteur 1 comportant un fond avec un dispositif de fermeture 2 dont l'obturation plus ou moins complète permet de réguler le débit vers une busette de coulée 3. A ce stade, la température de l'acier liquide ne doit pas être trop importante. On verra en effet plus loin que les additions effectuées au sein de la busette à jet creux doivent être effectuées à une température présentant un écart limité par rapport à la température de liquidus (désignée par Tliquidus) de l'acier.The ladle is poured into a tundish 1 comprising a bottom with a closure device 2 whose more or less complete closure makes it possible to regulate the flow towards a casting nozzle 3. At this stage, the temperature of the liquid steel must not be too important. It will be seen later that the additions made in the hollow jet nozzle must be carried out at a temperature having a limited distance from the liquidus temperature (referred to as T liquidus ) of the steel.

Au moyen de ses connaissances générales et des spécificités du dispositif de coulée qui conditionnent la perte de température entre le panier répartiteur et la busette, l'homme du métier saura ajuster la température de coulée en fonction des caractéristiques de l'invention exposées ci-dessous.By means of his general knowledge and the specificities of the casting device which condition the temperature loss between the tundish and the nozzle, the skilled person will be able to adjust the casting temperature according to the characteristics of the invention set out below. .

Comme on l'a exposé, le procédé selon l'invention nécessite l'utilisation d'une busette à jet creux. Cette busette comporte un dôme répartiteur 4 en matériau réfractaire percé d'un ou de plusieurs canaux d'injection qui débouchent dans la partie inférieure centrale du dôme sous forme de tubes d'injection 5. Il est ainsi possible d'ajouter une poudre métallique entrainée par un gaz vecteur. La poudre injectée 6 se mélange avec le métal liquide qui a été dévié par la partie supérieure du dôme vers les parois de la busette ou d'un organe tubulaire intermédiaire entre la busette proprement dite et le panier répartiteur.As has been explained, the process according to the invention requires the use of a hollow jet nozzle. This nozzle comprises a distribution dome 4 made of refractory material pierced with one or more injection channels which open into the central lower part of the dome in the form of injection tubes 5. It is thus possible to add a metallic powder entrained by a vector gas. The injected powder 6 mixes with the liquid metal which has been deflected by the upper part of the dome towards the walls of the nozzle or of an intermediate tubular member between the nozzle proper and the tundish.

L'alimentation en poudre est réalisée par un ou plusieurs tubes 7 reliés eux-mêmes à un ou plusieurs réservoirs 8. La partie supérieure 9 de ces réservoirs de poudre est mise sous pression grâce à un gaz neutre vecteur tel que l'argon, ce qui permet de protéger la poudre de l'oxydation. Un débit de gaz adapté force la poudre à s'écouler vers la busette à jet creux avec un débit correspondant à la quantité que l'on souhaite ajouter. L'écoulement de la poudre peut être également facilité par un dispositif mécanique tel qu'une vis sans fin. La granulométrie de la poudre doit être choisie de manière à assurer un écoulement facile entre les réservoirs et la busette ainsi qu'une fusion quasi-immédiate dans le métal liquide. Une granulométrie sphérique, de taille comprise entre 100 et 200 micromètres, est ainsi bien adaptée à ces exigences.The powder supply is carried out by one or more tubes 7 themselves connected to one or more tanks 8. The upper part 9 of these powder reservoirs is pressurized with a carrier neutral gas such as argon, which helps protect the powder from oxidation. A flow adapted gas forces the powder to flow to the hollow jet nozzle with a flow rate corresponding to the amount that it is desired to add. The flow of the powder can also be facilitated by a mechanical device such as a worm. The particle size of the powder must be chosen so as to ensure easy flow between the tanks and the nozzle and a near-immediate melting in the liquid metal. A spherical particle size of between 100 and 200 micrometers is well adapted to these requirements.

Cette poudre contient un ou plusieurs éléments métalliques destinés à assurer la stabilisation de l'acier inoxydable, ainsi :

  • le titane, qui peut être utilisé pur ou sous forme de ferro-titane pour des raisons de coût. Ces additions sont destinées à former des nitrures de titane TiN d'une grande stabilité ou des carbonitrures Ti(C,N)
  • le zirconium, formant lui aussi des nitrures et des carbonitrures très stables.
  • le niobium, destiné essentiellement à former des carbonitrures Nb(C,N)
  • le vanadium, formant également des carbonitrures
This powder contains one or more metallic elements intended to ensure the stabilization of the stainless steel, thus:
  • titanium, which can be used pure or in the form of ferro-titanium for cost reasons. These additions are intended to form titanium nitrides TiN of high stability or carbonitrides Ti (C, N)
  • zirconium, also forming nitrides and carbonitrides very stable.
  • niobium, essentially intended to form carbonitrides Nb (C, N)
  • vanadium, also forming carbonitrides

Des poudres de ces éléments métalliques peuvent être naturellement mélangées de façon à réaliser une combinaison particulière telle que par exemple une bi-stabilisation titane-niobium. Il est également possible de mélanger les poudres ci-dessus avec des ferro-alliages ou de la poudre de fer dans le but de diminuer la température de surchauffe à la sortie de la busette à jet creux de façon à augmenter la fraction de zone équiaxe du demi-produit après solidification.Powders of these metal elements can be naturally mixed so as to achieve a particular combination such as, for example, titanium-niobium bi-stabilization. It is also possible to mix the above powders with ferroalloys or iron powder in order to reduce the overheating temperature at the outlet of the hollow jet nozzle so as to increase the equiaxed zone fraction of the semi-finished product after solidification.

Simultanément à la coulée, on effectue l'addition de la poudre comportant le ou les éléments stabilisants dans un métal liquide à une température comprise entre Tliquidus+10°C et Tliquidus+40°C. Cette gamme particulière de température d'addition permet tout à la fois :

  • d'obtenir une précipitation fine et intense de nitrures et de carbonitrures
  • de favoriser la solidification sous une forme équiaxe.
Simultaneously with the casting, the addition of the powder comprising the stabilizing element or elements in a liquid metal is carried out at a temperature of between T liquidus + 10 ° C. and T liquidus + 40 ° C. This particular range of addition temperature allows all at once:
  • to obtain a fine and intense precipitation of nitrides and carbonitrides
  • to promote solidification in an equiaxial form.

Lorsque la température d'addition est trop élevée par rapport au liquidus, le temps qui s'écoule entre la formation des nitrures ou des carbonitrures et la fin de la solidification s'accroît, ce qui entraine une augmentation de leur taille, phénomène non désiré.When the addition temperature is too high compared to the liquidus, the time elapsing between the formation of the nitrides or carbonitrides and the end of the solidification increases, which leads to an increase in size, unwanted phenomenon.

Par contre, lorsque la température d'addition est trop faible par rapport au liquidus, le procédé devient plus sensible à une variation intempestive des paramètres de fabrication, il existe un risque de bouchage de la busette.On the other hand, when the addition temperature is too low compared to the liquidus, the process becomes more sensitive to an untimely variation of the manufacturing parameters, there is a risk of clogging of the nozzle.

Dès l'addition au sein de la busette à jet creux, l'élément stabilisant est fondu par contact du métal liquide en quelques dixièmes de seconde. La poudre étant protégée de l'oxydation par le gaz neutre jusqu'à son contact avec le métal liquide, le rendement de l'addition est élevé.Upon addition to the hollow jet nozzle, the stabilizing element is melted by contacting the liquid metal within a few tenths of a second. Since the powder is protected from oxidation by the neutral gas until it comes into contact with the liquid metal, the yield of the addition is high.

On ajoute une quantité suffisante d'éléments stabilisants pour que l'azote et le carbone soient totalement précipités et pour que le produit de solubilité correspondant à la formation de ces précipités soit atteint ou dépassé à la température où on réalise l'addition. Les nitrures et/ou les carbonitrures précipitent alors immédiatement sous une forme très fine.Sufficient stabilizing elements are added so that nitrogen and carbon are completely precipitated and the solubility product corresponding to the formation of these precipitates is reached or exceeded at the temperature at which the addition is made. The nitrides and / or carbonitrides then immediately precipitate in a very fine form.

Après addition, on débute la solidification du métal liquide en moins de 2 secondes, celle-ci débutant sur les parois de la lingotière 10. Ce temps de maintien très limité des précipités dans le métal liquide permet d'éviter une augmentation de leur taille. L'homme du métier saura adapter les différents paramètres à sa disposition tels que : hauteur du dispositif d'injection par rapport à la lingotière, débit d'injection, mise en oeuvre plus ou moins importante des échangeurs thermiques, vitesse d'extraction du demi-produit, température de surchauffe, injection complémentaire de poudre de ferro-alliage pour accélérer la solidification, pour que le délai entre l'addition et le début de la solidification soit inférieur à 2 secondes.After addition, the solidification of the liquid metal is begun in less than 2 seconds, the latter starting on the walls of the mold 10. This very limited hold time of the precipitates in the liquid metal makes it possible to avoid an increase in their size. Those skilled in the art will be able to adapt the various parameters at their disposal such as: height of the injection device with respect to the mold, injection flow rate, more or less significant implementation of the heat exchangers, extraction speed of the half -product, overheating temperature, complementary injection of ferroalloy powder to accelerate the solidification, so that the delay between the addition and the beginning of the solidification is less than 2 seconds.

Un mode préféré de réalisation repose sur l'utilisation de titane dans le but de former une précipitation de nitrures et/ou de carbonitrures fins et dispersés. Selon l'invention, les teneurs en titane, en carbone et en azote de l'acier inoxydable, exprimées en pourcentage massique, sont telles que :

  • Ti≥0,15 +4(C+N). Dans ces conditions, la quantité de titane ajoutée permet une stabilisation totale de l'acier.
A preferred embodiment relies on the use of titanium for the purpose of forming a precipitation of fine and dispersed nitrides and / or carbonitrides. According to the invention, the titanium, carbon and nitrogen contents of the stainless steel, expressed as a percentage by weight, are such that:
  • Ti≥0.15 +4 (C + N). Under these conditions, the amount of titanium added allows total stabilization of the steel.

Une particularité des aciers inoxydables obtenus selon l'invention réside dans la grande homogénéité de la dispersion des nitrures et des carbonitrures avec une plus faible distance moyenne inter-précipités, si bien qu'une éventuelle sensibilisation en raison d'une zone appauvrie localement est réduite.A particularity of the stainless steels obtained according to the invention lies in the great homogeneity of the dispersion of the nitrides and carbonitrides with a smaller mean inter-precipitate distance, so that possible awareness due to a depleted area locally is reduced.

Selon un autre mode préféré de réalisation de l'invention, les paramètres ci-dessus, et notamment le débit d'injection de poudre et la température de surchauffe, sont adaptés de façon à obtenir une structure de solidification totalement équiaxe sur demi-produit. Ce dernier terme désigne par exemple une brame (épaisseur de l'ordre de 200mm), une brame mince (épaisseur de l'ordre de 50-80mm), une bande mince (épaisseur de l'ordre de 1-3 mm), une billette, non encore déformée mécaniquement à chaud. Une telle structure équiaxe est particulièrement avantageuse dans le domaine des aciers inoxydables ferritiques pour minimiser le défaut de chiffonnage (« roping ») On sait que ce défaut se manifeste par la formation d'irrégularités de surface après emboutissage parallèles à la direction de laminage. Il est dû à la présence de structures hétérogènes avant laminage à froid et recuit, elles-mêmes résultant de structures de solidification colonnaire.According to another preferred embodiment of the invention, the above parameters, and in particular the powder injection rate and the superheating temperature, are adapted so as to obtain a completely equiaxed semi-finished solidification structure. This last term designates for example a slab (thickness of the order of 200mm), a slab (thickness of the order of 50-80mm), a thin strip (thickness of the order of 1-3 mm), a slab billet, not yet mechanically deformed hot. Such an equiaxed structure is particularly advantageous in the field of ferritic stainless steels to minimize the ragging defect. It is known that this defect is manifested by the formation of surface irregularities after stamping parallel to the rolling direction. It is due to the presence of heterogeneous structures before cold rolling and annealing, themselves resulting from columnar solidification structures.

L'addition de poudre s'avère avantageuse pour obtenir une structure totalement équiaxe car les précipités jouent le rôle de sites de germination, empêchant ainsi la formation d'une solidification de type colonnaire ou basaltique moins favorable. L'invention permet donc d'éviter éventuellement de mettre en oeuvre des techniques de brassage électromagnétique qui sont utilisées usuellement à ce dessein.The addition of powder proves to be advantageous for obtaining a totally equiaxed structure because the precipitates act as germination sites, thus preventing the formation of a less favorable columnar or basaltic type solidification. The invention thus makes it possible to avoid possibly implementing electromagnetic stirring techniques which are usually used for this purpose.

Après fabrication du demi-produit, celui-ci peut être laminé à chaud ou à froid, décapé, recuit, selon les procédés conventionnels, pour obtenir de la sorte un produit qui peut prendre des formes variées telles que bande à chaud, tôle mince, ou produit long de formes diverses.After manufacture of the semi-finished product, it can be hot-rolled or cold-rolled, pickled, annealed, according to conventional methods, to thereby obtain a product which can take various forms such as hot strip, thin sheet, or long product of various forms.

En l'absence de traitement de remise en solution, les caractéristiques de la précipitation sont pratiquement identiques sur les demi-produits et les produits obtenus à partir de ces demi-produits. Les avantages conférés par l'invention sur les demi-produits se retrouvent donc sur les produits obtenus. A titre d'exemple non limitatif, les résultats suivants vont montrer les caractéristiques avantageuses conférées par l'invention.In the absence of a redissolution treatment, the characteristics of the precipitation are practically identical on the semi-finished products and the products obtained from these semi-finished products. The advantages conferred by the invention on the semi-finished products are thus found on the products obtained. By way of non-limiting example, the following results will show the advantageous characteristics conferred by the invention.

Exemple:Example:

On a élaboré deux coulées d'acier inoxydable ferritique stabilisées au titane dont les compositions, exprimées en pourcentage pondéral, figurent au tableau 1. L'acier A a été élaboré selon l'invention dans des conditions qui vont être exposées, l'acier B a été fabriqué selon une technique de coulée continue conventionnelle. Tableau 1 Compositions des aciers C Mn Si Cr Cu Ni S Ti V N A 0,016 0,34 0,38 16,27 0,05 0,10 0,006 0,30 0,12 0,015 B 0,02 0,34 0,38 16,16 0,04 0,16 0,006 0,45 0,08 0,012 A= Fabriqué selon l'invention
B= Fabriqué selon une technique conventionnelle Two titanium stabilized ferritic stainless steel castings have been developed whose compositions, expressed in weight percent, are shown in Table 1. Steel A was produced according to the invention under conditions to be exposed, steel B being was manufactured using a conventional continuous casting technique. Table 1 Compositions of steels VS mn Yes Cr Cu Or S Ti V NOT AT 0.016 0.34 0.38 16.27 0.05 0.10 0.006 0.30 0.12 0,015 B 0.02 0.34 0.38 16.16 0.04 0.16 0.006 0.45 0.08 0.012 A = Made according to the invention
B = Made according to a conventional technique

Dans la nuance B, l'addition de titane a été réalisée en poche, sous forme d'éponge de titane.In grade B, the addition of titanium was carried out in pocket, in the form of a titanium sponge.

Dans l'élaboration de la nuance A selon l'invention, le métal liquide dans le panier répartiteur ne contient pas de titane. Cet élément a été ajouté au sein d'une busette à jet creux sous forme de poudre de ferro-titane (titane 70%-fer 30%) de granulométrie comprise entre 100 et 200 micromètres. La température d'addition de la poudre est de Tliquidus+35°C. La solidification du métal débute moins de deux secondes après addition, sur les parois de la lingotière. Différentes coulées sous forme de brames ont été effectuées selon l'invention sans rencontrer de problème de bouchage de busette. Ceci est une conséquence de la précipitation tardive caractéristique du procédé, du faible temps de maintien des précipités au sein du métal liquide et un avantage vis-à-vis des procédés d'addition conventionnels.In the preparation of the grade A according to the invention, the liquid metal in the tundish does not contain titanium. This element was added in a hollow jet nozzle in the form of ferro-titanium powder (titanium 70% -fer 30%) with a particle size of between 100 and 200 microns. The addition temperature of the powder is T liquidus + 35 ° C. The solidification of the metal begins less than two seconds after addition, on the walls of the mold. Different slab-shaped castings have been made according to the invention without encountering a problem of nozzle plugging. This is a consequence of the characteristic late precipitation of the process, the low retention time of the precipitates within the liquid metal and an advantage over conventional addition processes.

Après laminage à chaud des brames pour obtenir des bandes de 3 mm d'épaisseur, on a relevé la présence de précipités de nitrures de titane sur des coupes polies. La distribution de taille de ces précipités est mesurée par analyse d'images selon la procédure définie dans la norme ASTM E1245. La densité des précipités est exprimée en nombre de précipités par cm2.After hot rolling of the slabs to obtain strips 3 mm thick, the presence of titanium nitride precipitates was noted on polished sections. The size distribution of these precipitates is measured by image analysis according to the procedure defined in ASTM E1245. The density of the precipitates is expressed in number of precipitates per cm 2 .

On a également mesuré la distance moyenne inter-précipités. Les résultats de ces mesures sont les suivants : Tableau 2 : Caractéristiques de répartition des précipités Valeurs soulignées : non conformes à l'invention Nombre de TiN de taille inférieure à 2,5µm (N/cm2) Nombre de TiN de taille supérieure à 10pm (N/cm2) Distance moyenne inter-précipités (micromètres) Acier A (invention) 17560 30 14,2 Acier B (référence) 9320 110 24.6 The mean inter-precipitate distance was also measured. The results of these measurements are as follows: Table 2: Precipitation distribution characteristics Underlined values: not in accordance with the invention Number of TiNs less than 2.5μm (N / cm 2 ) Number of TiNs larger than 10pm (N / cm 2 ) Mean inter-precipitate distance (micrometers) Steel A (invention) 17560 30 14.2 Steel B (reference) 9320 110 24.6

Une densité de fins précipités (<2,5µm) supérieure à 15000/cm2 garantit une répartition très homogène des nitrures de titane. De la sorte, le piégeage du carbone et de l'azote est assuré de façon très complète et uniforme.A density of fine precipitates (<2.5 μm) greater than 15000 / cm 2 guarantees a very homogeneous distribution of titanium nitrides. In this way, the trapping of carbon and nitrogen is ensured in a very complete and uniform manner.

Une densité de précipités grossiers (>10µm) inférieure à 50/cm2 permet d'assurer qu'un amorçage de la rupture n'intervient pas de façon prématurée lors d'une sollicitation mécanique.A density of coarse precipitates (> 10 μm) less than 50 / cm 2 makes it possible to ensure that failure initiation does not occur prematurely during mechanical stressing.

Ces deux caractéristiques sont observées pour l'acier fabriqué selon le procédé de l'invention. Par rapport à un procédé conventionnel, l'invention permet de multiplier par un facteur d'environ 2 le nombre de fins précipités et de diviser par un facteur d'environ 3 le nombre de précipités grossiers.These two characteristics are observed for the steel manufactured according to the method of the invention. Compared to a conventional method, the invention makes it possible to multiply by a factor of about 2 the number of fine precipitates and to divide the number of coarse precipitates by a factor of about 3.

Des observations ont été effectuées sur une section transversale par rapport à la direction de coulée sur une bande de 1 m de large et de 3 mm d'épaisseur fabriquée selon l'invention. Les mesures réalisées au centre, 1/3 largeur, 2/3 largeur et au bord de la bande révèlent que la précipitation est très uniforme. En particulier, la distance moyenne inter-précipités est pratiquement identique entre le centre et la rive de la bande. Les demi-produits ou les produits fabriqués selon l'invention présentent donc une grande homogénéité de structures et de propriétés.Observations were made on a cross-section with respect to the casting direction on a strip 1 m wide and 3 mm thick made according to the invention. Measurements made in the center, 1/3 width, 2/3 width and at the edge of the band reveal that the precipitation is very uniform. In particular, the average inter-precipitate distance is virtually identical between the center and the bank of the strip. The semi-finished products or products manufactured according to the invention thus have a high homogeneity of structures and properties.

De plus, la structure de solidification examinée sur des coupes polies et attaquées transversales de brames est totalement équiaxe. L'absence de zones colonnaires se révèle favorable pour éviter le défaut de chiffonnage.In addition, the solidification structure examined on polished and etched transverse cuts of slabs is totally equiaxed. The absence of columnar areas is favorable to avoid the lack of crumpling.

Le rendement de l'addition de titane (rapport entre le titane présent dans le produit final et le titane ajouté sous forme de poudre) est de 95 à 100% dans le procédé selon l'invention. Ce rendement est donc très supérieur à celui du procédé conventionnel, de l'ordre de 60%.The yield of the addition of titanium (ratio of the titanium present in the final product to the titanium added in the form of a powder) is 95 to 100% in the process according to the invention. This yield is therefore much higher than that of the conventional process, of the order of 60%.

Le procédé selon l'invention permet donc de fabriquer de façon économique et fiable des nuances d'acier inoxydables stabilisées présentant une dispersion très fine de nitrures ou de carbonitrures.The method according to the invention thus makes it possible to economically and reliably produce stabilized stainless steel grades having a very fine dispersion of nitrides or carbonitrides.

Claims (9)

Procédé de fabrication d'un demi-produit en acier inoxydable stabilisé comportant une étape de coulée au moyen d'une busette à jet creux disposée entre un panier répartiteur (1) et une lingotière de coulée continue (10), ladite busette comprenant dans sa partie supérieure un organe répartiteur (4) permettant de dévier le métal liquide arrivant à l'entrée de ladite busette, définissant ainsi un volume intérieur sans métal liquide, caractérisé en ce que : - on approvisionne, sous forme de métal liquide dans ledit panier répartiteur, un acier inoxydable non stabilisé ne contenant pas de précipités de nitrures, de carbures et de carbonitrures, puis - on coule ledit métal liquide au moyen de ladite busette en effectuant simultanément une addition de poudre métallique (6) dans ledit volume intérieur dudit jet creux, ladite poudre métallique contenant au moins un élément permettant la stabilisation dudit acier inoxydable, ladite addition étant réalisée à une température de l'acier liquide comprise entre Tliquidus+10°C et Tliquidus+40°C, et - on solidifie ledit demi-produit A process for producing a stabilized stainless steel semi-finished product comprising a pouring step by means of a hollow jet nozzle disposed between a distributor basket (1) and a continuous casting mold (10), said nozzle comprising in its upper part a distribution member (4) for diverting the liquid metal arriving at the inlet of said nozzle, thereby defining an internal volume without liquid metal, characterized in that : - It provides, in the form of liquid metal in said tundish, an unstabilized stainless steel containing no precipitates of nitrides, carbides and carbonitrides, then said liquid metal is cast by means of said nozzle while simultaneously adding metal powder (6) into said interior volume of said hollow jet, said metal powder containing at least one element enabling the stabilization of said stainless steel, said addition being carried out at a temperature of the liquid steel between T liquidus + 10 ° C and T liquidus + 40 ° C, and solidifying said semi-finished product Procédé selon la revendication 1, caractérisé en ce qu'on débute la solidification dudit métal liquide moins de 2 secondes à partir de ladite addition pour obtenir ledit demi-produitProcess according to claim 1, characterized in that the solidification of said liquid metal begins less than 2 seconds from said addition to obtain said semi-finished product Procédé selon la revendication 1 ou 2, caractérisé en ce que ledit élément permettant la stabilisation est choisi parmi un ou plusieurs des éléments suivants : titane, niobium, zirconium, vanadiumProcess according to claim 1 or 2, characterized in that said stabilizing element is selected from one or more of the following elements: titanium, niobium, zirconium, vanadium Procédé suivant la revendication 3, caractérisé en ce que ledit élément permettant la stabilisation est le titane, les teneurs en titane, carbone et azote dudit acier inoxydable satisfaisant à, les teneurs étant exprimées en pourcentage massique : Ti ≥ 0 , 15 + 4 ⁢ C + N
Figure imgb0001
 Process according to claim 3, characterized in that said stabilizing element is titanium, the contents of titanium, carbon and nitrogen of said stainless steel satisfying, the contents being expressed as a percentage by mass: Ti ≥ 0 , 15 + 4 ⁢ VS + NOT
Figure imgb0001
 Procédé selon l'une quelconque des revendications 1 à 4, caractérisé en ce que ledit acier est un acier inoxydable ferritique, ou inoxydable austénitique, ou inoxydable martensitique ou inoxydable austéno-ferritiqueProcess according to any one of Claims 1 to 4, characterized in that the said steel is a ferritic stainless steel, or austenitic stainless steel, or a martensitic stainless or austeno-ferritic stainless steel. Demi-produit fabriqué par un procédé selon l'une quelconque des revendications 1 à 5, caractérisé en ce que sa structure de solidification est totalement équiaxeSemi-finished product manufactured by a method according to any one of claims 1 to 5, characterized in that its solidification structure is completely equiaxed Produit en acier inoxydable fabriqué à partir d'un demi-produit élaboré par un procédé selon l'une quelconque des revendications 1 à 5, caractérisé en ce que l'élément permettant la stabilisation est le titane, et que le nombre de nitrures ou de carbonitrures de titane de taille inférieure à 2,5 micromètres, est supérieur à 15000/cm2 A stainless steel product made from a semi-finished product made by a process according to any one of claims 1 to 5, characterized in that the stabilizing element is titanium, and the number of nitrides or titanium carbonitrides smaller than 2.5 micrometres is greater than 15000 / cm 2 Produit selon la revendication 7, caractérisé en ce que le nombre de nitrures ou de carbonitrures de titane de taille supérieure à 10 micromètres, est inférieur à 50/cm2 Product according to Claim 7, characterized in that the number of titanium nitrides or carbonitrides greater than 10 microns in size is less than 50 / cm 2 Produit selon la revendication 7 ou 8, caractérisé en ce que la distance moyenne interprécipités est inférieure à 15 micromètresProduct according to claim 7 or 8, characterized in that the average distance interprecipitated is less than 15 micrometers
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ES08860262.8T ES2690310T3 (en) 2007-10-10 2008-09-23 Manufacturing process of stainless steels of fine carbonitrides, and of the product obtained from this procedure
US12/682,380 US20100278684A1 (en) 2007-10-10 2008-09-23 Process for manufacturing stainless steel containing fine carbonitrides, and product obtained from this process
PCT/FR2008/001320 WO2009074736A1 (en) 2007-10-10 2008-09-23 Method for making stainless steel comprising fine carbonitrides and product obtained by said method
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