EP0732413B1 - Process for manufacturing grain oriented electrical steel sheets for transformers - Google Patents

Abstract

Electrical steel sheet is made by concasting steel of composition in wt. %:- 0.02-0.09 C; 2.5-4 Si; 0.027-0.17 Mn; 0.007-0.02 S; 0.01-0.03 Al; 0.004-0.012 N; 0.06-5 Cu; balance Fe and impurities. The concast slab or strip is reheated to 1200-1300 deg. C before being hot rolled to a thickness of 1-5 mm and then coiled at 500-700 deg. C. The strip is then cold rolled to below 0.5 mm, annealed to produce a primary recrystallisation and a decarburisation in a moist atmosphere and then coated with MgO. Finally it is annealed to produce a secondary recrystallisation and coated with an insulation layer which is fired.

Description

• une coulée d'un acier en continu sous forme de brame ou de bande d'acier contenant notamment dans sa composition moins de 0,1 % de carbone, de 2,5 à 4 % de silicium et au moins les éléments aluminium, azote, manganèse, soufre, cuivre, destinés à former des composés inhibiteurs de croissance normale,
• un réchauffage de la brame ou de la bande,
• un laminage à chaud de la brame ou de la bande pour l'obtention d'une tôle d'épaisseur comprise entre 1 et 5 mm,
• un bobinage à chaud de la tôle laminée à chaud,
• un recuit de la tôle laminée à chaud,
• un laminage à froid à une épaisseur finale inférieure à 0,5 mm,
• un recuit de recristallisation primaire et de décarburation en atmosphère humide,
• une application, sur au moins une face de la tôle décarburée, de magnésie MgO,
• un recuit final de recristallisation secondaire et d'épuration,
• une application d'un revêtement isolant et un recuit final de cuisson du revêtement,

The present invention relates to a method of manufacturing an electric steel sheet with oriented grains for the production in particular of magnetic circuits of transformers comprising, successively:

• a continuous casting of a steel in the form of a slab or a steel strip containing in particular in its composition less than 0.1% carbon, 2.5 to 4% silicon and at least the aluminum, nitrogen elements, manganese, sulfur, copper, intended to form compounds inhibiting normal growth,
• reheating of the slab or strip,
• hot rolling of the slab or strip to obtain a sheet of thickness between 1 and 5 mm,
• hot winding of the hot rolled sheet,
• annealing the hot-rolled sheet,
• cold rolling to a final thickness of less than 0.5 mm,
• a primary recrystallization and decarburization annealing in a humid atmosphere,
• an application, on at least one face of the decarburized sheet, of MgO magnesia,
• a final annealing of secondary recrystallization and purification,
• an application of an insulating coating and a final baking annealing of the coating,

The texture of a grain oriented electrical steel sheet is a so-called Goss texture symbolized by MILLER indices, {110} <001>, that the axis <001>, which is an axis of easy magnetization, is substantially parallel to the rolling direction and the plane {110} is a plane substantially parallel to the surface of the sheet. This texture gives grain oriented electrical steel sheet with good properties magnetic in the rolling direction which is substantially the direction of easy magnetization. B800 induction measurements acquired under a magnetic field of 800 A / m and energy losses W (1.7 / 50) of the sheet steel for a working induction of 1.7 Tesla (T) at a frequency of 50 Hertz are used in practice to assess the magnetic quality samples taken parallel to the direction of rolling of the sheet.

• Les tôles dites classiques sont caractérisées par une induction B800 inférieure à 1,86 Tesla ; elles sont obtenues par un procédé qui comprend notamment deux opérations de laminage à froid séparées par un recuit intermédiaire, le taux de réduction du second laminage étant généralement inférieur à 70 %.
• Les tôles dites à haute perméabilité sont caractérisées par une induction B800 supérieure à 1,88 Tesla ; elles sont obtenues par un procédé qui comprend notamment une seule opération de laminage à froid ou deux opérations de laminage à froid avec recuit intermédiaire, le taux de réduction du laminage à froid en une opération ou du deuxième laminage à froid étant généralement supérieur à 80 %.

Two types of oriented grain steel sheets are sold.

• The so-called conventional sheets are characterized by a B800 induction of less than 1.86 Tesla; they are obtained by a process which notably comprises two cold rolling operations separated by an intermediate annealing, the reduction rate of the second rolling being generally less than 70%.
• The so-called high permeability sheets are characterized by a B800 induction greater than 1.88 Tesla; they are obtained by a process which notably comprises a single cold rolling operation or two cold rolling operations with intermediate annealing, the reduction rate of cold rolling in one operation or of the second cold rolling generally being greater than 80% .

The steel of conventional sheets contains, before hot rolling, manganese, sulfur and copper. High permeability sheet steel contains, before hot rolling, aluminum, manganese, copper, sulfur and nitrogen.

A method of producing a sheet of grain steel is known. oriented, having substantially the texture {110} <001>, with high permeability in which the steel contains aluminum, manganese, copper, sulfur and nitrogen. Aluminum combines with nitrogen to form aluminum nitride AIN, and manganese and copper combine with the sulfur to form manganese sulfide MnS and copper sulfide CuS. The precipitates of aluminum nitride, manganese sulfide and copper sulfide inhibit normal growth of primary grains during static texturing annealing while allowing development of secondary recrystallization grains having the texture of Goss desired.

In known manner, the heating of the steel slab before hot rolling is operated at a temperature above 1300 ° C, around 1350 to 1400 ° C to completely re-dissolve the AIN, MnS and CuS precipitates alone or in combination. Their size in the state rough casting, generally greater than 1 micrometer, is too coarse to allow the development of secondary recrystallization. The compounds AIN, MnS and CuS reprecipitate alone or in combination in the state fine particles, with an average size of less than 150 nanometers (nm) at hot rolling and annealing before cold rolling one operation.

Reheating of the slab or strip of grain-oriented steel a main disadvantage of a temperature above 1300 ° C. the formation of liquid oxides which requires periodic shutdown of the oven specially adapted to this production with a view to cleaning it.

To obtain a magnetic quality equivalent to that of the products from the reheating of the slabs in the temperature range 1350-1400 ° C, several metallurgical processes using a reheating slabs to temperatures below 1300 ° C have been proposed:

For example, after cold rolling and decarburization, nitriding is used which has as its object the formation of fine precipitates of silicon nitride and aluminum (Si, Al) N before the start of secondary recrystallization. Nitriding is carried out either by an additional heat treatment in a gaseous atmosphere containing ammonia NH ₃ , or by addition of a compound containing nitrogen, such as manganese nitrides MnN, ferro-manganese FeMnN, chromium CrN, with an annealing separator mainly consisting of MgO magnesia. The slabs generally contain aluminum, and optionally titanium Ti, chromium Cr, boron B, elements known for their ability to form nitrides TiN, CrN, BN. Since this process aims at precipitating fine particles (Si, Al) N at the stage of secondary recrystallization annealing, the previous presence of fine precipitates MnS and AIN is not necessary. Consequently, the re-solution of the coarse particles MnS and AIN is incomplete during the reheating of the slabs, preceding the hot rolling. In addition, an incomplete re-solution of the aluminum is essential for the precipitation of the silicon nitride.

• Un réchauffage des brames à une température qui est inférieure à la température de solubilité du sulfure de manganèse et qui est supérieure à la température de solubilité du sulfure de cuivre.

The German patents DE 43 111 51 and European patent EP 0 619 376 describe a process which comprises:

• Reheating the slabs to a temperature which is lower than the solubility temperature of manganese sulfide and which is higher than the solubility temperature of copper sulfide.

• Par suite de la précipitation, au cours du laminage à chaud, de plus de 60 % de l'azote sous forme de particules grossières AIN, les précipités de nitrure d'aluminium ne jouent pas non plus le rôle d'inhibiteur.

Under these conditions, the copper sulfide precipitates are dissolved. On the other hand, the manganese sulfide precipitates are not redissolved and, being in the form of coarse particles, no longer play the role of inhibitor.

• As a result of the precipitation, during hot rolling, of more than 60% of the nitrogen in the form of coarse particles AIN, the precipitates of aluminum nitride also do not play the role of inhibitor.

In the process described in these documents, fine particles of copper sulfide CuS, which precipitate during the annealing of the rolled sheet are the essential inhibitor.

• une coulée d'un acier en continu sous forme de brame ou de bande d'acier contenant notamment dans sa composition pondérale moins de 0,1 % de carbone, de 2,5 à 4 % de silicium et au moins les éléments aluminium, azote, manganèse, soufre, cuivre, destinés à former des précipités inhibiteurs de croissance normale, procédé dans lequel l'acier de composition pondérale suivante:
• de 0,02 à 0,09 % de carbone,
• de 2,5 à 4 % de silicium
• de 0,027 à 0,17 % de manganèse
• de 0,007 à 0,020 % de soufre
• de 0,010 à 0,030 % d'aluminium
• de 0,004 à 0,012 % d'azote
• de 0,06 à 0,50 % de cuivre

de manière facultative, jusqu'à 0,15% d'étain,
le reste étant le fer et les impuretés parmi lesquelles moins de 0,015% de phosphore, est soumis successivement après l'élaboration de la brame ou de la bande par coulée continue à,

• un réchauffage à une température supérieure à 1200°C et inférieure ou égale à 1300°C, afin que les précipités AIN, MnS, et CuS pris seul ou en combinaison puissent être remis en solution en quantité suffisante,
• un laminage à chaud de la brame ou de la bande pour l'obtention d'une tôle d'épaisseur comprise entre 1 et 5 mm pendant lequel les précipités CuS piègent une partie du soufre de l'acier,
• un bobinage à chaud de la tôle laminée à chaud entre 500°C et 700°C,
• un recuit de la tôle laminée à chaud,
• un laminage à froid à une épaisseur finale inférieure à 0,5 mm,
• un recuit de recristallisation primaire et de décarburation en atmosphère humide,
• une application, sur au moins une face de la tôle décarburée, d'un séparateur à base de magnésie MgO,
• un recuit final de recristallisation secondaire et d'épuration, avant lequel, lors de la montée à température de ce recuit final de recristallisation secondaire, les fins précipités CuS se dissolvent progressivement, la libération du soufre s'accompagnant d'une fine précipitation de particules MnS,
• une application d'un revêtement isolant et le recuit final de cuisson du revêtement.

The subject of the present invention is a method of manufacturing electrical sheet with oriented grains for the production in particular of magnetic circuits of transformers comprising successively:

• a continuous casting of steel in the form of a slab or steel strip containing in particular in its weight composition less than 0.1% carbon, 2.5 to 4% silicon and at least the elements aluminum, nitrogen , manganese, sulfur, copper, intended to form precipitates inhibiting normal growth, process in which the steel of following composition by weight:
• from 0.02 to 0.09% carbon,
• 2.5 to 4% silicon
• 0.027 to 0.17% manganese
• 0.007 to 0.020% sulfur
• from 0.010 to 0.030% aluminum
• from 0.004 to 0.012% nitrogen
• from 0.06 to 0.50% copper

optionally, up to 0.15% tin,
the remainder being iron and impurities, of which less than 0.015% of phosphorus, is subjected successively after the slab or the strip is produced by continuous casting,

• reheating at a temperature higher than 1200 ° C. and lower than or equal to 1300 ° C., so that the precipitates AIN, MnS, and CuS taken alone or in combination can be redissolved in sufficient quantity,
• hot rolling of the slab or strip to obtain a sheet of thickness between 1 and 5 mm during which the CuS precipitates trap part of the sulfur of the steel,
• hot winding of the hot rolled sheet between 500 ° C and 700 ° C,
• annealing the hot-rolled sheet,
• cold rolling to a final thickness of less than 0.5 mm,
• a primary recrystallization and decarburization annealing in a humid atmosphere,
• an application, on at least one face of the decarburized sheet, of a MgO magnesia-based separator,
• a final annealing of secondary recrystallization and purification, before which, during the rise in temperature of this final annealing of secondary recrystallization, the fine CuS precipitates dissolve gradually, the release of the sulfur accompanied by a fine precipitation of particles MnS,
• an application of an insulating coating and the final baking annealing of the coating.

• on décarbure la bande de façon à ce que la teneur en oxygène du film d'oxyde formé soit inférieure à 800 10^-4 %,
• le produit résultant de la multiplication de la teneur en soufre par la teneur en manganèse est inférieur à 160.10^-5,
• le produit résultant de la multiplication de la teneur en azote par la teneur en aluminium est inférieur à 240.10^-6,
• l'acier est laminé à chaud de façon à faire précipiter l'azote sous forme de fines particules contenant notamment de l'azote et de l'aluminium de diamètre moyen inférieur à 100 nanomètres, le pourcentage d'azote précipité étant inférieur à 40 %,
• l'acier laminé à chaud est recuit de façon à faire précipiter l'azote sous forme de fines particules contenant notamment de l'azote et de l'aluminium de diamètre moyen inférieur à 100 nanomètres, le pourcentage d'azote précipité étant supérieur à 60 %,
• l'acier est laminé à chaud de façon à faire précipiter le soufre sous forme de particules dont le diamètre moyen est inférieur à 100 nanomètres,
• après recuit de la tôle laminée à chaud à une température comprise entre 850°C et 1150°C pendant 1 à 10 minutes et refroidissement, à une vitesse supérieure à 10°C par seconde à partir de 800°C, le laminage à froid à une épaisseur finale inférieure à 0,5 mm est effectué, en une seule opération comprenant plusieurs passes de laminage, avec un taux de réduction global supérieur à 70 %, la température de la tôle étant comprise entre 100°C et 300°C pendant au moins une passe de laminage,
• le laminage à froid, à une épaisseur finale inférieure à 0,5 mm, est effectué en deux opérations avec un recuit intermédiaire, à une température comprise entre 850°C et 1150°C pendant 1 à 10 minutes, suivi d'un refroidissement, à une vitesse supérieure à 10°C par seconde à partir de 800°C, le taux de réduction du second laminage à froid étant supérieur à 40 %, la température de la tôle étant comprise entre 100°C et 300°C pendant au moins une passe de laminage à froid lorsque le taux de réduction du second laminage à froid est supérieur à 70 %,
• préalablement au laminage à froid en deux opérations, la tôle est soumise à un recuit à une température comprise entre 850°C et 1150°C pendant 1 à 10 minutes, notamment si l'épaisseur finale de la tôle est inférieure à 0,27 mm,
• la magnésie contient, en plus des additions facultatives de dioxyde de titane, de bore ou d'un composé boré, au moins un composé soufré et/ou un composé soufré et azoté et/ou un composé de l'antimoine pris seuls ou en combinaison.
• la magnésie contient, en plus des additions facultatives de dioxyde de titane, de bore ou d'un composé contenant du bore, du soufre ou un ou plusieurs composés soufrés ou azotés choisis parmi le sulfate de magnésium, le sulfate de manganèse, l'urée, le thiosulfate de sodium.
• la magnésie contient, en plus des additions facultatives de dioxyde de titane, de bore ou d'un composé boré, au moins un composé soufré et azoté choisi parmi le sulfate d'ammonium, l'acide amidosulfurique (sulphamic acid), le thiosulfate d'ammonium.
• la magnésie contient, en plus des additions facultatives de dioxyde de titane, de bore ou d'un composé boré, du chlorure d'antimoine,

The other characteristics of the invention are:

• the strip is decarbonized so that the oxygen content of the oxide film formed is less than 800 10 ^-4 %,
• the product resulting from the multiplication of the sulfur content by the manganese content is less than 160.10 ^-5 ,
• the product resulting from the multiplication of the nitrogen content by the aluminum content is less than 240.10 ^-6 ,
• the steel is hot rolled so as to precipitate nitrogen in the form of fine particles containing in particular nitrogen and aluminum with an average diameter of less than 100 nanometers, the percentage of precipitated nitrogen being less than 40% ,
• hot rolled steel is annealed so as to precipitate nitrogen in the form of fine particles containing in particular nitrogen and aluminum with an average diameter of less than 100 nanometers, the percentage of nitrogen precipitated being greater than 60 %,
• the steel is hot rolled so as to precipitate the sulfur in the form of particles whose average diameter is less than 100 nanometers,
• after annealing the hot-rolled sheet at a temperature between 850 ° C and 1150 ° C for 1 to 10 minutes and cooling, at a speed greater than 10 ° C per second from 800 ° C, cold rolling to a final thickness of less than 0.5 mm is carried out, in a single operation comprising several rolling passes, with an overall reduction rate greater than 70%, the temperature of the sheet being between 100 ° C and 300 ° C during minus one rolling pass,
• cold rolling, to a final thickness of less than 0.5 mm, is carried out in two operations with intermediate annealing, at a temperature between 850 ° C and 1150 ° C for 1 to 10 minutes, followed by cooling, at a speed greater than 10 ° C per second from 800 ° C, the reduction rate of the second cold rolling being greater than 40%, the temperature of the sheet being between 100 ° C and 300 ° C for at least a cold rolling pass when the reduction rate of the second cold rolling is greater than 70%,
• prior to cold rolling in two operations, the sheet is subjected to annealing at a temperature between 850 ° C and 1150 ° C for 1 to 10 minutes, especially if the final thickness of the sheet is less than 0.27 mm ,
• magnesia contains, in addition to optional additions of titanium dioxide, boron or a boron compound, at least one sulfur compound and / or a sulfur and nitrogen compound and / or an antimony compound taken alone or in combination .
• magnesia contains, in addition to optional additions of titanium dioxide, boron or a compound containing boron, sulfur or one or more sulfur or nitrogen compounds selected from magnesium sulfate, manganese sulfate, urea , sodium thiosulfate.
• magnesia contains, in addition to optional additions of titanium dioxide, boron or a boron compound, at least one sulfur and nitrogen compound chosen from ammonium sulfate, amidosulfuric acid (sulphamic acid), thiosulfate d 'ammonium.
• magnesia contains, in addition to optional additions of titanium dioxide, boron or a boron compound, antimony chloride,

The following description and the attached figures, all given as non-limiting example, will make the invention well understood.

Figure 1 is a curve showing the loss of mass as a function the slab reheating temperature and illustrating the formation of fusible oxides above 1300 ° C.

Figure 2 shows, after hot rolling, the existing relationship between the average diameter of the precipitates and the sulfur percentage of steel.

Figure 3 presents after decarburization, the precipitate densities depending on the stop temperature of the recrystallization annealing secondary.

The present invention relates to the use of a steel of composition following determined weight: carbon of between 0.02 and 0.09%, silicon between 2.5 and 4%, copper between 0.06 and 0.50% and a selection, manganese between 0.027 and 0.17%, sulfur between 0.007 and 0.020%, aluminum between 0.010 and 0.030%, nitrogen between 0.004 and 0.012%, the rest being iron and impurities, composition submitted, after preparation by casting continues from the slab or strip to a full reheating to a temperature equal to or less than 1300 ° C. Indeed, according to this invention, there is no longer formation of fusible surface oxides below of this temperature as shown in Figure 1 for a total cycle of reheating of the slabs from 4 h 30, the time to maintain the temperature reheating target being one hour. The reheating temperature of the slab is greater than 1200 ° C. according to the invention so that the precipitates AIN, MnS and CuS taken alone or in combination can be re-established sufficient solution when reheating the slab in order to allow secondary recrystallization and obtaining good quality magnetic.

According to the present invention, the manganese, sulfur, aluminum and nitrogen are chosen in very narrow ranges, which allows the almost total solution and sufficient quantity of AIN, MnS and CuS precipitates taken alone or in combination, during reheating of the slabs, before hot rolling, to an equal temperature or lower than 1300 ° C which prevents the surface formation of oxides fuses.

According to the present invention, the precipitates containing sulfur and or most of the nitrogen is put back into solution when the slabs as a result of the adaptation of the chemical composition to the lower reheating temperature. The main inhibitor is nitride of aluminum which precipitates little during hot rolling and essentially during the annealing of the hot-rolled sheet in the form fine particles with an average diameter of less than 100 nanometers. The manganese sulfide is a complementary inhibitor. Copper has in particular a refining effect of the size of these AIN and MnS precipitates with which it can be associated. CuS precipitates that trap part of the sulfur of the steel at the hot rolling stage contributes to the reduction of the mean diameter of the precipitates as shown in Figure 2. These precipitates CuS, in small quantities, after annealing the hot-rolled sheet, compared to that of the AIN precipitates, are re-dissolved during the rise in temperature during secondary recrystallization annealing. They only participate therefore not significantly inhibition. The addition of one or several sulfur-containing and / or nitrogen-containing magnesia compounds with which the sheet metal after decarburization enhances inhibition by AIN and MnS.

• coulée continue de l'acier sous forme de brames d'épaisseur comprise entre 150 et 300 mm,
• réchauffage des brames à coeur à une température comprise entre 1200 °C et 1300°C,
• laminage à chaud à une épaisseur comprise entre 1 et 5 mm,
• bobinage entre 500°C et 700°C de la tôle laminée à chaud,
• recuit de la tôle laminée à chaud à une température comprise entre 850°C et 1150°C pendant 1à 10 minutes suivi d'un refroidissement, à une vitesse supérieure à 10°C par seconde à partir de 800°C,
• laminage à froid à une épaisseur finale inférieure à 0,5 mm en une opération comprenant plusieurs passes avec un taux de réduction global supérieur à 70 %, la température de la tôle étant comprise entre 100°C et 300°C pendant au moins une passe de laminage à froid,
• ou laminage à froid à une épaisseur finale inférieure à 0,5 mm en deux opérations avec un recuit intermédiaire, effectué à une température comprise entre 850°C et 1150°C pendant 1 à 10 minutes suivi d'un refroidissement, à une vitesse supérieure à 10°C par seconde à partir de 800°C, le taux de réduction du second laminage à froid étant supérieur à 40 %, la température de la tôle étant comprise entre 100°C et 300°C pendant au moins une passe lorsque le taux de réduction global de ce second laminage à froid est supérieur à 70 %,

The grain-oriented steel sheets according to the invention are produced from the following successive stages:

• continuous casting of steel in the form of slabs of thickness between 150 and 300 mm,
• reheating the core slabs to a temperature between 1200 ° C and 1300 ° C,
• hot rolling to a thickness between 1 and 5 mm,
• winding between 500 ° C and 700 ° C of the hot rolled sheet,
• annealing hot rolled sheet at a temperature between 850 ° C and 1150 ° C for 1 to 10 minutes followed by cooling, at a speed greater than 10 ° C per second from 800 ° C,
• cold rolling to a final thickness of less than 0.5 mm in an operation comprising several passes with an overall reduction rate of more than 70%, the temperature of the sheet being between 100 ° C and 300 ° C for at least one pass cold rolling,
• or cold rolling to a final thickness of less than 0.5 mm in two operations with intermediate annealing, carried out at a temperature between 850 ° C and 1150 ° C for 1 to 10 minutes followed by cooling, at a higher speed at 10 ° C per second from 800 ° C, the reduction rate of the second cold rolling being greater than 40%, the temperature of the sheet being between 100 ° C and 300 ° C for at least one pass when the overall reduction rate of this second cold rolling is greater than 70%,

• recuit de recristallisation primaire et de décarburation en atmosphère humide contenant de l'hydrogène et de l'azote à l'épaisseur finale,
• application sur au moins une face de la tôle d'un agent anti-collage constitué principalement de magnésie,
• recuit final de recristallisation secondaire et d'épuration du métal,
• application d'un revêtement isolant et recuit final de cuisson du revêtement.

In the case of two cold rolling operations with intermediate annealing, an optional short-term annealing, from 1 to 10 minutes, between 850 ° C and 1150 ° C before the first cold rolling is likely to stabilize the secondary recrystallization especially if the final thickness of the sheet is less than 0.27 mm; the cooling rate may be slower.

• annealing of primary recrystallization and decarburization in a humid atmosphere containing hydrogen and nitrogen at the final thickness,
• application on at least one face of the sheet of an anti-sticking agent consisting mainly of magnesia,
• final annealing of secondary recrystallization and purification of the metal,
• application of an insulating coating and final baking annealing of the coating.

The oriented grain steel according to the invention, having undergone the stages of manufacturing described above, contains from 0.02 to 0.09% carbon, 2.5 to 4% silicon, 0.027 to 0.17% manganese, 0.007 to 0.020% sulfur, 0.010 to 0.030% aluminum, 0.004 to 0.012 % nitrogen, 0.06% to 0.50% copper, and optionally up to 0.15% tin, the rest being iron and impurities.

The product resulting from the multiplication of the sulfur content by the manganese content is less than or equal to 160.10 ^-5 : (% S) x (% Mn) <160.10 -5 The product resulting from the multiplication of the nitrogen content by the aluminum content is less than 240.10 ^-6 : (% N) x (% Al) <240.10 -6 The percentage of nitrogen precipitated after hot rolling, in the form of fine particles with an average diameter of less than 100 nanometers, is less than 40%.
The percentage of nitrogen precipitated, after hot rolling and annealing, in the form of fine particles with an average diameter of less than 100 nanometers, is greater than 60%.
The magnesia used as a separator during the annealing of secondary recrystallization and of purification at high temperature may contain, alone or as a mixture, sulfur or one or more sulfur or nitrogen compounds chosen from magnesium sulfate and / or manganese sulfate and / or sodium thiosulfate, and / or urea, one or more sulfur and nitrogen compounds chosen from, amidosulfuric acid, (sulphamic acid) and / or ammonium sulfate and / or ammonium thiosulfate, antimony chloride, boron or a compound of boron and titanium dioxide.

Figure 2 shows, after hot rolling, the relationship between the average diameter of the precipitates and the percentage of sulfur in the steel, in the case of an almost total solution of all the precipitates when reheating the slab. In order to obtain a fine precipitation after hot rolling, the sulfur content according to the present invention is limited to 0.020%.

The fine precipitates MnS playing an active role of secondary inhibitor during secondary recrystallization annealing, the sulfur content must be at less than 0.007% to obtain a sufficient amount of these rushed.

The manganese content according to the present invention must be higher at 0.027% to obtain precipitation of a sufficient amount of ends MnS precipitates exerting an inhibitory effect and having an availability in free manganese in the case of sulfur supply via the additive channel to the magnesia for strengthening the inhibitory power of MnS precipitates. It is limited to 0.17% to avoid the presence of precipitation coarse MnS in slabs and incomplete solution during reheating between 1200 and 1300 ° C before hot rolling.

According to the present invention, compliance with the condition [% S] x [% Mn] <160.10 ^-5 promotes the presence of fine MnS precipitates in the slabs and their redissolution between 1200 ° C and 1300 ° C before hot rolling . In the process according to the present invention, the nitrogen content must be greater than 0.004% in order to obtain sufficient precipitation of fine precipitates AIN, main inhibitor, during the annealing of the hot-rolled sheet. The nitrogen content is limited to 0.012% to avoid the formation of blistering on the surface of the steel. The condition (% N) x (% Al) <240.10 ^-6 allows almost complete dissolution of the AIN precipitates when reheating the slabs between 1200 ° C and 1300 ° C before hot rolling.

According to the present invention, the aluminum content must be equal to or greater than 0.010% on the one hand, so that the amount of AIN precipitates formed during the annealing of the hot-rolled sheet is sufficient, AIN being the main inhibitor and on the other hand, to have an availability in free aluminum, in the case of nitrogen supply via the additive channel to the magnesia for strengthening the inhibitory power of AIN precipitates. Aluminum content is less than 0.030% to avoid precipitation coarse AIN particles during the final stage of hot rolling.

In addition to the elements mentioned above, steel can contain up to 0.15% tin which exerts a beneficial effect on inhibition.

In the process according to the present invention which uses sulfur and aluminum lower than in the heating process of slabs at higher temperature, the density of inhibitory precipitates containing either sulfur and manganese or nitrogen and aluminum, may be insufficient to obtain recrystallization complete secondary and homogeneous magnetic quality. To to increase the stability of the secondary recrystallization and thus avoid a dispersion of the values of the magnetic characteristics, it is, of preferably, added to magnesia one or more compounds containing sulfur and or nitrogen or antimony which allow the formation of a complement of inhibitors, either based on sulfur and manganese, or based nitrogen and aluminum, either antimony-based during the rise temperature preceding the start of secondary recrystallization.

The present invention is illustrated from the observations and following examples, table 1 giving the chemical composition of tested steels. Steels 2 to 5 and 7 are steels according to the present invention. Steels 1, 6, 8 and 9 are reference steels. Content phosphorus, residual element, according to the present invention, is less than 0.015%.

Steel No. 1 is a reference steel containing 0.021% sulfur and 0.030% aluminum (Steel No. 1, Table 1), a slab of which has been reheated to 1400 ° C before hot rolling, so as to dissolve the majority of AIN, MnS, CuS precipitates of coarse size. The cold rolling was carried out according to the invention, in a single operation after annealing of the hot rolled sheet at 1120 ° C. The magnetic properties obtained at the final thickness of 0.285 mm are:
W (1.7 / 50) = 1.03 Watt / kg; B800 = 1.91 T

Under the same conditions, but with a heating temperature of the steel slab N ° 1 of 1280 ° C, the magnetic properties obtained at the thickness of 0.285 mm are poor because the product of the percentage of sulfur by the percentage of manganese is greater than 160.10 ^-5 :
W (1.7 / 50) = 1.65 Watt / kg; B800 = 1.72 T

• coulée continue de brames d'acier d'épaisseur 210 mm,
• réchauffage de brame à coeur à une température comprise entre 1200°C et 1300°C, avec une montée en température en 3 heures 30 minutes et un maintien à la température visée de 1 heure,
• laminage à chaud préliminaire jusqu'à l'épaisseur de 45 mm en 5 passes,
• laminage à chaud de finition en 7 passes jusqu'à l'épaisseur de 2,3 mm, la température de début du laminage à chaud de finition étant comprise entre 1070°C et 1000°C, la température finale de laminage à chaud étant comprise entre 965°C et 915°C,
• bobinage de la tôle laminée à chaud à la température de 530 °C ou 640°C,
• recuit de la tôle laminée à chaud avec une montée en température d'environ 60 secondes, maintien à 950°C pendant 160 secondes, refroidissement de la tôle laminée à chaud avec un temps de passage entre 700°C et 300°C inférieur à 15 secondes,
• laminage à froid jusqu'à l'épaisseur finale en six passes correspondant à des taux de réduction à froid successifs d'environ 30 %, la température de laminage atteignant la température de 230°C à la troisième passe, (pour les exemples 2 à 9)
• recristallisation et décarburation en atmosphère N2/H2 humide entre 800°C et 850°C, la durée du traitement thermique étant inférieure à 500 secondes,
• enduction de la tôle avec un lait de magnésie et séchage,

Le lait de magnésie est constitué de 150 g de MgO par litre d'eau. A ce lait on ajoute les additifs.In the context of the examples presented below describing the present invention, the steel sheets were produced in the following manner:

• continuous casting of 210 mm thick steel slabs,
• reheating of the slab to the core at a temperature between 1200 ° C. and 1300 ° C., with a rise in temperature in 3 hours 30 minutes and maintaining at the target temperature for 1 hour,
• preliminary hot rolling up to the thickness of 45 mm in 5 passes,
• 7-pass finishing hot rolling up to the thickness of 2.3 mm, the temperature of the start of the hot finishing rolling being between 1070 ° C and 1000 ° C, the final temperature of the hot rolling being between 965 ° C and 915 ° C,
• winding of hot rolled sheet at a temperature of 530 ° C or 640 ° C,
• annealing of the hot-rolled sheet with a temperature rise of approximately 60 seconds, holding at 950 ° C for 160 seconds, cooling of the hot-rolled sheet with a passage time between 700 ° C and 300 ° C of less than 15 seconds,
• cold rolling to the final thickness in six passes corresponding to successive cold reduction rates of approximately 30%, the rolling temperature reaching the temperature of 230 ° C. on the third pass, (for examples 2 to 9)
• recrystallization and decarburization in a humid N2 / H2 atmosphere between 800 ° C and 850 ° C, the duration of the heat treatment being less than 500 seconds,
• coating the sheet with a milk of magnesia and drying,

Milk of magnesia consists of 150 g of MgO per liter of water. To this milk are added the additives.

• recuit lent de recristallisation secondaire avec une montée en température de 15°C/h sous atmosphère 25 % N2-75 % H2 entre 650°C et 1200°C et épuration du métal à 1200°C sous hydrogène,
• application d'un revêtement isolant et cuisson du revêtement.

Le revêtement est composé de silice, d'un phosphate d'aluminium et d'acide chromiqueThe percentage of an element of magnesia additive (Ti, B, S, Sb, N) is the quotient of the mass of the element by the mass of dry magnesia multiplied by 100.

• slow secondary recrystallization annealing with a temperature rise of 15 ° C / h under 25% N2-75% H2 atmosphere between 650 ° C and 1200 ° C and purification of the metal at 1200 ° C under hydrogen,
• application of an insulating coating and curing of the coating.

The coating is composed of silica, aluminum phosphate and chromic acid

Example 1

Pertes (1,7 / 50)

= 1,27 W/kg

B 800

= 1,85 T

After reheating the slab to 1280 ° C, hot rolling and coiling of the sheet to 530 ° C, the steel No. 2 was cold rolled to the intermediate thickness of 0.74 mm, underwent annealing 90 seconds at 1050 ° C followed by cooling, very rapid from 800 ° C with a passage time between 700 ° C and 300 ° C less than 15 seconds, and was then rolled to the final thickness of 0.285 mm which corresponds to a rate of second cold reduction of 61%. After further processing as indicated above, the magnetic characteristics obtained are as follows:

Losses (1.7 / 50)

= 1.27 W / kg

B 800

= 1.85 T

Example 2

Pertes (1,7 / 50)

= 1,03 W/kg

B 800

= 1,93 T

After reheating the slab to 1280 ° C, hot rolling and coiling to 530 ° C, annealing hot rolled sheet at 950 ° C for 160 seconds, cooling, very fast from 800 ° C, cold rolling to '' at the final thickness of 0.285 mm, which corresponds to a cold reduction rate of 87%, and continuation of the treatment as indicated previously, the magnetic characteristics obtained for steel No. 3 are as follows:

Losses (1.7 / 50)

= 1.03 W / kg

B 800

= 1.93 T

From these two examples, it appears that the reduction rate of final cold rolling should be more than 70% to get the best magnetic properties.

In Examples 1 and 2, the annealing separator consisted of magnesia containing 0.080% boron and 1.2% of the titanium element in the form of titanium dioxide TiO ₂ .

Example 3

Under the conditions of Example 2, 10% of sulfur in the form of ammonium sulfate are added to the magnesia. The following magnetic characteristics are obtained for steel 2:

Example 4

Pertes (1,7 / 50)

= 1,02 W/kg

B 800

= 1,94 T

Under the conditions of Example 2, 3% by mass of sulfur in the form of magnesium sulfate are added to the magnesia. The magnetic characteristics obtained for steel No. 2 are as follows:

Losses (1.7 / 50)

= 1.02 W / kg

B 800

= 1.94 T

Example 5

Pertes (1,7 / 50)

= 1,01 W/kg

B 800

= 1,94 T

Under the conditions of Example 2, 1% of sulfur in the form of amidosulfuric acid is added to the magnesia. The magnetic characteristics obtained for steel No. 2 are as follows:

Losses (1.7 / 50)

= 1.01 W / kg

B 800

= 1.94 T

Example 6

Pertes (1,7 / 50)

= 1,03 W/kg

B 800

= 1,92 T

Under the conditions of Example 2, 0.026% of the antimony element in the form of antimony chloride is added to the magnesia. The magnetic characteristics obtained for steel No. 2 are as follows:

Losses (1.7 / 50)

= 1.03 W / kg

B 800

= 1.92 T

Example 7

Pertes (1,7 / 50) =

1,06 W/kg

B 800 =

1,91 T

Under the conditions of Example 2 with an annealing temperature of the hot-rolled strip of 1050 ° C., 0.93% of nitrogen in the urea state is added to the magnesia. The characteristics obtained for steel No. 7 are as follows:

Losses (1.7 / 50) =

1.06 W / kg

B 800 =

1.91 T

Example 8

Pertes (1,7 / 50)

= 0,98 W/kg

B 800

= 1,93 T

Under the conditions of Example 2, but with an annealing temperature of the hot-rolled strip of 1080 ° C and a magnesia containing 3.6% of the titanium element in the state of titanium dioxide, 0.10 % boron and free from sulfur and or nitrogen additives and antimony chloride, the characteristics obtained for steel No. 8 are as follows:

Losses (1.7 / 50)

= 0.98 W / kg

B 800

= 1.93 T

Example 9

Pertes (1,7 / 50)

= 1,03 W/kg

B 800

= 1,92 T

Under the conditions of Example 2, but with a reheating temperature of the slab of 1240 ° C., an annealing temperature of the hot-rolled strip equal to 1050 ° C., and a magnesia containing 1.5% of sulfur at the state of amidosulfuric acid, the magnetic characteristics obtained for steel No. 9 were as follows:

Losses (1.7 / 50)

= 1.03 W / kg

B 800

= 1.92 T

The addition of sulfur or a sulfur compound (magnesium sulfate, manganese, sodium thiosulfate) to magnesia helps strengthen inhibition by precipitates containing manganese and sulfur at during secondary recrystallization annealing.

The addition to magnesia of a nitrogenous compound (urea) allows to introduce nitrogen into the steel which reinforces the inhibition by the precipitates containing nitrogen and aluminum.

The addition to magnesia of a sulfur and nitrogen compound (ammonium thiosulfate, amidosulfuric acid which contains both 33% sulfur and 14% nitrogen) allows sulfur to be introduced into the steel and nitrogen to reinforce the inhibition by the precipitates containing, on the one hand, manganese and sulfur and, on the other hand, nitrogen and aluminum. The sulfur and nitrogen diffusing into the steel, this results in additional precipitation of very fine inhibitory particles before the start of secondary recrystallization.
The beneficial effect of nitrogen associated with sulfur is illustrated by the fact that the percentage of sulfur used in example 5 is lower than that used in example 4.
The addition of ammonium sulfate to magnesia also allows a simultaneous supply of sulfur and nitrogen. The addition of antimony chloride to magnesia allows the introduction into the steel of the antimony element, which by segregating at the grain boundaries, acts as an inhibitor. The addition of a water-soluble sulfur compound is preferred to the possible addition of insoluble elemental sulfur since the dispersion in milk of magnesia is more homogeneous. The addition, to magnesia, of compounds containing sulfur, nitrogen and antimony promotes the obtaining of a homogeneous magnetic quality over the length of the strip of coiled sheet.

Table 2 shows that according to the invention, the percentage of nitrogen precipitated in the hot rolled sheet is less than 40%. Lowering winding temperature significantly reduces the percentage of precipitated nitrogen, up to less than 5% in the case of steel n ° 3 reheated to 1280 ° C, hot rolled and coiled to 530 ° C. At this winding temperature, the percentage of precipitated nitrogen remains very low when the slab reheating temperature drops from 1280 ° C to 1240 ° C, usual temperature for reheating carbon steels.

Generally speaking, the lowering of the temperature winding below 600 ° C prevents precipitation of particles coarse with no inhibitory effect.

The amount of nitrogen combined with aluminum was determined from dosing of precipitated aluminum. By reheating the slabs between 1200 ° C and 1300 ° C according to the present invention, it is possible to obtain, thanks to the strict control of the contents of Mn, S, Al and N, a low percentage of nitrogen precipitated in the hot rolled state, which does not allow not the higher contents characterizing the known reheating process slabs at a temperature above 1350 ° C.

Table 3 shows that according to the invention, the percentage of nitrogen precipitate is greater than 60% after annealing the hot-rolled sheet to 950 ° C.

Table 4 shows that, according to the invention, the average diameter precipitates containing nitrogen and aluminum, obtained by annealing 160 seconds from hot rolled sheet steel n ° 2, wound at 530 ° C, is less than 50 nanometers in a wide temperature range of annealed.

Nitrogen and aluminum precipitates can therefore play an active role as an inhibitor.

The effect of copper has been analyzed in the context of this invention.

Table 5 gives the average diameter and density of the precipitates after heating the steel slab n ° 2 to 0.15% copper to 1280 ° C, hot rolling 2.3 mm thick and coiling at 640 ° C.

By way of comparison, the characteristics of the sheet precipitates hot rolled to the thickness 2.3 mm and wound at 640 ° C from the steel of reference n ° 1 with 0.09% of copper are presented. It appears, from the table n ° 5, that the increase in the copper content results in a increase in the density of CuS and INCINC precipitates and a decrease of their average diameter. Combinations of precipitated compounds are designated by the nature of the constituent elements without taking into account proportions.

In No. 2 raw hot-rolled steel sheet, the CuS and INAC particles of small diameter are the majority, those containing manganese are in the minority. The addition of copper and the sulfur trapping by copper avoids, during rolling when hot, the formation of coarse particles containing manganese, diameter too large to act as an inhibitor. The review of Table 4 shows a decrease in the number of particles containing copper when the annealing temperature of hot rolled sheet increases, highlighting the partial dissolution of the particles containing Al, N, Cu, S.

Figure 3 shows the evolution of the density of the CuS precipitates and MnCuS after decarburization and during recrystallization annealing secondary of steel n ° 6 which does not contain aluminum, composition chosen to facilitate counting of precipitates by microscopy electronic transmission. This steel, whose slabs were reheated to 1400 ° C, underwent two cold rolling operations with intermediate annealing at 950 ° C, the reduction rate of the second rolling at cold being 60%.

The fine CuS precipitates dissolve gradually before secondary recrystallization which occurs around 950 ° C, the release of sulfur accompanied by a fine precipitation of MnS particles. The particles identified under the electron microscope are MnCuS because copper precipitates on the MnS particles during cooling. According to this invention, fine CuS particles do not play a decisive inhibitory role for the development of secondary recrystallization.

In steel n ° 2, the percentage of CuS precipitates in diameter mean less than 100 nm is less than 3% of the total population, after annealing of hot rolled sheet. These are MnS precipitates formed after decarburization and before secondary recrystallization which reinforce inhibition by precipitates containing nitrogen and aluminum.

According to the present invention, the copper content must be greater than 0.06% to obtain fine precipitation at the hot rolled stages and hot rolled and annealed. The increase in the copper content favors refinement of precipitation. The copper content is limited to 0.50% to avoid the problems of pickling the sheet obtained.

As has been demonstrated, it is possible to manufacture, by reheating slabs at a temperature above 1200 ° C and equal to or below 1300 ° C, grain oriented electrical steel sheets which have either the properties magnetic of conventional sheets (B 800 <1.86 T) by rolling with cold in two operations separated by an intermediate annealing and a rate of reduction in the second cold rolling between 40 and 70%, i.e. magnetic properties of sheets with high permeability (B 800> 1.88 T) thanks to in a single cold rolling operation preceded by annealing or in a rolling cold in two operations separated by an intermediate annealing, the rate of reduction of the final cold rolling operation being greater than 70%. Lowering the reheating temperature compared to the process known avoids the formation of liquid oxides fouling the oven.

In the case of final cold rolling, with a reduction rate greater than 70%, we can obtain either the magnetic properties of the sheets conventional (B800 <1.86 T), i.e. the magnetic properties of the sheets high permeability. (B800> 1.88 T)

It has been noted that the level of losses decreases and that the level of B800 increases as a function of the reduction in the mass oxygen content of the surface oxide film formed during the decarburization operation. The lowering of the oxygen content of the surface oxide film, mainly consisting of silica and containing less than 20% of iron oxide, below 800.10 ^-4 % (approximately 1.8 g of oxygen per m ² ) allows an improvement in magnetic properties, all the more marked as this reduction is high.

The method of the present invention described for slabs of continuous casting with thickness between 150 and 300 mm can be applied to thinner slabs, thickness between 15 and 100 mm approx.

The method of the present invention can also be applied to thin strips obtained by casting liquid steel between two rollers, thicker than 2 mm, the strips being heated between 1200 ° C and 1300 ° C, before undergoing hot rolling.

The number of passes of preliminary hot rolling and finishing hot rolling is a function of the thickness of the product cast in continuous and the thickness referred to in the hot rolled state. If the thickness of the continuously cast product is sufficiently low, hot rolling preliminary can be deleted.

The total duration of the heating cycle for the continuously cast product is a function of its thickness. The smaller this thickness, the faster the reheating temperature is reached at heart. Chemical analysis (mass percentage) of the slabs Steel mark Yes VS Mn S Al NOT Cu Sn 1 3.16 0.060 0.081 0.021 0.030 0.0074 0.09 <0.015 2 3.15 0.044 0.080 0.011 0.025 0.0086 0.151 <0.015 3 3.18 0.057 0.081 0.015 0.021 0.0072 0.154 <0.015 4 3.16 0.059 0.079 0.014 0.026 0.0073 0.150 <0.015 5 3.15 0.058 0.079 0.012 0.021 0.0093 0.151 <0.015 6 3.21 0.042 0.058 0.020 0.001 0.0042 0.205 <0.015 7 3.12 0.058 0.080 0.015 0.024 0.0074 0.149 <0.015 8 3.26 0.054 0.079 0.015 0.019 0.0066 0.010 0.069 9 3.12 0.056 0.078 0.011 0.019 0.0068 0.014 <0.015 Percentage of nitrogen precipitated after hot rolling Steel mark Slab reheating temperature (° C) Winding temperature (° C) % N precipitated 1 1400 640 40 2 1280 530 21 3 1280 640 24 530 3 4 1280 640 37 530 12 5 1245 640 34 8 1240 530 12 1280 530 7 9 1240 530 9 1280 530 3 Percentage of nitrogen precipitated after annealing the hot rolled sheet. Steel mark Slab reheating temperature (° C) Winding temperature (° C) % N precipitated 2 1280 530 75 3 1280 640 74 530 75 4 1280 640 77 530 82

Claims (14)

1. Process for manufacturing an oriented-grain electric steel sheet for producing, in particular, the magnetic circuits of transformers, comprising in succession:

   continuous casting of a steel in the form of a slab or strip of steel containing, in particular, in its composition by weight less than 0.1% of carbon, 2.5 to 4% of silicon and at least the elements aluminium, nitrogen, manganese, sulphur, copper, these being intended to form precipitates inhibiting normal growth,
   in which process the steel, of the following weight composition:

   0.02 to 0.09% of carbon,

   2.5 to 4% of silicon,

   0.027 to 0.17% of manganese,

   0.007 to 0.020% of sulphur,

   0.010 to 0.030% of aluminium,

   0.004 to 0.012% of nitrogen,

   0.06 to 0.50% of copper,

   optionally, up to 0.15% of tin,
   the balance being iron and impurities, among which less than 0.015% of phosphorus, is, after production of the slab or sheet by continuous casting, is subjected in succession to:

   reheating at a temperature greater than 1200°C and less than or equal to 1300°C, so that the AlN, MnS and CuS precipitates, taken singly or in combination, may go back into solution in a sufficient amount,

   hot rolling of the slab or strip in order to obtain a sheet having a thickness of between 1 and 5 mm during which the CuS precipitates trap some of the sulphur of the steel;

   hot coiling of the hot-rolled sheet between 500°C and 700°C;

   annealing of the hot-rolled sheet;

   cold rolling to a final thickness of less than 0.5 mm;

   primary recrystallization annealing and decarburization in a wet atmosphere;

   applying a separator based on magnesia MgO on at least one side of the decarburized sheet;

   final secondary recrystallization and purification annealing, before which, during the rise in temperature for this final secondary recrystallization annealing, the fine CuS precipitates gradually dissolve, the release of the sulphur being accompanied by a fine precipitation of MnS particles;

   applying an insulating coating and final baking annealing of the coating.
2. Process according to Claim 1, characterized in that the strip is decarburized so that the oxygen content of the oxide film formed is less than 800 × 10^-4%.
3. Process according to Claim 1, characterized in that the product resulting from multiplying the sulphur content by the manganese content is less than 160 × 10^-5.
4. Process according to Claim 1, characterized in that the product resulting from multiplying the nitrogen content by the aluminium content is less than 240 × 10^-6.
5. Process according to Claims 1 to 4, characterized in that the steel is hot rolled so as to precipitate the nitrogen in the form of fine particles, containing in particular nitrogen and aluminium, having an average diameter of less than 100 nanometres, the percentage of nitrogen precipitated being less than 40%.
6. Process according to Claims 1 to 5, characterized in that the hot-rolled steel is annealed so as to precipitate the nitrogen in the form of fine particles, containing in particular nitrogen and aluminium, having an average diameter of less than 100 nanometres, the percentage of nitrogen precipitated being greater than 60%.
7. Process according to Claims 1 to 6, characterized in that the steel is hot rolled so as to precipitate the sulphur in the form of particles, the average diameter of which is less than 100 nanometres.
8. Process according to Claims 1 to 7, characterized in that, after annealing the hot-rolled sheet at a temperature of between 850°C and 1150°C for 1 to 10 minutes and cooling, at a rate greater than 10°C per second, from 800°C, the cold rolling to a final thickness of less than 0.5 mm is carried out in a single operation comprising several passes, with an overall reduction ratio of greater than 70%, the temperature of the sheet being between 100 and 300°C for at least one cold- rolling pass.
9. Process according to Claims 1 to 7, characterized in that cold rolling, to a final thickness of less than 0.5 mm, is carried out in two operations with an intermediate anneal, at a temperature of between 850°C and 1150°C for 1 to 10 minutes, followed by cooling, at a rate greater than 10°C per second, from 800°C, the reduction ratio of the second cold-rolling operation being greater than 40%, the temperature of the sheet being between 100°C and 300°C for at least one cold-rolling pass when the reduction ratio of the second cold-rolling operation is greater than 70%.
10. Process according to Claim 9, characterized in that, prior to cold rolling in two operations, the sheet is annealed at a temperature of between 850°C and 1150°C for 1 to 10 minutes, in particular if the thickness of the sheet is less than 0.27 mm.
11. Process according to Claims 1 to 10, characterized in that the magnesia contains, as well as the optional additions of titanium dioxide, boron or a boron compound, at least one compound containing sulphur or nitrogen, a compound containing sulphur and nitrogen, or an antimony compound, these being taken alone or in combination.
12. Process according to Claims 1 to 10, characterized in that the magnesia contains, as well as the optional additions of titanium dioxide, boron or a boron-containing compound, sulphur or one or more sulphur compounds chosen from magnesium sulphate, manganese sulphate and sodium thiosulphate.
13. Process according to Claims 1 to 10, characterized in that the magnesia contains, as well as the optional additions of titanium dioxide, boron or a boron compound, at least one compound containing sulphur and/or nitrogen chosen from ammonium sulphate, ammonium thiosulphate, sulphamic acid and urea.
14. Process according to Claims 1 to 10, characterized in that the magnesia contains, as well as the optional additions of titanium dioxide, boron and/or a boron compound, antimony chloride.

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