EP0609190A1 - Method of manufacturing a hot rolled steel sheet with high magnetic properties - Google Patents

Method of manufacturing a hot rolled steel sheet with high magnetic properties Download PDF

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Publication number
EP0609190A1
EP0609190A1 EP94870013A EP94870013A EP0609190A1 EP 0609190 A1 EP0609190 A1 EP 0609190A1 EP 94870013 A EP94870013 A EP 94870013A EP 94870013 A EP94870013 A EP 94870013A EP 0609190 A1 EP0609190 A1 EP 0609190A1
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Prior art keywords
temperature
hot
sheet
slab
process according
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EP94870013A
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German (de)
French (fr)
Inventor
Pierre Messien
Jean-Claude Herman
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Centre de Recherches Metallurgiques CRM ASBL
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Centre de Recherches Metallurgiques CRM ASBL
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1216—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties the working step(s) being of interest
    • C21D8/1222—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1216—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties the working step(s) being of interest
    • C21D8/1233—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1244—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties the heat treatment(s) being of interest

Definitions

  • the present invention relates to a method of manufacturing a hot-rolled steel sheet, having high magnetic properties, in particular very low watt losses.
  • the watt losses, or magnetic losses, of a steel are constituted by the energy dissipated in the form of heat in the steel when the latter is subjected to a variable magnetic field. These losses occur in electrical machines such as motors and transformers, where the magnetic field varies in an alternating cyclic fashion. Watts losses are broken down into hysteresis losses, which are proportional to the area of the hysteresis curve, and eddy current losses, which are proportional to the square of the induction.
  • the magnetic properties of a steel depend on its chemical as well as physical characteristics.
  • the chemical composition has a significant influence on the magnetic permeability and the coercive field.
  • the effect of alloying elements is well known in the art. For information, we will briefly recall here the action of the alloying elements most commonly encountered in steels for magnetic sheets. Carbon has a predominant effect, which is particularly marked for contents lower than 0.006%; it decreases the magnetic permeability and it increases the coercive field, and therefore the losses in watts. Carbon is also harmful not only in the form of interstitial atoms, but also in the form of iron carbides of reduced size and finely dispersed. Nitrogen plays essentially the same role as carbon, both if it is in solution and if it is present in the form of fine precipitates.
  • the elements present in solid solution have various effects. Phosphorus increases magnetic losses; its action is very marked for contents less than 0.02%. At contents between 0.5% and 5%, silicon reduces magnetic losses thanks to its double action of reducing losses by hysteresis and eddy currents by increasing the resistivity of steel. Overall, manganese reduces total magnetic losses by reducing losses by eddy currents by an increase in resistivity, despite an increase in losses by hysteresis. In its frequent form of MnS sulfide, sulfur increases magnetic losses; on the other hand, if it is in solution, the sulfur contributes to reducing the losses by eddy currents. Aluminum, partially present in the form of oxide (Al2O3) or nitride (AlN), has the same effect as sulfur. Chromium reduces total magnetic losses, as long as it does not form carbides.
  • the magnetic properties also depend to a large extent on the heterogeneities present in the steel.
  • the presence of inclusions or precipitates, as well as a finer grain, have the effect of increasing the losses in watts and the coercive field and of decreasing the relative magnetic permeability.
  • the optimal grain size is approximately 100 ⁇ m, because the losses by hysteresis increase and that the losses by eddy currents decrease when the size of the grains decreases.
  • internal stresses, crystal boundaries and crystal orientation of the grains play an important role, due to the anisotropy of the phenomena. In this regard, the crystal texture (100) of the steel is best suited to minimize the watt losses.
  • a steel having a high magnetic permeability is used.
  • steels having magnetic properties adapted to each type of application are currently used, by varying the chemical composition of the steel, by modifying its crystalline texture and by seeking a high grain size.
  • the object of the present invention is to propose a method for manufacturing a hot-rolled steel sheet having magnetic properties, the values of which, but also the uniformity, are clearly improved compared with the magnetic steels of the prior art.
  • a method of manufacturing a hot-rolled steel sheet, having high magnetic properties which comprises a finishing rolling operation in the ferritic field, is characterized in that a steel slab up to a temperature less than or equal to 1100 ° C., in that this slab is subjected to a roughing rolling operation in the austenitic domain up to an intermediate thickness, in that one cools said slab to a temperature below Ac3 to effect the phase transformation of austenite into ferrite, in that said slab is subjected to a finishing rolling operation in the ferritic field by means of rolling cylinders hot lubricated by a lubricant resistant to high temperature, to form a hot sheet having a thickness between 0.4 mm and 10 mm, in that one coils this sheet to c haud at a temperature below 850 ° C, and in that said coil is then cooled to room temperature.
  • a slab of continuously cast steel is generally used.
  • the reheating temperature limited to 1100 ° C. makes it possible to limit the re-solution of the nitrides AlN in the steels calmed with aluminum, the dissolution of the sulphide MnS in the steels with low manganese content as well as its reprecipitation in fine and dispersed form. , which would be penalizing for the subject of the invention.
  • the roughing rolling in the austenitic field has the effect of giving the product an intermediate thickness, which depends in particular on the thickness of the initial slab and the thickness of the final sheet.
  • the intermediate thickness can be between 8 mm and 40 mm.
  • phase transformation of austenite into ferrite takes place during the transfer of the product from the roughing rolling train to the finishing rolling train. In general, this transformation takes place completely for ultra-low carbon steels, without the need for additional cooling. In some cases, it may not be complete, in particular in steels with a higher carbon content; the cooling of the product can then be accelerated by any suitable means, in order to complete the phase transformation before the product enters the hot finish rolling train.
  • the finishing hot rolling must be mainly carried out in the ferritic field.
  • the optimal temperature range is between Ac3 and 650 ° C and preferably between 850 ° C and 750 ° C.
  • the reduction rate is at least 90%
  • the range of rolling temperatures in the ferritic region as well as the work hardenings carried out in the successive rolling passes are optimally chosen so that, on the coiled sheet, a microstructure with large ferritic grains, the size of which develops is preferably between 50 ⁇ m and 120 ⁇ m and on the other hand a majority crystalline texture (100).
  • Rolling in the ferritic field with lubricated rolling rolls makes it possible to very significantly reduce the shearing of the sheet metal surfaces in contact with the rolls. Therefore, dynamic recrystallization under the surface of the laminated sheet is largely avoided and the texture (110) is no longer created in this area.
  • the conditions of application of the lubricating agent as well as the quantities used can be optimized as a function of the temperature of the product, in order to eliminate the aforementioned shearing and its consequences, and thus to produce a sheet having a microstructure and a homogeneous crystalline texture on the surface. 'thickness.
  • the sheet is rolled in the ferritic region to a thickness of between 0.4 mm and 1.5 mm.
  • the winding temperature of the hot-rolled sheet is advantageously between 800 ° C and 600 ° C, and more preferably between 750 ° C and 700 ° C.
  • the winding in this temperature range allows, after recrystallization of the deformed ferrite, to achieve the growth of the ferrite grains to an optimal size.
  • This winding also makes it possible to ensure the precipitation of carbides and nitrides - for the fractions which may be dissolved - in a coarse form, which is not detrimental to the magnetic properties of the sheet.
  • the winding is advantageously carried out immediately after the finishing rolling, to avoid the risk of uncontrolled deformation of the sheet, in particular for sheets of very small thickness.
  • the sheet can be hot-rolled at temperatures which can be markedly lower, that is to say between 650 ° C. and 20 ° C., to produce a steel not recrystallized. This is then, without intermediate work hardening, annealed at a high temperature in the ferritic region, that is to say between 600 ° C and 800 ° C, and preferably between 700 ° C and 750 ° C, then the sheet metal is cooled to room temperature. It is thus possible to develop the optimal microstructure for the magnetic properties.
  • the aforementioned annealing of the sheet can be carried out either continuously, optionally combining a pickling of the sheets with hydrogen, or in coils.
  • the hot-rolled sheet which is in a coil in the recrystallized or work-hardened state, can be subjected to a limited work hardening, the rate of which is between 1% and 35% and preferably between 5% and 20%, then a final annealing according to the process described in the previous variant.
  • the process of the invention is preferably applied to steels of the ELC type, that is to say with a carbon content of less than 0.03%, and of the ULC type, that is to say with a carbon content of less than 0.005%.
  • Such low carbon contents are generally obtained by a vacuum decarburization operation at the steelworks.
  • columns 9 to 12 have two values, which reflect the situation respectively in the skin and in the heart of a sheet having a heterogeneous structure.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Thermal Sciences (AREA)
  • Manufacturing & Machinery (AREA)
  • Electromagnetism (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Soft Magnetic Materials (AREA)
  • Manufacturing Of Steel Electrode Plates (AREA)

Abstract

A slab of steel is heated up to a temperature less than or equal to 1100@C, this slab is subjected to a roughing rolling operation in the austenitic region down to an intermediate thickness of between 8 mm and 40 mm, and this slab is cooled down to a temperature below Ac3 in order to carry out the austenite-to-ferrite phase transformation. Next, the slab is subjected to a finishing rolling operation in the ferritic range by means of hot-rolling rolls lubricated with a lubricant able to withstand high temperature, in order to form a hot-rolled sheet having a thickness of between 0.4 mm and 10 mm, this hot-rolled sheet is coiled at a temperature below 850@C, and the coil is then cooled down to ambient temperature.

Description

La présente invention concerne un procédé de fabrication d'une tôle d'acier laminée à chaud, présentant des propriétés magnétiques élevées, en particulier des pertes en watts très faibles.The present invention relates to a method of manufacturing a hot-rolled steel sheet, having high magnetic properties, in particular very low watt losses.

On sait que les pertes en watts, ou pertes magnétiques, d'un acier sont constituées par l'énergie dissipée sous forme de chaleur dans l'acier lorsque celui-ci est soumis à un champ magnétique variable. Ces pertes se produisent dans les machines électriques telles que les moteurs et les transformateurs, où le champ magnétique varie de façon cyclique alternée. Les pertes en watts se décomposent en pertes par hystérésis, qui sont proportionnelles à l'aire de la courbe d'hystérésis, et en pertes par courants de Foucault, qui sont proportionnelles au carré de l'induction.It is known that the watt losses, or magnetic losses, of a steel are constituted by the energy dissipated in the form of heat in the steel when the latter is subjected to a variable magnetic field. These losses occur in electrical machines such as motors and transformers, where the magnetic field varies in an alternating cyclic fashion. Watts losses are broken down into hysteresis losses, which are proportional to the area of the hysteresis curve, and eddy current losses, which are proportional to the square of the induction.

Les propriétés magnétiques d'un acier dépendent de ses caractéristiques tant chimiques que physiques. La composition chimique exerce en effet une influence sensible sur la perméabilité magnétique et le champ coercitif. L'effet des éléments d'alliage est bien connu dans la technique. A titre d'information, on rappellera brièvement ici l'action des éléments d'alliage le plus couramment rencontrés dans les aciers pour tôles magnétiques. Le carbone a un effet prépondérant, qui est particulièrement marqué pour des teneurs inférieures à 0,006 %; il diminue la perméabilité magnétique et il augmente le champ coercitif, et donc les pertes en watts. Le carbone est d'ailleurs néfaste non seulement sous la forme d'atomes interstitiels, mais aussi sous la forme de carbures de fer de taille réduite et finement dispersés. L'azote joue sensiblement le même rôle que le carbone, aussi bien s'il se trouve en solution que s'il est présent sous la forme de fins précipités. Les éléments présents en solution solide ont des effets variés. Le phosphore augmente les pertes magnétiques; son action est très marquée pour des teneurs inférieures à 0,02 %. A des teneurs comprises entre 0,5 % et 5 %, le silicium diminue les pertes magnétiques grâce à sa double action de diminution des pertes par hystérésis et par courants de Foucault en augmentant la résistivité de l'acier. Globalement, le manganèse diminue les pertes magnétiques totales en diminuant les pertes par courants de Foucault par une augmentation de la résistivité, malgré une augmentation des pertes par hystérésis. Sous sa forme fréquente de sulfure MnS, le soufre augmente les pertes magnétiques; en revanche, s'il se trouve en solution, le soufre contribue à réduire les pertes par courants de Foucault. L'aluminium, présent partiellement sous forme d'oxyde (Al₂O₃) ou de nitrure (AlN), a le même effet que le soufre. Le chrome diminue les pertes magnétiques totales, pour autant qu'il ne forme pas de carbures.The magnetic properties of a steel depend on its chemical as well as physical characteristics. The chemical composition has a significant influence on the magnetic permeability and the coercive field. The effect of alloying elements is well known in the art. For information, we will briefly recall here the action of the alloying elements most commonly encountered in steels for magnetic sheets. Carbon has a predominant effect, which is particularly marked for contents lower than 0.006%; it decreases the magnetic permeability and it increases the coercive field, and therefore the losses in watts. Carbon is also harmful not only in the form of interstitial atoms, but also in the form of iron carbides of reduced size and finely dispersed. Nitrogen plays essentially the same role as carbon, both if it is in solution and if it is present in the form of fine precipitates. The elements present in solid solution have various effects. Phosphorus increases magnetic losses; its action is very marked for contents less than 0.02%. At contents between 0.5% and 5%, silicon reduces magnetic losses thanks to its double action of reducing losses by hysteresis and eddy currents by increasing the resistivity of steel. Overall, manganese reduces total magnetic losses by reducing losses by eddy currents by an increase in resistivity, despite an increase in losses by hysteresis. In its frequent form of MnS sulfide, sulfur increases magnetic losses; on the other hand, if it is in solution, the sulfur contributes to reducing the losses by eddy currents. Aluminum, partially present in the form of oxide (Al₂O₃) or nitride (AlN), has the same effect as sulfur. Chromium reduces total magnetic losses, as long as it does not form carbides.

Les propriétés magnétiques dépendent également, dans une large mesure, des hétérogénéités présentes dans l'acier. La présence d'inclusions ou de précipités, ainsi qu'un grain plus fin, ont pour effet d'augmenter les pertes en watts et le champ coercitif et de diminuer la perméabilité magnétique relative. On sait en particulier que la taille de grain optimale est d'environ 100 µm, parce que les pertes par hystérésis augmentent et que les pertes par courants de Foucault diminuent lorsque la taille des grains décroît. De même, les contraintes internes, les frontières cristallines et l'orientation cristalline des grains jouent un rôle important, en raison de l'anisotropie des phénomènes. A cet égard, la texture cristalline (100) de l'acier est la mieux appropriée pour minimiser les pertes en watts.The magnetic properties also depend to a large extent on the heterogeneities present in the steel. The presence of inclusions or precipitates, as well as a finer grain, have the effect of increasing the losses in watts and the coercive field and of decreasing the relative magnetic permeability. It is known in particular that the optimal grain size is approximately 100 μm, because the losses by hysteresis increase and that the losses by eddy currents decrease when the size of the grains decreases. Likewise, internal stresses, crystal boundaries and crystal orientation of the grains play an important role, due to the anisotropy of the phenomena. In this regard, the crystal texture (100) of the steel is best suited to minimize the watt losses.

Dans les conditions actuelles, un acier placé dans un environnement magnétique doit présenter des propriétés adaptées à chaque application particulière.Under current conditions, a steel placed in a magnetic environment must have properties adapted to each particular application.

Pour assurer un effet de blindage, on utilise de préférence un acier ayant une perméabilité magnétique élevée. Par contre, dans une machine tournante, on cherchera à minimiser les pertes en watts totales, de façon à réduire la puissance installée.To ensure a shielding effect, preferably a steel having a high magnetic permeability is used. On the other hand, in a rotating machine, we will seek to minimize the total watt losses, so as to reduce the installed power.

Pour atteindre ces objectifs, on utilise actuellement des aciers présentant des propriétés magnétiques adaptées à chaque type d'application, en faisant varier la composition chimique de l'acier, en modifiant sa texture cristalline et en recherchant une taille de grains élevée.To achieve these objectives, steels having magnetic properties adapted to each type of application are currently used, by varying the chemical composition of the steel, by modifying its crystalline texture and by seeking a high grain size.

La présente invention a pour objet de proposer un procédé de fabrication d'une tôle d'acier laminée à chaud présentant des propriétés magnétiques dont non seulement les valeurs mais aussi l'homognéité sont nettement améliorées par rapport aux aciers magnétiques de la technique antérieure.The object of the present invention is to propose a method for manufacturing a hot-rolled steel sheet having magnetic properties, the values of which, but also the uniformity, are clearly improved compared with the magnetic steels of the prior art.

Conformément à la présente invention, un procédé de fabrication d'une tôle d'acier laminée à chaud, présentant des propriétés magnétiques élevées, qui comprend une opération de laminage de finition dans le domaine ferritique, est caractérisé en ce que l'on réchauffe une brame d'acier jusqu'à une température inférieure ou égale à 1100°C, en ce que l'on soumet cette brame à une opération de laminage de dégrossissage dans le domaine austénitique jusqu'à une épaisseur intermédiaire, en ce que l'on refroidit ladite brame jusqu'à une température inférieure à Ac₃ pour opérer la transformation de phase de l'austénite en ferrite, en ce que l'on soumet ladite brame à une opération de laminage de finition dans le domaine ferritique au moyen de cylindres de laminage à chaud lubrifiés par un lubrifiant résistant à haute température, pour former une tôle à chaud ayant une épaisseur comprise entre 0,4 mm et 10 mm, en ce que l'on bobine cette tôle à chaud à une température inférieure à 850°C, et en ce qu'on refroidit ensuite ladite bobine jusqu'à la température ambiante.According to the present invention, a method of manufacturing a hot-rolled steel sheet, having high magnetic properties, which comprises a finishing rolling operation in the ferritic field, is characterized in that a steel slab up to a temperature less than or equal to 1100 ° C., in that this slab is subjected to a roughing rolling operation in the austenitic domain up to an intermediate thickness, in that one cools said slab to a temperature below Ac₃ to effect the phase transformation of austenite into ferrite, in that said slab is subjected to a finishing rolling operation in the ferritic field by means of rolling cylinders hot lubricated by a lubricant resistant to high temperature, to form a hot sheet having a thickness between 0.4 mm and 10 mm, in that one coils this sheet to c haud at a temperature below 850 ° C, and in that said coil is then cooled to room temperature.

Pour la mise en oeuvre de ce procédé, on utilise généralement une brame d'acier coulée en continu.For the implementation of this process, a slab of continuously cast steel is generally used.

Après la coulée de la brame, on peut appliquer les techniques récentes dites de l'enfournement chaud ou du laminage direct. Il s'est cependant avéré préférable d'enfourner la brame à basse température dans le four de réchauffage, en vue de minimiser la dispersion des précipités.After pouring the slab, recent techniques known as hot charging or direct rolling can be applied. However, it has proved preferable to put the slab at low temperature in the reheating oven, in order to minimize the dispersion of the precipitates.

La température de réchauffage limitée à 1100°C permet de limiter la remise en solution des nitrures AlN dans les aciers calmés à l'aluminium, la dissolution du sulfure MnS dans les aciers à basse teneur en manganèse ainsi que sa reprécipitation sous forme fine et dispersée, qui serait pénalisante pour l'objet de l'invention.The reheating temperature limited to 1100 ° C. makes it possible to limit the re-solution of the nitrides AlN in the steels calmed with aluminum, the dissolution of the sulphide MnS in the steels with low manganese content as well as its reprecipitation in fine and dispersed form. , which would be penalizing for the subject of the invention.

Le laminage de dégrossissage dans le domaine austénitique a pour effet de donner au produit une épaisseur intermédiaire, qui dépend notamment de l'épaisseur de la brame initiale et de l'épaisseur de la tôle finale. Pour la gamme précitée des épaisseurs de tôle, l'épaisseur intermédiaire peut être comprise entre 8 mm et 40 mm.The roughing rolling in the austenitic field has the effect of giving the product an intermediate thickness, which depends in particular on the thickness of the initial slab and the thickness of the final sheet. For the aforementioned range of sheet thicknesses, the intermediate thickness can be between 8 mm and 40 mm.

La transformation de phase de l'austénite en ferrite s'opère pendant le transfert du produit depuis le train de laminage de dégrossissage jusqu'au train de laminage de finition. En général, cette transformation s'opère complètement pour les aciers à ultra-basse teneur en carbone, sans qu'il soit nécessaire d'appliquer un refroidissement supplémentaire. Dans certains cas, elle peut ne pas être complète, en particulier dans des aciers dont la teneur en carbone est plus élevée; on peut alors accélérer le refroidissement du produit par tout moyen approprié, afin d'achever la transformation de phase avant l'entrée du produit au train de laminage à chaud de finition.The phase transformation of austenite into ferrite takes place during the transfer of the product from the roughing rolling train to the finishing rolling train. In general, this transformation takes place completely for ultra-low carbon steels, without the need for additional cooling. In some cases, it may not be complete, in particular in steels with a higher carbon content; the cooling of the product can then be accelerated by any suitable means, in order to complete the phase transformation before the product enters the hot finish rolling train.

Le laminage à chaud de finition doit être réalisé pour l'essentiel dans le domaine ferritique. L'intervalle de température optimal est compris entre Ac₃ et 650°C et de préférence entre 850°C et 750°C. Le taux de réduction est au moins égal à 90 %The finishing hot rolling must be mainly carried out in the ferritic field. The optimal temperature range is between Ac₃ and 650 ° C and preferably between 850 ° C and 750 ° C. The reduction rate is at least 90%

L'intervalle des températures de laminage dans le domaine ferritique ainsi que les écrouissages réalisés dans les passes de laminage successives sont choisis de manière optimale pour que se développent, dans la tôle bobinée, d'une part une microstructure à gros grains ferritiques dont la taille est de préférence comprise entre 50 µm et 120 µm et d'autre part une texture cristalline (100) majoritaire.The range of rolling temperatures in the ferritic region as well as the work hardenings carried out in the successive rolling passes are optimally chosen so that, on the coiled sheet, a microstructure with large ferritic grains, the size of which develops is preferably between 50 μm and 120 μm and on the other hand a majority crystalline texture (100).

On a cependant constaté que l'optimisation des conditions du laminage à chaud dans le domaine ferritique n'était pas suffisante pour obtenir une tôle à chaud présentant une taille de grains et une texture cristalline homogènes suivant l'épaisseur. On a en effet observé que l'écrouissage de l'acier dans le domaine ferritique provoquait une hétérogénéité de la microstructure et de la texture cristalline suivant l'épaisseur de la tôle laminée, suivant un mécanisme qui n'est pas établi avec certitude. Il semble bien qu'au contact des cylindres, les surfaces du produit laminé dans le domaine ferritique soient déformées essentiellement par cisaillement, et que ce cisaillement soit d'autant plus important que la friction au contact avec les cylindres est plus élevée. L'importance de ce cisaillement des surfaces laminées augmente lorsque croît l'écrouissage dans la ferrite; les hétérogénéités de la microstructure et de la texture cristalline qui en résultent ont alors tendance à envahir le coeur du produit laminé. Le cisaillement augmente également lorsque la température de laminage dans le domaine ferritique diminue. Sous l'effet de ce cisaillement, il apparaît dans la surface et immédiatement sous la surface une concentration de déformations, qui induit une recristallisation dynamique dans les régions considérées. Il en résulte, dans la zone superficielle de la tôle, une stabilisation d'une couche de grains très fins, très différents par leur taille des grains produits au coeur de la tôle, qui est déformé essentiellement par compression. Le cisaillement exerce un second effet, à savoir la production en surface d'une texture (110) de cisaillement. Cette texture est moins favorable, en ce qui concerne les propriétés magnétiques, que la texture (100) du coeur déformé en compression.However, it has been found that the optimization of the conditions of hot rolling in the ferritic field was not sufficient to obtain a hot sheet having a grain size and a crystalline texture homogeneous depending on the thickness. It has in fact been observed that the hardening of steel in the ferritic region causes heterogeneity of the microstructure and of the crystal texture according to the thickness of the rolled sheet, according to a mechanism which is not established with certainty. It seems that in contact with the cylinders, the surfaces of the rolled product in the ferritic domain are deformed essentially by shearing, and that this shearing is all the more important as the friction contact with the cylinders is higher. The importance of this shearing of the laminated surfaces increases when the work hardening in the ferrite increases; the heterogeneities of the microstructure and of the crystalline texture which result therefrom then tend to invade the core of the laminated product. The shear also increases when the rolling temperature in the ferritic region decreases. Under the effect of this shearing, there appears in the surface and immediately below the surface a concentration of deformations, which induces a dynamic recrystallization in the regions considered. As a result, in the surface zone of the sheet, a stabilization of a layer of very fine grains, very different in size from the grains produced in the core of the sheet, which is deformed essentially by compression. Shearing has a second effect, namely the production on the surface of a shear texture (110). This texture is less favorable, as regards the magnetic properties, than the texture (100) of the core deformed in compression.

Le laminage dans le domaine ferritique avec des cylindres de laminage lubrifiés, préconisé par l'invention, permet de réduire très nettement le cisaillement des surfaces de la tôle au contact des cylindres. De ce fait, la recristallisation dynamique sous la surface de la tôle laminée est largement évitée et la texture (110) ne se crée plus dans cette zone. Les conditions d'application de l'agent lubrifiant ainsi que les quantités utilisées peuvent être optimisées en fonction de la température du produit, pour supprimer le cisaillement précité et ses conséquences, et ainsi produire une tôle présentant une microstructure et une texture cristalline homogènes sur l'épaisseur.Rolling in the ferritic field with lubricated rolling rolls, recommended by the invention, makes it possible to very significantly reduce the shearing of the sheet metal surfaces in contact with the rolls. Therefore, dynamic recrystallization under the surface of the laminated sheet is largely avoided and the texture (110) is no longer created in this area. The conditions of application of the lubricating agent as well as the quantities used can be optimized as a function of the temperature of the product, in order to eliminate the aforementioned shearing and its consequences, and thus to produce a sheet having a microstructure and a homogeneous crystalline texture on the surface. 'thickness.

De préférence, on lamine la tôle dans le domaine ferritique jusqu'à une épaisseur comprise entre 0,4 mm et 1,5 mm.Preferably, the sheet is rolled in the ferritic region to a thickness of between 0.4 mm and 1.5 mm.

La température de bobinage de la tôle laminée à chaud est avantageusement comprise entre 800°C et 600°C, et de préférence encore entre 750°C et 700°C. Le bobinage dans cette gamme de température permet, après la recristallisation de la ferrite déformée, de réaliser la croissance des grains de ferrite jusqu'à une taille optimale. Ce bobinage permet encore d'assurer la précipitation des carbures et des nitrures - pour les fractions éventuellement remises en solution - sous une forme grossière, qui n'est pas pénalisante pour les propriétés magnétiques de la tôle. Le bobinage est avantageusement effectué immédiatement après le laminage de finition, pour éviter le risque de déformations incontrôlées de la tôle, en particulier pour les tôles de très faible épaisseur.The winding temperature of the hot-rolled sheet is advantageously between 800 ° C and 600 ° C, and more preferably between 750 ° C and 700 ° C. The winding in this temperature range allows, after recrystallization of the deformed ferrite, to achieve the growth of the ferrite grains to an optimal size. This winding also makes it possible to ensure the precipitation of carbides and nitrides - for the fractions which may be dissolved - in a coarse form, which is not detrimental to the magnetic properties of the sheet. The winding is advantageously carried out immediately after the finishing rolling, to avoid the risk of uncontrolled deformation of the sheet, in particular for sheets of very small thickness.

Suivant une variante intéressante du procédé de l'invention, on peut bobiner la tôle à chaud à des températures qui peuvent être nettement moins élevées, c'est-à-dire comprises entre 650°C et 20°C, pour produire un acier non recristallisé. Celui-ci est ensuite, sans écrouissage intermédiaire, recuit à une température élevée dans le domaine ferritique, c'est-à-dire entre 600°C et 800°C, et de préférence entre 700°C et 750°C, puis la tôle est refroidie jusqu'à la température ambiante. On peut ainsi développer la microstructure optimale pour les propriétés magnétiques.According to an advantageous variant of the process of the invention, the sheet can be hot-rolled at temperatures which can be markedly lower, that is to say between 650 ° C. and 20 ° C., to produce a steel not recrystallized. This is then, without intermediate work hardening, annealed at a high temperature in the ferritic region, that is to say between 600 ° C and 800 ° C, and preferably between 700 ° C and 750 ° C, then the sheet metal is cooled to room temperature. It is thus possible to develop the optimal microstructure for the magnetic properties.

Dans le cadre de cette variante, le recuit précité de la tôle peut être réalisé soit en continu, en y associant éventuellement un décapage des tôles par l'hydrogène, soit en bobines.In the context of this variant, the aforementioned annealing of the sheet can be carried out either continuously, optionally combining a pickling of the sheets with hydrogen, or in coils.

Suivant une autre variante du procédé de l'invention, la tôle laminée à chaud, qui se trouve en bobine à l'état recristallisé ou écroui, peut être soumise à un écrouissage limité, dont le taux est compris entre 1 % et 35 % et de préférence entre 5 % et 20 %, puis à un recuit final suivant le processus décrit dans la variante précédente.According to another variant of the process of the invention, the hot-rolled sheet, which is in a coil in the recrystallized or work-hardened state, can be subjected to a limited work hardening, the rate of which is between 1% and 35% and preferably between 5% and 20%, then a final annealing according to the process described in the previous variant.

Le procédé de l'invention s'applique de préférence à des aciers du type ELC, c'est-à-dire avec une teneur en carbone inférieure à 0,03 %, et de type ULC, c'est-à-dire avec une teneur en carbone inférieure à 0,005 %. Des teneurs en carbone aussi basses sont généralement obtenues par une opération de décarburation sous vide à l'aciérie.The process of the invention is preferably applied to steels of the ELC type, that is to say with a carbon content of less than 0.03%, and of the ULC type, that is to say with a carbon content of less than 0.005%. Such low carbon contents are generally obtained by a vacuum decarburization operation at the steelworks.

Comme on l'a indiqué plus haut, il est essentiel que les aciers présentent le moins possible de sources d'hétérogénéité, comme des inclusions ou des précipités. A cet effet, il est préférable que leur composition chimique respecte les conditions suivantes, en poids :

Carbone :
< 0,03 %, de préférence < 0,005 %
Azote :
< 0,004 %, de préférence < 0,002 %
Soufre :
< 0,010 %, de préférence < 0,003 %
Phosphore :
< 0,010 %
Manganèse :
< 0,2 %
Silicium :
< 0,5 %, de préférence < 0,3 %
Aluminium total :
< 0,05 %, de préférence < 0,003 %
Fer :
< reste.
As indicated above, it is essential that the steels have as few sources of heterogeneity as possible, such as inclusions or precipitates. For this purpose, it is preferable that their chemical composition meets the following conditions, by weight:
Carbon:
<0.03%, preferably <0.005%
Nitrogen:
<0.004%, preferably <0.002%
Sulfur:
<0.010%, preferably <0.003%
Phosphorus:
<0.010%
Manganese:
<0.2%
Silicon:
<0.5%, preferably <0.3%
Total aluminum:
<0.05%, preferably <0.003%
Iron :
<stay.

Ces teneurs sont aussi basses que possible; elles peuvent ête nulles, à l'exception de la teneur en silicium.These contents are as low as possible; they can be zero, with the exception of the silicon content.

L'amélioration des propriétés magnétiques de tôles d'acier laminées à chaud obtenues par le procédé de l'invention est illustrée par des exemples comparatifs rassemblés dans le tableau 1.The improvement of the magnetic properties of hot-rolled steel sheets obtained by the process of the invention is illustrated by comparative examples collected in Table 1.

Ces exemples correspondent aux conditions suivantes, reprises dans les diverses colonnes du tableau : Colonne 1 : type d'acier : ELC - ULC 2 : épaisseur de la tôle, en mm 3 : obtenu suivant l'invention : OUI (+) - NON (-) 4 : type de laminage à chaud : austénite (A) - ferrite (F) 5 : cylindres de laminage lubrifiés : OUI (+) - NON (-) 6 : température de bobinage; en °C 7 : laminage à froid éventuel; taux de réduction en % 8 : recuit éventuel; température en °C 9 : taille des grains, en µm 10 : orientation principale (110) dans le plan de laminage 11 : orientation principale (100) dans le plan de laminage 12 : orientation principale (111) dans le plan de laminage 13 : induction magnétique maximum B50, en Tesla 14 : pertes magnétiques W15/50, en W/kg. These examples correspond to the following conditions, listed in the various columns of the table: Column 1: steel type: ELC - ULC 2: sheet thickness, in mm 3: obtained according to the invention: YES (+) - NO (-) 4: type of hot rolling: austenite (A) - ferrite (F) 5: lubricated rolling cylinders: YES (+) - NO (-) 6: winding temperature; in ° C 7: possible cold rolling; reduction rate in% 8: possible annealing; temperature in ° C 9: grain size, in µm 10: main orientation (110) in the rolling plane 11: main orientation (100) in the rolling plane 12: main orientation (111) in the rolling plane 13: maximum magnetic induction B50, in Tesla 14: magnetic losses W15 / 50, in W / kg.

Pour des aciers ULC, les colonnes 9 à 12 comportent deux valeurs, qui traduisent la situation respectivement en peau et au coeur d'une tôle présentant une structure hétérogène.For ULC steels, columns 9 to 12 have two values, which reflect the situation respectively in the skin and in the heart of a sheet having a heterogeneous structure.

L'examen de ce tableau indique que les tôles produites par le procédé de l'invention (signe + dans la colonne 3) présentent une induction magnétique accrue (B50 - colonne 13) et des pertes magnétiques réduites (W15/50 - colonne 14) par rapport aux tôles produites par d'autres méthodes.

Figure imgb0001
Examination of this table indicates that the sheets produced by the process of the invention (sign + in column 3) exhibit increased magnetic induction (B50 - column 13) and reduced magnetic losses (W15 / 50 - column 14) compared to sheets produced by other methods.
Figure imgb0001

Claims (10)

Procédé de fabrication d'une tôle d'acier laminée à chaud, présentant des propriétés magnétiques élevées, qui comprend une opération de laminage de finition dans le domaine ferritique, caractérisé en ce que l'on réchauffe une brame d'acier jusqu'à une température inférieure ou égale à 1100°C, en ce que l'on soumet cette brame à une opération de laminage de dégrossissage dans le domaine austénitique jusqu'à une épaisseur intermédiaire, en ce que l'on refroidit ladite brame jusqu'à une température inférieure à Ac₃ pour opérer la transformation de phase de l'austénite en ferrite, en ce que l'on soumet ladite brame à une opération de laminage de finition dans le domaine ferritique au moyen de cylindres de laminage à chaud lubrifiés par un lubrifiant résistant à haute température, pour former une tôle à chaud ayant une épaisseur comprise entre 0,4 mm et 10 mm, en ce que l'on bobine cette tôle à chaud à une température inférieure à 850°C, et en ce qu'on la refroidit ensuite jusqu'à la température ambiante.Method for manufacturing a hot-rolled steel sheet, having high magnetic properties, which comprises a finishing rolling operation in the ferritic field, characterized in that a steel slab is heated to a temperature less than or equal to 1100 ° C., in that this slab is subjected to a roughing rolling operation in the austenitic domain to an intermediate thickness, in that said slab is cooled to a temperature lower than Ac₃ to effect the phase transformation of austenite into ferrite, in that said slab is subjected to a finishing rolling operation in the ferritic field by means of hot rolling cylinders lubricated with a lubricant resistant to high temperature, to form a hot sheet having a thickness of between 0.4 mm and 10 mm, in that the sheet is hot rolled at a temperature below 850 ° C., and in that it is then cooled to room temperature. Procédé suivant la revendication 1, caractérisé en ce que ladite épaisseur intermédiaire est comprise entre 8 mm et 40 mm.Method according to claim 1, characterized in that said intermediate thickness is between 8 mm and 40 mm. Procédé suivant l'une ou l'autre des revendications 1 et 2, caractérisé en ce que l'on opère ledit laminage de finition à une température comprise entre Ac₃ et 650°C.Method according to either of claims 1 and 2, characterized in that said finishing rolling is carried out at a temperature between Ac₃ and 650 ° C. Procédé suivant l'une ou l'autre des revendications 1 à 3, caractérisé en ce que l'on opère ledit laminage de finition à une température comprise entre 850°C et 750°C.Process according to either of Claims 1 to 3, characterized in that the said finishing rolling is carried out at a temperature between 850 ° C and 750 ° C. Procédé suivant l'une ou l'autre des revendications 1 à 4, caractérisé en ce que l'on opère ledit laminage de finition avec un taux de réduction au moins égal à 90 %.Process according to either of Claims 1 to 4, characterized in that the said finishing rolling is carried out with a reduction rate at least equal to 90%. Procédé suivant l'une ou l'autre des revendications 1 à 5, caractérisé en ce que l'on bobine la tôle à chaud à une température comprise entre 800°C et 600°C.Process according to either of Claims 1 to 5, characterized in that the sheet is hot-rolled at a temperature between 800 ° C and 600 ° C. Procédé suivant l'une ou l'autre des revendications 1 à 5, caractérisé en ce qu'on bobine la tôle à chaud à une température comprise entre 650°C et 20°C, en ce qu'on la recuit dans le domaine ferritique à une température comprise entre 600°C et 800°C et en ce qu'on la refroidit jusqu'à la température ambiante.Process according to either of Claims 1 to 5, characterized in that the sheet is hot-rolled at a temperature between 650 ° C and 20 ° C, in that it is annealed in the ferritic domain at a temperature between 600 ° C and 800 ° C and in that it is cooled to room temperature. Procédé suivant l'une ou l'autre des revendications 1 à 5, caractérisé en ce que l'on soumet la tôle à chaud, après le bobinage, à un écrouissage dont le taux est compris entre 1 % et 35 %, en ce qu'on la recuit dans le domaine ferritique à une température comprise entre 600°C et 800°C et en ce qu'on la refroidit jusqu'à la température ambiante.Process according to either of Claims 1 to 5, characterized in that the hot sheet, after winding, is subjected to work hardening, the rate of which is between 1% and 35%, in that 'It is annealed in the ferritic area at a temperature between 600 ° C and 800 ° C and in that it is cooled to room temperature. Procédé suivant l'une ou l'autre des revendications 7 et 8, caractérisé en ce que ledit recuit est effectué en continu sous une atmosphère d'hydrogène.Process according to either of Claims 7 and 8, characterized in that the said annealing is carried out continuously under a hydrogen atmosphere. Procédé suivant l'une ou l'autre des revendications 1 à 9, caractérisé en ce que l'on bobine ladite tôle immédiatement après le laminage à chaud de finition.Process according to either of Claims 1 to 9, characterized in that the said sheet is wound immediately after the hot finish rolling.
EP94870013A 1993-01-29 1994-01-27 Method of manufacturing a hot rolled steel sheet with high magnetic properties Withdrawn EP0609190A1 (en)

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EP0651061A1 (en) * 1993-11-01 1995-05-03 EKO Stahl GmbH Process for producing grain-oriented electrical strips and magnetic cores produced therefrom
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CN112176147A (en) * 2020-10-13 2021-01-05 五矿营口中板有限责任公司 Manufacturing method of normalized thick steel plate suitable for large-wire welding
CN112176147B (en) * 2020-10-13 2021-06-08 五矿营口中板有限责任公司 Manufacturing method of normalized thick steel plate suitable for large-wire welding

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