EP1187691B1 - Method for continuously casting ferritic stainless steel strips free of microcracks - Google Patents

Method for continuously casting ferritic stainless steel strips free of microcracks Download PDF

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Publication number
EP1187691B1
EP1187691B1 EP00915238A EP00915238A EP1187691B1 EP 1187691 B1 EP1187691 B1 EP 1187691B1 EP 00915238 A EP00915238 A EP 00915238A EP 00915238 A EP00915238 A EP 00915238A EP 1187691 B1 EP1187691 B1 EP 1187691B1
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EP
European Patent Office
Prior art keywords
cylinders
steel
liquid metal
microcracks
ferritic stainless
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EP00915238A
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German (de)
French (fr)
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EP1187691A1 (en
Inventor
Frédéric Mazurier
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USINOR SA
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USINOR SA
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Classifications

    • 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
    • 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
    • B22D11/0648—Casting surfaces
    • B22D11/0651—Casting wheels
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
    • B22D11/0637—Accessories therefor
    • B22D11/0697—Accessories therefor for casting in a protected atmosphere
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D27/00—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting
    • B22D27/003—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting by using inert gases
    • 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten

Definitions

  • the invention relates to the continuous casting of metals, and more precisely the casting continuous, directly from liquid metal, of stainless steel strips of the type ferritic whose thickness is of the order of a few mm, by the process known as "casting between cylinders ".
  • the process mainly used today is pouring 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 is desired impart to the cast strip (for example a few mm).
  • the casting space containing liquid steel is defined by the lateral surfaces of the cylinders, on which the solidification of the strip, and by side refractory closing plates applied against the ends of the cylinders.
  • the liquid metal initiates its solidification on contact with external surfaces of the cylinders, on which it forms solidified "skins", of which ensures that they meet at the "neck", that is to say the area where the distance between cylinders is minimum.
  • microcracks are cracks of small dimensions which are nevertheless sufficient to make them unfit for use cold processed products produced therefrom. They form during solidification steel and have a depth of about 40 ⁇ m and an opening of about 20 ⁇ m. Their appearance is linked to the conditions of contact, during solidification, between the steel and the surface of the cylinders along the length of their contact arc. These conditions can be described as comprising two successive stages. The first step concerns contact initial between the liquid steel and the surface of the cylinder, which leads to the formation of a skin solid steel 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 constitute the fully solidified strip.
  • Contact between steel and cylinder surface is conditioned by the topography of the surface of the casting cylinders, combined with the nature of the inerting gas and the chemical composition of the steel. All these parameters involved in establishing heat transfers between the steel and the cylinder and govern the conditions of solidification of the skins.
  • document EP-A-0 796 685 teaches to cast a steel whose Cr eq / Ni eq ratio is greater than 1.55 so as to minimize the phase changes at high temperature, and to carry out this casting by using cylinders the surface of which comprises contiguous dimples of diameter 100-1500 ⁇ m and depth 20-150 ⁇ m and by inerting the casting space with a gas soluble in steel, or a mixture of gases mainly composed of such a soluble gas.
  • document JP-A-5337612 proposes cast a steel with low carbon contents (less than 0.05%) and nitrogen (less than 0.05%) and containing niobium (0.1 to 5%) and titanium. It is also necessary to cool the output strip cylinders at high speed, and then control the winding temperature of the bandaged. These preparation and pouring conditions are costly and restrictive, and the particular characteristics of the nuances required limit the fields of use of products thus obtained.
  • the object of the invention is to propose a method for casting thin strips in ferritic stainless steel, the surface of which would be free of microcracks. This process does not would not require particularly restrictive casting conditions for its implementation work, and could be applied to a wide range of grades of such steels.
  • the invention consists in combining conditions on the composition of the metal, governing the possibilities of high austenite formation temperature after metal solidification, a condition on roughness minimum pour surfaces and a condition on the composition of the inerting gas. In respecting this combination, we manage to avoid the formation of micro-cracks on the surface of the band without having to impose too restrictive limitations on the process without overly restricting the areas of use of the products which will be made from cast strips.
  • This phase transformation causes local contractions of the metal, resulting from the density differences between these two phases which are sensitive to the level microscopic. These contractions can be significant enough to cause local loss of contact between the solidified skin and the surface of the cylinder. As we understands, these losses of contact radically modify the local conditions of transfers thermal. In conjunction with the surface condition of the cylinders and the nature of the inerting gas present in the depressions of said surface, the magnitude of this phase transformation, linked to the composition of the metal, therefore influences the intensity of heat transfers.
  • ⁇ p The extent of the phase transformation ⁇ ⁇ ⁇ in ferritic stainless steels can be described by the index ⁇ p .
  • ⁇ p constituted a good indicator of the level of the heat flux extracted by the casting rolls during solidification, all other things being equal.
  • the heat flux extracted from the metal by the cylinders can be quantified experimentally by an average value, calculated from a measurement of the heating of the coolant of the cylinders.
  • the average heat flux extracted from the metal by the cylinders is lower the higher the index ⁇ p .
  • the inerting gas surrounding the surface of the liquid metal in the vicinity of the meniscus contains a gas soluble in l steel, or is entirely constituted by such a gas.
  • Nitrogen is conventionally used for this purpose, but the use of hydrogen, ammonia or CO 2 would also be possible.
  • insoluble gas ensuring the possible complement to 100% of the inerting atmosphere argon is conventionally used, but the use of another insoluble gas, such as helium, would also be possible.
  • This last parameter is represented by the average roughness Ra, defined according to standard ISO 4287-1997 by the arithmetic mean of the deviations of the roughness profile on the average line within the measurement run 1 m .
  • the mean line is defined as the line, produced by filtering, which cuts the palpated profile so that the surfaces which are greater than it are equal to those which are less than it.
  • the microcracks are absent when the nitrogen content of the inerting gas (which is a nitrogen-argon mixture) is at least 60%. All these steels have a ⁇ p index of 45.7 to 53.4%, and were cast with cylinders having an Ra of 7 or 11 ⁇ m.
  • the experiment carried out on steel C shows that, even with a Ra of 8.5 ⁇ m and an inert gas rich in nitrogen, microcracks are systematically obtained when a steel is cast with a low ⁇ p index. (29.5%).
  • the flow must first thermal extract during the first contact between the metal and the cylinder is high. If the gas is not sufficiently soluble in steel, the average heat flux extracted is too weak, the steel does not solidify fairly uniformly and this favors the appearance of micro cracks. From this point of view, it would a priori also be desirable to have a low cylinder roughness. But if the roughness Ra is too low, the number and the area total of the solidification initiation sites becomes very high, which leads to a Too brutal cooling which causes the appearance of micro cracks. In addition, it is also necessary take into account the conditions required by the following stages of the solidification process and cooling the skins. Experience shows that by combining a gas content soluble at least 60% in the inerting gas and a roughness of the cylinders Ra greater at 5 ⁇ m, satisfactory results are obtained.
  • the minimum roughness Ra of 5 ⁇ m is justified in that the roughness peaks serve as sites for initiation and development of solidification, and the parties hollow, in which the metal penetrates without necessarily going to the bottom of the hollow, act as contraction joints, absorbing changes in skin volume during its solidification and cooling. It is, however, not advisable to have a roughness Ra greater than 20 ⁇ m, because otherwise the roughness which is printed "in negative "on the tape surface is high, and will be difficult to reduce during the steps subsequent cold rolling and processing. We would therefore risk ending up with a final product whose surface appearance would not be satisfactory.
  • the roughness of the cylinders sought can be obtained by any known means for this purpose, such as shot blasting, laser machining, photoengraving, EDM, etc.
  • a high value of the index ⁇ p imposed by the composition of the metal amplifies the transformation ⁇ ⁇ ⁇ over the entire contact arc.
  • the solidified skins are therefore subjected, on said contact arc, to detachments which moderate the extracted heat flux and maintain it at a suitable level, without however leading to microcracks which would be due to the fragility of the skin, when this- this is already sufficiently solidified.
  • the lower limit to set for the index ⁇ p is 35%. Beyond a ⁇ p index of 60%, the detachments caused by the transformation ⁇ ⁇ ⁇ become too large, and lead to the appearance of microcracks by excessive embrittlement of the skins.
  • the invention therefore achieves a compromise between requirements sometimes contradictory, dictated by the need to avoid the presence on the casting tape of surface micro-cracks, with multiple formation mechanisms. She permits to dispense with the compulsory presence of expensive alloying elements (elements stabilizers such as aluminum, titanium, zirconium, niobium may be present optionally). Similarly, it does not require cooling conditions and particular winding of the strip after it has left the cylinders.

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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)
  • Treatment Of Steel In Its Molten State (AREA)
  • Refinement Of Pig-Iron, Manufacture Of Cast Iron, And Steel Manufacture Other Than In Revolving Furnaces (AREA)
  • Electroplating Methods And Accessories (AREA)
  • Coating With Molten Metal (AREA)
  • Heat Treatment Of Steel (AREA)
  • Moulds For Moulding Plastics Or The Like (AREA)

Abstract

The invention concerns a method for continuously casting a ferritic stainless steep strip with thickness not more than 10 mm directly from liquid metal between two cooled rolls with horizontal axes and driven in rotation, characterized in that: the liquid metal composition in weight percentages is as follows: % C+% N<=0.12; % Mn<=1; % P<=0.4; % Si<=1; % Mo<=2.5; % Cr between 11 and 19; A1<=1%; % Ti+%Nb+% Zr<=1; the rest being iron and the impurities resulting from preparation; the Upsilp index of the liquid metal ranges between 35% and 60%, Upsilp being defined by the formula: gammap=420% C+470% N+23% Ni+9% Cu+7% Mn 11.5% Cr 11.5% Si 12% Mo 23% V 47% Nb 49% Ti 52% A1+189: the surface roughness of said rolls being more than 5 mum; in the proximity of the meniscus metal liquid present between the rolls an inerting gas is used consisting of at least 60% by volume of a gas soluble in steel.

Description

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 ferritique dont l'épaisseur est de l'ordre de quelques mm, par le procédé dit de «coulée entre cylindres ».The invention relates to the continuous casting of metals, and more precisely the casting continuous, directly from liquid metal, of stainless steel strips of the type ferritic whose thickness is of the order of a few mm, by the process known as "casting between cylinders ".

Ces dernières années ont vu s'accomplir des progrès sensibles dans le développement des procédés de coulée de minces bandes d'acier 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 last few years have seen significant progress in development of processes for casting thin strips of carbon or stainless steel directly from liquid metal. The process mainly used today is pouring 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 is desired impart to the cast strip (for example a few mm). The casting space containing liquid steel is defined by the lateral surfaces of the cylinders, on which the solidification of the strip, and by side refractory closing plates applied against the ends of the cylinders. The liquid metal initiates its solidification on contact with external surfaces of the cylinders, on which it forms solidified "skins", of which ensures that they meet at the "neck", that is to say the area where the distance between cylinders is minimum.

Un des principaux problèmes rencontrés lors de la fabrication de bandes minces d'acier inoxydable ferritique 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 conditions de contact, lors de la solidification, entre l'acier et la surface des cylindres sur la longueur de leur arc de contact. Ces conditions peuvent être décrites comme comportant deux étapes successives. La première étape concerne le contact initial entre l'acier liquide et la surface du cylindre, qui entraíne 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 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. One of the main problems encountered in the manufacture of thin strips of ferritic stainless steel by casting between cylinders is the significant risk of appearance on the strip of surface defects called microcracks. These are cracks of small dimensions which are nevertheless sufficient to make them unfit for use cold processed products produced therefrom. They form during solidification steel and have a depth of about 40 µm and an opening of about 20 µm. Their appearance is linked to the conditions of contact, during solidification, between the steel and the surface of the cylinders along the length of their contact arc. These conditions can be described as comprising two successive stages. The first step concerns contact initial between the liquid steel and the surface of the cylinder, which leads to the formation of a skin solid steel 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 constitute the fully solidified strip. Contact between steel and cylinder surface is conditioned by the topography of the surface of the casting cylinders, combined with the nature of the inerting gas and the chemical composition of the steel. All these parameters involved in establishing heat transfers between the steel and the cylinder and govern the conditions of solidification of the skins.

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.Various attempts have been made to develop methods of casting between cylinders enabling bands to be obtained reliably free from defects in unacceptable surfaces such as micro cracks.

Les solutions évoquées dans le cas des aciers au carbone s'appuient sur la nécessité d'une bonne maítrise des échanges thermiques entre l'acier et la surface des cylindres. On cherche, en particulier, à augmenter le flux thermique extrait de l'acier, à l'amorce de sa solidification, par les cylindres de coulée. Dans ce but, le document EP-A-0 732 163 propose d'utiliser des cylindres à très faible rugosité (Ra inférieur à 5 µm), en les associant à une composition de l'acier et à des conditions d'élaboration qui favorisent la formation, au sein du métal, d'oxydes liquides qui viennent mouiller les interfaces surface de l'acier/cylindre. En ce qui concerne les aciers inoxydables austénitiques, 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 l'espace de coulée avec un gaz soluble dans l'acier, ou un mélange de gaz composé majoritairement d'un tel gaz soluble.The solutions mentioned in the case of carbon steels are based on the need for good control of the heat exchanges between the steel and the surface of the cylinders. It is sought, in particular, to increase the heat flux extracted from the steel, at the start of its solidification, by the casting rolls. To this end, the document EP-A-0 732 163 proposes using cylinders with very low roughness (Ra less than 5 μm), by associating them with a composition of the steel and with working conditions which favor the formation, within the metal, of liquid oxides which wet the steel surface / cylinder interfaces. With regard to austenitic stainless steels, document EP-A-0 796 685 teaches to cast a steel whose Cr eq / Ni eq ratio is greater than 1.55 so as to minimize the phase changes at high temperature, and to carry out this casting by using cylinders the surface of which comprises contiguous dimples of diameter 100-1500 μm and depth 20-150 μm and by inerting the casting space with a gas soluble in steel, or a mixture of gases mainly composed of such a soluble gas.

Pour les aciers inoxydables ferritiques, le document JP-A-5337612 propose de couler un acier à basses teneurs en carbone (moins de 0,05%) et azote (moins de 0,05%) et contenant du niobium (0,1 à 5%) et du titane. Il faut également refroidir la bande en sortie des cylindres à une vitesse élevée, et contrôler ensuite la température de bobinage de la bande. Ces conditions d'élaboration et de coulée sont coûteuses et contraignantes, et les caractéristiques particulières des nuances exigées limitent les domaines d'emploi des produits ainsi obtenus.For ferritic stainless steels, document JP-A-5337612 proposes cast a steel with low carbon contents (less than 0.05%) and nitrogen (less than 0.05%) and containing niobium (0.1 to 5%) and titanium. It is also necessary to cool the output strip cylinders at high speed, and then control the winding temperature of the bandaged. These preparation and pouring conditions are costly and restrictive, and the particular characteristics of the nuances required limit the fields of use of products thus obtained.

Le but de l'invention est de proposer un procédé de coulée de bandes minces en acier inoxydable ferritique dont la surface serait exempte de microcriques. Ce procédé ne nécessiterait pas de conditions de coulée particulièrement contraignantes pour sa mise en oeuvre, et pourrait s'appliquer à une large gamme de nuances de tels aciers.The object of the invention is to propose a method for casting thin strips in ferritic stainless steel, the surface of which would be free of microcracks. This process does not would not require particularly restrictive casting conditions for its implementation work, and could be applied to a wide range of grades of such steels.

A cet effet, l'invention a pour objet un procédé de coulée continue d'une bande d'acier inoxydable ferritique d'épaisseur inférieure ou égale à 10 mm directement à partir de métal liquide entre deux cylindres à axes horizontaux refroidis et mis en rotation, caractérisé en ce que :

  • le métal liquide a la composition en pourcentages pondéraux C% + N% ≤ 0,12, Mn% ≤ 1, P% ≤ 0,04, Si% ≤ 1, Mo% ≤ 2,5, Cr% compris entre 11 et 19, Al ≤ 1%, Ti% + Nb% + Zr% ≤ 1, le reste étant du fer et des impuretés résultant de l'élaboration ;
  • l'indice γp du métal liquide est compris entre 35% et 60%, γp étant défini par la formule : γp = 420 C% + 470 N% + 23 Ni% + 9 Cu% + 7 Mn% - 11,5 Cr% - 11,5 Si% - 12 Mo% - 23 V% - 47 Nb% - 49 Ti% - 52 Al% + 189 ;
  • la rugosité Ra des surfaces desdits cylindres est supérieure à 5 µm ;
  • on utilise au voisinage du ménisque du métal liquide présent entre les cylindres un gaz d'inertage composé à au moins 60% en volume d'un gaz soluble dans l'acier.
To this end, the subject of the invention is a process for the continuous casting of a strip of ferritic stainless steel of thickness less than or equal to 10 mm directly from liquid metal between two cylinders with horizontal axes cooled and rotated , characterized in that:
  • the liquid metal has the composition in percentages by weight C% + N% ≤ 0.12, Mn% ≤ 1, P% ≤ 0.04, Si% ≤ 1, Mo% ≤ 2.5, Cr% between 11 and 19 , Al ≤ 1%, Ti% + Nb% + Zr% ≤ 1, the remainder being iron and impurities resulting from the production;
  • the index γ p of the liquid metal is between 35% and 60%, γ p being defined by the formula: γ p = 420 C% + 470 N% + 23 Ni% + 9 Cu% + 7 Mn% - 11.5 Cr% - 11.5 Si% - 12 Mo% - 23 V% - 47 Nb% - 49 Ti% - 52 Al% + 189;
  • the roughness Ra of the surfaces of said cylinders is greater than 5 μm;
  • an inert gas composed at least 60% by volume of a gas soluble in steel is used in the vicinity of the meniscus of the liquid metal present between the cylinders.

Comme on l'aura compris, l'invention consiste à combiner des conditions sur la composition du métal, régissant les possibilités de formation d'austénite à haute température postérieurement à la solidification du métal, une condition sur la rugosité minimale des surfaces de coulée et une condition sur la composition du gaz d'inertage. En respectant cette combinaison, on parvient à éviter la formations de microcriques en surface de la bande sans pour autant devoir imposer des limitations trop contraignantes au process de coulée et sans restreindre trop largement les domaines d'utilisation des produits qui seront fabriqués à partir des bandes coulées.As will be understood, the invention consists in combining conditions on the composition of the metal, governing the possibilities of high austenite formation temperature after metal solidification, a condition on roughness minimum pour surfaces and a condition on the composition of the inerting gas. In respecting this combination, we manage to avoid the formation of micro-cracks on the surface of the band without having to impose too restrictive limitations on the process without overly restricting the areas of use of the products which will be made from cast strips.

L'invention sera mieux comprise à la lecture de la description détaillée qui suit.The invention will be better understood on reading the detailed description which follows.

Un des paramètres essentiels de la réussite d'une coulée de bandes minces entre cylindres est la maítrise des échanges thermiques entre la bande en cours de solidification et les cylindres. Une bonne maítrise de ces transferts exige que les conditions d'adhérence des peaux solidifiées sur les parois des cylindres soient connues et reproductibles. Or, lors de la coulée des bandes d'acier inoxydable ferritique contenant 11 à 19% de chrome, il se produit après la solidification complète de la peau contre le cylindre le phénomène suivant. La peau solidifiée présente d'abord une structure entièrement ferritique (phase δ), puis au cours de son refroidissement, alors qu'elle adhère toujours à la surface du cylindre, elle subit une transformation de phase ferrite δ-austénite γ dans une gamme de températures de 1300-1400°C. Cette transformation de phase provoque des contractions locales du métal, résultant des différences de densité entre ces deux phases qui sont sensibles au niveau microscopique. Ces contractions peuvent être suffisamment importantes pour entraíner des pertes locales de contact entre la peau solidifiée et la surface du cylindre. Comme on le comprend, ces pertes de contact modifient radicalement les conditions locales des transferts thermiques. Conjointement à l'état de surface des cylindres et à la nature du gaz d'inertage présent dans les dépressions de ladite surface, l'ampleur de cette transformation de phase, liée à la composition du métal, influence donc l'intensité des transferts thermiques.One of the essential parameters for the success of a thin strip casting between cylinders is the control of heat exchanges between the strip being solidified and the cylinders. A good control of these transfers requires that the conditions of adhesion skins solidified on the walls of the cylinders are known and reproducible. Now, when from the casting of ferritic stainless steel strips containing 11 to 19% of chromium, it produced after the complete solidification of the skin against the cylinder the following phenomenon. The solidified skin first has an entirely ferritic structure (phase δ), then at during its cooling, while it still adheres to the surface of the cylinder, it undergoes a δ-austenite γ ferrite phase transformation in a temperature range of 1300-1400 ° C. This phase transformation causes local contractions of the metal, resulting from the density differences between these two phases which are sensitive to the level microscopic. These contractions can be significant enough to cause local loss of contact between the solidified skin and the surface of the cylinder. As we understands, these losses of contact radically modify the local conditions of transfers thermal. In conjunction with the surface condition of the cylinders and the nature of the inerting gas present in the depressions of said surface, the magnitude of this phase transformation, linked to the composition of the metal, therefore influences the intensity of heat transfers.

L'ampleur de la transformation de phase δ → γ dans les aciers inoxydables ferritiques peut être décrite par l'indice γp. Celui-ci représente la quantité maximale d'austénite présente dans le métal à haute température. Cet indice γp est calculé, de manière connue, à partir de la composition du métal, selon la relation dite « de Tricot et Castro » (les pourcentages sont des pourcentages pondéraux) : γp = 420 C% + 470 N% + 23 Ni% + 9 Cu% + 7 Mn% - 11,5 Cr% - 11,5 Si% - 12 Mo% - 23 V% - 47 Nb% - 49 Ti% - 52 Al% + 189 The extent of the phase transformation δ → γ in ferritic stainless steels can be described by the index γ p . This represents the maximum amount of austenite present in the metal at high temperature. This index γ p is calculated, in a known manner, from the composition of the metal, according to the so-called "knitting and caster" relation (the percentages are percentages by weight): γ p = 420 C% + 470 N% + 23 Ni% + 9 Cu% + 7 Mn% - 11.5 Cr% - 11.5 Si% - 12 Mo% - 23 V% - 47 Nb% - 49 Ti% - 52 Al% + 189

Lors des études qui ont conduit à l'invention, on s'est aperçu que la valeur de γp constituait un bon indicateur du niveau du flux thermique extrait par les cylindres de coulée lors de la solidification, toutes choses étant égales par ailleurs. Le flux thermique extrait du métal par les cylindres peut être quantifié expérimentalement par une valeur moyenne, calculée à partir d'une mesure de l'échauffement du fluide de refroidissement des cylindres. L'expérience montre que le flux thermique moyen extrait du métal par les cylindres est d'autant plus faible que l'indice γp a une valeur élevée.During the studies which led to the invention, it was found that the value of γ p constituted a good indicator of the level of the heat flux extracted by the casting rolls during solidification, all other things being equal. The heat flux extracted from the metal by the cylinders can be quantified experimentally by an average value, calculated from a measurement of the heating of the coolant of the cylinders. Experience shows that the average heat flux extracted from the metal by the cylinders is lower the higher the index γ p .

Une condition nécessaire pour éviter l'apparition de criques sur des bandes minces en acier inoxydable ferritique coulées entre cylindres est que, lors du contact initial entre le métal liquide et les cylindres, le flux thermique extrait soit élevé. A cet effet, il est préférable que le gaz d'inertage environnant la surface du métal liquide au voisinage du ménisque (nom que l'on donne à l'intersection entre les surfaces du métal liquide et des cylindres) contienne un gaz soluble dans l'acier, ou soit entièrement constitué par un tel gaz. On utilise classiquement à cet effet de l'azote, mais l'utilisation d'hydrogène, d'ammoniac ou de CO2 serait aussi envisageable. Comme gaz insoluble assurant l'éventuel complément à 100% de l'atmosphère d'inertage, on utilise classiquement de l'argon, mais l'utilisation d'un autre gaz insoluble, tel que l'hélium, serait aussi envisageable. Avec un gaz majoritairement soluble dans l'acier, on réalise un meilleur contact entre l'acier et le cylindres, car un gaz insoluble modère davantage qu'un gaz soluble la pénétration du métal dans les dépressions de la surface du cylindre. De même, une faible rugosité de la surface des cylindres procure un flux thermique élevé car il en résulte un contact étroit entre le cylindre et le métal.A necessary condition to avoid the appearance of cracks on thin strips of ferritic stainless steel cast between cylinders is that, during the initial contact between the liquid metal and the cylinders, the extracted heat flux is high. For this purpose, it is preferable that the inerting gas surrounding the surface of the liquid metal in the vicinity of the meniscus (name given to the intersection between the surfaces of the liquid metal and the cylinders) contains a gas soluble in l steel, or is entirely constituted by such a gas. Nitrogen is conventionally used for this purpose, but the use of hydrogen, ammonia or CO 2 would also be possible. As insoluble gas ensuring the possible complement to 100% of the inerting atmosphere, argon is conventionally used, but the use of another insoluble gas, such as helium, would also be possible. With a gas which is mainly soluble in steel, better contact is made between the steel and the cylinders, since an insoluble gas moderates more than a soluble gas the penetration of the metal into the depressions on the surface of the cylinder. Likewise, a low roughness of the surface of the cylinders provides a high heat flux since this results in close contact between the cylinder and the metal.

Toutefois, postérieurement à l'initiation de la solidification, un flux thermique moyen très élevé augmente les risques d'hétérogénéités entre les valeurs locales de ce flux. Or, ces hétérogénéités peuvent être à l'origine de criques superficielles sur la bande, car elles provoquent des tensions entre les différentes zones de la surface, qui est encore fragile. Il y aurait donc, si possible, un compromis à trouver entre les différents impératifs à respecter sur les conditions de coulée, si on veut éviter la formation de microcriques lors de toutes les étapes de la solidification et du refroidissement des peaux contre les cylindres.However, after the initiation of solidification, a thermal flux very high average increases the risk of heterogeneity between the local values of this flow. However, these heterogeneities can cause surface cracks on the strip, because they cause tension between the different areas of the surface, which is still brittle. There would therefore be, if possible, a compromise to be found between the different imperatives to respect on the casting conditions, if we want to avoid the formation of microcracks during all stages of solidification and cooling of the skins against the cylinders.

A cet effet, on a expérimenté différentes conditions de coulée de bandes d'acier inoxydable ferritique à partir de métal liquide. Les expériences ont eu lieu en coulant des bandes de 2,9 à 3,4 mm d'épaisseur entre des cylindres dont les surfaces extérieures refroidies par circulation interne d'eau étaient en cuivre et revêtues de nickel. Le tableau 1 suivant montre les compositions du métal coulé lors des différents essais (désignées de A à F), et les valeurs de l'indice γp correspondantes, et le tableau 2 présente les résultats obtenus lors des différents essais, en termes de qualité de surface obtenue, en fonction de la composition de l'acier, de la composition du gaz d'inertage et de la rugosité des cylindres. Ce dernier paramètre est représenté par la rugosité moyenne Ra, définie selon la norme ISO 4287-1997 par la moyenne arithmétique des écarts du profil de rugosité sur la ligne moyenne au sein de la course de mesure 1m. La ligne moyenne est définie comme étant la ligne, produite par filtrage, qui coupe le profil palpé de telle sorte que les surfaces qui lui sont supérieures soient égales à celles qui lui sont inférieures. Selon cette définition :

Figure 00050001
Figure 00050002
Influence des paramètres de coulée sur la présence de microcriques Acier γp (%) N2% dans gaz d'inertage Ra (µm) Qualité de surface A 20 microcriques A 52,1 50 7 microcriques A 60 pas de microcriques A 95 pas de microcriques B 20 microcriques B 45,7 50 11 microcriques B 60 pas de microcriques B 95 pas de microcriques C 20 microcriques C 29,5 60 8,5 microcriques C 95 microcriques D 62,0 90 7,5 microcriques E 42,3 90 4 microcriques F 53,4 60 7 pas de microcriques To this end, various conditions for casting ferritic stainless steel strips from liquid metal were tested. The experiments were carried out by pouring bands of 2.9 to 3.4 mm thick between cylinders whose outer surfaces cooled by internal circulation of water were made of copper and coated with nickel. Table 1 below shows the compositions of the metal cast during the various tests (designated from A to F), and the corresponding values of the index γ p , and Table 2 presents the results obtained in the various tests, in terms of quality. surface area obtained, depending on the composition of the steel, the composition of the inerting gas and the roughness of the cylinders. This last parameter is represented by the average roughness Ra, defined according to standard ISO 4287-1997 by the arithmetic mean of the deviations of the roughness profile on the average line within the measurement run 1 m . The mean line is defined as the line, produced by filtering, which cuts the palpated profile so that the surfaces which are greater than it are equal to those which are less than it. According to this definition:
Figure 00050001
Figure 00050002
Influence of the casting parameters on the presence of microcracks Steel γ p (%) N 2 % in inerting gas Ra (µm) Surface quality AT 20 microcracks AT 52.1 50 7 microcracks AT 60 no micro cracks AT 95 no micro cracks B 20 microcracks B 45.7 50 11 microcracks B 60 no micro cracks B 95 no micro cracks VS 20 microcracks VS 29.5 60 8.5 microcracks VS 95 microcracks D 62.0 90 7.5 microcracks E 42.3 90 4 microcracks F 53.4 60 7 no micro cracks

Pour les aciers A, B et F, les microcriques sont absentes lorsque la teneur en azote du gaz d'inertage (qui est un mélange azote-argon) est d'au moins 60%. Tous ces aciers ont un indice γp de 45,7 à 53,4%, et ont été coulés avec des cylindres ayant un Ra de 7 ou 11 µm.For steels A, B and F, the microcracks are absent when the nitrogen content of the inerting gas (which is a nitrogen-argon mixture) is at least 60%. All these steels have a γ p index of 45.7 to 53.4%, and were cast with cylinders having an Ra of 7 or 11 µm.

L'expérience menée sur l'acier C montre que, même avec un Ra de 8,5 µm et un gaz d'inertage riche en azote, on obtient systématiquement des microcriques lorsqu'on coule un acier dont l'indice γp est bas (29,5%). L'expérience menée sur l'acier D, dont l'indice γp est de 62,0%, montre qu'à l'inverse, on obtient aussi des microcriques lorsque l'acier coulé a un indice γp très élevé.The experiment carried out on steel C shows that, even with a Ra of 8.5 μm and an inert gas rich in nitrogen, microcracks are systematically obtained when a steel is cast with a low γ p index. (29.5%). The experiment carried out on steel D, whose index γ p is 62.0%, shows that, conversely, microcracks are also obtained when the cast steel has a very high γ p index.

L'expérience menée sur l'acier E montre que même lorsque les conditions de composition de l'acier et d'inertage sont convenables au vu des essais précédents, une faible rugosité des cylindres (Ra de 4 µm) conduit à l'apparition de microcriques.Experience with steel E shows that even when the conditions of steel composition and inerting are suitable in view of the previous tests, a low roughness of the cylinders (Ra of 4 µm) leads to the appearance of microcracks.

On explique ces différents résultats de la manière suivante.These different results are explained as follows.

Pour obtenir une bande exempte de criques, il faut en premier lieu que le flux thermique extrait lors du premier contact entre le métal et le cylindre soit élevé. Si le gaz d'inertage n'est pas suffisamment soluble dans l'acier, le flux thermique moyen extrait est trop faible, l'acier ne se solidifie pas de façon assez homogène et cela favorise l'apparition de microcriques. De ce point de vue, il serait a priori également désirable d'avoir une rugosité des cylindres faible. Mais si la rugosité Ra est trop faible, le nombre et la surface totale des sites d'amorçage de la solidification devient très élevé, ce qui conduit à un refroidissement trop brutal qui provoque l'apparition de microcriques. De plus, il faut aussi tenir compte des conditions requises par les étapes suivantes du processus de solidification et de refroidissement des peaux. L'expérience montre qu'en combinant une teneur en gaz soluble d'au moins 60% dans le gaz d'inertage et une rugosité des cylindres Ra supérieure à 5 µm, on obtient des résultats satisfaisants.To obtain a strip free from cracks, the flow must first thermal extract during the first contact between the metal and the cylinder is high. If the gas is not sufficiently soluble in steel, the average heat flux extracted is too weak, the steel does not solidify fairly uniformly and this favors the appearance of micro cracks. From this point of view, it would a priori also be desirable to have a low cylinder roughness. But if the roughness Ra is too low, the number and the area total of the solidification initiation sites becomes very high, which leads to a Too brutal cooling which causes the appearance of micro cracks. In addition, it is also necessary take into account the conditions required by the following stages of the solidification process and cooling the skins. Experience shows that by combining a gas content soluble at least 60% in the inerting gas and a roughness of the cylinders Ra greater at 5 μm, satisfactory results are obtained.

Dans la suite du processus de solidification et de refroidissement des peaux contre les cylindres, il faut, comme on l'a dit, éviter d'avoir un flux extrait trop intense afin d'éviter les hétérogénéités thermiques, qui sont elles aussi sources de microcriques. De ce point de vue, la rugosité minimale Ra de 5 µm se justifie en ce que les pics de rugosité servent de sites d'amorçage et de développement de la solidification, et les parties en creux, dans lesquelles le métal pénètre sans forcêment aller jusqu'au fond des creux, agissent comme des joints de contraction, absorbant les variations de volume de la peau lors de sa solidification et de son refroidissement. Il n'est, cependant, pas conseillé d'avoir une rugosité Ra supérieure à 20 µm, car sinon la rugosité qui se retrouve imprimée «en négatif» sur la surface de la bande est élevée, et sera difficile à réduire lors des étapes ultérieures de laminage et transformation à froid. On risquerait, donc, de se retrouver avec un produit final dont l'aspect de surface ne serait pas satisfaisant. La rugosité des cylindres recherchée peut être obtenue par tout moyen connu à cet effet, tel qu'un grenaillage, un usinage laser, une opération de photogravure, d'électroérosion, etc.In the continuation of the process of solidification and cooling of the skins against the cylinders, we must, as we said, avoid having an excessively intense extract flow in order avoid thermal heterogeneities, which are also sources of micro-cracks. From this point of view, the minimum roughness Ra of 5 µm is justified in that the roughness peaks serve as sites for initiation and development of solidification, and the parties hollow, in which the metal penetrates without necessarily going to the bottom of the hollow, act as contraction joints, absorbing changes in skin volume during its solidification and cooling. It is, however, not advisable to have a roughness Ra greater than 20 µm, because otherwise the roughness which is printed "in negative "on the tape surface is high, and will be difficult to reduce during the steps subsequent cold rolling and processing. We would therefore risk ending up with a final product whose surface appearance would not be satisfactory. The roughness of the cylinders sought can be obtained by any known means for this purpose, such as shot blasting, laser machining, photoengraving, EDM, etc.

Une forte valeur de l'indice γp imposée par la composition du métal, amplifie la transformation δ → γ sur l'ensemble de l'arc de contact. Les peaux solidifiées sont donc soumises, sur ledit arc de contact, à des décollements qui modèrent le flux thermique extrait et le maintiennent à un niveau convenable, sans pour autant conduire à des microcriques qui seraient dues à la fragilité de la peau, lorsque celle-ci est déjà suffisamment solidifiée. L'expérience montre que la limite inférieure à fixer pour l'indice γp est de 35%. Au-delà d'un indice γp de 60%, les décollements provoqués par la transformation δ → γ deviennent trop importants, et conduisent à l'apparition de microcriques par fragilisation excessive des peaux.A high value of the index γ p imposed by the composition of the metal, amplifies the transformation δ → γ over the entire contact arc. The solidified skins are therefore subjected, on said contact arc, to detachments which moderate the extracted heat flux and maintain it at a suitable level, without however leading to microcracks which would be due to the fragility of the skin, when this- this is already sufficiently solidified. Experience shows that the lower limit to set for the index γ p is 35%. Beyond a γ p index of 60%, the detachments caused by the transformation δ → γ become too large, and lead to the appearance of microcracks by excessive embrittlement of the skins.

L'invention réalise donc un compromis entre des exigences parfois contradictoires, dictées par la nécessité d'éviter la présence sur la bande coulée de microcriques superficielles, dont les mécanismes de formation sont multiples. Elle permet de se passer de la présence obligatoire d'éléments d'alliage coûteux (des éléments stabilisants tels que l'aluminium, le titane, le zirconium, le niobium peuvent être présents de manière optionnelle). De même elle ne nécessite pas de conditions de refroidissement et de bobinage particulières de la bande après que celle-ci a quitté les cylindres.The invention therefore achieves a compromise between requirements sometimes contradictory, dictated by the need to avoid the presence on the casting tape of surface micro-cracks, with multiple formation mechanisms. She permits to dispense with the compulsory presence of expensive alloying elements (elements stabilizers such as aluminum, titanium, zirconium, niobium may be present optionally). Similarly, it does not require cooling conditions and particular winding of the strip after it has left the cylinders.

Claims (3)

  1. A method of continuously casting a ferritic stainless steel strip of a thickness less than or equal to 10 mm, directly from liquid metal, between two cooled rotated cylinders with horizontal axes, characterised in that:
    the liquid metal has the composition in percent by weight C% + N% ≤ 0.12, Mn% ≤ 1, P% ≤ 0.04, Si% ≤ 1, Mo% ≤ 2.5, Cr% between 11 and 19, Al ≤ 1%, Ti% + Nb% + Zr% ≤ 1, the remainder being iron and impurities resulting from preparation;
    the γp index of the liquid metal is between 35% and 60%, γp being defined by the formula: γp = 420 C% + 470 N% + 23 Ni% + 9 Cu% + 7 Mn% - 11.5 Cr% - 11.5 Si% - 12 Mo%- 23 V% - 47 Nb% - 49 Ti% - 52 Al% + 189;
    the roughness Ra of the surfaces of the said cylinders is greater than 5 µm;
    in the proximity of the meniscus of the liquid metal present between the cylinders, use is made of an inert gas formed of at least 60% by volume of a gas soluble in steel.
  2. A method according to Claim 1 or 2, characterised in that the inert gas is a mixture of nitrogen and argon, in respective percentages of 60-100% and 0-30%.
  3. A method according to any one of Claims 1 to 3, characterised in that the roughness Ra of the surfaces of the cylinders is between 5 and 20 µm.
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