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

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

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 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.

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.

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.

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.

• 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 .

The invention also relates to tapes capable of being produced by this. process, as well as the casting cylinders necessary for its implementation.

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.

• 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.

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.

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.

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

Yp_i (i variant de 1 à 5) étant la hauteur des 5 saillies les plus élevées et Yv_i (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(R_Z1+R_Z2+R_Z3+R_Z4+R_Z5)On a given length L _i equal to 1/5 of the circumference of the cylinder (i varying from 1 to 5),

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 )

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.

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.

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. 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).

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.

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.

• 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%.

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.

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.

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.

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. 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%.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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)

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. Method according to claim 1, characterized in that said liquid steel (4) contains less than 100 ppm total oxygen. Method according to claim 2, characterized in that said liquid steel (4) contains 30 to 70 ppm of total oxygen. 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 '). 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. 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. Method according to one of claims 1 to 6, characterized in that said strip (7) then undergoes hot rolling. Method according to claim 7, characterized in that said hot rolling is carried out online, after the casting of said strip (7). 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. 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. Cylinder (1, 1 ') according to claim 9, characterized in that said dimples (2) are formed on said surface by projection of balls. Cylinder (1, 1 ') according to claim 10 or 11, characterized in that said dimples (2) are distributed randomly over its surface.

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