DE102011107304A1 - Method for producing a grain-oriented electrical steel flat product intended for electrotechnical applications - Google Patents

Abstract

The invention relates to a method for producing a grain-oriented steel flat product for electrotechnical applications in which a melt cast into a strand, divided from the cast strand a thin slab, heated the thin slab and hot rolled into a hot strip, the hot strip cooled, coiled and cold rolled into a cold strip , decarburizing and annealing the cold strip, applying an annealing separator to the surface of the cold strip and finally annealing the cold strip to form a Gosstexture. The decarburizing and nitriding annealing step is performed in two stages, wherein the first annealing step comprises a first time interval comprising heating the cold strip from a start temperature to a first target annealing temperature and holding at that target annealing temperature and the second stage of annealing extends over a second time interval by heating the cold strip to a second desired annealing temperature and then maintaining it at that desired annealing temperature. The first target annealing temperature is 10-50 ° C lower than the second target annealing temperature and the duration of the first time interval is 30-70% of the total duration of the annealing treatment comprising the first and second time intervals.

Description

The invention relates to a method for producing grain-oriented electrical steel flat products intended for electrotechnical applications. Electrical steel flat products of this type are also referred to in practice as grain-oriented "electrical sheets" or grain-oriented "electrical tapes".

Grain-oriented electrical steel flat products have special magnetic properties and are produced by a complex manufacturing process. Base material for electrical steel flat products is a silicon steel sheet. The metallurgical properties of the material, the degree of deformation of the rolling processes and the parameters of the heat treatment steps are coordinated so that targeted recrystallization processes proceed. These recrystallization processes lead to a typical for the material "Goss texture" in which the direction of the easiest magnetization in the rolling direction of the finished strips.

To be distinguished from grain-oriented electrical sheet or strip of the type in question here are electrical steel flat products in which the grains have no pronounced orientation. In such non-grain oriented electrical steel or sheet, the magnetic flux is not fixed in any particular direction, so that the same magnetic properties are formed in all directions (isotropic magnetization).

Grain-oriented electrical steel or sheet of the type in question, on the other hand, has a strongly anisotropic magnetic behavior. This is due to a uniform orientation of the grains (crystallites) of the structure. This crystallographic texture is achieved by effective grain growth selection by taking appropriate measures in the manufacturing process. The aim is to obtain, after a final annealing at the end of the production process, an electrical steel flat product in which the grains have a slight misorientation and, consequently, an almost ideal texture.

Grain-oriented electrical steel is particularly suitable for applications in which particularly high demands are placed on the magnetic properties, as is the case, for example, in the construction of transformers.

Methods of producing high quality grain oriented electrical steel are known in greater numbers.

With the in the EP 0 910 676 B1 described, so-called "low-heating process" can produce highly permeable, grain-oriented electrical sheets with an optimized distribution of properties. Characteristic of this process is a slab heating temperature below 1250 ° C. As a result of this comparatively low temperature, aluminum nitrides, which are completely dissolved in the high-temperature annealing step at the end of the production process, are only partially dissolved and removed again. As a result, electrical steel produced by the low-heating process has weaker inherent inhibition than material produced by the high temperature slab heating conventional process path.

The purpose of the particle inhibition is to suppress the grain growth in the primary structure of the cold strip after and during the decarburization annealing. Only during the final annealing, in which the cold strips are annealed at temperatures of up to 1200 ° C, a controlled abnormal grain growth in the temperature range of 950-1100 ° C made in order to allow a high texture sharpness with Gossorientierung [001] (110) ,

In order for optimal, abnormal grain growth with high texture sharpness to begin, it is necessary to set an ideal equilibrium state between driving and restoring forces after decarburization annealing. The driving force for grain growth during annealing is the grain boundary energy stored in the microstructure. This is essentially determined by the particle size after the primary recrystallization.

Due to the weaker inherent inhibition in the low-heating process, the average primary grain size after a decarburization annealing is greater than in the conventional process and is subject to greater fluctuations due to the cold process. The driving force for abnormal grain growth is thus generally lower. On the other hand, a repulsive force opposing the abnormal grain growth is determined by the non-magnetic precipitates (inhibitors) precipitated in the cold strip. Therefore, it is important to have many finely divided particles. In the low-heating process will be However, the relevant particles are not produced in the hot strip but before, after or during the decarburization annealing or during the heating phase of the final annealing in the course of various nitriding processes.

In the course of in the EP 0 950 119 B1 and the EP 0 950 120 B1 described method is set by the hot rolling process an inhibiting strength Iz by nitrides and sulfides so that the primary grain growth is inhibited in the cold process, even at higher temperatures. The slabs are heated to temperatures of 1100 ° C to 1320 ° C before hot rolling. A nitriding treatment carried out simultaneously with the decarburization annealing at temperatures between 850 and 1050 ° C. in an ammonia-containing atmosphere enables the direct formation of aluminum nitrides. The subsequent high-temperature annealing need not be modified compared to the conventional production method of grain-oriented electrical steel production.

The opposite is in the in the EP 0 219 611 B1 Nitriding is carried out after the primary recrystallization but before the onset of abnormal grain growth. The nitration can be carried out here by an atmosphere with nitriding ability or by a nitrogen-donating adhesive protection additive.

Especially in the case of the method with an ammonia-containing atmosphere in which the nitriding temperature is below 850 ° C., after the nitration near the surface, silicon-manganese nitrides are present ( Materials Science Forum 204-206 (1996), 143-154 ). Due to their lower thermodynamic stability they dissolve during the heating of the annealing. The nitrogen then diffuses into the steel matrix and recombines with the free aluminum present there to aluminum nitride ( Materials Science Forum 204-206 (1996), 593-598 ). The resulting aluminum nitrides are then the inhibitors effective for secondary grain growth. Although the inhibition is weaker compared to the conventional process used for the production of grain-oriented electrical steel sheet, but allows a complete secondary recrystallization at higher temperatures with a larger secondary grain size in the finished strip ( TMS Proceedings 3 (2008), 49-54 ).

A disadvantage of this approach, however, is that a modified time-temperature cycle of the annealing is required. The dissolution of the silicon-manganese nitrides and the new formation of AlN by nitrogen diffusion occur at temperatures between 700 to 800 ° C. In order to make this crucial process step completely possible, an isothermal holding step of at least four hours during the heating phase of the high-temperature annealing is required in carrying out the method explained above. This not only requires a significant extension of the entire process duration, but also leads to increased costs in the production.

In addition to the above-described prior art is from the EP 1 752 549 A1 a method for the production of high-quality grain-oriented electrical steel based on thin slab continuous casting is known, in which the steps are coordinated so that using conventional aggregates an electrical sheet with optimized magnetic properties is obtained. The aim is to avoid the formation of nitridic precipitates before hot rolling and during hot rolling as possible in order to use the possibility of controlled production of such precipitates in the cooling of the hot strip on a large scale. Specifically, for this purpose, a steel is first melted, which in addition to iron and unavoidable impurities (in mass%) Si: 2.5-4.0%, C: 0.02-0.10%, Al: 0.01-0.065 % N: 0.003-0.015% optionally up to 0.30% Mn, up to 0.05% Ti, up to 0.3% P, one or more elements from the group S, Se in contents whose sum is at most 0, 04%, one or more elements from the group As, Sn, Sb, Te, Bi with contents of up to 0.2%, one or more elements from the group Cu, Ni, Cr, Co, Mo with contents of each containing up to 0.5% and one or more elements from the group B, V, Nb at levels of up to 0.012% each. The thus composed melt is then treated secondary metallurgically in a vacuum plant or a ladle furnace and then cast continuously into a strand. From the strand thus obtained thin slabs are divided, which are subsequently heated in a standing oven to a temperature between 1050 ° C and 1300 ° C. The residence time in the oven is at most 60 min. Following the heating of the thin slabs, the thin slabs are hot rolled into a hot strip of thickness 0.5-4.0 mm in a multi-stand hot bar mill in line. During hot rolling, the first forming pass is carried out at a temperature of 900-1200 ° C with a degree of deformation of more than 40%. Furthermore, during hot rolling, at least the two forming passes subsequent to rolling at 900-1200 ° C are rolled in the two-phase mixing region (α-γ). Finally, in the last cold forming of hot rolling, the reduction is not more than 30%. Following hot rolling, the resulting hot strip is cooled and coiled into a coil. Optional can then annealing of the hot strip after reeling or before cold rolling can be performed. Subsequently, the hot strip is cold rolled to a cold strip having a final thickness of 0.15 mm to 0.50 mm. The resulting cold strip is then annealed recrystallizing and decarburizing. In addition to the decarburization annealing, nitriding of the strip in a NH ₃ -containing atmosphere can also be carried out at temperatures above 850 ° C. After an annealing separator has subsequently been applied to the surface of the annealed cold-rolled strip, the cold-rolled strip coated in this way is finally re-crystallized to form a Gosstexture. Also optionally, the finally annealed cold strip can then be provided with an electrical insulation and finally annealed stress-free.

In the EP 0 378 131 B1 attention is drawn to the importance of mean grain size but also its variance. In addition to an optimal average grain size is therefore of particular importance that the deviation from the mean grain size in the sheet is low. This results from the fact that grain growth processes are more uncontrolled due to the lower inhibition ( Materials Science Forum 204-206 (1996), 623-628 ). As a result, under unfavorable process conditions, grains that are not goss-oriented but can not grow at high temperatures can contribute to fine grain formation.

In the EP 0 392 534 B1 Finally, the candidate atmospheres of decarburization annealing are described in detail. In this context, it should be noted that at the beginning of the decarburization and nitriding annealing, the partial pressure p _H2O / p _H2 must be lowered in order to set a suitable oxide layer. The result of this process is a satisfactory formation of the glass film during the annealing.

Against the background of the prior art explained above, the object of the invention was to provide a method with which it is possible in a simple manner to produce grain-oriented electrical steel flat products with an optimally uniform distribution of the grain size.

This object has been achieved by a method which comprises the measures and features specified in claim 1.

Advantageous embodiments of the invention are specified in the dependent claims and are explained below as the general inventive concept in detail.

• a) Erzeugen einer Stahlschmelze, die neben Eisen und unvermeidbaren Verunreinigungen (in Gew.-%) Si: 2,5 4,0%, C: 0,02–0,1%, Al: 0,01–0,065%, N: 0,003–0,015% sowie jeweils wahlweise bis zu 0,30% Mn, bis zu 0,05% Ti, bis zu 0,3% P, eines oder mehrere Elements aus der Gruppe S, Se in Gehalten, deren Summe höchstens 0,04% beträgt, eines oder mehrere Elemente aus der Gruppe As, Sn, Sb, Te, Bi mit Gehalten von jeweils bis zu 0,2%, eines oder mehrere Elemente aus der Gruppe Cu, Ni, Cr, Co, Mo mit Gehalten von jeweils bis zu 0,5%, eines oder mehrere Elemente aus der Gruppe B, V, Nb mit Gehalten von jeweils bis zu 0,012%, enthält,
• b) Vergießen der Schmelze zu einem Strang in einer Stranggussmaschine,
• c) Abteilen mindestens einer Dünnbramme von dem gegossenen Strang,
• d) Erwärmen der Dünnbramme auf eine Temperatur zwischen 1050°C und 1300°C,
• e) Warmwalzen der Dünnbramme in einer Warmwalzstraße zu einem Warmband mit einer Dicke von 0,5–4,0 mm,
• f) Abkühlen des Warmbands,
• g) Haspeln des Warmbands zu einem Coil,
• h) Kaltwalzen des Warmbands zu einem Kaltband mit einer Enddicke von 0,15–0,50 mm,
• i) entkohlendes und nitrierendes Glühen des erhaltenen Kaltbands,
• j) Auftrag eines Glühseparators auf die Oberfläche des geglühten Kaltbands, und
• k) Schlussglühen des mit dem Glühseparator versehenen Kaltbands zur Ausprägung einer Gosstextur.

In accordance with the prior art explained above, a method according to the invention for producing a grain-oriented electrical steel flat product intended for electrical applications comprises the following steps:

• a) producing a molten steel, which in addition to iron and unavoidable impurities (in wt .-%) Si: 2.5 4.0%, C: 0.02-0.1%, Al: 0.01-0.065%, N 0.003-0.015% and each optionally up to 0.30% Mn, up to 0.05% Ti, up to 0.3% P, one or more elements from the group S, Se in contents whose sum is at most 0, 04%, one or more elements from the group As, Sn, Sb, Te, Bi with contents of up to 0.2%, one or more elements from the group Cu, Ni, Cr, Co, Mo with contents of each containing up to 0.5%, one or more elements from the group B, V, Nb, each containing up to 0.012%,
• b) casting the melt into a strand in a continuous casting machine,
• c) separating at least one thin slab from the cast strand,
• d) heating the thin slab to a temperature between 1050 ° C and 1300 ° C,
• e) hot rolling the thin slab in a hot rolling mill to a hot strip having a thickness of 0.5-4.0 mm,
• f) cooling the hot strip,
• g) coiling the hot strip into a coil,
• h) cold rolling the hot strip to a cold strip having a final thickness of 0.15-0.50 mm,
• i) decarburizing and nitriding annealing of the cold strip obtained,
• j) applying an annealing separator to the surface of the annealed cold-rolled strip, and
• k) final annealing of the cold strip provided with the annealing separator for embossing a Gosstextur.

Of course, in the production of the electrical steel flat product additional operations may be performed which are usually required in the conventional production of grain-oriented electrical tapes or sheets. These include, for example, a one- or multi-stage hot strip annealing performed between steps g) and h), a thermal straightening of the cold strip and the order of an insulating layer, which can be carried out in the context of the inventive method using and taking into account the parameters known from the prior art.

Essential for the invention is that the cold strip in the course of the process step i) "decarburizing and nitriding annealing of the obtained cold strip" in at least two stages decarburizing and nitriding annealed.

According to the invention, the first stage of this annealing extends over a first time interval, which comprises heating the cold strip starting from a starting temperature to a first desired annealing temperature and then holding it at this desired annealing temperature.

The second stage of the annealing extends in a corresponding manner according to the invention over a second time interval, within which the cold strip is first heated to a second target annealing temperature and then maintained at this target annealing temperature.

According to the invention, the first target annealing temperature is 10-50 ° C. lower than the second target annealing temperature. At the same time, according to the invention, the duration of the first time interval is 30-70% of the total duration of the annealing treatment comprising the first time interval and the second time interval.

The invention is based on the recognition that can be produced by an at least two-stage "step annealing" during the step i) a cold strip in which on the one hand the grains have an optimal mean grain size and on the other hand, the deviation of the grain size of the individual grains of the average grain size is low.

In practice, this can be achieved by passing the cold strip for decarburizing and nitriding annealing obtained after cold rolling through a continuous annealing furnace divided into at least two zones, setting a target annealing temperature in its first zone passed through first in accordance with the invention is, which is 10-50 ° C lower than the target annealing temperature in the subsequently passed by the cold strip second zone of the furnace, wherein the duration of the time interval within which the first stage of the annealing runs, 30-70% of the total duration of the decarburizing and nitriding annealing is. Due to the inventively predetermined temperature difference between the first and second stages of the decarburizing and nitriding annealing and the inventively provided for the two stages of this annealing times an excessive grain growth of unfavorable for the Gosstexturausbildung orientations is suppressed. The cold band structure obtained after annealing thus has a significantly smaller variance with the same mean grain size set by the higher anneal temperature anneal in the back furnace zone, thus allowing a homogeneous annealing during the final annealing performed at a high temperature secondary grain growth.

With the method according to the invention, it is thus possible to minimize a variance of the grain sizes which has occurred in the course of the cold rolling process. Overall, the result of the previous cold process is stabilized against fluctuations in the particle size distribution. An electric flat steel product produced according to the invention thus has a crystallographic texture after the annealing treatment carried out after at least two stages following the cold rolling, which optimally ensures homogeneous secondary grain growth during the final high-temperature annealing.

The invention combines in this way the known from the low-heating process approach with a modern thin-slab production, which takes place according to the known, characterized by a continuous manufacturing process casting-rolling process. As a result, an electrical steel flat product is available in accordance with the method according to the invention which has optimum magnetic properties in relation to the uses typical of grain-oriented electrical sheets or tapes.

If a nitriding and decarburizing annealing (working step i) carried out according to the invention in at least two stages is mentioned here, this does not mean that a combined nitriding and decarburization always necessarily has to take place in both stages of this annealing.

Instead, the first stage of this annealing carried out according to the invention can also be carried out as a pure heating stage and the decarburization and nitration take place in the second stage. It is also conceivable to carry out a decarburization over the two annealing stages and then in another Annealing to perform a residual decarburization and nitriding. Alternatively, the decarburization and nitriding can take place in succession distributed over the at least two stages of the annealing carried out according to the invention. Finally, it is also conceivable to have at least one of the inventive annealing stages carried out without decarburization or nitriding and to finish the decarburization and nitriding only in an annealing step following the two stages of the annealing according to the invention.

Accordingly, in the context of the invention in practice in step i) 1.i) the first and second stages of the annealing can be completed following each other and subsequently a further annealing step are carried out in which the cold strip is subjected to a decarburizing and nitriding annealing. The first and second stages of the annealing in step i) can be carried out taking into account the invention for these annealing stages in terms of the position of the temperature levels and the time portion of the first annealing stage based on the total time of the annealing stages. Subsequently, a further annealing step takes place in which decarburization and nitriding are carried out in a conventional manner. Overall, in this variant of the invention in the course of the step i) so at least three Teilglühschritte completed successively, apply to the first two annealing steps, the specifications of the invention, and the third, nitriding comprehensive step is completed in a conventional manner.

Practical tests have shown that optimum properties of an electrical steel flat product produced according to the invention result when the target first stage annealing temperature is 10-30 ° C. lower than the target second stage annealing temperature of the anneal.

Likewise, it has a favorable effect on the result of the at least two-stage annealing step carried out according to the invention if the duration of the first time interval is limited to 30-60% of the total duration of the annealing treatment.

The heating of the cold strip to the desired temperature of the first annealing stage should be as fast as possible. During the heat-up phase of decarburization and nitriding annealing, the cold-worked strip first undergoes recovery. Then the primary recrystallization begins. At higher temperatures and longer annealing times, grain growth processes are added. In order to provide as much stored energy as possible for recrystallization, the temperature range of the recovery should be run through quickly. For this purpose, an advantageous embodiment of the invention provides that the heating rate at which the cold strip is heated in the first stage of the annealing of the starting temperature to the first target annealing temperature, 25-500 ° C / s. In a conventional heating, the heating rate is typically 30-70 ° C / s. In view of a particularly good primary recrystallization and, consequently, optimal work results, it may also be advantageous to set particularly fast heating rates of 200-500 ° C / s. In practice, such a rapid heating rate can be achieved, in particular in the case of continuous production, in that an inductive rapid heating takes place at the entrance of the respective continuous furnace, in which the cold strip is heated by the action of an electromagnetic field induced in the strip.

The invention will be explained in more detail by means of exemplary embodiments. Show it:

Diag. 1 shows a schematic representation of the temperature curve T over the annealing time t for a conventionally annealed electrical steel strip (curve A) and an electrical steel strip according to the invention (curve B);

Diag. 2 shows the polarization at 800 A / m in Tesla for two differently composed electrical steel sheets S1, S2, plotted over the ratio t ₁ / t _{2 of} the duration of the time interval t _{1 of} the annealing stage according to the invention for the first annealing stage to the total duration t _{2 of} the annealing.

In a conventional continuous casting machine, four S1-S4 molten steels having the compositions shown in Table 1 were continuously poured into a 63 mm thick strand following a secondary metallurgical treatment carried out in a ladle furnace and a vacuum plant.

From the strand thin slabs have also been divided in a conventional manner. After an equalizing annealing in an equalizing furnace at 1165 ° C, these thin slabs were descaled and hot rolled in the finishing train to a final thickness of 2.34 mm and coiled into a coil.

Example 1:

The hot strips produced in the manner described above have been subjected to a two-stage hot strip annealing. The annealing temperature in the first stage of the hot strip annealing was 1090 ° C, while the annealing temperature in the second stage was 850 ° C. Instead of a two-stage hot strip annealing, a single-stage hot strip annealing with a uniform uniform annealing temperature could have been carried out.

After the hot strip annealing, the annealed hot strip was cold rolled in one stage with a degree of deformation of 87% to a final thickness of 0.285 mm. Sheet samples have been separated from the cold strips thus obtained.

A comparison group A of these sheet samples has been annealed in a continuous annealing furnace. Here, in a first furnace section first passed through, a 150 second annealing step was first carried out at a temperature of 860 ° C. under a moist atmosphere formed from a hydrogen / nitrogen mixture (p _H2O / p _H2 = 0.50). Then, in a second furnace section passed after the first furnace section, a second annealing step lasting 30 seconds was carried out under a humid atmosphere composed of an ammonia / hydrogen / nitrogen mixture to effect residual decarburization and nitriding. The temperature of the annealing was always 910 ° C. Corresponding to the above-mentioned, for practice important embodiment of the invention was thus here the annealing in step i) of the method according to the invention divided into two annealing steps, of which the first annealing step according to the invention predetermined subdivision has been performed again in two annealing stages on the following has been completed as a second annealing step a conventional decarburizing and nitriding annealing. Overall, the step i) has thus been completed here in three consecutive parts.

In a corresponding sequence of operations, a second group B of the sheet metal samples has first been annealed in the course of the first annealing step in two consecutive annealing stages according to the invention and then restet carburized and nitrided in a second annealing step. Five variants Ba) -Be) of the two-stage annealing according to the invention have been investigated. In the first annealing stage, which proceeds over a first duration t ₁ , in each case one target annealing temperature T ₁ and in the second annealing stage a respective target annealing temperature T ₂ have been set. The total duration t _{2 of} the two consecutively completed annealing stages was also 150 s in this case. The first stage of the first annealing section also comprised a rapid heating to the respective desired annealing temperature T ₁ taking place at a heating rate of 40 ° C./sec.

In Diag. 1, the temperature profile during the annealing in the first annealing step is shown in a dashed line over the annealing time t on the one hand for the group A electrical sheet samples produced for comparison in a solid line and on the other hand for one of the variants B.a) -B.e).

Accordingly, the first two annealing stages of the variant exemplified here of the method according to the invention are mainly used for decarburization and are optimized in this regard with respect to gas composition and temperature. The Entkohlungsglühung takes place in two stages with respect to the temperature control in such a way that in the first passed first section is first gently decarburized to avoid grain enlargements as possible, and in the subsequently passed section at the optimal temperature for the effectiveness of the decarburization decarburization is continued and terminated.

In contrast, the third annealing step of the process according to the invention is optimized with respect to nitriding. At the same time a residual decarburization occurs here to a small extent. The optimization of the third annealing stage in relation to the nitriding is done essentially by the choice of an optimized gas composition, but may also mean a temperature adjustment. In Diag. By way of example, FIG. 1 shows a correspondingly performed temperature control on a small temperature jump which occurs after the annealing time t _{2 has} elapsed.

Specifically, for the implementation of the inventive Glühbehandlungsvarianten Ba) -Be), the first furnace section of the continuous annealing furnace has been divided into two equal-length temperature zones, for their passage each required to be glowing sheet metal samples so each 75 s. Accordingly, in these tests, the duration t _{1 of} the first annealing stage was 50% of the total duration t ₂ of 150 s.

In the first temperature zone of the first furnace section, which is first passed through by the respective sample, the desired annealing temperature of the variant is sufficient for carrying out the experiments according to the invention Variant has been changed, while in the second temperature zone in the implementation of the second annealing stage in each case a constant, 860 ° C amounting target annealing temperature has been set. The two annealing stages carried out according to the invention in the first furnace section of the continuous annealing furnace have each been carried out under a moist atmosphere formed from a hydrogen / nitrogen mixture (p _H2O / p _H2 = 0.50), as in the processing of the sheet samples of group A.

Then, as in the case of treating the comparative samples of group A in the second furnace section adjoining the first furnace section, a decarburizing and nitriding annealing was carried out for 30 seconds under a humid atmosphere consisting of an ammonia / hydrogen / nitrogen mixture. In the meantime, the target annealing temperature in the second annealing step was 910 ° C.

After annealing, the samples were subsequently coated with magnesia and final annealed under an annealing atmosphere consisting of 50% by volume of H2 and 50% by volume of N2.

In Table 2, for each variant a) -e) of the heat treatment according to the invention, the target annealing temperature T ₁ set in the first annealing stage, the difference ΔT between the first target annealing temperature and the target annealing temperature of the second annealing stage and the polarization J ₈₀₀ at 800 A / m, given in Tesla, and the loss of magnetization P _1.7 given in W / kg at a polarization of 1.7 T and a respective frequency of 50 Hz. It turns out that the electrical steel sheets produced according to the invention, irrespective of which of the variants a) -e) they are produced, have better properties than the samples which have been heat-treated in a conventional manner.

Example 2:

Hot strips produced from the melt 1 as discussed above were subjected to a two-stage hot strip annealing at 1130 ° C / 900 ° C, hot strips of the melt 2 to a one step hot strip annealing at 980 ° C. Subsequently, the hot-rolled strip was cold rolled with a degree of deformation of 87% in one stage to 0.285 mm thick cold strips. From the obtained cold tapes sheet samples have been divided.

In this case as well, for comparison, a group A of electric sheet samples obtained from the cold tapes is held for 150 seconds at a temperature of 840 ° C in a wet hydrogen / nitrogen mixed atmosphere (p _H2O / p _H2 = 0.45). been annealed. This was followed by annealing in a humid ammonia / hydrogen / nitrogen mixture at 860 ° C. for 30 seconds, with residual decarburization and nitriding. Subsequently, as in Example 1, nitrided at 910 ° C and restentarbohlt.

A second group B of samples was annealed in the same atmosphere according to the invention in two stages in the first process part of the continuous furnace used. The temperature of the first furnace zone was set at 810 ° C (ΔT = 30 ° C). Also in this case, five variants Ba) -Be) were shown. The annealing time t ₁ to increase the target annealing temperature in the second part of the annealing to 840 ° C was in the variant Ba) 120 s (annealing time ratio t ₁ / t ₂ = 80%), in the variant Bb) 90 s (t ₁ / t ₂ = 60%), in variant Bc) 75 s (t ₁ / t ₂ = 50%), in variant Bd) 45 s (t ₁ / t ₂ = 30%) and in variant Be) 30 s (t ₁ / t ₂ = 20%). Subsequently, as in Example 1, nitriding and residual decarburization were also carried out at 910.degree.

The electric sheet samples were then subsequently coated with magnesium oxide and finally annealed under an annealing atmosphere consisting of 50% by volume of H2 and 50% by volume of N2.

In Diag. 2, the polarization J _{800 is} plotted over the annealing time t _{1 of} the first stage of the annealing according to the invention for the samples prepared from the melts 1 and 2 in the manner according to the invention.

Example 3:

Hot melts of melts 1 and 2 were subjected to a single-stage hot strip annealing at 950 ° C. This was followed by one-stage cold rolling to cold-rolled strip with a final thickness of 0.165 mm. From the obtained cold tapes sheet samples have been divided.

A first group A of the sample sheets separated from the cold strip was annealed for a period of 130 seconds at a temperature of 880 ° C in a wet hydrogen / nitrogen mixture atmosphere (p _H2O / p _H2 = 0.44). This was followed by annealing at 900 ° C. for 30 seconds in a humid ammonia / hydrogen / nitrogen atmosphere. In the course of this second annealing step, on the one hand, residual carbon was used and, on the other hand, nitrided.

A second group B of sheet metal samples was annealed in the same atmosphere in the first part of the process used in the experiments reported here in two stages, during the first, until the 70th second (t ₁ / t ₂ ~ 55%) continuous annealing stage of the annealing a target annealing temperature of 850 ° C and then in the second, from the 70th to the 130th second annealing stage a target annealing temperature of 880 ° C was set. Subsequently, as in Example 1, nitriding and residual decarburization were also carried out at 900 ° C. in each case.

After this annealing treatment of the electric sheet samples, they are each subsequently coated with magnesium oxide and finally annealed under an annealing atmosphere consisting of 50% by volume of H2 and 50% by volume of N2.

The magnetic properties J ₈₀₀ and P _{1.7 of} the samples produced according to the invention and for comparison are summarized in Table 3. It also shows here the superiority of the products produced according to the invention.

Example 4:

Hot rolled strip produced from the melt 3 in the above-described manner has undergone a two-stage hot strip annealing at 1070 ° C./950 ° C. and cold rolled in one step to a cold strip having a final thickness of 0.215 mm. From the obtained cold tapes sheet samples have been divided.

A first group A of the sheet samples was annealed for a period of 120 seconds at a temperature of 870 ° C in an atmosphere consisting of a wet hydrogen / nitrogen mixture (p _H2O / p _H2 = 0.51). This was followed by annealing at 910 ° C. for 30 seconds under an atmosphere consisting of a moist ammonia / hydrogen / nitrogen mixture in which on the one hand a residual decarburization and on the other a nitration took place.

A second group B of sheet metal samples has been annealed in a moist hydrogen / nitrogen mixture with p _H2O / p _H2 = 0.51 according to the invention in a first annealing step divided in two stages according to the invention in the first furnace section of the continuous annealing furnace used here. In this case, in a first annealing stage lasting up to the 65th second, the setpoint annealing temperature was set to 850.degree. C., while the target annealing temperature in the second annealing stage, which lasted from the 70th to the 120th second, was 870.degree. After completion of the first two-stage annealing step completed in this manner in the first furnace section, the sheet samples were nitrided at 910 ° C in a wet ammonia / hydrogen / nitrogen mixture and restetalled.

All sheets were subsequently coated with magnesium oxide and finally annealed under an annealing atmosphere consisting of 50% by volume of H2 and 50% by volume of N2.

In the present example, the first annealing stage of the first annealing step, as in the above-described examples, involved rapid heating of the sheet samples to the desired annealing temperature of the first annealing stage. In order to show the influence of the heating rates "AHR", in the present example 4 the heating rates AHR were varied in four different experimental runs under otherwise unchanged conditions. (Experiment 4.1: AHR = 70 ° C / s, experiment 4.2: AHR = 150 ° C / s, experiment 4.3: AHR = 300 ° C / s, experiment 4.4: AHR = 500 ° C / s).

The magnetic characteristics of the electrical steel sheets thus obtained are summarized in Table 4. melt Si [%] C [ppm] Al [ppm] N [ppm] Mn [ppm] S [ppm] Cu [ppm] Sn [ppm] Cr [ppm] S1 3.10 470 260 93 1470 78 1950 580 1140 S2 3.32 580 287 105 1390 82 1810 720 780 S3 3.24 720 320 87 1580 92 1470 630 820 S4 2.90 450 348 112 1420 85 1610 1050 930 Table 1 In% by weight or by weight ppm;
Remaining iron and unavoidable impurities Variant Ba) according to the invention T ₁ : 800 ° C. ΔT: 60 ° C. Variant Bb) according to the invention T ₁ : 820 ° C. ΔT: 40 ° C. J ₈₀₀ [T] P _1.7 [W / kg] J ₈₀₀ [T] P _1.7 [W / kg] Melt 1 1,865 1,317 1,920 1,038 Melt 2 1,845 1,432 1,909 1,089 Melt 3 1,872 1,238 1,914 1,101 Melt 4 1,853 1,365 1,918 1,030 Variant Bc) According to the invention T ₁ : 830 ° C. ΔT: 30 ° C. Variant Bd) According to the invention T ₁ : 840 ° C. ΔT: 20 ° C. J ₈₀₀ [T] P _1.7 [W / kg] J ₈₀₀ [T] P _1.7 [W / kg] Melt 1 1,931 1,017 1,922 1,048 Melt 2 1,915 1,056 1,910 1,062 Melt 3 1,924 1,088 1,924 1,087 Melt 4 1,923 1,026 1,919 1,031 Variant Be) according to the invention T ₁ : 850 ° C. ΔT: 10 ° C. Comparative Experiment Not according to the invention T ₁ : 860 ° C. ΔT: 0 ° C. J ₈₀₀ [T] P _1.7 [W / kg] J ₈₀₀ [T] P _1.7 [W / kg] Melt 1 1,910 1,113 1,904 1,123 Melt 2 1,908 1,077 1,899 1,092 Melt 3 1,915 1,092 1,890 1,116 Melt 4 1,899 1,104 1,882 1,107 Table 2 Reference 880 ° C invention J ₈₀₀ [T] P _1.7 [W / kg] J ₈₀₀ [T] P _1.7 [W / kg] Melt 1 1,867 0.882 1,899 0.803 Melt 2 1,873 0.853 1,901 0.779 Table 3 J ₈₀₀ [T] P _1.7 [W / kg] Experiment 4.1 AHR = 70 ° C / s according to the invention 1,879 0.879 Experiment 4.2 AHR = 150 ° C / s according to the invention 1,885 0.853 Experiment 4.3 AHR = 300 ° C / s according to the invention 1,904 0.837 Experiment 4.4 AHR = 500 ° C / s according to the invention 1,903 0.842 Comparative samples not according to the invention 1,872 0.894 Table 4

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 0910676 B1 [0007]
• EP 0950119 B1 [0011]
• EP 0950120 B1 [0011]
• EP 0219611 B1 [0012]
• EP 1752549 A1 [0015]
• EP 0 378 131 B1 [0016]
• EP 0392534 B1 [0017]

Cited non-patent literature

• Materials Science Forum 204-206 (1996), 143-154 [0013]
• Materials Science Forum 204-206 (1996), 593-598 [0013]
• TMS Proceedings 3 (2008), 49-54 [0013]
• Materials Science Forum 204-206 (1996), 623-628 [0016]

Claims (8)

A method for producing a grain-oriented electrical steel flat product intended for electrotechnical applications, comprising the following steps: a) producing a molten steel which, in addition to iron and unavoidable impurities (in% by weight) Si: 2.5-4.0%, C: 0.02 -0.1%, Al: 0.01-0.065%, N: 0.003-0.015%, optionally - up to 0.30% Mn, - up to 0.05% Ti, - up to 0.3% P, One or more elements from the group S, Se in contents of which the sum is not more than 0.04%, one or more elements from the group As, Sn, Sb, Te, Bi in each case up to 0.2% , One or more elements from the group Cu, Ni, Cr, Co, Mo with contents of in each case up to 0.5%, one or more elements from the group B, V, Nb with contents of up to 0.012% each , b) pouring the melt into a strand in a continuous casting machine, c) separating at least one thin slab from the cast strand, d) heating the thin slab to a temperature ur between 1050 ° C and 1300 ° C, e) hot rolling the thin slab in a hot rolling mill to a hot strip having a thickness of 0.5-4.0 mm, f) cooling the hot strip, g) coiling the hot strip into a coil, h Cold rolling of the hot strip to a cold strip having a final thickness of 0.15-0.50 mm, i) decarburizing and nitriding annealing of the obtained cold strip, j) application of an annealing separator to the surface of the annealed cold strip, k) final annealing of the annealing separator provided Cold strip for embossing a Gosstextur, characterized in that - the cold strip in the course of the step i) is annealed in at least two stages, - that the first stage of this annealing extends over a first time interval and a heating of the cold strip, starting from a starting temperature to a first target annealing temperature and then holding the heated cold strip at the target annealing temperature comprises, - that the second stage of the annealing over a z extends a long time interval within which the cold strip is first heated to a second target annealing temperature and then maintained at this target annealing temperature, - that the first target annealing temperature is 10-50 ° C lower than the second target annealing temperature, and the duration of the first time interval is 30-70% of the total duration of the annealing treatment comprising the first time interval and the second time interval. Method according to one of the preceding claims, characterized in that the first target annealing temperature is lower by 10-30 ° C than the second target annealing temperature. Method according to one of the preceding claims, characterized in that the duration of the first time interval amounts to 30-60% of the total duration of the annealing treatment. Method according to one of the preceding claims, characterized in that the heating rate at which the cold strip is heated in the first stage of the annealing of the starting temperature to the first target annealing temperature, 25-500 ° C / s. A method according to claim 4, characterized in that the heating rate is at least 200 ° C. A method according to claim 5, characterized in that the heating of the cold strip is effected inductively. Method according to one of the preceding claims, characterized in that in step i) 1.i) the first and second stages of the annealing are completed following each other and subsequently a further annealing step is carried out in which subjected the cold strip of a decarburizing and nitriding annealing becomes. A method according to claim 7, characterized in that the first and second stage of the annealing in step i) are carried out as pure decarburization annealing.

Images