EP1185387A1 - Method for continuously casting between two rolls austenitic stainless steel strips with excellent surface quality and resulting strips - Google Patents

Method for continuously casting between two rolls austenitic stainless steel strips with excellent surface quality and resulting strips

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Publication number
EP1185387A1
EP1185387A1 EP00915237A EP00915237A EP1185387A1 EP 1185387 A1 EP1185387 A1 EP 1185387A1 EP 00915237 A EP00915237 A EP 00915237A EP 00915237 A EP00915237 A EP 00915237A EP 1185387 A1 EP1185387 A1 EP 1185387A1
Authority
EP
European Patent Office
Prior art keywords
steel
cylinders
gas
strips
austenitic stainless
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
EP00915237A
Other languages
German (de)
French (fr)
Other versions
EP1185387B1 (en
Inventor
Christian Marchionni
Frédéric Mazurier
Jean-Michel Damasse
Frédéric Descaves
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USINOR SA
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USINOR SA
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Publication date
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Priority to SI200030035T priority Critical patent/SI1185387T1/en
Publication of EP1185387A1 publication Critical patent/EP1185387A1/en
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Publication of EP1185387B1 publication Critical patent/EP1185387B1/en
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Classifications

    • 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
    • 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
    • B22D11/001—Continuous casting of metals, i.e. casting in indefinite lengths of specific alloys
    • B22D11/002—Stainless steels
    • 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
    • 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
    • B22D11/0697—Accessories therefor for casting in a protected atmosphere
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel

Definitions

  • the invention relates to the continuous casting of metals, and more precisely the continuous casting, directly from liquid metal, of stainless steel strips of the austenitic type, the thickness of which is of the order of a few mm, by the so-called process. of "casting between cylinders".
  • process. of "casting between cylinders" significant progress has been made in the development of processes for casting thin strips of carbon or stainless steel directly from liquid metal.
  • the method mainly used today is the casting of said liquid metal between two internally cooled cylinders, rotating around their horizontal axes in opposite directions, and arranged opposite one another, the minimum distance between their surfaces being substantially equal to the thickness that you want to give to the cast strip (for example a few mm).
  • the pouring space containing the liquid steel is defined by the lateral surfaces of the cylinders, on which the solidification of the strip is initiated, and by refractory side closure plates applied against the ends of the cylinders.
  • the liquid metal initiates its solidification on contact with the outer surfaces of the cylinders, on which it forms solidified "skins", which are made to meet at the "neck", that is to say the area where the distance between the cylinders is minimal.
  • microcracks appearing on the strip. These are small cracks which are nevertheless sufficient to make the cold processed products which result from them unfit for use. They form during the solidification of the steel and have a depth of the order of 40 ⁇ m and an opening of approximately 20 ⁇ m. Their appearance is linked to the contractions of the metal during the solidification of the skins in contact with the cylinders, over the length of their contact arc. This solidification can be described as comprising two successive stages. The first stage occurs during the initial contact between the liquid steel and the surface of the cylinder, which results in the formation of a solid steel skin on the surface of the cylinders.
  • the second step concerns the growth of this skin up to the neck, where, as we said, it joins the skin formed on the other cylinder to form the fully solidified strip.
  • the contact between the steel and the surface of the cylinder is conditioned by the topography of the surface of the casting rolls, combined with the nature of the inerting gas surrounding the casting space and with the chemical composition of the steel. All these parameters intervene in the establishment of heat transfers between the steel and the cylinder and govern the conditions of solidification of the skins.
  • solidification and cooling of the skins they undergo contractions. They depend in particular on the extent of the phase transformation ⁇ - » ⁇ , which takes place with a significant variation in the density of the metal, at the microscopic level. It is determined by the composition of the cast metal.
  • Ni eq (%) Ni% + 0.31 Mn% + 22 C% + 14.2 N% + Cu%
  • EP-A-0 409 645 It combines a defined geometry of the "dimples" (hollow engravings of roughly circular or elliptical shape) present on the surface of the cylinders with the use as inerting gas of a gas mixture containing 30 90% of a gas soluble in steel, which fills the dimples at the time of the first cylinder / liquid steel contact.
  • the dimples can conventionally be produced by shot blasting or laser machining. In all the preceding documents, it is required that these dimples are separated from each other.
  • Document EP-A-0 679 114 proposes the use of circumferential grooves formed on the surface of the cylinders, which give said surface a roughness Ra of 2.5 to
  • Document EP-A-0 796 685 teaches to cast a steel whose Cr e q / Ni e q ratio is greater than 1.55 so as to minimize the phase changes at high temperature, and to carry out this casting using cylinders the surface of which has joined dimples of diameter 100-1500 ⁇ m and depth 20-150 ⁇ m and by inerting the vicinity of the meniscus (the intersection between the surface of the liquid steel and the surface of the cylinders) with a soluble gas in steel, or a mixture of gases mainly composed of such a soluble gas.
  • the roughness peaks serve as priming sites for solidification, while the roughness hollows constitute contraction joints of the metal during solidification, and allow a better distribution of the stresses.
  • the object of the invention is to propose a process for casting thin strips of austenitic stainless steel, the surface of which would be free from microcracks and other major defects, not requiring particularly restrictive casting conditions for its implementation, and making it possible to cast steels with a Cr eq / Ni eq ratio which is more extensive than in existing processes.
  • the subject of the invention is a process for the continuous casting of a strip of austenitic stainless steel of thickness less than or equal to 10 mm, directly from liquid metal, between two cooled horizontal cylinders, characterized in that than :
  • composition of said steel in weight percent, comprises: C% ⁇ 0.08; If% ⁇ 1; P% ⁇ 0.04; Mn% ⁇ 2; Cr% between 17 and 20; Ni% between 8 and 10.5; S% between 0.007 and 0.040; the remainder being iron and impurities resulting from processing;
  • the surface of the cylinders has contiguous dimples of approximately circular or elliptical section, with a diameter of 100 to 1500 ⁇ m and a depth of 20 to 150 ⁇ m;
  • the inerting gas surrounding the meniscus is a gas soluble in steel or a mixture of such gases, or consists at least of 50% by volume of such a gas or mixture of gases.
  • the invention also relates to bands which can be produced by this method.
  • the invention consists in combining conditions relating to the composition of the cast metal, the surface condition of the cylinders and the composition of the meniscus inerting gas, so as to obtain a strip-free surface. of micro cracks.
  • the main originality of the required composition is that the metal must contain a quantity of sulfur greater than the quantities most commonly encountered (without however being so great as to compromise the resistance of the products to corrosion), and that this content must be combined with a precise range of Cr eq / Ni e q ratios.
  • the conditions of the first contact between the liquid steel and the cylinders constitute a very important factor in the process of solidification of the strip, and notably influence the surface quality of the latter. Their good control is therefore very important to guarantee the absence of microcracks on the casting strip.
  • the inevitable fluctuations in the level of the surface of the liquid metal present between the cylinders complicate this control, in particular in that they are a source of irregularities in the heat exchanges which take place in this zone of first contact.
  • Other such irregularities are due, during the later stages of the solidification of the skins, to the contractions of the metal during solidification, which result in particular from phase transformations at high temperature characteristic of austenitic stainless steels. These contractions can cause micro cracks.
  • FIG. 1 shows a micrograph produced on a sample of a thin strip 1 of austenitic stainless steel, seen in longitudinal section.
  • This strip 1 has on its surface 2 a microcrack 3, of the type that the invention seeks precisely to avoid.
  • the metallographic attack carried out on the sample highlights a clear area 4 located around the microcrack 3 and in its extension: it corresponds to a segregated area enriched with certain elements such as nickel and manganese.
  • the curve in FIG. 2 translates the results of these investigations, which were carried out on strips of thickness 3 mm cast at a speed of 50 m / min.
  • the surfaces casting rolls were roughened by contiguous dimples of average depth 80 ⁇ m and average diameter 1000 ⁇ m.
  • the composition of the cast steels fell within the limits: C: 0.02-0.06%; Mn: 1.3-1.6%; P: 0.019-0.024%; If: 0.34-0.45%; Cr: 18.0-18.7%; Ni: 8.6-9.8%; S: 0.0005-0.446%.
  • the Cr eq / Ni eq ratios of these steels ranged from 1.79 to 1.85.
  • the inert gas surrounding the meniscus contained 60% by volume of nitrogen and 40% by volume of argon.
  • the inventors have determined a set of conditions to be respected so that the casting of austenitic stainless steels in thin strips takes place without the formation of microcracks on the surface of the strips, and they have been mentioned above. They are justified by the following considerations.
  • the sulfur content is less than 0.007%, the fluctuations in the level of the meniscus become too great, and the irregularities of the heat transfers which result therefrom cause the formation of microcracks, in particular when the Cr eq / Ni eq ratio is greater than 1 , 70.
  • the upper limit of the sulfur content is set at 0.04% because beyond this value, the influence of the sulfur content on the stability of the meniscus no longer increases significantly, and on the other hand, there is an increased risk of deterioration in the resistance to corrosion by pitting of the finished product made from these strips.
  • the phosphorus content must be kept below 0.04%, in order to avoid risks of hot cracking of the bands when the Cr eq / Ni eq ratio is close to 1.55, that is to say when solidification takes place partially in primary austenite, and not mainly in primary ferrite.
  • the Cr eq / Ni eq ratio must be at least 1.55, in that below this value, the steel solidifies at least partially into primary austenite, which increases the sensitivity to cracking of the strip and promotes the appearance of longitudinal cracks, which must be absolutely avoided, too.
  • a Cr eq / Ni eq ratio greater than 1.90 the contraction linked to the ferrite-austenite transformation becomes too great, and the microcracks are then inevitable.
  • the ferrite level in the strip becomes too high, which can lead to ruptures after the shaping of the finished products made from the strips thus cast.
  • the other analytical conditions on cast steel are conventional on the most common austenitic stainless steels, in particular those of type 304 and related. It is understood that elements other than those explicitly mentioned in the foregoing may be present in the steel, as impurities or alloying elements in small quantities, insofar as they would not significantly modify the solidification conditions and the surface tension of the liquid steel on the surface of the cylinders, which would be confirmed by the absence of microcracks on the strips produced.
  • the nature of the inerting gas surrounding the meniscus has a strong influence on the conditions of contact between the steel and the surface of the cylinders, in particular on the manner in which the transfer takes place "in negative" the roughness of the cylinders on the surface of the strip, and the risks of microcracks forming.
  • a gas wholly or mainly insoluble in steel such as argon or helium
  • the steel in the course of solidification does not penetrate little or little into the depressions of the surface of the cylinder. Heat extraction is therefore practically only carried out at the roughness peaks, which makes it very heterogeneous on the surface of the cylinder. This heterogeneity is favorable to the appearance of numerous microcracks.
  • an inerting gas containing a significant quantity of a gas soluble in steel such as nitrogen, hydrogen, ammonia, CO, a fortiori if it is entirely constituted by a such gas or a mixture of such gases
  • the steel penetrates well into the depressions of the surface of the cylinders, and the extraction of heat at the first contact is important.
  • this decreases the heterogeneity of heat extraction at the level of peaks and depressions. All this goes in the direction of limiting the risks of micro-crack formation.
  • the lower limit of the content of the inerting gas in a gas (or mixture of gas) soluble in steel taking into account the other casting conditions required for the composition of the metal and the roughness of the surfaces of the rolls.
  • Strips of austenitic stainless steel 3 mm thick were poured between cylinders.
  • the cylinder surfaces included joined dimples with an average diameter of 1000 ⁇ m and an average depth of 100 ⁇ m.
  • the inert gas surrounding the meniscus contained 40% argon and 60% nitrogen.
  • the composition of the steel varied within the following limits: C: 0.02-0.06%; Mn: 1.3-1.6%; P: 0.019-0.024%; If: 0.34-0.45%; Cr: 18.0-18.7%; Ni: 8.6-9.8%; S: 0.0005-0.0446%.
  • the Cr eq / Ni e q ratio of cast steels varied from 1.79 to 1.85.
  • the surface density of the microcracks on the strips thus cast was measured, and the results of these measurements were compared with the sulfur contents in the cast steels. Table 1 presents the conclusions of these tests.
  • Strips of thickness 3.8 mm in austenitic stainless steel were poured between cylinders, the compositions of which are shown in Table 2.
  • the cylinders had surface roughnesses characterized by the presence of contiguous dimples of average diameter 1000 ⁇ m and average depth 120 ⁇ m.
  • Table 3 Influence of the composition of the inerting gas on the surface density of the strip microcracks, according to the sulfur content and the Cr eq / Ni eq ratio of the cast steel
  • the composition of the inerting gas is a parameter on which we often wish to play to control the intensity of the heat transfers between the cylinders and the metal, for example to vary the bulging of the cylinders which affects the shape of the strip. (see document EP-A-0 736 350).
  • the results obtained with steel C therefore lead to the conclusion that a sulfur content of 0.005% cannot fall within the scope of the invention.
  • the strips cast with steels B and D do not exhibit microcracks as long as the nitrogen content of the inerting gas is at least 50%.
  • Their sulfur contents are respectively 0.019 and 0.039%, and their Cr eq / Ni eq ratios are respectively 1.82 and 1.64.
  • the invention applies preferably to the case of steels having a Cr e q / Ni eq ratio of between 1.70 and 1.90, since this range corresponds to steels in which have added less gammagenic elements (such as nickel) than for steels with a lower Cr eq / Ni eq ratio, and which are therefore more economical to manufacture.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Chemical & Material Sciences (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Continuous Casting (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
  • Metal Extraction Processes (AREA)
  • Heat Treatment Of Steel (AREA)
  • Heat Treatment Of Articles (AREA)
  • Pressure Welding/Diffusion-Bonding (AREA)

Abstract

The invention concerns a method for continuously casting an austenitic stainless steel strip with a thickness not more than 10 mm, directly from liquid metal, between two cooled horizontal rolls, characterised in that: said steel composition in weight proportions comprises: %C<=0.08%; %Si<=1; %P<=0.04; %Mn<=2; %Cr between 17 and 20; %Ni between 8 and 10.5; %S between 0.007 and 0.040; the rest being iron and the impurities resulting from preparation; the ratio Creq/Nieq ranges between 1.55 and 1.90 with: Creq(%)=%Cr+1.37 %Mo+1.5 %Si+2%Nb+3 %Ti; Nieq(%)=%Ni+0.31 %Mn+22 %C+14.2 %N+Cu; the surface of the rolls comprises contiguous dimples with more or less circular or elliptical cross-section, of diameter between 100 and 1500 mum and depth between 20 and 150 mum; the inerting gas surrounding the meniscus is a gas soluble in steel or a mixture of such gases, or consists of at least 50% by volume of such a gas or mixture of gases.

Description

PROCEDE DE COULEE CONTINUE ENTRE CYLINDRES DE BANDES CONTINUOUS CASTING PROCESS BETWEEN TAPE CYLINDERS
D'ACIER INOXYDABLE AUSTENITIQUE D'EXCELLENTE QUALITE DEAUSTENITIC STAINLESS STEEL OF EXCELLENT QUALITY
SURFACE, ET BANDES AINSI OBTENUESSURFACE, AND BANDS THUS OBTAINED
L'invention concerne la coulée continue des métaux, et plus précisément la coulée continue, directement à partir de métal liquide, de bandes d'acier inoxydable de type austénitique dont l'épaisseur est de l'ordre de quelques mm, par le procédé dit de « coulée entre cylindres ». Ces dernières années ont vu s'accomplir des progrès sensibles dans le développement des procédés de coulée de bandes minces d'acier au carbone ou inoxydable 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 invention relates to the continuous casting of metals, and more precisely the continuous casting, directly from liquid metal, of stainless steel strips of the austenitic type, the thickness of which is of the order of a few mm, by the so-called process. of "casting between cylinders". In recent years, significant progress has been made in the development of processes for casting thin strips of carbon or stainless steel directly from liquid metal. The method mainly used today is the casting of said liquid metal between two internally cooled cylinders, rotating around their horizontal axes in opposite directions, and arranged opposite one another, the minimum distance between their surfaces being substantially equal to the thickness that you want to give to the cast strip (for example a few mm). The pouring space containing the liquid steel is defined by the lateral surfaces of the cylinders, on which the solidification of the strip is initiated, and by refractory side closure plates applied against the ends of the cylinders. The liquid metal initiates its solidification on contact with the outer surfaces of the cylinders, on which it forms solidified "skins", which are made to meet at the "neck", that is to say the area where the distance between the cylinders is minimal.
Un des principaux problèmes rencontrés lors de la fabrication de bandes minces d'acier inoxydable par coulée entre cylindres est le risque important d'une apparition sur la bande de défauts de surface appelés microcriques. Il s'agit de fissures de faibles dimensions qui sont néanmoins suffisantes pour rendre impropres à l'utilisation les produits transformés à froid qui en sont issus. Elles se forment au cours de la solidification de l'acier et ont une profondeur de l'ordre de 40 μm et une ouverture d'environ 20 μm. Leur apparition est liée aux contractions du métal lors de la solidification des peaux au contact des cylindres, sur la longueur de leur arc de contact. Cette solidification peut être décrite comme comportant deux étapes successives. La première étape se produit lors du contact initial entre l'acier liquide et la surface du cylindre, qui entraîne la formation d'une peau d'acier solide à la surface des cylindres. La seconde étape concerne la croissance de cette peau jusqu'au col, où comme on l'a dit, elle rejoint la peau formée sur l'autre cylindre pour constituer la bande entièrement solidifiée. Le contact entre l'acier et la surface du cylindre est conditionné par la topographie de la surface des cylindres de coulée, conjuguée à la nature du gaz d'inertage environnant l'espace de coulée et à la composition chimique de l'acier. Tous ces paramètres interviennent dans l'établissement des transferts thermiques entre l'acier et le cylindre et régissent les conditions de solidification des peaux. Lors de la solidification et du refroidissement des peaux, celles-ci subissent des contractions. Elles dépendent notamment de l'ampleur de la transformation de phase δ -» γ, qui s'effectue avec une variation sensible de la densité du métal, au niveau microscopique. Elle est déterminée par la composition du métal coulé. Ces contractions vont, elles aussi, modifier les conditions de solidification et de refroidissement des peaux.One of the main problems encountered during the production of thin strips of stainless steel by casting between rolls is the significant risk of surface defects called microcracks appearing on the strip. These are small cracks which are nevertheless sufficient to make the cold processed products which result from them unfit for use. They form during the solidification of the steel and have a depth of the order of 40 μm and an opening of approximately 20 μm. Their appearance is linked to the contractions of the metal during the solidification of the skins in contact with the cylinders, over the length of their contact arc. This solidification can be described as comprising two successive stages. The first stage occurs during the initial contact between the liquid steel and the surface of the cylinder, which results in the formation of a solid steel skin on the surface of the cylinders. The second step concerns the growth of this skin up to the neck, where, as we said, it joins the skin formed on the other cylinder to form the fully solidified strip. The contact between the steel and the surface of the cylinder is conditioned by the topography of the surface of the casting rolls, combined with the nature of the inerting gas surrounding the casting space and with the chemical composition of the steel. All these parameters intervene in the establishment of heat transfers between the steel and the cylinder and govern the conditions of solidification of the skins. When solidification and cooling of the skins, they undergo contractions. They depend in particular on the extent of the phase transformation δ - »γ, which takes place with a significant variation in the density of the metal, at the microscopic level. It is determined by the composition of the cast metal. These contractions will also modify the conditions of solidification and cooling of the skins.
Le rapport Creq/Nieq est classiquement considéré comme représentatif du chemin de solidification des aciers inoxydables austénitiques. On le calcule, selon la relation de Hammar et Swensson, au moyen des formules (les pourcentages sont des pourcentages pondéraux) : Creq (%) = Cr% + 1 ,37 Mo% + 1 ,5 Si% + 2 Nb% + 3 Ti%The Cr eq / Ni e q ratio is conventionally considered to be representative of the solidification path of austenitic stainless steels. It is calculated, according to Hammar and Swensson's relationship, using the formulas (the percentages are weight percentages): Cr eq (%) = Cr% + 1.37 MB% + 1.5 Si% + 2 Nb% + 3 Ti%
Nieq (%) = Ni% + 0,31 Mn% + 22 C% + 14,2 N% + Cu% Diverses tentatives ont été faites pour mettre au point des procédés de coulée entre cylindres permettant l'obtention, de manière fiable, de bandes exemptes de défauts de surface rédhibitoires tels que des microcriques. En ce qui concerne les aciers inoxydables austénitiques, on peut citer le documentNi eq (%) = Ni% + 0.31 Mn% + 22 C% + 14.2 N% + Cu% Various attempts have been made to develop casting processes between cylinders allowing obtaining, reliably , strips free from unacceptable surface defects such as microcracks. With regard to austenitic stainless steels, the document can be cited
EP-A-0 409 645. Il conjugue une géométrie définie des « fossettes » (gravures en creux de forme grossièrement circulaire ou elliptique) présentes à la surface des cylindres avec l'utilisation comme gaz d'inertage d'un mélange gazeux contenant 30 à 90% d'un gaz soluble dans l'acier, qui garnit les fossettes au moment du premier contact cylindres/acier liquide. Le document EP-A-0 481 481 conjugue une composition chimique, où l'indice δ- Fecaι défini par δ-Feca, = 3 (Cr% + 1,5 Si% + Mo%) - 2,8 (Ni% + 0,5 Mn% + 0,5 Cu%) - 84 (C% + N%) - 19,8 est compris entre 5 et 9%, avec une géométrie de fossettes sur les cylindres, de manière à favoriser la solidification en ferrite primaire δ — > δ + γ. Les fossettes peuvent classiquement être réalisées par grenaillage ou usinage laser. Dans tous les documents précédents, il est exigé que ces fossettes soient disjointes les unes des autres.EP-A-0 409 645. It combines a defined geometry of the "dimples" (hollow engravings of roughly circular or elliptical shape) present on the surface of the cylinders with the use as inerting gas of a gas mixture containing 30 90% of a gas soluble in steel, which fills the dimples at the time of the first cylinder / liquid steel contact. Document EP-A-0 481 481 combines a chemical composition, where the index δ- Fe ca ι defined by δ-Fe ca , = 3 (Cr% + 1.5 Si% + Mo%) - 2.8 ( Ni% + 0.5 Mn% + 0.5 Cu%) - 84 (C% + N%) - 19.8 is between 5 and 9%, with a geometry of dimples on the cylinders, so as to favor the solidification in primary ferrite δ -> δ + γ. The dimples can conventionally be produced by shot blasting or laser machining. In all the preceding documents, it is required that these dimples are separated from each other.
Le document EP-A-0 679 114 propose l'utilisation de rainures circonférencielles ménagées sur la surface des cylindres, qui donnent à ladite surface une rugosité Ra de 2,5 àDocument EP-A-0 679 114 proposes the use of circumferential grooves formed on the surface of the cylinders, which give said surface a roughness Ra of 2.5 to
15 μm. Elle est conjuguée à une composition chimique de l'acier permettant une solidification en austénite primaire, caractérisée par un rapport Creq/Nieq inférieur à 1,60. Toutefois, la solidification en austénite primaire augmente la sensibilité à la fissuration à chaud des aciers inoxydables et les risques de formation de criques longitudinales sur la bande.15 μm. It is combined with a chemical composition of the steel allowing solidification into primary austenite, characterized by a Cr eq / Ni eq ratio of less than 1.60. However, solidification into primary austenite increases the sensitivity to hot cracking of stainless steels and the risks of longitudinal cracks forming on the strip.
Le document EP-A-0 796 685 enseigne de couler un acier dont le rapport Creq/Nieq est supérieur à 1,55 de manière à minimiser les changements de phase à haute température, et de réaliser cette coulée en utilisant des cylindres dont la surface comporte des fossettes jointives de diamètre 100-1500 μm et de profondeur 20-150 μm et en inertant le voisinage du ménisque (l'intersection entre la surface de l'acier liquide et la surface des cylindres) avec un gaz soluble dans l'acier, ou un mélange de gaz composé majoritairement d'un tel gaz soluble. Les pics de rugosité servent de sites d'amorçage à la solidification, alors que les creux de la rugosité constituent des joints de contraction du métal au cours de la solidification, et permettent une meilleure répartition des contraintes. Toutefois, lorsque le rapport Creq/Nieq est supérieur à 1,70, on ne peut pas toujours éviter la présence de quelques microcriques. Le but de l'invention est de proposer un procédé de coulée de bandes minces en acier inoxydable austénitique dont la surface serait exempte de microcriques et d'autres défauts majeurs, ne nécessitant pas de conditions de coulée particulièrement contraignantes pour sa mise en œuvre, et permettant de couler des aciers ayant un rapport Creq/Nieq plus étendu que dans les procédés existants. A cet effet, l'invention a pour objet un procédé de coulée continue d'une bande d'acier inoxydable austénitique d'épaisseur inférieure ou égale à 10 mm, directement à partir de métal liquide, entre deux cylindres horizontaux refroidis, caractérisé en ce que :Document EP-A-0 796 685 teaches to cast a steel whose Cr e q / Ni e q ratio is greater than 1.55 so as to minimize the phase changes at high temperature, and to carry out this casting using cylinders the surface of which has joined dimples of diameter 100-1500 μm and depth 20-150 μm and by inerting the vicinity of the meniscus (the intersection between the surface of the liquid steel and the surface of the cylinders) with a soluble gas in steel, or a mixture of gases mainly composed of such a soluble gas. The roughness peaks serve as priming sites for solidification, while the roughness hollows constitute contraction joints of the metal during solidification, and allow a better distribution of the stresses. However, when the Cr eq / Ni eq ratio is greater than 1.70, it is not always possible to avoid the presence of a few microcracks. The object of the invention is to propose a process for casting thin strips of austenitic stainless steel, the surface of which would be free from microcracks and other major defects, not requiring particularly restrictive casting conditions for its implementation, and making it possible to cast steels with a Cr eq / Ni eq ratio which is more extensive than in existing processes. To this end, the subject of the invention is a process for the continuous casting of a strip of austenitic stainless steel of thickness less than or equal to 10 mm, directly from liquid metal, between two cooled horizontal cylinders, characterized in that than :
- la composition dudit acier, en pourcentage pondéraux, comprend : C% < 0,08 ; Si% <1 ; P% < 0,04 ; Mn% < 2 ; Cr% compris entre 17 et 20 ; Ni% compris entre 8 et 10,5 ; S% compris entre 0,007 et 0,040 ; le reste étant du fer et des impuretés résultant de l'élaboration ;- The composition of said steel, in weight percent, comprises: C% <0.08; If% <1; P% <0.04; Mn% <2; Cr% between 17 and 20; Ni% between 8 and 10.5; S% between 0.007 and 0.040; the remainder being iron and impurities resulting from processing;
- le rapport Creq/Nieq est compris entre 1,55 et 1,90, avec :- the Cr eq / Ni e q ratio is between 1.55 and 1.90, with:
Creq (%) = Cr% + 1,37 Mo% + 1,5 Si% + 2 Nb% + 3 Ti% et Nieq (%) = Ni% + 0,31 Mn% + 22 C% + 14,2 N% + Cu% ;Cr eq (%) = Cr% + 1.37 Mo% + 1.5 Si% + 2 Nb% + 3 Ti% and Nieq (%) = Ni% + 0.31 Mn% + 22 C% + 14.2 N% + Cu%;
- la surface des cylindres comporte des fossettes jointives de section approximativement circulaire ou elliptique, de diamètre 100 à 1500 μm et de profondeur 20 à 150 μm ;- The surface of the cylinders has contiguous dimples of approximately circular or elliptical section, with a diameter of 100 to 1500 μm and a depth of 20 to 150 μm;
- le gaz d'inertage environnant le ménisque est un gaz soluble dans l'acier ou un mélange de tels gaz, ou est constitué au moins par 50% en volume d'un tel gaz ou mélange de gaz.the inerting gas surrounding the meniscus is a gas soluble in steel or a mixture of such gases, or consists at least of 50% by volume of such a gas or mixture of gases.
L'invention a également pour objet des bandes pouvant être produites par ce procédé.The invention also relates to bands which can be produced by this method.
Comme on l'aura compris, l'invention consiste à combiner des conditions portant sur la composition du métal coulé, l'état de surface des cylindres et la composition du gaz d'inertage du ménisque, de manière à obtenir une surface de bande exempte de microcriques. La principale originalité de la composition requise est que le métal doit contenir une quantité de soufre supérieure aux quantités les plus usuellement rencontrées (sans être cependant importante au point de compromettre la résistance des produits à la corrosion), et que cette teneur doit être combinée à une gamme précise de rapports Creq/Nieq.As will be understood, the invention consists in combining conditions relating to the composition of the cast metal, the surface condition of the cylinders and the composition of the meniscus inerting gas, so as to obtain a strip-free surface. of micro cracks. The main originality of the required composition is that the metal must contain a quantity of sulfur greater than the quantities most commonly encountered (without however being so great as to compromise the resistance of the products to corrosion), and that this content must be combined with a precise range of Cr eq / Ni e q ratios.
L'invention sera mieux comprise à la lecture de la description qui suit, donnée en référence aux figures annexées suivantes : - la figure 1 qui montre vue en coupe une bande d'acier inoxydable austénitique coulée entre cylindres selon l'art antérieur, et qui met en évidence la morphologie des microcriques que l'on veut éviter ;The invention will be better understood on reading the description which follows, given with reference to the following appended figures: - Figure 1 which shows a sectional view of a strip of austenitic stainless steel poured between cylinders according to the prior art, and which highlights the morphology of the microcracks that we want to avoid;
- la figure 2 qui est une courbe montrant l'influence de la teneur en soufre du métal sur la présence de microcriques à la surface de la bande coulée.- Figure 2 which is a curve showing the influence of the sulfur content of the metal on the presence of microcracks on the surface of the cast strip.
Les conditions du premier contact entre l'acier liquide et les cylindres constituent un facteur très important dans le processus de solidification de la bande, et influent notablement sur la qualité de surface de celle-ci. Leur bonne maîtrise est donc très importante pour garantir l'absence de microcriques sur la bande coulée. Cependant, les inévitables fluctuations du niveau de la surface du métal liquide présent entre les cylindres compliquent cette maîtrise, notamment en ce qu'ils sont source d'irrégularités dans les échanges thermiques qui ont lieu dans cette zone de premier contact. D'autres telles irrégularités sont dues, lors des stades ultérieurs de la solidification des peaux, aux contractions du métal lors de la solidification, qui résultent notamment des transformations de phase à haute température caractéristiques des aciers inoxydables austénitiques. Ces contractions peuvent être à l'origine de microcriques. La figure 1 montre une micrographie réalisée sur un échantillon d'une bande mince 1 d'acier inoxydable austénitique, vue en coupe longitudinale. Cette bande 1 présente sur sa surface 2 une microcrique 3, du type de celles que l'invention cherche précisément à éviter. L'attaque métallographique réalisée sur l'échantillon met en évidence une plage claire 4 située autour de la microcrique 3 et dans son prolongement : elle correspond à une zone ségrégée enrichie en certains éléments tels que le nickel et le manganèse.The conditions of the first contact between the liquid steel and the cylinders constitute a very important factor in the process of solidification of the strip, and notably influence the surface quality of the latter. Their good control is therefore very important to guarantee the absence of microcracks on the casting strip. However, the inevitable fluctuations in the level of the surface of the liquid metal present between the cylinders complicate this control, in particular in that they are a source of irregularities in the heat exchanges which take place in this zone of first contact. Other such irregularities are due, during the later stages of the solidification of the skins, to the contractions of the metal during solidification, which result in particular from phase transformations at high temperature characteristic of austenitic stainless steels. These contractions can cause micro cracks. FIG. 1 shows a micrograph produced on a sample of a thin strip 1 of austenitic stainless steel, seen in longitudinal section. This strip 1 has on its surface 2 a microcrack 3, of the type that the invention seeks precisely to avoid. The metallographic attack carried out on the sample highlights a clear area 4 located around the microcrack 3 and in its extension: it corresponds to a segregated area enriched with certain elements such as nickel and manganese.
On a découvert que l'addition au métal liquide d'éléments tensioactifs, tels que le soufre, qui agissent sur la tension superficielle de l'acier liquide à la surface des cylindres, a une influence sensible sur les conditions du premier contact entre le métal et les cylindres de coulée. En particulier, une telle addition permet de stabiliser très sensiblement la forme du ménisque du métal liquide, grâce à un meilleur mouillage de la surface du cylindre. Il en résulte une amélioration significative de l'homogénéité et de la régularité dans le temps des échanges thermiques entre le métal liquide et la surface des cylindres lors de leur premier contact. Ces effets ont été mis en évidence par les inventeurs à partir de mesures de la régularité des épaisseurs de peaux colonnaires réalisées sur des coupes métallographiques en sens travers de bandes minces brutes de coulée en acier inoxydable austénitique de type 304. Une irrégularité des ces épaisseurs se traduit par une propension élevée de la bande coulée à présenter des microcriques sur sa surface. En revanche une épaisseur régulière de la partie colonnaire de la peau solidifiée, qui est l'indice que le niveau du ménisque n'a que peu varié pendant la coulée, va de pair avec une absence de microcriques à la surface de la bande.It has been discovered that the addition to the liquid metal of surfactant elements, such as sulfur, which act on the surface tension of the liquid steel on the surface of the cylinders, has a significant influence on the conditions of the first contact between the metal. and the casting cylinders. In particular, such an addition makes it possible to very substantially stabilize the shape of the meniscus of the liquid metal, thanks to better wetting of the surface of the cylinder. This results in a significant improvement in the uniformity and regularity over time of the heat exchanges between the liquid metal and the surface of the cylinders during their first contact. These effects have been demonstrated by the inventors from measurements of the regularity of the thicknesses of columnar skins carried out on metallographic sections in the transverse direction of raw thin strips of austenitic stainless steel casting of type 304. An irregularity of these thicknesses occurs. results in a high propensity of the cast strip to present micro cracks on its surface. On the other hand, a regular thickness of the columnar part of the solidified skin, which is the indication that the level of the meniscus has changed little during casting, goes hand in hand with an absence of microcracks on the surface of the strip.
La courbe de la figure 2 traduit les résultats de ces investigations, qui ont été menées sur des bandes d'épaisseur 3 mm coulées à une vitesse de 50 m/min. Les surfaces des cylindres de coulée étaient rendues rugueuses par des fossettes jointives de profondeur moyenne 80 μm et de diamètre moyen 1000 μm. La composition des aciers coulés s'inscrivait dans les limites : C : 0,02-0,06% ; Mn : 1,3-1,6% ; P : 0,019-0,024% ; Si : 0,34- 0,45% ; Cr : 18,0-18,7% ; Ni : 8,6-9,8% ; S: 0,0005-0,446%. Les rapports Creq/Nieq de ces aciers variaient de 1,79 à 1,85. Le gaz d'inertage environnant le ménisque contenait 60% en volume d'azote et 40% en volume d'argon. En abscisses figure la teneur en soufre du métal, en ordonnées un indice représentatif de l'ampleur des fluctuations du niveau du ménisque lors de la coulée, qui représente l' écart-type sur l'épaisseur des zones colonnaires observées sur la structure de solidification de la bande. On voit qu'à conditions de coulée égales, plus la teneur en soufre du métal est élevée, alors que par ailleurs les teneurs des autres éléments restent similaires, plus les fluctuations du niveau du ménisque ont une ampleur réduite. A partir d'une teneur en soufre de 0,007%, cette influence diminue très sensiblement, alors qu'elle est très nette pour les teneurs plus basses. On se rend également compte que la présence de microcriques à la surface de la bande est directement liée à ces fluctuations, et que la limite inférieure de 0,007% pour la teneur en soufre correspond également au minimum nécessaire pour éviter la formation de microcriques.The curve in FIG. 2 translates the results of these investigations, which were carried out on strips of thickness 3 mm cast at a speed of 50 m / min. The surfaces casting rolls were roughened by contiguous dimples of average depth 80 μm and average diameter 1000 μm. The composition of the cast steels fell within the limits: C: 0.02-0.06%; Mn: 1.3-1.6%; P: 0.019-0.024%; If: 0.34-0.45%; Cr: 18.0-18.7%; Ni: 8.6-9.8%; S: 0.0005-0.446%. The Cr eq / Ni eq ratios of these steels ranged from 1.79 to 1.85. The inert gas surrounding the meniscus contained 60% by volume of nitrogen and 40% by volume of argon. On the abscissa is the sulfur content of the metal, on the ordinate a representative index of the magnitude of the fluctuations in the level of the meniscus during casting, which represents the standard deviation over the thickness of the columnar zones observed on the solidification structure. Of the band. We see that at equal casting conditions, the higher the sulfur content of the metal, while moreover the contents of the other elements remain similar, the more the fluctuations in the level of the meniscus have a reduced magnitude. From a sulfur content of 0.007%, this influence decreases very appreciably, whereas it is very clear for the lower contents. We also realize that the presence of microcracks on the surface of the strip is directly linked to these fluctuations, and that the lower limit of 0.007% for the sulfur content also corresponds to the minimum necessary to avoid the formation of microcracks.
De manière générale, les inventeurs ont déterminé un ensemble de conditions à respecter pour que la coulée d'aciers inoxydables austénitiques en bandes minces s'effectue sans formation de microcriques à la surface des bandes, et elles ont été citées plus haut. Elles se justifient par les considérations suivantes.In general, the inventors have determined a set of conditions to be respected so that the casting of austenitic stainless steels in thin strips takes place without the formation of microcracks on the surface of the strips, and they have been mentioned above. They are justified by the following considerations.
Lorsque la teneur en soufre est inférieure à 0,007%, les fluctuations du niveau du ménisque deviennent trop importantes, et les irrégularités des transferts thermiques qui en résultent provoquent la formation de microcriques, en particulier lorsque le rapport Creq/Nieq est supérieur à 1,70. On fixe à 0,04% la limite supérieure de la teneur en soufre parce qu'au-delà de cette valeur, l'influence de la teneur en soufre sur la stabilité du ménisque n'augmente plus significativement, et qu'en revanche, on observe un accroissement des risques de dégradation de la résistance à la corrosion par piqûres du produit fini fabriqué à partir de ces bandes.When the sulfur content is less than 0.007%, the fluctuations in the level of the meniscus become too great, and the irregularities of the heat transfers which result therefrom cause the formation of microcracks, in particular when the Cr eq / Ni eq ratio is greater than 1 , 70. The upper limit of the sulfur content is set at 0.04% because beyond this value, the influence of the sulfur content on the stability of the meniscus no longer increases significantly, and on the other hand, there is an increased risk of deterioration in the resistance to corrosion by pitting of the finished product made from these strips.
La teneur en phosphore doit être maintenue à moins de 0,04%, afin d'éviter des risques de fissuration à chaud des bandes lorsque le rapport Creq/Nieq est proche de 1,55, c'est à dire lorsque la solidification s'effectue partiellement en austénite primaire, et pas majoritairement en ferrite primaire.The phosphorus content must be kept below 0.04%, in order to avoid risks of hot cracking of the bands when the Cr eq / Ni eq ratio is close to 1.55, that is to say when solidification takes place partially in primary austenite, and not mainly in primary ferrite.
Le rapport Creq/Nieq doit être d'au moins 1,55, en ce qu'en dessous de cette valeur, l'acier se solidifie au moins partiellement en austénite primaire, ce qui augmente la sensibilité à la fissuration de la bande et favorise l'apparition de criques longitudinales, qui doivent être absolument évitées, elles aussi. Pour un rapport Creq/Nieq supérieur à 1,90, la contraction liée à la transformation ferrite-austénite devient trop importante, et les microcriques sont alors inévitables. De plus, le taux de ferrite dans la bande devient trop élevé, ce qui peut entraîner des ruptures après la mise en forme des produits finis réalisés à partir des bandes ainsi coulées.The Cr eq / Ni eq ratio must be at least 1.55, in that below this value, the steel solidifies at least partially into primary austenite, which increases the sensitivity to cracking of the strip and promotes the appearance of longitudinal cracks, which must be absolutely avoided, too. For a Cr eq / Ni eq ratio greater than 1.90, the contraction linked to the ferrite-austenite transformation becomes too great, and the microcracks are then inevitable. In addition, the ferrite level in the strip becomes too high, which can lead to ruptures after the shaping of the finished products made from the strips thus cast.
Les autres conditions analytiques sur l'acier coulé sont classiques sur les aciers inoxydables austénitiques les plus courants, notamment ceux du type 304 et apparentés. Il est entendu que d'autres éléments que ceux cités explicitement dans ce qui précède peuvent être présents dans l'acier, au titre d'impuretés ou d'éléments d'alliage en faible quantité, dans la mesure où ils ne modifieraient pas notablement les conditions de solidification et la tension superficielle de l'acier liquide à la surface des cylindres, ce que confirmerait l'absence de microcriques sur les bandes produites. Comme on l'a dit, la nature du gaz d'inertage environnant le ménisque a une forte influence sur les conditions de contact entre l'acier et la surface des cylindres, notamment sur la manière dont s'effectue le transfert « en négatif» de la rugosité des cylindres sur la surface de la bande, et sur les risques de formation de microcriques. Avec un gaz totalement ou majoritairement insoluble dans l'acier, tel que l'argon ou l'hélium, l'acier en cours de solidification ne pénètre pas ou peu dans les dépressions de la surface du cylindre. L'extraction de chaleur ne s'effectue donc pratiquement qu'au droit des pics de rugosité, ce qui la rend très hétérogène sur la surface du cylindre. Cette hétérogénéité est favorable à l'apparition de nombreuses microcriques. En revanche, avec un gaz d'inertage contenant une quantité notable d'un gaz soluble dans l'acier tel que l'azote, l'hydrogène, l'ammoniac le CO , a fortiori s'il est constitué dans sa totalité par un tel gaz ou un mélange de tels gaz, l'acier pénètre bien dans les dépressions de la surface des cylindres, et l'extraction de chaleur au premier contact est importante. De plus, cela diminue l'hétérogénéité de l'extraction de la chaleur au droit des pics et des dépressions. Tout cela va dans le sens d'une limitation des risques de formation de microcriques. Dans la pratique, compte tenu des autres conditions de coulée exigées pour la composition du métal et la rugosité des surfaces des cylindres, on fixe à 50% en volume la limite inférieure de la teneur du gaz d'inertage en un gaz (ou mélange de gaz) soluble dans l'acier.The other analytical conditions on cast steel are conventional on the most common austenitic stainless steels, in particular those of type 304 and related. It is understood that elements other than those explicitly mentioned in the foregoing may be present in the steel, as impurities or alloying elements in small quantities, insofar as they would not significantly modify the solidification conditions and the surface tension of the liquid steel on the surface of the cylinders, which would be confirmed by the absence of microcracks on the strips produced. As has been said, the nature of the inerting gas surrounding the meniscus has a strong influence on the conditions of contact between the steel and the surface of the cylinders, in particular on the manner in which the transfer takes place "in negative" the roughness of the cylinders on the surface of the strip, and the risks of microcracks forming. With a gas wholly or mainly insoluble in steel, such as argon or helium, the steel in the course of solidification does not penetrate little or little into the depressions of the surface of the cylinder. Heat extraction is therefore practically only carried out at the roughness peaks, which makes it very heterogeneous on the surface of the cylinder. This heterogeneity is favorable to the appearance of numerous microcracks. On the other hand, with an inerting gas containing a significant quantity of a gas soluble in steel such as nitrogen, hydrogen, ammonia, CO, a fortiori if it is entirely constituted by a such gas or a mixture of such gases, the steel penetrates well into the depressions of the surface of the cylinders, and the extraction of heat at the first contact is important. In addition, this decreases the heterogeneity of heat extraction at the level of peaks and depressions. All this goes in the direction of limiting the risks of micro-crack formation. In practice, taking into account the other casting conditions required for the composition of the metal and the roughness of the surfaces of the rolls, the lower limit of the content of the inerting gas in a gas (or mixture of gas) soluble in steel.
Les conditions qui viennent d'être décrites conduisent aux résultats souhaités dans le cas où les cylindres ont sur leur surface des fossettes jointives d'un diamètre compris entre 100 et 1500 μm et une profondeur comprise entre 20 et 150 μm.The conditions which have just been described lead to the desired results in the case where the cylinders have joined dimples on their surface with a diameter between 100 and 1500 μm and a depth between 20 and 150 μm.
Des exemples d'application vont à présent illustrer l'invention et justifier ses exigences.Application examples will now illustrate the invention and justify its requirements.
Exemple 1 :Example 1:
On a coulé entre cylindres des bandes d'acier inoxydable austénitique de 3 mm d'épaisseur. Les surfaces des cylindres comportaient des fossettes jointives de diamètre moyen 1000 μm et de profondeur moyenne 100 μm. Le gaz d'inertage environnant le ménisque comportait 40% d'argon et 60% d'azote. La composition de l'acier variait dans les limites suivantes : C : 0,02-0,06% ; Mn : 1,3-1,6% ; P : 0,019-0,024% ; Si : 0,34- 0,45% ; Cr : 18,0-18,7% ; Ni : 8,6-9,8% ; S : 0,0005-0,0446%. Le rapport Creq/Nieq des aciers coulés variait de 1,79 à 1,85. On a mesuré la densité surfacique des microcriques sur les bandes ainsi coulées, et on a confronté les résultats de ces mesures avec les teneurs en soufre dans les aciers coulés. Le tableau 1 présente les conclusions des ces essais.Strips of austenitic stainless steel 3 mm thick were poured between cylinders. The cylinder surfaces included joined dimples with an average diameter of 1000 μm and an average depth of 100 μm. The inert gas surrounding the meniscus contained 40% argon and 60% nitrogen. The composition of the steel varied within the following limits: C: 0.02-0.06%; Mn: 1.3-1.6%; P: 0.019-0.024%; If: 0.34-0.45%; Cr: 18.0-18.7%; Ni: 8.6-9.8%; S: 0.0005-0.0446%. The Cr eq / Ni e q ratio of cast steels varied from 1.79 to 1.85. The surface density of the microcracks on the strips thus cast was measured, and the results of these measurements were compared with the sulfur contents in the cast steels. Table 1 presents the conclusions of these tests.
Tableau 1 : Effet de la teneur en soufre de l'acier sur la densité surfacique de microcriques Table 1: Effect of the sulfur content of steel on the surface density of microcracks
Dans ces exemples, où le rapport Creq/Nieq des aciers coulés était de 1,79 à 1,85 (et ne variait donc que dans des limites très étroites), il apparaît clairement que la densité de microcriques observées dépend fortement de la teneur en soufre de l'acier. Pour des teneurs en soufre supérieures à 0,007%, on n'observe pas de microcriques, alors que pour les teneurs basses et très basses en soufre, les microcriques sont présentes de manière très significative. C'est à partir de ces résultats qu'a été établie la courbe de la figure 2.In these examples, where the Cr eq / Ni e q ratio of the cast steels was 1.79 to 1.85 (and therefore varied only within very narrow limits), it is clear that the density of observed microcracks strongly depends on the sulfur content of the steel. For sulfur contents greater than 0.007%, no microcracks are observed, while for low and very low sulfur contents, the microcracks are present very significantly. It is from these results that the curve in Figure 2 was established.
Exemple 2 :Example 2:
On a coulé entre cylindres des bandes d'épaisseur 3,8 mm en acier inoxydable austénitique, dont les compositions figurent dans le tableau 2. Les cylindres avaient des rugosités superficielles caractérisées par la présence de fossettes jointives de diamètre moyen 1000 μm et de profondeur moyenne 120 μm.Strips of thickness 3.8 mm in austenitic stainless steel were poured between cylinders, the compositions of which are shown in Table 2. The cylinders had surface roughnesses characterized by the presence of contiguous dimples of average diameter 1000 μm and average depth 120 μm.
Tableau 2 : Composition chimique des aciers de l'exemple 2Table 2: Chemical composition of the steels of Example 2
Au cours de la coulée de ces aciers, on a fait varier la composition du gaz d'inertage présent au voisinage du ménisque en modulant ses proportions respectives en argon et azote, et on a mesuré sur les bandes coulées la densité surfacique de microcriques observées, pour les différentes compositions du gaz d'inertage employées. Les résultats sont regroupés dans le tableau 3 :During the casting of these steels, the composition of the inerting gas present in the vicinity of the meniscus was varied by modulating its respective proportions in argon and nitrogen, and the surface density of microcracks was measured on the cast strips. observed, for the different compositions of the inerting gas used. The results are collated in Table 3:
Tableau 3 : Influence de la composition du gaz d'inertage sur la densité surfacique des microcriques de la bande, selon la teneur en soufre et le rapport Creq/Nieq de l'acier couléTable 3: Influence of the composition of the inerting gas on the surface density of the strip microcracks, according to the sulfur content and the Cr eq / Ni eq ratio of the cast steel
Ces essais montrent que l'acier A, qui a un rapport Creq/Nieq satisfaisant, mais une teneur en soufre faible, conduit systématiquement à la formation de microcriques en quantités importantes, quelle que soit la composition du gaz d'inertage. L'acier C a une teneur en soufre légèrement plus forte, et cela suffit à améliorer sensiblement la qualité de surface de la bande, puisqu'on n'observe pas de microcriques lorsque la teneur en azote du gaz d'inertage est d'au moins 80%. Cependant, ce résultat ne peut être considéré comme entièrement satisfaisant, car cette nécessité de maintenir la teneur en azote du gaz d'inertage à un niveau élevé réduit les possibilités, pour les opérateurs, de piloter le fonctionnement de l'installation de coulée de façon fine. En effet, la composition du gaz d'inertage est un paramètre sur lequel on désire souvent jouer pour maîtriser l'intensité des transferts thermiques entre les cylindres et le métal, par exemple pour faire varier le bombé des cylindres qui affecte la forme de la bande (voir le document EP-A-0 736 350). Les résultats obtenus avec l'acier C font donc conclure qu'une teneur en soufre de 0,005% ne peut entrer dans le cadre de l'invention.These tests show that steel A, which has a satisfactory Cr eq / Ni eq ratio, but a low sulfur content, systematically leads to the formation of microcracks in large quantities, whatever the composition of the inerting gas. Steel C has a slightly higher sulfur content, and this is enough to significantly improve the surface quality of the strip, since no microcracks are observed when the nitrogen content of the inerting gas is at least minus 80%. However, this result cannot be considered entirely satisfactory, since this need to keep the nitrogen content of the inerting gas at a high level reduces the possibilities for operators to control the operation of the casting installation in such a way. fine. Indeed, the composition of the inerting gas is a parameter on which we often wish to play to control the intensity of the heat transfers between the cylinders and the metal, for example to vary the bulging of the cylinders which affects the shape of the strip. (see document EP-A-0 736 350). The results obtained with steel C therefore lead to the conclusion that a sulfur content of 0.005% cannot fall within the scope of the invention.
En revanche, les bandes coulées avec les aciers B et D ne présentent pas de microcriques tant que la teneur en azote du gaz d'inertage est d'au moins 50%. Leurs teneurs en soufre sont respectivement de 0,019 et 0,039%, et leurs rapports Creq/Nieq sont respectivement de 1,82 et 1,64. Ces exemples entrent donc bien dans le cadre de l'invention. L'invention s'applique préférentiellement au cas des aciers présentant un rapport Creq/Nieq compris entre 1,70 et 1,90, car cette gamme correspond à des aciers dans lesquels on a ajouté moins d'éléments gammagènes (tels que le nickel) que pour les aciers présentant un rapport Creq/Nieq plus faible, et qui sont donc plus économiques à fabriquer. On the other hand, the strips cast with steels B and D do not exhibit microcracks as long as the nitrogen content of the inerting gas is at least 50%. Their sulfur contents are respectively 0.019 and 0.039%, and their Cr eq / Ni eq ratios are respectively 1.82 and 1.64. These examples therefore fall well within the scope of the invention. The invention applies preferably to the case of steels having a Cr e q / Ni eq ratio of between 1.70 and 1.90, since this range corresponds to steels in which have added less gammagenic elements (such as nickel) than for steels with a lower Cr eq / Ni eq ratio, and which are therefore more economical to manufacture.

Claims

REVENDICATIONS
1) Procédé de coulée continue d'une bande d'acier inoxydable austénitique d'épaisseur inférieure ou égale à 10 mm, directement à partir de métal liquide, entre deux cylindres horizontaux refroidis, caractérisé en ce que :1) Process for the continuous casting of a strip of austenitic stainless steel of thickness less than or equal to 10 mm, directly from liquid metal, between two cooled horizontal cylinders, characterized in that:
- la composition dudit acier, en pourcentage pondéraux, comprend : C% < 0,08 ; Si% ≤l ; P% < 0,04 ; Mn% < 2 ; Cr% compris entre 17 et 20 ; Ni% compris entre 8 et 10,5 ; S% compris entre 0,007 et 0,040 ; le reste étant du fer et des impuretés résultant de l'élaboration ; - le rapport Creq/Nieq est compris entre 1,55 et 1,90, avec :- The composition of said steel, in weight percent, comprises: C% <0.08; If% ≤l; P% <0.04; Mn% <2; Cr% between 17 and 20; Ni% between 8 and 10.5; S% between 0.007 and 0.040; the remainder being iron and impurities resulting from processing; - the Cr e q / Ni eq ratio is between 1.55 and 1.90, with:
Creq (%) = Cr% + 1,37 Mo% + 1,5 Si% + 2 Nb% + 3 Ti% etCr eq (%) = Cr% + 1.37 Mo% + 1.5 Si% + 2 Nb% + 3 Ti% and
Nieq (%) = Ni% + 0,31 Mn% + 22 C% + 14,2 N% + Cu% ;Ni eq (%) = Ni% + 0.31 Mn% + 22 C% + 14.2 N% + Cu%;
- la surface des cylindres comporte des fossettes jointives de section approximativement circulaire ou elliptique, de diamètre 100 à 1500 μm et de profondeur- the surface of the cylinders has contiguous dimples of approximately circular or elliptical section, with a diameter of 100 to 1500 μm and a depth
20 à 150 μm ;20 to 150 μm;
- le gaz d'inertage environnant le ménisque est un gaz soluble dans l'acier ou un mélange de tels gaz, ou est constitué au moins par 50% en volume d'un tel gaz ou mélange de gaz. 2) Procédé selon la revendication 1, caractérisé en ce que le rapport Creq/Nieq est compris entre 1,70 et 1,90.the inerting gas surrounding the meniscus is a gas soluble in steel or a mixture of such gases, or consists at least of 50% by volume of such a gas or mixture of gases. 2) Method according to claim 1, characterized in that the Cr e q / Ni eq ratio is between 1.70 and 1.90.
3) Procédé selon la revendication 1 ou 2, caractérisé en ce que le gaz d'inertage est composé d'un mélange azote 50-100%-argon 50-0% en volume.3) Method according to claim 1 or 2, characterized in that the inerting gas is composed of a nitrogen mixture 50-100% -argon 50-0% by volume.
4) Bandes d'acier inoxydable austénitique, caractérisées en ce qu'elles sont susceptibles d'être obtenues par le procédé selon l'une des revendications 1 à 3. 4) Austenitic stainless steel strips, characterized in that they are capable of being obtained by the method according to one of claims 1 to 3.
EP00915237A 1999-04-22 2000-04-12 Method for continuously casting between two rolls austenitic stainless steel strips with excellent surface quality and resulting strips Expired - Lifetime EP1185387B1 (en)

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FR9905052A FR2792560B1 (en) 1999-04-22 1999-04-22 CONTINUOUS CASTING PROCESS BETWEEN AUSTENITIC STAINLESS STEEL TAPE CYLINDERS OF EXCELLENT SURFACE QUALITY AND BANDS THUS OBTAINED
FR9905052 1999-04-22
PCT/FR2000/000780 WO2000064612A1 (en) 1999-04-22 2000-04-12 Method for continuously casting between two rolls austenitic stainless steel strips with excellent surface quality and resulting strips

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CN100493745C (en) * 2006-06-23 2009-06-03 宝山钢铁股份有限公司 Method for continuously casting sheet band by dual roller
DE102007003548B3 (en) * 2007-01-24 2008-09-04 Thyssenkrupp Nirosta Gmbh Casting roll for a two-roll caster and two-roll caster
US7975754B2 (en) * 2007-08-13 2011-07-12 Nucor Corporation Thin cast steel strip with reduced microcracking
AU2008100847A4 (en) 2007-10-12 2008-10-09 Bluescope Steel Limited Method of forming textured casting rolls with diamond engraving
KR101239589B1 (en) * 2010-12-27 2013-03-05 주식회사 포스코 High corrosion resistance martensite stainless steel and method of manufacturing the same
WO2013075092A1 (en) 2011-11-17 2013-05-23 Nucor Corporation Method of continuous casting thin steel strip
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CN1187148C (en) 2005-02-02
ES2188523T3 (en) 2003-07-01
DE60000997D1 (en) 2003-01-23
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CZ296650B6 (en) 2006-05-17
PL194231B1 (en) 2007-05-31
CN1351528A (en) 2002-05-29
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KR100636868B1 (en) 2006-10-19
WO2000064612A1 (en) 2000-11-02
ATE229391T1 (en) 2002-12-15
DK1185387T3 (en) 2003-03-24
JP2002542039A (en) 2002-12-10
JP4454868B2 (en) 2010-04-21
RU2239518C2 (en) 2004-11-10
CZ20013776A3 (en) 2002-03-13
FR2792560A1 (en) 2000-10-27
SI1185387T1 (en) 2003-04-30
DE60000997T2 (en) 2003-07-17
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AU767990B2 (en) 2003-11-27
TW434058B (en) 2001-05-16

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