EP2942417A1 - Method for producing high permeability grain-oriented electrical strip - Google Patents

Abstract

The invention relates to a method for the production of grain-oriented electrical steel, in which a steel containing (in mass%) 2.5-4.0% Si, 0.01-0.10% C and optional contents of other alloying elements and as Contains residual Fe and unavoidable impurities, is cast into thin slabs, which are processed in a conventional manner to a cold-rolled strip with a final thickness of 0.15 to 0.50 mm. The cold strip is recrystallized and decarburizing annealed and subjected to a nitriding treatment at 850-1050 ° C. Subsequently, an annealing separator is applied to the cold strip and the other typical for the production of electrical steel finishing work steps go through. According to the invention, after the nitriding and before the application of the annealing separator, a post-oxidation annealing takes place in which the cold strip is annealed for 15-180 s at 750-900 ° C. under a hydrogen-containing annealing atmosphere whose partial pressure ratio p H2O / p H2 lies within an area. which is defined by the vertices P1 (T NO = 750 ° C, p H2O / p H2 = 0.1), P2 (T NO = 900 ° C, p H2O / p H2 = 0.2), P3 (T NO = 750 ° C; p H2O / p H2 = 0.42) and P4 (T NO = 900 ° C; p H2O / p H2 = 0.52).

Description

For the production of highly permeable grain-oriented electrical steel, in technical language also referred to as "KO-Elektroband", different methods exist.

Grain-oriented electrical tapes or sheets are characterized by a particularly sharp {110} <001> texture, which has an easy magnetization direction parallel to the rolling direction. Such a texture is called after their discoverer also "Goss texture".

The formation of the Goss texture occurs via a selective anomalous grain growth, which is also called secondary recrystallization. In this case, the natural tendency of a metallic matrix to increase grain size is suppressed by the presence of grain growth inhibitors, which in technical language are also called "inhibitors" or "inhibitor phase" for short.

The inhibitor phase consists of very fine and homogeneously distributed particles of one or more foreign phases. The particles in question have at their Interface to the matrix already has a natural interface energy. This obstructs a grain boundary moving across it, because the further savings in interfacial energy in the overall system are greatly reduced.

The inhibitor phase therefore has a central importance for the formation of the Goss texture and, consequently, for the magnetic properties of the respective material.

Purpose of particle inhibition, it is possible 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, controlled abnormal grain growth at 950 - 1100 ° C to allow a high texture sharpness with Gossorientierung {110} <001>.

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 decarburization annealing is greater than in the conventional process and subject greater fluctuations due to the cold process. The driving force for abnormal grain growth is thus generally lower.

On the other hand, the restoring force is determined by the non-magnetic precipitates (inhibitors) precipitated in the cold strip. It is important to have many finely divided particles present.

One of the methods for producing KO electrical steel is known as "low-heating method". In this process, a suitably composite steel is cast into slabs which, after solidification, are annealed at comparatively low slab preheating temperatures below 1300 ° C, typically at 1250 ° C or below. In the so-called low-heating method, therefore, 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 with a variety of nitriding.

In the in the EP 0 950 119 B1 and EP 0 950 120 B1 described method is set through 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. Before hot rolling, the slabs are heated to 1100 - 1320 ° C. A nitriding treatment carried out simultaneously with the decarburization annealing at temperatures of 850 ° -1050 ° C. in an atmosphere containing ammonia allows the direct Formation of aluminum nitrides. The subsequent high-temperature annealing does not have to be modified compared with the conventional production method of KO electrical steel production.

The opposite is in the EP 0 219 611 B1 A method is described in which 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 when nitriding under an ammonia-containing atmosphere and at a nitriding temperature of less than 850 ° C, silicon nitride nitrides are present near the surface after nitriding ( Materials Science Forum 204-206 (1996), 143-154 ). Due to the lower thermodynamic stability they dissolve during the heating of the annealing. The nitrogen 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 aluminum nitrides are then the effective inhibitors of secondary grain growth.

Nevertheless, the inhibition effected in this way is weaker in comparison with the conventional KO process, but allows a complete secondary recrystallization at higher temperatures with a larger secondary particle size in the finished strip ( TMS Proceedings 3 (2008), 49-54 ). The disadvantage of this approach is the need for a modified time-temperature cycle of high annealing. The dissolution of silicon-manganese nitrides or the formation of new AlN by nitrogen diffusion takes place at temperatures between 700 to 800 ° C. To fully facilitate this critical process step, an isothermal hold of at least four hours is required during the heat up phase of the anneal.

Another method for producing grain-oriented electrical steel is from the EP 1 752 548 A1 known. In this process, so-called CGO material ( C onventional G rain O riented - material) based on thin slab continuous casting is produced by the fact that based on a molten steel with (in mass%) Si: 2.5 - 4.0% , C: 0.01-0.10%, Mn: 0.02-0.50%, S and Se in contents whose sum is 0.005 to 0.04%, remainder iron and unavoidable impurities, the steps "secondary metallurgical treatment the melt in a vacuum plant and / or a ladle furnace "," continuous casting of the melt into a strand "," dividing the strand into thin slabs "," heating the thin slabs in a line furnace "," continuous hot rolling of the thin slabs in a "Hot rolled strip multistage hot rolling mill", "hot strip cooling, hot strip rolling", "cold rolling of hot strip to cold strip", "recrystallizing and decarburizing annealing of cold strip", "annealing separator annealing", "final annealing of the annealed annealing strip Cold tapes for expressing a Gosstextur "are so coordinated that an electric sheet with optimized electro-magnetic properties is obtained using conventional aggregates.

A comparable procedure is from the EP 1 752 549 A1 for the production of so-called HGO material (H ighly G rain O riented - material) are known, in which case from a steel alloy having (in mass%) Si: 2,5 - 4,0%, C: 0.02 - 0.10%, Al: 0.01-0.065%, N: 0.003-0.015%, balance iron and unavoidable impurities.

A general disadvantage of all variants of low-heating processes described here, in which the inhibition by ammonia-containing atmospheres is set, is the composition and morphology of the oxide layer obtained after the nitriding process. It is irrelevant at which temperatures previously decarburized and nitrided.

During nitriding, nitrides are precipitated in the near-surface layers and in the oxide layer because the solubility limit of nitrogen in Fe-Si is exceeded. In general, the nitrogen excreted in the nitration zone has a high recombination pressure, so that z. T. molecular nitrogen is precipitated in the grain boundaries of the oxide / nitriding layer ( Z. Werkst. Wärmebeh. Manufacturing 63 (2008), 3 ). The consequence of this phenomenon are cracks and pores in the oxide layer, which adversely affect the oxidation during the glass film formation in the course of the cold strip process.

Another disadvantage of in EP 0 219 611 B1 or EP 0 494 730 B1 The process described is the longer time required to allow the dissolution and re-precipitation of nitrides via diffusion processes in the annealing. For this either the heating rate must be lowered or a holding step at medium temperatures of 500 - 800 ° C are performed.

Against the background of the prior art discussed above, the object of the invention was to provide a method which enables the production of grain-oriented electrical steel with improved surface properties and with which, moreover, the dissolution of the nitrides during the heating phase of the high-annealing can be accelerated.

To solve this problem, the invention proposes the method 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.

The invention is therefore based on the conventional processes for the production of KO electrical steel, in which a suitably composed melt is cast into thin slabs and in which the nitration is carried out simultaneously to the decarburization in an ammonia-containing atmosphere at temperatures above 850 ° C. Appropriate procedures are in the EP 1 752 548 A1 , of the EP 1 752 549 A1 , of the EP 0 950 119 B1 and the EP 0 950 120 B1 , the contents of which are hereby incorporated into the present disclosure. From these publications, the usual steps and parameters are known, which is applied and considered by a person skilled in the production of grain-oriented electrical steel and which are also relevant here.

According to the invention, the processes described in the above-mentioned publications are supplemented by post-nitriding after nitriding, in which the oxide layer present on the surface and the precipitation zone of the KO electrical strip produced according to the invention are passivated.

1. a) Erschmelzen eines Stahls, der neben Eisen und unvermeidbaren Verunreinigungen (in Masse-%) Si: 2,5 - 4,0 %, C: 0,01 - 0,10 % und zusätzlich jeweils optional eines oder mehrere der nachfolgend aufgezählten Elemente mit folgender Maßgabe enthalten kann: bis zu 0,50 % Mn, 0,003 - 0,04 % S, 0,003 - 0,04 % Se, bis zu 0,07 % Al, bis zu 0,015 % N, bis zu 0,05 % Ti, bis zu 0,3 % P, 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 %;
2. b) kontinuierliches Abgießen der Schmelze zu einem Strang;
3. c) Abteilen von Dünnbrammen von dem gegossenen Strang;
4. d) Glühen der Dünnbrammen in einem in Linie stehenden Ofen bei einer Dünnbrammentemperatur von 1050 - 1300 °C;
5. e) kontinuierliches Warmwalzen der Dünnbrammen in einer in Linie stehenden mehrgerüstigen Warmwalzstraße zu einem Warmband mit einer Dicke von 0,5 - 4,0 mm;
6. f) Abkühlen des Warmbands;
7. g) Haspeln des Warmbands zu einem Coil;
8. h) optional: Glühen des Warmbands nach dem Haspeln bzw. vor dem Kaltwalzen;
9. i) Kaltwalzen des Warmbandes zu einem Kaltband mit einer Enddicke von 0,15 mm bis 0,50 mm;
10. j) rekristallisierendes und entkohlendes Glühen des Kaltbands;
11. k) während oder nach dem entkohlenden Glühen erfolgendes Nitrieren des Kaltbands unter einer ammoniakhaltigen Atmosphäre bei einer 850 - 1050 °C betragenden Nitriertemperatur;
12. l) Auftrag eines Glühseparators auf die Bandoberfläche;
13. m) Schlussglühen des rekristallisierend und entkohlend geglühten Kaltbands zur Ausprägung einer Gosstextur;
14. n) optional: Beschichten des schlussgeglühten Kaltbands mit einer elektrischen Isolierung und anschließendes Spannungsfreiglühen des beschichteten Kaltbands
    und
15. o) optional: Domainenverfeinerung des beschichteten Kaltbandes.

Thus, according to the invention, at least the following work steps are carried out in the production of grain-oriented electrical steel based on thin-slab continuous casting:

1. a) melting a steel, in addition to iron and unavoidable impurities (in mass%) Si: 2.5 - 4.0%, C: 0.01 - 0.10% and in addition each optionally one or more of the enumerated below elements up to 0,50% Mn, 0,003 - 0,04% S, 0,003 - 0,04% Se, up to 0,07% Al, up to 0,015% N, up to 0,05% Ti, up to 0.3% P, one or more elements from the group As, Sn, Sb, Te, Bi with contents of up to 0.2% each, one or more elements from the group Cu, Ni, Cr, Co, Mo at levels of up to 0.5% each, one or more of Group B, V, Nb at levels of up to 0.012% each;
2. b) continuously pouring the melt into a strand;
3. c) separating thin slabs from the cast strand;
4. d) annealing the thin slabs in a line oven at a thin slab temperature of 1050-1300 ° C;
5. e) continuously hot rolling the thin slabs in a stand-alone multistage hot rolling mill to a hot strip having a thickness of 0.5-4.0 mm;
6. f) cooling the hot strip;
7. g) coiling the hot strip into a coil;
8. h) optional: annealing of the hot strip after coiling or before cold rolling;
9. i) cold rolling the hot strip into a cold strip having a final thickness of 0.15 mm to 0.50 mm;
10. j) recrystallizing and decarburizing annealing of the cold strip;
11. k) nitriding the cold-rolled strip during or after the decarburizing annealing under an ammonia-containing atmosphere at a nitriding temperature of 850 - 1050 ° C;
12. l) applying an annealing separator to the strip surface;
13. m) final annealing of the recrystallizing and decarburizing annealed cold-rolled strip to form a Gosstextur;
14. n) optional: coating the final annealed cold-rolled strip with electrical insulation and then stress-relieving the coated cold-rolled strip
    and
15. o) optional: domain refinement of the coated cold-rolled strip.

According to the invention, the cold strip after nitriding (step k) and before application of the annealing separator (step l) undergoes a post-oxidation annealing at an annealing time of 15-180 s and a post-oxidation annealing temperature T _NO of 750-900 ° C. under a hydrogen-containing annealing atmosphere whose partial pressure ratio p _H2O / p _H2 plotted over the respective temperature T _NO within an area defined by the vertices P1 (T _NO = 750 ° C, p _H2O / p _H2 = 0.1), P2 (T _NO = 900 ° C; P _H2O / P _H2 = 0.2), P3 (T _NO = 750 ° C, P _H2O / P _H2 = 0.42) and P4 (T _NO = 900 ° C; P _{H2O / PH2} = 0.52) is.

FIG. 1 shows schematically according to the invention as a function of the respective Nachoxidationsglühtemperatur for the annealing atmosphere of Nachoxidationsbehandlung to be observed partial pressure range in the stability diagram of the electroband typical oxides fayalite (Fe ₂ SiO ₄ ) and Wüstit (FeO) (see also ISIJ 9 (1969), 66).

By means of a post-oxidation annealing performed according to the invention in addition to the conventional procedure, an oxide layer which has penetrated into the zone of nitride precipitation by internal oxidation of the base material is present on the cold strip provided for the annealing separator in step l). The vast majority of previously precipitated nitrides now have an oxidic environment, as shown by the FIGS. 2a, 2b clarified. There are the oxide layer morphologies ( Fig. 2a ) and after post-oxidation annealing ( Fig. 2b ) illustrates schematically. As in Fig. 2b shown, after the post-oxidation annealing according to the invention between the crack-and pore-prone oxide layer by internal oxidation, a nitriding layer formed. This passivates the base material against the cracks and pores previously created by the nitriding process in the oxide layer.

In order for sufficient oxygen transport through the oxide layer to take place in the base material, it has been found that the equilibrium curve of fayalite must be exceeded in the oxidation annealing. On the other hand, excessive oxidation should be avoided, so the upper limit of the process is the decomposition pressure of wustite.

Due to the post-oxidation of the base layer, most of the nitrides are now present in an oxidic environment after annealing. The oxidic environment of the nitrides causes a catalytically driven dissolution during the heating phase of the annealing, whereby a uniform distribution of the nitrogen and thus the inhibitor particles is achieved. The uniform inhibitor distribution has the consequence that the secondary recrystallization is stabilized.

• a) eine Verbesserung der Ausbildung des im Zuge der Schlussglühung ausgebildeten Glasfilms durch eine zusätzliche passivierende innere Oxidationszone und
• b) eine Verbesserung der magnetischen Eigenschaften durch die katalytisch bedingte Homogenisierung der Inhibitorteilchen.

The post-oxidation carried out according to the invention accordingly causes two things:

• a) an improvement in the formation of the formed during the final annealing glass film through an additional passivating inner oxidation zone and
• b) an improvement of the magnetic properties by the catalytically induced homogenization of the inhibitor particles.

The operating parameters set in steps a) to o) of the method according to the invention correspond to the specifications known from the prior art. For example, the temperature of the hot strip during reeling can be 520 - 720 ° C.

In the case where the recrystallizing and decarburizing annealing (step j) is carried out simultaneously with the nitriding treatment (k), it may be desirable to carry out the recrystallizing and decarburizing annealing and nitriding of the cold strip at a temperature of 800 - 1050 ° C under an annealing atmosphere consisting of 0.1-10% by volume of NH ₃ , 30-75% by volume of H ₂ and 30-75% by volume of N ₂ , the dew point of which is 5-75 ° C. over one Duration of 60 - 300 s is performed.

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

Two steel melts S1, S2 were melted after secondary metallurgical treatment in a ladle furnace and a vacuum system having the compositions shown in Table 1. Table 1, balance iron and unavoidable impurities Si [%] C [ppm] Al [ppm] N [ppm] Mn [ppm] S [ppm] Cu [ppm] Sn [ppm] Cr [ppm] S1 3.10 350 260 70 1200 70 2000 580 1000 S2 3.32 500 320 100 1500 100 1800 720 800

The thus composed and treated melts S1, S2 have been continuously poured into a 63 mm thick strand.

From the strand thin slabs have then been separated, which have been fed to a compensation annealing in a compensation furnace at temperatures of 1120 ° C to 1190 ° C for a period of 10-90 min, the average residence time was typically 20- 35 minutes.

After the equalizing annealing, the thin slabs were descaled and hot rolled to a final thickness of 2.3 mm in seven hot rolling steps in a finish hot rolling mill.

example 1

The hot strips produced from the melt S2 were wound into a coil at a temperature of 630 ° C. and then subjected to a two-stage hot-rolled annealing. The annealing temperature in the first stage of the hot strip annealing was 1100 ° C with an annealing time of 90 s and the annealing temperature in the second stage was 900 ° C with an annealing time of 200 s. Alternatively, a single-stage hot strip annealing at 1050 ° C over a period of 350 s would have been possible.

After the hot strip annealing, the hot strip was cold rolled in one stage to a cold strip having a final thickness of 0.285 mm.

Subsequently, plates separated from the cold strip were annealed for a period of 150 s at 870 ° C. in a moist hydrogen / nitrogen mixture (p _H20 / p _H2 = 0.50). This was followed by annealing at 900 ° C. for 30 seconds under a humid ammonia-hydrogen-nitrogen atmosphere (carrier gas N ₂ / H ₂ 50:50 with 5% NH ₃ and partial pressure p _H2O / p _H2 = 0.1), so that the respective sheet metal sample was completely decarburized and nitrided on the other hand.

Following the combined decarburization and nitriding treatment, the sheet samples have undergone post-oxidation annealing (also called "passivation annealing" due to the effect achieved by this annealing) at T _NO = 860 ° C. under a likewise humid hydrogen / nitrogen annealing atmosphere. The partial pressure ratio of the annealing atmosphere p _H2O / p _H2 varies in the range of 0.006-0.44.

The oxidation degree changes OO _initially obtained in dependence on the partial pressure ratio p _H2O / p _H2 are in FIG. 3 shown. O _initial refers to the oxygen content of the sheets without Nachoxidationsglühung. As you can see, the post-oxidation annealing has a positive effect on the oxygen content only if the partial pressure ratio p _H2O / p _{H2 is} greater than 0.16.

Fig. 4a shows by way of example a sectional image of the oxide layer before the post-oxidation treatment, while Fig. 4b a sectional image of the oxide layer after the post-oxidation treatment at p _{H2O /} p _H2 = 0.44 shows, each in 10,000 magnification. The oxide layer becomes more compact through post-oxidation annealing and penetrates into the region of nitridic precipitates.

The sheets are then coated with magnesium oxide and finally annealed under an atmosphere consisting of 75% hydrogen and 25% nitrogen. The re-magnetization losses as a function of the partial pressure ratio p _H2O / p _H2 are in FIG. 5 shown.

Example 2

The 2.3 mm thick hot strips produced from each of the melts S1 and S2 were subjected to a two-stage hot strip annealing after hot rolling. The temperature of the hot strip annealing in the first stage was also in this case at an annealing time of 90 s here 1100 ° C, whereas the annealing temperature in the 200 second second stage of the hot strip annealing was 900 ° C. Again, alternatively, a single-stage hot strip annealing at 1050 ° C over a period of 350 s would have been possible.

After the hot strip annealing, the hot strips were cold rolled in one stage to form a cold strip. The final thickness of the cold strip was 0.175 mm.

Subsequently, sheet metal samples obtained from the cold strip were annealed for a period of 150 s at a temperature of 850 ° C. in a moist hydrogen / nitrogen mixture (p _H2O / p _H2 = 0.50).

Annealing was then carried out at 900 ° C. for 30 seconds in a moist ammonia / hydrogen / nitrogen mixture, whereby the samples were firstly decarburized and, secondly, nitrided.

Some of the sheet samples have subsequently undergone post-oxidation annealing at T _NO = 770 ° C in a humidified hydrogen / nitrogen atmosphere. In this case, a partial pressure ratio of p _H2O / P _H2 = 0.238 was selected for the post-oxidation.

The sheet samples were subsequently coated with magnesia and finally annealed under an atmosphere consisting of half hydrogen and half nitrogen. The typical polarizations J800 determined for the sheet metal samples obtained in this way and the reversal magnetization losses P _1.7,50 likewise determined at 50 Hz are given in Table 2. It shows here a clear superiority of the inventively produced samples. Table 2 Samples without post-oxidation annealing Samples with post-oxidation annealing melt J ₈₀₀ (T) P _1.7,50 (Wkg ^-1 ) J ₈₀₀ (T) P _1.7,50 (Wkg ^-1 ) S1 1,853 0.973 1,889 0.863 S2 1,861 0.962 1,903 0.841

Claims (4)

Process for the production of grain-oriented electrical steel based on thin slab continuous casting, comprising the following steps: a) Melting of a steel, in addition to iron and unavoidable impurities (in mass%) Si: 2.5-4.0%, C: 0.01-0.10%, each optional - up to 0,50% Mn, - 0.003 - 0.04% S, - 0.003 - 0.04% Se, - up to 0.07% Al, - up to 0.015% N, - up to 0.05% Ti, - up to 0.3% P, one or more elements of the group As, Sn, Sb, Te, Bi, each containing 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 of group B, V, Nb, each containing up to 0.012%, contains b) continuous pouring of the melt into a strand, c) separating thin slabs from the cast strand, d) annealing the thin slabs in a line oven at a thin slab temperature of 1050-1300 ° C, e) continuous hot rolling of the thin slabs in a stand-alone multistage 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) optional: annealing of the hot strip after reeling or before cold rolling, i) cold rolling the hot strip into a cold strip having a final thickness of 0.15 mm to 0.50 mm, j) recrystallizing and decarburizing annealing of the cold strip, (k) during or after decarburizing annealing, nitriding of the cold-rolled strip under an ammonia-containing atmosphere at a nitriding temperature of 850 - 1050 ° C, l) applying an annealing separator to the strip surface, m) final annealing of the recrystallizing and decarburizing annealed cold-rolled strip to form a Gosstextur, n) optional: coating the final annealed cold-rolled strip with electrical insulation and then stress-relieving the coated cold-rolled strip, o) optional: domain refinement of the coated cold-rolled strip
characterized in that the cold strip after nitriding (step k) and before the order of Glühseparators (step l) undergoes a Nachoxidationsglühung at an annealing time of 15 - 180 s and a Nachoxidationsglühtemperatur T _NO of 750 - 900 ° C under a hydrogen-containing Glühatmosphäre the partial pressure ratio p _H2O / p _H2 plotted against the respective temperature T _NO lies within an area which is defined by the vertices P1 (T _NO = 750 ° C., p _H2O / p _H2 = 0.1), P2 (T _NO = 900 ° C, p _H2O / p _H2 = 0.2), P3 (T _NO = 750 ° C, p _H2O / p _H2 = 0.42) and P4 (T _NO = 900 ° C; p _H2O / p _H2 = 0, 52) is determined. A method according to claim 1, characterized in that the temperature of the hot strip during the coiling is 520 - 720 ° C. Method according to one of the preceding claims, characterized in that the recrystallizing and decarburization annealing (step j) is carried out simultaneously with the nitriding treatment (k). A method according to claim 3, characterized in that the recrystallizing and decarburization annealing and the simultaneous nitriding of the cold strip at a temperature of 800 - 1050 ° C under a Glühatmosphäre, the 1-10 vol .-% NH ₃ , 30 - 75 vol % H ₂ and 30-75 vol.% N ₂ and whose dew point is 5 - 75 ° C, over a period of 60 - 300 s is performed.

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