EP1038612A1 - Twin roll continuous casting of strips of carbon steel - Google Patents

Twin roll continuous casting of strips of carbon steel Download PDF

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Publication number
EP1038612A1
EP1038612A1 EP00400573A EP00400573A EP1038612A1 EP 1038612 A1 EP1038612 A1 EP 1038612A1 EP 00400573 A EP00400573 A EP 00400573A EP 00400573 A EP00400573 A EP 00400573A EP 1038612 A1 EP1038612 A1 EP 1038612A1
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Prior art keywords
cylinders
casting
dimples
liquid steel
steel
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EP00400573A
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German (de)
French (fr)
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EP1038612B1 (en
Inventor
Francois Poirier
Patrice Vicente
Francoise Commarieu
Michel Faral
Christian Marchionni
Jacques Selaries
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Sollac SA
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Sollac SA
Lorraine de Laminage Continu SA SOLLAC
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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
    • B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
    • B22D11/0637—Accessories therefor
    • B22D11/0648—Casting surfaces
    • B22D11/0651—Casting wheels
    • 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
    • B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
    • B22D11/0622—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars formed by two casting wheels

Definitions

  • the invention relates to the continuous casting of metals. It concerns more particularly the casting of thin metallic carbon steel strips on installations of the type called "casting between cylinders".
  • the process mainly used today is the casting of said metal liquid between two internally cooled cylinders, rotating around their axes horizontal in opposite directions, and arranged opposite one another, the distance minimum between their surfaces being substantially equal to the thickness which it is desired to impart to the cast strip (for example a few mm).
  • the casting space containing the liquid steel is defined by the lateral surfaces of the cylinders, on which the solidification of the strip, and by refractory side closure plates applied against the ends of cylinders.
  • Liquid metal initiates solidification on contact with surfaces outside of the cylinders, on which it forms solidified "skins", which are made of so that they meet at the level of the "collar", ie of the zone where the distance between the cylinders is minimal.
  • the object of the invention is to provide steelmakers with a method of manufacturing thin carbon steel strips by casting between cylinders leading to obtaining reliable of bands free from superficial cracks, this method should not limit excessive scope of products, nor necessarily lead to the use of long and complex methods for the preparation of the surface of cylinders.
  • the invention also relates to tapes capable of being produced by this. process, as well as the casting cylinders necessary for its implementation.
  • the invention consists in combining conditions particulars on the composition of the steel, the surface condition of the cylinders and the composition of the atmosphere surrounding the surface of the liquid steel present between the cylinders.
  • the inventors have found that the claimed combination leads to regular obtaining of products with the required surface qualities.
  • the conditions for implementing this process also have the advantage of not leading to a substantially casting process more restrictive than the usual processes.
  • the method of the invention does not require no actions on the composition of the metal which would significantly increase the price of returns from the strip, since it is not necessary to add elements to the liquid metal of expensive alloy or to impose on the liquid metal unusually low contents in certain elements.
  • an essential condition for obtaining a strip pouring between cylinders free from surface cracks is good anchoring strip skins on all the surfaces of the rolls during solidification.
  • a such anchoring guarantees that the various constraints linked to thermal contractions and possible phase changes undergone by the skins are distributed uniformly, while heterogeneities in these constraints can cause cracks superficial. They therefore recommend using casting rolls whose surface has a relatively large roughness, in the form of "dimples".
  • These dimples as is known (see for example document EP-A - 0 796 685), are depressions of roughly circular or oval shapes, which can be formed on the surface of the cylinder by projection of metal or ceramic balls. In these conditions, during solidification, the metal skins penetrate inside the dimples, the more deeply as these have a large diameter. So they behave as anchors for the skins on the cylinders.
  • Ra and Rz are, according to the standards in force, calculated as follows.
  • these dimples must be contiguous, it is say that their peripheries are not systematically separated by flat areas. Indeed, the alternation of large flat areas, where the contact between the metal and the cylinder is narrow, and dimples where this contact is more relaxed, may be unfavorable for obtaining a crack-free strip surface, because in this case the areas with strong cooling are present in a proportion which may be too great.
  • the dimples are distributed randomly. In addition, the absence of beaches planes increases the number of anchor points for the solidified skin.
  • Conditions for wetting the surface of the cylinder with liquid steel at the level meniscus are also very important for establishing transfers thermal. They depend in particular on the composition of the liquid metal.
  • FIG. 1 schematically represents the pouring space of an installation for continuous casting of thin metal strips between two parallel cylinders 1, 1 'cooled internally, in rotation around their axes kept horizontal.
  • Dimples 2 are formed on their external surfaces 3, 3 'which are made of copper or copper alloy.
  • the liquid steel 4 is present in the casting space defined by the surfaces 3, 3 ′, where it is brought from a container called a distributor by means of a refractory nozzle (not shown).
  • the liquid steel 4 solidifies to form skins 5, 5 'whose thickness increases as they progress downwards the casting space under the effect of the rotation of the cylinders 1, 1 '.
  • the scale of the different parts of the installation was not respected.
  • the cylinders 1, 1 ′ generally have a diameter from 500 to 1500 mm, the diameter and depth of the dimples 2 are of the order of a few tens to several hundred ⁇ m, and the thickness of the strips is a few mm (up to 10 mm, usually 2 to 6 mm).
  • a preferred condition for obtaining the desired result is to perform the casting without using any covering material, so leaving the metal surface bare liquid 4, so as not to hinder evaporation and deposition on the cylinders 1, 1 'of volatile elements.
  • the process according to the invention is compatible with the presence of aluminum in steel.
  • this presence of aluminum can be accompanied by a addition of a few tens of ppm of calcium intended to transform the inclusions from alumina to aluminates of lime liquid at the melting temperature of steel.
  • the method according to the invention has the advantage of not requiring the presence within the liquid metal 4 of a quantity relatively large number of oxidized inclusions which could be unfavorable for many uses of the final metal.
  • these oxide inclusions would risk form plaques near the meniscus that could be trapped by skins 5, 5 '. This would deteriorate the quality of the surface of the strip 7. It is considered that it It is preferable not to exceed a content of 100 ppm of total oxygen (i.e.
  • This oxygen content total largely depends on the dissolved oxygen content which is determined by the chemical balances between the liquid steel 4 and its environment, and in particular by the contents of liquid steel 4 in deoxidizing elements: manganese, silicon and possibly aluminum.
  • a classic, fast and inexpensive method for forming dimples 2 on surfaces 3, 3 'of the cylinders 1, 1' is to project metal or ceramic balls onto said surfaces 3, 3 '. By playing on the number, materials, diameters and pressure of projection of the balls, one can obtain the desired configurations for said dimples 2.
  • Other methods using a laser or chemical attack or EDM of surfaces 3, 3 ', or a marking by knurling of surfaces 3, 3') are also possible.
  • the invention therefore makes it possible to achieve good anchoring of the skins 5, 5 ′ during solidification on the surfaces 3, 3 'of the cylinders to avoid cracks which would be due to excessive fragility of the skins 5, 5 '.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Continuous Casting (AREA)
  • Treatment Of Steel In Its Molten State (AREA)
  • Heat Treatment Of Strip Materials And Filament Materials (AREA)

Abstract

Carbon steel strips (7) of thickness not more than 10 mm are produced by continuous casting of molten steel (4) containing Mn and Si between the lateral copper or copper alloy surfaces (3) of two internally cooled and rotating horizontal rolls (1, 1'), in an atmosphere of 40-100 % nitrogen, the remainder being an inert gas insoluble in the molten steel, at the meniscus (8, 8'). The molten steel contains (in weight %): C ≤ 0.5%, Mn 0.2-2%, Si ≤ 0.2%, the ratio Mn/Si being 3-16, optionally Al+Ti+Zr ≤ 0.10%, O 100 ppm, preferably 30-700 ppm, and iron and unavoidable impurities the remainder. The lateral surfaces (3, 3') of the rolls (1, 1') have contacting pits (2) providing the surfaces with roughness Rz 40-200 microns and roughness Ra 10-40 microns . The inert gas insoluble in the molten steel can be a mixture of such gases. The cast strip is subjected to in-line hot rolling after casting. Independent claims are given for: (a) a carbon steel strip of thickness not greater than 10 mm and produced by the above process; and (b) the rolls, as above, used for the continuous casting of thin metal strips.

Description

L'invention concerne la coulée continue des métaux. Elle concerne plus particulièrement la coulée de bandes minces métalliques en acier au carbone sur des installations du type appelé « coulées entre cylindres ».The invention relates to the continuous casting of metals. It concerns more particularly the casting of thin metallic carbon steel strips on installations of the type called "casting between cylinders".

Ces dernières années ont vu s'accomplir des progrès sensibles dans le développement des procédés de coulée de minces bandes d'acier directement à partir de métal liquide. Le procédé principalement utilisé aujourd'hui est la coulée dudit métal liquide entre deux cylindres refroidis intérieurement, tournant autour de leurs axes horizontaux dans des sens opposés, et disposés en regard l'un de l'autre, la distance minimale entre leurs surfaces étant sensiblement égale à l'épaisseur que l'on désire conférer à la bande coulée (par exemple quelques mm). L'espace de coulée renfermant l'acier liquide est défini par les surfaces latérales des cylindres, sur lesquelles s'initie la solidification de la bande, et par des plaques de fermeture latérale en réfractaire appliquées contre les extrémités des cylindres. Le métal liquide initie sa solidification au contact des surfaces extérieures des cylindres, sur lesquelles il forme des « peaux » solidifiées, dont on fait en sorte qu'elles se rejoignent au niveau du « col », c'est à dire de la zone où la distance entre les cylindres est minimale.The last few years have seen significant progress in development of processes for casting thin steel strips directly from liquid metal. The process mainly used today is the casting of said metal liquid between two internally cooled cylinders, rotating around their axes horizontal in opposite directions, and arranged opposite one another, the distance minimum between their surfaces being substantially equal to the thickness which it is desired to impart to the cast strip (for example a few mm). The casting space containing the liquid steel is defined by the lateral surfaces of the cylinders, on which the solidification of the strip, and by refractory side closure plates applied against the ends of cylinders. Liquid metal initiates solidification on contact with surfaces outside of the cylinders, on which it forms solidified "skins", which are made of so that they meet at the level of the "collar", ie of the zone where the distance between the cylinders is minimal.

Ces procédés de coulée concernent aussi bien la coulée d'acier au carbone que la coulée d'aciers inoxydables ou d'autres alliages ferreux. Cependant, l'application industrielle de la coulée entre cylindres aux aciers au carbone ne peut s'envisager de manière acceptable que si on parvient à obtenir de manière constante une qualité de surface des bandes suffisante pour que les traitements ultérieurs subis par ces bandes (laminages à froid, traitements de surface...) soient possibles et conduisent à l'obtention de produits exempts de défauts rédhibitoires. Il est, en particulier, primordial que les bandes issues d'une installation de coulée entre cylindres soient exemptes de fissures superficielles, appelées criques, car sinon de graves incidents peuvent se produire lors de leur laminage à froid.These casting processes relate to both the casting of carbon steel and the casting of stainless steels or other ferrous alloys. However, the application industrial casting between cylinders with carbon steels cannot be envisaged acceptable way that if we can consistently achieve a surface quality bands sufficient for the subsequent treatments undergone by these bands (rolling to cold, surface treatments ...) are possible and lead to obtaining products free from unacceptable defects. It is, in particular, essential that the bands from of a casting installation between cylinders are free from surface cracks, called cracks, because otherwise serious incidents can occur during their rolling at cold.

Pour tenter d'éviter ces criques, on a déjà proposé des solutions mettant en jeu une texturation particulière de la surface des cylindres de coulée, à savoir une succession de rainures contigües et parallèles, associées de préférence à un métal coulé calmé silicium-manganèse à teneur en soufre élevée, supérieure à 0,02% (document EP-A-0 740 972). Mais cette solution complique la préparation des cylindres par rapport aux modes de préparation plus classiques, et dans ses conditions optimales, limite le domaine d'application des produits coulés à celles où la teneur en soufre élevée du métal est tolérable.To try to avoid these cracks, we have already proposed solutions involving a particular texturing of the surface of the casting rolls, namely a succession of Contiguous and parallel grooves, preferably associated with a calm silicon-manganese cast metal with a high sulfur content, greater than 0.02% (document EP-A-0 740 972). But this solution complicates the preparation of the cylinders compared to the modes of more classic preparation, and in its optimal conditions, limits the area application of cast products to those where the high sulfur content of the metal is tolerable.

Le but de l'invention est de procurer aux aciéristes un procédé de fabrication de bandes minces en acier au carbone par coulée entre cylindres conduisant à l'obtention fiable de bandes exemptes de criques superficielles, cette méthode ne devant pas limiter de manière excessive le champ d'application des produits, ni conduire forcément à l'utilisation de méthodes longues et complexes pour la préparation de la surface des cylindres.The object of the invention is to provide steelmakers with a method of manufacturing thin carbon steel strips by casting between cylinders leading to obtaining reliable of bands free from superficial cracks, this method should not limit excessive scope of products, nor necessarily lead to the use of long and complex methods for the preparation of the surface of cylinders.

A cet effet, l'invention a pour objet un procédé de fabrication de bandes d'épaisseur inférieure ou égale à 10 mm en acier au carbone directement à partir d'acier liquide, par coulée dudit acier liquide entre les surfaces latérales en cuivre ou alliage de cuivre de deux cylindres horizontaux en rotation refroidis intérieurement, caractérisé en ce que :

  • ledit acier a la composition, en pourcentages pondéraux : carbone ≤ 0,5%, manganèse de 0,2 à 2%, silicium ≤ 2%, le rapport Mn% / Si% étant compris entre 3 et 16, optionnellement aluminium + titane + zirconium ≤ 0,10%, le reste étant du fer et des impuretés habituelles ;
  • lesdites surfaces latérales des cylindres comportent des fossettes jointives, imposant auxdites surfaces une rugosité Rz comprise entre 40 et 200 µm et une rugosité Ra comprise entre 10 et 40 µm ;
  • et l'atmosphère environnant le ménisque de l'acier liquide présent entre les cylindres comprend entre 40 et 100% d'azote, le restant étant composé d'un gaz neutre insoluble dans l'acier liquide ou d'un mélange de tels gaz neutres.
To this end, the subject of the invention is a method of manufacturing strips of thickness less than or equal to 10 mm in carbon steel directly from liquid steel, by casting said liquid steel between the lateral surfaces of copper or alloy of copper from two internally cooled horizontal rotating cylinders, characterized in that:
  • said steel has the composition, in weight percentages: carbon ≤ 0.5%, manganese from 0.2 to 2%, silicon ≤ 2%, the Mn% / Si% ratio being between 3 and 16, optionally aluminum + titanium + zirconium ≤ 0.10%, the remainder being iron and usual impurities;
  • said side surfaces of the cylinders have joined dimples, imposing on said surfaces a roughness Rz of between 40 and 200 μm and a roughness Ra of between 10 and 40 μm;
  • and the atmosphere surrounding the meniscus of the liquid steel present between the cylinders comprises between 40 and 100% nitrogen, the remainder being composed of a neutral gas insoluble in liquid steel or a mixture of such neutral gases .

L'invention concerne également des bandes susceptibles d'être produites par ce procédé, ainsi que les cylindres de coulée nécessaires à sa mise en pratique.The invention also relates to tapes capable of being produced by this. process, as well as the casting cylinders necessary for its implementation.

Comme on l'aura compris, l'invention consiste à combiner des conditions particulières sur la composition de l'acier, l'état de surface des cylindres et la composition de l'atmosphère environnant la surface de l'acier liquide présent entre les cylindres. Les inventeurs ont constaté que la combinaison revendiquée conduisait à l'obtention régulière de produits ayant les qualités de surface requises. Les conditions de mise en oeuvre de ce procédé ont également l'avantage de ne pas conduire à un process de coulée sensiblement plus contraignant que les process habituels. De plus, le procédé de l'invention ne nécessite pas d'actions sur la composition du métal qui augmenteraient sensiblement le prix de revient de la bande, puisqu'il n'est pas nécessaire d'ajouter au métal liquide d'éléments d'alliage coûteux ou d'imposer au métal liquide des teneurs inhabituellement basses en certains éléments. De même, les gammes d'utilisation des produits fabriqués à partir des bandes ainsi produites ne sont pas limitées par la présence en quantités plus élevées que d'ordinaire d'éléments tels que le soufre, qui pourraient altérer radicalement les propriétés mécaniques desdits produits. Enfin, cette invention n'implique pas d'augmentation des flux de chaleur à extraire du métal liquide par les cylindres, alors qu'une telle augmentation serait dommageable pour la durée d'utilisation de la surface externe refroidie desdits cylindres. As will be understood, the invention consists in combining conditions particulars on the composition of the steel, the surface condition of the cylinders and the composition of the atmosphere surrounding the surface of the liquid steel present between the cylinders. The inventors have found that the claimed combination leads to regular obtaining of products with the required surface qualities. The conditions for implementing this process also have the advantage of not leading to a substantially casting process more restrictive than the usual processes. In addition, the method of the invention does not require no actions on the composition of the metal which would significantly increase the price of returns from the strip, since it is not necessary to add elements to the liquid metal of expensive alloy or to impose on the liquid metal unusually low contents in certain elements. Likewise, the ranges of use of products made from bands thus produced are not limited by the presence in higher quantities than usually elements such as sulfur, which could drastically alter the properties mechanical of said products. Finally, this invention does not imply an increase in flows heat to extract liquid metal through the cylinders, while such an increase would be harmful for the duration of use of the cooled external surface of said cylinders.

L'invention sera mieux comprise à la lecture de la description qui suit, donnée en référence aux figures annexées suivantes :

  • la figure 1, qui représente schématiquement, vu de face et en coupe transversale, l'espace de coulée d'une installation de coulée entre cylindres, en y montrant le comportement du ménisque de l'acier liquide présent entre les cylindres de coulée dans des conditions correspondant à celles de l'art antérieur ;
  • la figure 2, qui représente schématiquement le comportement du ménisque dans le cas de l'invention.
The invention will be better understood on reading the description which follows, given with reference to the following appended figures:
  • Figure 1, which schematically shows, seen from the front and in cross section, the casting space of a casting installation between cylinders, showing there the behavior of the meniscus of the liquid steel present between the casting cylinders in conditions corresponding to those of the prior art;
  • FIG. 2, which schematically represents the behavior of the meniscus in the case of the invention.

Selon les inventeurs, une condition essentielle pour l'obtention d'une bande coulée entre cylindres exempte de criques superficielles est l'obtention d'un bon ancrage des peaux de la bande sur l'ensemble des surfaces des cylindres lors de la solidification. Un tel ancrage garantit que les différentes contraintes liées aux contractions thermiques et aux éventuels changements de phase subis par les peaux sont répartis de manière uniforme, alors que des hétérogénéités dans ces contraintes peuvent provoquer des criques superficielles. Ils préconisent donc d'utiliser des cylindres de coulée dont la surface présente une rugosité relativement importante, sous forme de « fossettes ». Ces fossettes, comme il est connu (voir par exemple le document EP- A - 0 796 685), sont des dépressions de formes grossièrement circulaires ou ovales, que l'on peut ménager sur la surface du cylindre par projection de billes en métal ou en céramique. Dans ces conditions, au cours de la solidification, les peaux métalliques pénètrent à l'intérieur des fossettes, d'autant plus profondément que celles-ci ont un fort diamètre. Elles se comportent donc comme des points d'ancrage des peaux sur les cylindres.According to the inventors, an essential condition for obtaining a strip pouring between cylinders free from surface cracks is good anchoring strip skins on all the surfaces of the rolls during solidification. A such anchoring guarantees that the various constraints linked to thermal contractions and possible phase changes undergone by the skins are distributed uniformly, while heterogeneities in these constraints can cause cracks superficial. They therefore recommend using casting rolls whose surface has a relatively large roughness, in the form of "dimples". These dimples, as is known (see for example document EP-A - 0 796 685), are depressions of roughly circular or oval shapes, which can be formed on the surface of the cylinder by projection of metal or ceramic balls. In these conditions, during solidification, the metal skins penetrate inside the dimples, the more deeply as these have a large diameter. So they behave as anchors for the skins on the cylinders.

Toutefois, cette pénétration ne doit pas être trop importante, pour plusieurs raisons. Si les fossettes sont plus profondes que 200 µm en moyenne et si l'acier en cours de solidification les remplit totalement, la surface de la bande comportera « en négatif» l'image en relief des fossettes, autrement dit un bosselage, qui n'est généralement pas désirable sur le produit final. Il faudra l'écraser lors des laminages ultérieurs que subira la bande, et un tel écrasement n'est pas toujours possible de manière suffisante à l'aide des taux de réduction habituellement pratiqués sur les bandes coulées entre cylindres. D'autre part, il n'est pas toujours souhaitable que le contact entre la peau en cours de solidification et la surface du cylindre soit très étroit, car cela conduirait à un flux de chaleur extrait du métal par le cylindre très important. Il contribue à dégrader rapidement la surface des cylindres, en y accentuant les phénomènes de fatigue. Des indices classiques décrivant la rugosité de la surface des cylindres sont l'indice Ra et l'indice Rz. Si on considère que la surface des cylindres est une succession de saillies et de creux par rapport à un niveau moyen, et que ces saillies et creux ont une hauteur ou une profondeur y, Ra et Rz sont, selon les normes en vigueur, calculés de la manière suivante. However, this penetration should not be too great, for many reasons. If the dimples are deeper than 200 µm on average and if the steel in progress of solidification completely fills them, the surface of the strip will have "negative" the relief image of the dimples, in other words a bump, which is generally not desirable on the final product. It will have to be crushed during the subsequent rolling that will undergo the tape, and such crushing is not always possible sufficiently using the reduction rates usually practiced on strips cast between cylinders. Else apart, it is not always desirable that the contact between the skin during solidification and the cylinder surface is very narrow, as this would lead to a heat flux extracted from the metal by the very important cylinder. It contributes to rapidly degrading the surface of cylinders, accentuating the phenomena of fatigue. Classic clues describing the surface roughness of the cylinders are the Ra index and the Rz index. If we consider that the surface of the cylinders is a succession of projections and depressions with respect to a level medium, and that these projections and hollows have a height or a depth y, Ra and Rz are, according to the standards in force, calculated as follows.

Sur une longueur L donnée (égale à la circonférence du cylindre dans notre cas),

Figure 00040001
On a given length L (equal to the circumference of the cylinder in our case),
Figure 00040001

Sur une longueur donnée Li égale à 1/5 de la circonférence du cylindre (i variant de 1 à 5),

Figure 00040002
Ypi (i variant de 1 à 5) étant la hauteur des 5 saillies les plus élevées et Yvi (i variant de 1 à 5) étant la profondeur des 5 creux les plus profonds. On pose ensuite selon la norme DIN 4768 que Rz= 1 / 5(RZ1+RZ2+RZ3+RZ4+RZ5)On a given length L i equal to 1/5 of the circumference of the cylinder (i varying from 1 to 5),
Figure 00040002
Yp i (i varying from 1 to 5) being the height of the 5 highest projections and Yv i (i varying from 1 to 5) being the depth of the 5 deepest hollows. Then we ask according to DIN 4768 that Rz = 1/5 (R Z1 + R Z2 + R Z3 + R Z4 + R Z5 )

Dans le procédé selon l'invention, il faut que ces fossettes soient jointives, c'est à dire que leurs périphéries ne soient pas systématiquement séparées par des plages planes. En effet, l'alternance de larges plages planes, où le contact entre le métal et le cylindre est étroit, et de fossettes où ce contact est plus relâché, peut être défavorable à l'obtention d'une surface de bande exempte de criques, car, dans ce cas, les zones à fort refroidissement sont présentes dans une proportion qui peut être trop importante. De préférence, les fosssettes sont réparties de façon aléatoire. De plus, l'absence de plages planes permet d'augmenter le nombre de points d'ancrage de la peau solidifiée.In the method according to the invention, these dimples must be contiguous, it is say that their peripheries are not systematically separated by flat areas. Indeed, the alternation of large flat areas, where the contact between the metal and the cylinder is narrow, and dimples where this contact is more relaxed, may be unfavorable for obtaining a crack-free strip surface, because in this case the areas with strong cooling are present in a proportion which may be too great. Of preferably, the dimples are distributed randomly. In addition, the absence of beaches planes increases the number of anchor points for the solidified skin.

Un autre facteur à prendre en considération est la composition du gaz qui est utilisé pour inerter l'atmosphère entourant le ménisque, c'est à dire la périphérie de la surface du métal liquide présent entre les cylindres, au niveau de laquelle s'amorce la solidification des peaux. En effet, les fossettes, au moment ou elles viennent au contact du ménisque, contiennent du gaz, qui reste donc emprisonné entre le fond de chaque fossette et la peau en cours de solidification. En fonction de ses caractéristiques physico-chimiques, ce gaz va influer sur les conditions de formation des peaux. L'expérience montre que si ce gaz est insoluble dans l'acier (ce qui est le cas de l'argon et de l'hélium), il forme un « matelas» gazeux qui peut empêcher une pénétration profonde du métal dans les fossettes. Cela peut aller dans le sens d'une diminution de la formation des criques par l'effet des contraintes liées à la contraction de la peau lors de sa solidification et de son refroidissement. Mais on peut aussi observer la formation de criques par un autre mécanisme : la dilatation du gaz peut conduire localement à des pertes de contact entre la peau et le cylindre, ce qui ralentit exagérément les transferts thermiques. La peau s'en trouve fragilisée localement, ce qui est favorable à la formation de criques. On peut également dans certains cas retrouver sur la bande un marquage en creux, si la dilatation du gaz insoluble a été suffisamment importante pour repousser la peau en cours de solidification. La pénétration procure également un bon ancrage de la peau sur le cylindre, ce qui, comme on l'a dit, favorise l'homogénéité des transferts thermiques sur l'ensemble de la surface de la peau et va donc dans le sens d'une diminution de la formation de criques superficielles. Il n'y a pas, dans l'absolu, de composition de gaz d'inertage qui serait universellement idéale, et cette composition doit pouvoir être ajustée en fonction des autres conditions opératoires, au besoin pendant la coulée elle-même.Another factor to consider is the composition of the gas which is used to inert the atmosphere surrounding the meniscus, i.e. the periphery of the surface of the liquid metal present between the cylinders, at which the solidification of the skins. Indeed, the dimples, when they come into contact with the meniscus, contain gas, which therefore remains trapped between the bottom of each dimple and the skin being solidified. Depending on its physico-chemical characteristics, this gas will influence the conditions of skin formation. Experience shows that if this gas is insoluble in steel (which is the case of argon and helium), it forms a Gaseous "mattress" that can prevent deep penetration of the metal into the dimples. This can go in the direction of a reduction in the formation of cracks by the effect of stresses related to the contraction of the skin during its solidification and its cooling. But we can also observe the formation of cracks by another mechanism: gas expansion can locally lead to contact losses between the skin and cylinder, which excessively slows heat transfer. The skin gets finds it weakened locally, which is favorable for the formation of cracks. We can also in some cases find a hollow marking on the tape, if the expansion of the insoluble gas was important enough to repel the skin during solidification. Penetration also provides good anchoring of the skin on the cylinder, which, as we said, promotes the homogeneity of heat transfers over the whole of the surface of the skin and therefore goes in the direction of a decrease in the formation of cracks superficial. There is, in absolute terms, no composition of inerting gas which would be universally ideal, and this composition must be able to be adjusted according to the others operating conditions, if necessary during the casting itself.

Les conditions de mouillage de la surface du cylindre par l'acier liquide au niveau du ménisque sont également très importantes pour l'établissement des transferts thermiques. Elles dépendent notamment de la composition du métal liquide.Conditions for wetting the surface of the cylinder with liquid steel at the level meniscus are also very important for establishing transfers thermal. They depend in particular on the composition of the liquid metal.

La figure 1 représente schématiquement l'espace de coulée d'une installation de coulée continue de bandes minces métalliques entre deux cylindres 1, 1' parallèles refroidis intérieurement, en rotation autour de leurs axes maintenus horizontaux. Des fossettes 2 sont ménagées sur leurs surfaces externes 3, 3' qui sont en cuivre ou alliage de cuivre. De l'acier liquide 4 est présent dans l'espace de coulée défini par les surfaces 3, 3', où il est amené à partir d'un récipient appelé répartiteur au moyen d'une busette en réfractaire (non représentée). Au contact des surfaces 3, 3', l'acier liquide 4 se solidifie pour former des peaux 5, 5' dont l'épaisseur croít au fur et à mesure de leur progression vers le bas de l'espace de coulée sous l'effet de la rotation des cylindres 1, 1'. Les peaux 5, 5' se rejoignent au niveau du col 6 pour former une bande 7 entièrement solidifiée qui est extraite de l'espace de coulée par un dispositif classique non représenté, comportant par exemple des rouleaux pinceurs. Comme représenté, les surfaces des peaux 5, 5' pénètrent à l'intérieur des fossettes 2, ce qui procure à la surface de la bande 7 une forme légèrement bosselée. Pour des raisons de clarté de la figure, l'échelle des différentes parties de l'installation n'a pas été respectée. A titre indicatif, les cylindres 1, 1' ont généralement un diamètre de 500 à 1500 mm, le diamètre et la profondeur des fossettes 2 sont de l'ordre de quelques dizaines à plusieurs centaines de µm, et l'épaisseur des bandes est de quelques mm (jusqu'à 10 mm, couramment 2 à 6 mm).FIG. 1 schematically represents the pouring space of an installation for continuous casting of thin metal strips between two parallel cylinders 1, 1 'cooled internally, in rotation around their axes kept horizontal. Dimples 2 are formed on their external surfaces 3, 3 'which are made of copper or copper alloy. Of the liquid steel 4 is present in the casting space defined by the surfaces 3, 3 ′, where it is brought from a container called a distributor by means of a refractory nozzle (not shown). On contact with the surfaces 3, 3 ′, the liquid steel 4 solidifies to form skins 5, 5 'whose thickness increases as they progress downwards the casting space under the effect of the rotation of the cylinders 1, 1 '. The skins 5, 5 'are join at the neck 6 to form a fully solidified band 7 which is extracted from the casting space by a conventional device not shown, comprising by example of pinch rollers. As shown, the surfaces of the skins 5, 5 ′ penetrate the interior of the dimples 2, which gives the surface of the strip 7 a slightly bumpy. For reasons of clarity of the figure, the scale of the different parts of the installation was not respected. As an indication, the cylinders 1, 1 ′ generally have a diameter from 500 to 1500 mm, the diameter and depth of the dimples 2 are of the order of a few tens to several hundred µm, and the thickness of the strips is a few mm (up to 10 mm, usually 2 to 6 mm).

Habituellement, les conditions de mouillage des surfaces 3, 3' des cylindres 1, 1' par l'acier liquide 4, lors de la coulée d'aciers au carbone, font que le ménisque 8 prend la forme représentée sur la figure 1, à savoir que le contact acier 4 - surface 3, 3' peut s'effectuer avec un angle de raccordement aigu. Au niveau du ménisque 8, 8', il y a donc un interstice 9, 9' entre l'acier liquide 4 et la surface 3, 3' du cylindre 1, 1'. Sous l'effet de la rotation des cylindres 1, 1', on assiste donc à un entraínement (symbolisé par les flèches 10, 10') du gaz formant l'atmosphère environnant le ménisque 8, 8' à l'intérieur des fossettes 2. Un bon mouillage de la surface 3, 3' du cylindre 1, 1' par l'acier liquide inhibe l'entraínement de la couche limite de gaz présente au voisinage de la surface 3, 3' du cylindre 1, 1', entre le cylindre 1, 1' et la peau solidifiée 5, 5', ce qui permet d'éviter la possibilité de formation d'un marquage en creux sur la surface de la bande 7. La présence d'un tel angle de raccordement rend la forme et la position du ménisque peu stables, ce qui introduit des variations significatives dans la manière dont s'amorce la solidification des peaux 5, 5'. De plus, l'entraínement de gaz influe de manière plus ou moins aléatoire sur la composition et le volume du gaz formant le matelas gazeux présent dans les fossettes 2, ce qui provoque également des hétérogénéités dans le déroulement de la solidification des peaux 5, 5'. Il est probable que ces phénomènes au niveau du ménisque 8, 8' rendent difficile la recherche et le maintien permanent de conditions de coulée qui éviteraient l'apparition de criques superficielles.Usually, the wetting conditions of the surfaces 3, 3 'of the cylinders 1, 1' by the liquid steel 4, during the casting of carbon steels, cause the meniscus 8 to take the shape shown in Figure 1, namely that the contact steel 4 - surface 3, 3 'can take place with an acute connection angle. At the level of the meniscus 8, 8 ', there is therefore a gap 9, 9 'between the liquid steel 4 and the surface 3, 3' of the cylinder 1, 1 '. Under the influence of the rotation of the cylinders 1, 1 ', so we are witnessing a training (symbolized by the arrows 10, 10 ') of the gas forming the atmosphere surrounding the meniscus 8, 8' inside the dimples 2. Good wetting of the surface 3, 3 'of the cylinder 1, 1' by liquid steel inhibits the driving of the boundary layer of gas present in the vicinity of the surface 3, 3 'of the cylinder 1, 1 ', between cylinder 1, 1' and the solidified skin 5, 5 ', which avoids the possibility of forming a hollow marking on the surface of the strip 7. The presence such a connection angle makes the shape and position of the meniscus unstable, which introduces significant variations in the way in which the solidification of skins 5, 5 '. In addition, the gas entrainment influences more or less randomly the composition and volume of the gas forming the gas blanket present in the dimples 2, this which also causes heterogeneities in the course of the solidification of skins 5, 5 '. It is likely that these phenomena at the meniscus 8, 8 'make difficult to find and permanently maintain casting conditions which would avoid the appearance of superficial cracks.

Des méthodes ont déjà été proposées pour améliorer la stabilité des conditions au ménisque 8, 8'. Selon l'une d'entre elles on impose des vibrations au ménisque 8, 8', soit en faisant vibrer les cylindres 1, 1' (document EP - A - 0 670 757), soit en plongeant un émetteur ultrasonore dans l'acier liquide 4 (document EP - A - 0 684 098), soit en appliquant des champs électromagnétiques à l'acier liquide dans la zone du ménisque 8, 8' (document EP - A - 0 754 515). Cette méthode présente l'inconvénient de nécessiter des appareillages spéciaux qui compliquent la construction de l'installation de coulée. Selon une autre méthode (document EP - A - 0 732 163), on on utilise des cylindres à très faible rugosité (Ra < 5 µm), et on forme à l'intérieur du métal liquide 4, en agissant sur sa composition, des produits de désoxydation qui améliorent les conditions de mouillage au ménisque 8, 8'. Cette méthode impose cependant d'avoir au sein du métal liquide 4 une concentration d'inclusions oxydées relativement importante. Cela exclut de couler en bandes minces des aciers ayant l'excellente propreté inclusionnaire exigée par beaucoup d'utilisations des bandes d'acier au carbone. On a également proposé (document EP - A - 0 740 972 déjà cité) de coupler l'utilisation d'une surface des cylindres finement rainurée avec l'addition d'au moins 0, 02% de soufre pour obtenir des conditions de transfert thermique plus régulières. Ces méthodes ne s'appliquent que dans le cas où les cylindres 1, 1' ont une rugosité très faible (Ra de quelque µm), et elles ont pour but premier d'accroítre l'intensité des transferts thermiques entre les cylindres 1, 1' et le métal liquide 4. Or on a vu que de telles conditions peuvent causer l'apparition de criques superficielles. Ces méthodes de l'art antérieur ne peuvent donc résoudre le problème posé par cette apparition de criques. En outre, comme on l'a dit, des flux de chaleur extraits trop inportants raccourcissent la durée de vie des surfaces extérieures des cylindres, dont la fabrication et le montage sur les noyaux des cylindres sont des opérations très onéreuses. Enfin, la présence d'une quantité de soufre aussi importante dans le métal limite la gamme d'applications des produits qui seront fabriqués à partir des bandes coulées.Methods have already been proposed to improve the stability of the conditions at meniscus 8, 8 '. According to one of them, vibrations are imposed on the meniscus 8, 8 ′, or by vibrating the cylinders 1, 1 '(document EP - A - 0 670 757), or by plunging a ultrasonic transmitter in liquid steel 4 (document EP - A - 0 684 098), or in applying electromagnetic fields to liquid steel in the meniscus area 8, 8 ' (document EP - A - 0 754 515). This method has the disadvantage of requiring special equipment which complicates the construction of the casting installation. According to another method (document EP - A - 0 732 163), very low cylinders are used roughness (Ra <5 µm), and we form inside the liquid metal 4, acting on its composition, deoxidation products which improve the wetting conditions at meniscus 8, 8 '. This method however requires having within the liquid metal 4 a relatively high concentration of oxidized inclusions. This rules out sinking thin strips of steels having the excellent inclusiveness required by many uses of carbon steel strips. We also proposed (document EP - A - 0 740 972 already cited) to couple the use of a finely grooved surface of the cylinders with the addition of at least 0.02% sulfur to obtain transfer conditions more regular thermal. These methods only apply in the case where cylinders 1, 1 'have a very low roughness (Ra of some µm), and their primary purpose is to increase the intensity of the heat transfers between the cylinders 1, 1 ′ and the liquid metal 4. Now we have as such conditions can cause the appearance of superficial cracks. These methods of the prior art cannot therefore solve the problem posed by this appearance of coves. In addition, as we have said, excessively excessive extracting heat fluxes shorten the life of the exterior surfaces of cylinders, the manufacture and mounting on the cores of the cylinders are very expensive operations. Finally, the presence of such a large amount of sulfur in the metal limits the range of applications of the products which will be produced from the cast strips.

Selon l'invention, on parvient à limiter considérablement, voire à supprimer totalement cette apparition de criques lors de la coulée en bandes minces d'aciers au carbone en respectant simultanément les conditions suivantes :

  • la teneur en carbone de l'acier est inférieure à 0,5%, sa teneur en manganèse est comprise entre 0,4 et 2%, sa teneur en silicium est inférieure à 2%, le rapport Mn% / Si% est compris entre 3 et 16 (tous les pourcentages sont des pourcentages pondéraux) ;
  • les fossettes 2 sont jointives et réparties de manière aléatoire sur les surfaces 3, 3' des cylindres 1, 1', et confèrent à ces surfaces 3, 3' une rugosité Rz comprise entre 40 et 130 µm et une rugosité Ra comprise entre 10 et 20 µm ;
  • et l'atmosphère environnant le ménisque 8, 8' est composée d'azote pour au moins 40%.
According to the invention, it is possible to considerably limit, or even completely eliminate this appearance of cracks during the casting in thin strips of carbon steels by simultaneously respecting the following conditions:
  • the carbon content of the steel is less than 0.5%, its manganese content is between 0.4 and 2%, its silicon content is less than 2%, the Mn% / Si% ratio is between 3 and 16 (all percentages are weight percentages);
  • the dimples 2 are contiguous and distributed randomly over the surfaces 3, 3 'of the cylinders 1, 1', and give these surfaces 3, 3 'a roughness Rz of between 40 and 130 µm and a roughness Ra of between 10 and 20 µm;
  • and the atmosphere surrounding the meniscus 8, 8 'is composed of nitrogen for at least 40%.

En respectant ces conditions, on parvient à obtenir un ménisque 8, 8' qui a la configuration représentée sur la figure 2, à savoir un angle de raccordement obtus entre la surface 3, 3' du cylindre 1, 1' et l'acier liquide 4. On supprime ainsi les entraínements de gaz 10 entre le ménisque 8, 8' et la surface 3, 3' du cylindre 1, 1'. Le gaz présent dans les fossettes 2 lors de la formation des peaux 5, 5' était donc dans sa totalité déjà présent avant le contact avec le ménisque 8, 8', et on obtient ainsi des conditions d'initiation et de développement de la solidification des peaux 5, 5' plus stables et plus aisément reproductibles que dans les conditions de l'art antérieur. La stabilité de la position du ménisque 8, 8' est également meilleure, sans qu'il soit nécessaire à cet effet d'utiliser des moyens de mise en vibration du ménisque 8, 8' tels que ceux décrits précédemment.By respecting these conditions, we manage to obtain a meniscus 8, 8 'which has the configuration shown in Figure 2, namely an obtuse connection angle between the surface 3, 3 'of the cylinder 1, 1' and the liquid steel 4. This eliminates the entrainments of gas 10 between the meniscus 8, 8 'and the surface 3, 3' of the cylinder 1, 1 '. The gas present in the dimples 2 during the formation of the skins 5, 5 ′ was therefore in its entirety already present before contact with the meniscus 8, 8 ′, and thus conditions of initiation and of development of solidification of more stable and easier 5, 5 'skins reproducible only under the conditions of the prior art. The stability of the position of the meniscus 8, 8 'is also better, without the need for this purpose means for vibrating the meniscus 8, 8 ′ such as those described above.

Une possible explication à cette obtention de conditions de mouillage favorables est que dans le cas où l'acier a la composition citée, il se produit une évaporation importante de manganèse, et à un degré moindre, de silicium, qui viennent se déposer sur les surfaces 3, 3' des cylindres 1, 1'. Ce dépôt, qui est constamment renouvelé pendant la coulée, procure probablement aux interfaces cylindre - acier des propriétés favorables, sous réserve que l'atmosphère environnante soit pratiquement exempte d'oxygène qui pourrait se combiner aux éléments évaporés et modifier la composition du dépôt. En dessous d'un rapport Mn% / Si% de 3 et au dessus d'un rapport Mn% / Si% de 16, on n'obtient pas ces conditions de mouillage particulièrement favorables.A possible explanation for this obtaining favorable wetting conditions is that in the case where the steel has the composition cited, evaporation takes place significant manganese, and to a lesser degree, silicon, which deposit on the surfaces 3, 3 'of the cylinders 1, 1'. This deposit, which is constantly renewed during the casting, probably gives the cylinder - steel interfaces favorable properties, under provided that the surrounding atmosphere is practically free of oxygen which could combine with the evaporated elements and modify the composition of the deposit. Below a Mn% / Si% ratio of 3 and above an Mn% / Si% ratio of 16, we do not obtain these particularly favorable mooring conditions.

L'expérience des inventeurs montre que l'absence de criques superficielles sur la bande 7 est obtenue lorsqu'on associe ces conditions de mouillage à l'utilisation d'un environnement gazeux et de cylindres 1, 1' répondant aux critères précédemment précisés. Dans ces conditions, on obtient des transferts de rugosité entre les peaux 5, 5' et les cylindres 1, 1' dont la régularité est satisfaisante.The experience of the inventors shows that the absence of surface cracks on the strip 7 is obtained when these wetting conditions are combined with the use of a gaseous environment and cylinders 1, 1 ′ meeting the criteria previously specified. Under these conditions, roughness transfers are obtained between the skins 5, 5 ′ and the cylinders 1, 1 'whose regularity is satisfactory.

Une condition préférée pour l'obtention du résultat recherché est d'effectuer la coulée sans utiliser de matériau de couverture, donc en laissant nue la surface du métal liquide 4, de manière à ne pas gêner l'évaporation et le dépôt sur les cylindres 1, 1' des éléments volatils.A preferred condition for obtaining the desired result is to perform the casting without using any covering material, so leaving the metal surface bare liquid 4, so as not to hinder evaporation and deposition on the cylinders 1, 1 'of volatile elements.

Le procédé selon l'invention est compatible avec la présence d'aluminium dans l'acier. Comme il est connu, on peut accompagner cette présence d'aluminium par une addition de quelques dizaines de ppm de calcium destinée à transformer les inclusions d'alumine en aluminates de chaux liquides à la température de fusion de l'acier. On évite ainsi le bouchage des orifices de coulée du répartiteur par les inclusions d'alumine, et on confère aux inclusions oxydées une plasticité et une morphologie bien adaptées aux traitements thermomécaniques que la bande subira ultérieurement, ainsi qu'aux futures utilisations des produits qui en seront issus. On peut également envisager de remplacer partiellement ou totalement l'aluminium par d'autres éléments fortement désoxydants, tels que le titane et/ou le zirconium. La teneur maximale totale en ces désoxydants forts est de 0,1%.The process according to the invention is compatible with the presence of aluminum in steel. As is known, this presence of aluminum can be accompanied by a addition of a few tens of ppm of calcium intended to transform the inclusions from alumina to aluminates of lime liquid at the melting temperature of steel. We avoid thus plugging the pouring orifices of the distributor with the alumina inclusions, and gives oxidized inclusions plasticity and morphology well suited to thermomechanical treatments that the strip will undergo later, as well as in the future uses of the products which will result from it. We can also consider replacing partially or completely aluminum by other strongly deoxidizing elements, such as than titanium and / or zirconium. The maximum total content of these strong deoxidizers is 0.1%.

Par rapport aux autres méthodes précédemment proposées pour obtenir une bonne stabilité du ménisque 8, 8' (et qui, rappelons-le, ne permettaient pas d'obtenir de manière fiable une bande 7 exempte de criques superficielles), le procédé selon l'invention présente l'avantage de ne pas nécessiter la présence au sein du métal liquide 4 d'une quantité relativement importante d'inclusions oxydées qui pourraient être défavorables pour beaucoup d'utilisations du métal final. De plus, ces inclusions d'oxydes risqueraient de former des plaques au voisinage du ménisque qui pourraient être emprisonnées par les peaux 5, 5'. On détériorerait ainsi la qualité de la surface de la bande 7. On considère qu'il est préférable de ne pas dépasser une teneur de 100 ppm en oxygène total (c'est à dire présent soit sous forme dissoute, soit sous forme combinée dans des inclusions oxydées), et préférentiellement de maintenir cette teneur entre 30 et 70 ppm. Cette teneur en oxygène total dépend en grande partie de la teneur en oxygène dissous qui est déterminée par les équilibres chimiques entre l'acier liquide 4 et son environnement, et notamment par les teneurs de l'acier liquide 4 en éléments désoxydants : manganèse, silicium et éventuellement aluminium. Une façon d'obtenir une basse teneur en oxygène (donc une bonne propreté inclusionnaire du produit final) dans l'acier liquide 4 au moment de sa solidification, même s'il ne contient pas de désoxydants treès forts tels que l'aluminium ou le titane, est d'imposer une telle basse teneur lors de l'élaboration de l'acier en poche en réalisant un équilibre chimique entre le métal et un laitier fortement enrichi en chaux et appauvri en oxydes de silicium et de manganèse, et d'empêcher ensuite autant que possible l'oxygène atmosphérique de pénétrer dans l'acier liquide 4, par un inertage soigné de l'installation de coulée.Compared to the other methods previously proposed to obtain a good stability of the meniscus 8, 8 '(and which, it should be remembered, did not make it possible to obtain reliable a strip 7 free of surface cracks), the method according to the invention has the advantage of not requiring the presence within the liquid metal 4 of a quantity relatively large number of oxidized inclusions which could be unfavorable for many uses of the final metal. In addition, these oxide inclusions would risk form plaques near the meniscus that could be trapped by skins 5, 5 '. This would deteriorate the quality of the surface of the strip 7. It is considered that it It is preferable not to exceed a content of 100 ppm of total oxygen (i.e. present either in dissolved form or in combined form in oxidized inclusions), and preferably maintain this content between 30 and 70 ppm. This oxygen content total largely depends on the dissolved oxygen content which is determined by the chemical balances between the liquid steel 4 and its environment, and in particular by the contents of liquid steel 4 in deoxidizing elements: manganese, silicon and possibly aluminum. One way to get a low oxygen content (so a good inclusiveness of the final product) in the liquid steel 4 at the time of its solidification, even if it does not contain very strong deoxidizers such as aluminum or titanium, is to impose such a low content when developing the ladle steel in achieving a chemical balance between the metal and a slag highly enriched in lime and depleted in oxides of silicon and manganese, and then prevent as much as possible atmospheric oxygen to penetrate into the liquid steel 4, by careful inerting of the casting installation.

Il n'est pas, non plus, nécessaire d'imposer la présence d'une quantité élevée de soufre dans le métal liquide 4, qui conduirait à restreindre les possibilités d'utilisation des produits fabriqués à partir des bandes coulées. Le soufre est un élément dont on cherche à limiter la présence dans la plupart des aciers au carbone à caractéristiques mécaniques élevées. La présence de carbone à une teneur maximale de 0,5% n'est pas très contraignante, dans la mesure où la plupart des aciers au carbone que l'on peut désirer couler sous forme de bandes minces répondent à cette caractéristique.Nor is it necessary to impose the presence of a high amount of sulfur in the liquid metal 4, which would restrict the possibilities of using the products made from cast strips. Sulfur is an element which we seek to limit the presence in most carbon steels with mechanical characteristics high. The presence of carbon at a maximum content of 0.5% is not very binding, since most of the carbon steels that can be desired flow in the form of thin strips meet this characteristic.

Les meilleurs résultats sont obtenus avec une atmosphère environnant la surface libre de l'acier liquide 4 comportant 100% d'azote. Toutefois, cette teneur peut descendre de manière acceptable jusqu'à 40%, le restant étant composé d'un gaz inerte et insoluble dans l'acier (tel que l'argon ou l'hélium) ou d'un mélange de tels gaz. En jouant sur la composition du gaz d'inertage, on peut, comme il est connu, moduler l'intensité des transferts thermiques entre les cylindres 1, 1' et l'acier liquide 4, et agir sur la productivité de l'installation, ainsi que sur la forme des cylindres résultant de leur dilatation (document EP-A-0 736 350). The best results are obtained with an atmosphere surrounding the surface free of liquid steel 4 comprising 100% nitrogen. However, this content can go down acceptably up to 40%, the remainder being composed of an inert and insoluble gas in steel (such as argon or helium) or a mixture of such gases. Playing on the composition of the inerting gas, it is possible, as is known, to modulate the intensity of the heat transfers between cylinders 1, 1 'and liquid steel 4, and act on productivity of the installation, as well as on the shape of the cylinders resulting from their expansion (document EP-A-0 736 350).

Tous ces résultats sont obtenus dans le cas de l'utilisation de cylindres 1, 1' dont les surfaces 3, 3' sont en cuivre ou alliage de cuivre, éventuellement recouvertes par une pellicule d'alliage à base de nickel ou de chrome, comme c'est souvent le cas.All these results are obtained in the case of the use of cylinders 1, 1 'of which the surfaces 3, 3 ′ are made of copper or copper alloy, possibly covered by a alloy film based on nickel or chromium, as is often the case.

Une méthode classique, rapide et peu coûteuse pour former les fossettes 2 sur les surfaces 3, 3' des cylindres 1, 1' est de projeter des billes en métal ou en céramique sur lesdites surfaces 3, 3'. En jouant sur le nombre, les matériaux, les diamètres et la pression de projection des billes, on peut obtenir les configurations désirées pour lesdites fossettes 2. D'autres méthodes (utilisant un laser ou une attaque chimique ou une électroérosion des surfaces 3, 3', ou un marquage par moletage des surfaces 3, 3') sont également envisageables.A classic, fast and inexpensive method for forming dimples 2 on surfaces 3, 3 'of the cylinders 1, 1' is to project metal or ceramic balls onto said surfaces 3, 3 '. By playing on the number, materials, diameters and pressure of projection of the balls, one can obtain the desired configurations for said dimples 2. Other methods (using a laser or chemical attack or EDM of surfaces 3, 3 ', or a marking by knurling of surfaces 3, 3') are also possible.

Si les conditions de coulée conduisent à la présence à la surface de la bande 7 de reliefs un peu trop importants, suite à une relativement forte pénétration du métal liquide 4 dans les fossettes 2, on peut prévoir de laminer à chaud la bande 7 pour écraser ces reliefs, de préférence sur une installation disposée en ligne avec l'installation de coulée.If the casting conditions lead to the presence on the surface of the strip 7 of slightly too high reliefs, due to relatively strong penetration of liquid metal 4 in the dimples 2, provision may be made for hot rolling the strip 7 to crush these reliefs, preferably on an installation arranged in line with the casting installation.

A titre d'exemple, on peut citer le cas d'une coulée de bandes de 2,6 mm d'épaisseur réalisée sur un acier ayant la composition : C = 0,042% ; Mn = 0,816% ; P = 0,006% ; S = 0,005% ; Si = 0,220% ; Al = 0,002% ; Ni = 0,066% ; Cr = 0,126% ; Cu = 0,085% ; N = 0,0058%. La rugosité des cylindres était définie par un Ra de 21 pm et un Rz de 92 µm obtenus par projection de billes d'acier. Les caractéristiques de composition et de rugosité étaient donc conformes à l'invention (en particulier le rapport Mn% / Si% était égal à 3,7). Lorsque, selon l'invention, l'inertage de la surface du métal liquide était assuré par de l'azote pur ou par un mélange azote-argon 50-50%, on n'observait pas de criques sur la surface de la bande. En revanche, un inertage à 100% d'argon provoquait l'apparition de criques, toutefois en relativement faible nombre.As an example, we can cite the case of a 2.6 mm strip casting of thickness produced on a steel having the composition: C = 0.042%; Mn = 0.816%; P = 0.006%; S = 0.005%; If = 0.220%; Al = 0.002%; Ni = 0.066%; Cr = 0.126%; Cu = 0.085%; N = 0.0058%. The roughness of the cylinders was defined by a Ra of 21 pm and a Rz of 92 µm obtained by projection of steel balls. The characteristics of composition and roughness were therefore in accordance with the invention (in particular the Mn% / Si% ratio was equal to 3.7). When, according to the invention, inerting of the surface of the liquid metal was ensured by pure nitrogen or by a 50-50% nitrogen-argon mixture, no cracks were observed on the surface of the strip. On the other hand, 100% argon inerting caused the appearance of coves, however in relatively small numbers.

On prend également pour exemple une coulée de référence d'une bande de 2,6 mm d'épaisseur dont la composition était : C = 0,0426% ; Mn = 0,303% ; P = 0,004% ; S = 0,0007% ; Si = 0,186% ; Al = 0,003% ; Ni = 0,035% ; Cr = 0,075% ; Cu = 0,031% ; N = 0,0044%. Le rapport Mn% / Si% était cette fois de 1,6, donc non conforme à l'invention. La rugosité des cylindres était la même que pour la coulée précédente. L'inertage de la surface du métal liquide était assuré par un mélange argon-azote 70-30%. Ces deux dernières caractéristiques tombaient en dehors des exigences de l'invention. Dans ces conditions, on a constaté l'apparition significative de criques sur la surface de la bande.We also take for example a reference flow of a band of 2.6 mm of thickness whose composition was: C = 0.0426%; Mn = 0.303%; P = 0.004%; S = 0.0007%; If = 0.186%; Al = 0.003%; Ni = 0.035%; Cr = 0.075%; Cu = 0.031%; N = 0.0044%. The Mn% / Si% ratio was 1.6 this time, therefore not in accordance with the invention. The roughness of the cylinders was the same as for the previous casting. Inerting the liquid metal surface was provided by a 70-30% argon-nitrogen mixture. These two latter features fell outside the requirements of the invention. In these conditions, there was a significant appearance of cracks on the surface of the strip.

Une autre coulée de référence d'une bande de 2,6 mm d'épaisseur avait pour composition : C = 0,054% ; Mn = 0,601% ; P = 0,007% ; S = 0,004% ; Si = 0,320% ; Al = 0,003% ; Ni = 0,040% ; Cr = 0,100% ; Cu = 0,028% ; N = 0,0059%. Le rapport Mn% / Si% était de 1,9, donc non conforme à l'invention. Les cylindres avaient un Ra de 8 µm et un Rz de 35 µm, donc une rugosité insuffisamment prononcée pour être dans les conditions de l'invention. L'inertage était assuré à 100% par de l'azote. Là encore, on a observé des criques en nombre significatif sur la surface de la bande. Another reference run of a 2.6 mm thick strip was for composition: C = 0.054%; Mn = 0.601%; P = 0.007%; S = 0.004%; If = 0.320%; Al = 0.003%; Ni = 0.040%; Cr = 0.100%; Cu = 0.028%; N = 0.0059%. The Mn% / ratio If% was 1.9, therefore not in accordance with the invention. The cylinders had a Ra of 8 µm and a Rz of 35 µm, therefore a roughness insufficiently pronounced to be in the conditions of the invention. The inerting was 100% ensured by nitrogen. Again, we observed cracks in significant numbers on the surface of the strip.

Une autre coulée particulièrement intéressante a été réalisée pour obtenir une bande de 3,9 mm d'épaisseur. Sa composition était : C = 0,049% ; Mn = 0,791% ; P = 0,005% ; S = 0,006% ; Si = 0,200% ; Al = 0,003% ; Ni = 0,028% ; Cr = 0,049% ; Cu = 0,015% ; N = 0,0052%. Le rapport Mn% / Si% était de 4, donc conforme à l'invention. L'inertage de la surface du métal était assuré par de l'azote pur ou par un mélange azote-argon 50-50%. L'un des cylindres avait un Ra de 21 µm et un Rz de 92 µm (selon l'invention), et l'autre cylindre avait un Ra de 8 µm et un Rz de 35 µm (hors de l'invention). Il s'est avéré que la face de la bande qui s'est solidifiée contre le cylindre à forte rugosité conformément à l'invention était exempte de criques, alors que la face opposée de la bande, qui s'est solidifiée contre le cylindre à faible rugosité hors de l'invention présentait de nombreuses criques. Ce dernier exemple montre bien l'influence fondamentale de la rugosité des cylindres sur le résultat final, toutes choses étant égales par ailleurs.Another particularly interesting casting was carried out to obtain a 3.9 mm thick strip. Its composition was: C = 0.049%; Mn = 0.791%; P = 0.005%; S = 0.006%; If = 0.200%; Al = 0.003%; Ni = 0.028%; Cr = 0.049%; Cu = 0.015%; N = 0.0052%. The Mn% / Si% ratio was 4, therefore in accordance with the invention. The surface of the metal was inerted by pure nitrogen or by a nitrogen-argon mixture 50-50%. One of the cylinders had a Ra of 21 µm and an Rz of 92 µm (according to the invention), and the other cylinder had a Ra of 8 μm and an Rz of 35 μm (outside of the invention). It turned out that the side of the strip which solidified against the cylinder at high roughness according to the invention was free of cracks, while the face opposite of the strip, which solidified against the low roughness cylinder out of the invention had numerous cracks. This last example shows the influence fundamental of the roughness of the cylinders on the final result, all things being equal by elsewhere.

L'invention permet donc de réaliser un bon ancrage des peaux 5, 5' en cours de solidification sur les surfaces 3, 3' des cylindres pour éviter des criques qui seraient dues à une fragilité excessive des peaux 5, 5'.The invention therefore makes it possible to achieve good anchoring of the skins 5, 5 ′ during solidification on the surfaces 3, 3 'of the cylinders to avoid cracks which would be due to excessive fragility of the skins 5, 5 '.

Claims (12)

Procédé de fabrication de bandes (7) d'épaisseur inférieure ou égale à 10 mm en acier au carbone directement à partir d'acier liquide (4), par coulée dudit acier liquide (4) entre les surfaces latérales (3) en cuivre ou alliage de cuivre de deux cylindres horizontaux (1, 1') en rotation refroidis intérieurement, caractérisé en ce que : ledit acier liquide (4) a la composition, en pourcentages pondéraux carbone ≤ 0,5%, manganèse de 0,2 à 2%, silicium ≤ 2%, le rapport Mn% / Si% étant compris entre 3 et 16, optionnellement aluminium + titane + zirconium ≤ 0,10%, le reste étant du fer et des impuretés habituelles ; lesdites surfaces latérales (3, 3') des cylindres (1, 1') comportent des fossettes (2) jointives, imposant auxdites surfaces (3, 3') une rugosité Rz comprise entre 40 et 200 µm et une rugosité Ra comprise entre 10 et 40 µm ; et l'atmosphère environnant le ménisque (8, 8') de l'acier liquide (4) présent entre les cylindres (1, 1') comprend entre 40 et 100% d'azote, le restant étant composé d'un gaz neutre insoluble dans l'acier liquide (4) ou d'un mélange de tels gaz neutres. Method for manufacturing strips (7) of thickness less than or equal to 10 mm in carbon steel directly from liquid steel (4), by casting said liquid steel (4) between the lateral surfaces (3) of copper or copper alloy of two horizontal cylinders (1, 1 ') in rotation internally cooled, characterized in that: said liquid steel (4) has the composition, in weight percent carbon ≤ 0.5%, manganese from 0.2 to 2%, silicon ≤ 2%, the Mn% / Si% ratio being between 3 and 16, optionally aluminum + titanium + zirconium ≤ 0.10%, the rest being iron and usual impurities; said lateral surfaces (3, 3 ') of the cylinders (1, 1') have joined dimples (2), imposing on said surfaces (3, 3 ') a roughness Rz between 40 and 200 µm and a roughness Ra between 10 and 40 µm; and the atmosphere surrounding the meniscus (8, 8 ') of the liquid steel (4) present between the cylinders (1, 1') comprises between 40 and 100% nitrogen, the remainder being composed of a neutral gas insoluble in liquid steel (4) or a mixture of such neutral gases. Procédé selon la revendication 1, caractérisé en ce que ledit acier liquide (4) comporte moins de 100 ppm d'oxygène total.Method according to claim 1, characterized in that said liquid steel (4) contains less than 100 ppm total oxygen. Procédé selon la revendication 2, caractérisé en ce que ledit acier liquide (4) comporte de 30 à 70 ppm d'oxygène total.Method according to claim 2, characterized in that said liquid steel (4) contains 30 to 70 ppm of total oxygen. Procédé selon l'une des revendications 1 à 3, caractérisé en ce que lesdites fossettes (2) jointives sont réparties de manière aléatoire sur les surfaces (3, 3') des cylindres (1, 1').Method according to one of claims 1 to 3, characterized in that said contiguous dimples (2) are distributed randomly over the surfaces (3, 3 ') of the cylinders (1, 1 '). Procédé selon l'une des revendications 1 à 4, caractérisé en ce que l'atmosphère environnant le ménisque (8, 8') du métal liquide (4) présent entre les cylindres (1, 1') comprend 100% d'azote.Method according to one of claims 1 to 4, characterized in that the atmosphere surrounding the meniscus (8, 8 ') of the liquid metal (4) present between the cylinders (1, 1') contains 100% nitrogen. Procédé selon l'une des revendications 1 à 5, caractérisé en ce que la surface de l'acier liquide (4) présent entre les cylindres (1, 1') est exempte de matériau de couverture.Method according to one of claims 1 to 5, characterized in that the surface of the liquid steel (4) present between the cylinders (1, 1 ') is free of covering material. Procédé selon l'une des revendications 1 à 6, caractérisé en ce que ladite bande (7) subit ensuite un laminage à chaud.Method according to one of claims 1 to 6, characterized in that said strip (7) then undergoes hot rolling. Procédé selon la revendication 7, caractérisé en ce que ledit laminage à chaud est effectué en ligne, après la coulée de ladite bande (7).Method according to claim 7, characterized in that said hot rolling is carried out online, after the casting of said strip (7). Bande (7) en acier au carbone d'épaisseur inférieure ou égale à 10 mm, caractérisée en ce qu'elle est susceptible d'être obtenue par le procédé selon l'une des revendications 1 à 8.Strip (7) made of carbon steel with a thickness less than or equal to 10 mm, characterized in that it is capable of being obtained by the process according to one of the claims 1 to 8. Cylindre (1, 1') de coulée pour la coulée de bandes minces métalliques (7) dont la surface latérale (3, 3') en cuivre ou alliage de cuivre comporte des fossettes (2) jointives, caractérisée en ce que lesdites fossettes (2) imposent auxdites surfaces (3, 3') une rugosité Rz comprise entre 40 et 200 µm et une rugosité Ra comprise entre 10 et 40 µm. Casting cylinder (1, 1 ') for casting thin metal strips (7) the side surface (3, 3 ') of copper or copper alloy includes dimples (2) contiguous, characterized in that said dimples (2) impose on said surfaces (3, 3 ') a roughness Rz between 40 and 200 µm and a roughness Ra between 10 and 40 µm. Cylindre (1, 1') selon la revendication 9, caractérisé en ce que lesdites fossettes (2) sont ménagées sur ladite surface par projection de billes.Cylinder (1, 1 ') according to claim 9, characterized in that said dimples (2) are formed on said surface by projection of balls. Cylindre (1, 1') selon la revendication 10 ou 11, caractérisé en ce que lesdites fossettes (2) sont réparties de manière aléatoire sur sa surface.Cylinder (1, 1 ') according to claim 10 or 11, characterized in that said dimples (2) are distributed randomly over its surface.
EP00400573A 1999-03-26 2000-03-03 Twin roll continuous casting of strips of carbon steel Expired - Lifetime EP1038612B1 (en)

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FR9903778A FR2791286B1 (en) 1999-03-26 1999-03-26 PROCESS FOR PRODUCING CARBON STEEL STRIPS BY CONTINUOUS CASTING BETWEEN TWO CYLINDERS

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