RU2313584C2 - Method for producing of cold-rolled steel for cold pressing - Google Patents

Abstract

FIELD: metallurgy, in particular, manufacture of steel used in automobile building.

SUBSTANCE: method involves smelting steel containing components used in the following ratio, wt%: carbon 0.002-0.015; silicon 0.005-0.050; manganese 0.05-0.50; phosphor 0.005-0.09; sulfur 0.003-0.020; copper 0.1-0.6; aluminum 0.02-0.07; nitrogen 0.002- 0.007; titanium no more than 0.040, niobium no more than 0.060; iron and unavoidable impurities the balance, on condition that C_ef=[C]-C_Ti-C_Nb≥0.0006%, where C_ef is effective content of titanium or niobium non-bound carbon, [C] is total content of carbon in steel, C_Ti is content of titanium bound carbon, with titanium [Ti] to nitrogen [N] content ratio making [Ti]/[N]<3.43 C_Ti=0, with [Ti]/{N]≥3.43; C_Ti=([Ti]-3.43N)/4 C_Nb is content of niobium bound carbon, C_Nb=Nb/7.74; C_ef+0.05[P]≥0.003%, where [P] is phosphor content in steel; providing steel casting; hot rolling; winding strips into rolls at temperature not in the excess of 650 C; performing recrystallization on annealing in cover furnace at temperature of at least 700 C combined with regulated heating: first to temperature of 450-500 C at heating rate of at least 50 C/hour, then to temperature of up to 550-600 C at heating rate not in the excess of 30 C/hour, then at heating rate not in the excess of 50 C/hour to annealing temperature.

EFFECT: increased tendency to BH-effect, including after thermal processing in cover furnaces, and provision for keeping of high formability.

3 cl, 1 tbl, 6 ex

Description

The invention relates to the field of metallurgy, to methods for the production of cold rolled steel with high exhaust properties for cold stamping and can be used in the manufacture of steels used in the automotive industry.

Recently, in addition to the requirements for ensuring high punchability, more and more demands are being made on the increased level of strength, in particular, as a result of hardening during drying of coatings on finished parts - the BH-effect (bake-hardening effect). In this case, depending on the equipment of specific plants, mainly on the heat treatment modes, a certain steel alloying system and other technological parameters of production are selected. So, when using annealing in bell furnaces to ensure the required value of the HV effect, steel is often alloyed with an increased amount of phosphorus, which can lead to embrittlement of grain boundaries. Therefore, it is very important to choose the optimal chemical composition of steel and other technological parameters in order to ensure the highest complex of steel properties at its minimum cost. In addition, increasing the corrosion resistance of sheet steel, in particular under atmospheric conditions, is becoming increasingly important.

A known method of producing cold rolled steel for deep drawing, including the smelting of steel containing, wt.%:

carbon - 0.001 ÷ 0.006

silicon - 0.002 ÷ 0.020

Manganese - 0.07 ÷ 0.30

phosphorus - 0.005 ÷ 0.020

sulfur - 0.005 ÷ 0.010

aluminum - 0.015 ÷ 0.050

nitrogen - 0.002 ÷ 0.006

titanium - 0.02 ÷ 0.08

niobium - 0.005 ÷ 0.060

oxygen - 0.001 ÷ 0.005

iron and unavoidable impurities - the rest,

the total aluminum and titanium content is 0.07 ÷ 0.12%, the ratio of aluminum to oxygen content is at least 5.0, and the minimum titanium content is calculated from the ratio

(Ti _min ) = 3.43 (N) +2.4 (S),

casting, hot rolling, winding strips into rolls at 710-730 ° C, etching, cold rolling, annealing in bell-type furnaces at 700 ° C and training. Alternatively, after pickling and cold rolling, galvanizing, continuous annealing at 850 ° C and training are carried out.

The method is aimed at increasing the stampability of steel, regardless of the heat treatment and applying a protective coating, increasing corrosion resistance (RF Patent No. 2233905 IPC С22С 38/14, 08/10/2004).

The disadvantage of this method is the lack of a guaranteed value of the HV effect, especially after annealing in bell-type furnaces, as well as the relatively high cost of steel associated with the need to ensure ultra-low carbon content, alloying with titanium and niobium, as well as insufficient corrosion resistance of steel (without zinc coating) .

A known method for the production of sheet steel, including continuous casting of slabs from steel containing, wt.%:

carbon - 0.002 ÷ 0.007

silicon - 0.005 ÷ 0.050

Manganese - 0.08 ÷ 0.16

aluminum - 0.01 ÷ 0.05

titanium - 0.05 ÷ 0.12

phosphorus - more than 0.015

sulfur - not more than 0.010

chrome, nickel, copper - not more than 0.04 each

nitrogen - not more than 0.006

iron - the rest,

heating slabs up to 1150 ÷ 1240 ° С, hot rolling with a temperature of rolling end not lower than 870 ° С, cooling with water up to 550 ÷ 730 ° С, winding into coils, pickling, cold rolling with a total compression of at least 70%, annealing in a bell furnace at 700 ÷ 750 ° C for 11 ÷ 34 hours and training.

The method is aimed at improving the exhaust properties and increasing the yield of conditioned sheet steel (RF Patent No. 2197542 IPC С21D 8/04 01/27/2003).

The disadvantage of this method: a high titanium content, a low phosphorus content do not allow for the hardening of steel as a result of the HH effect. Steel also has insufficient resistance to atmospheric corrosion.

Closest to the claimed is a method for the production of cold rolled strips of ultra-low carbon steel, comprising the smelting of steel, containing, wt.%:

carbon - 0.006-0.10

Manganese - 0.01-0.15

phosphorus ≤0.07

nitrogen ≤0.0025

aluminum ≤0.04

niobium - 0,031 ÷ 0,06

sulfur ≤0.008

iron and unavoidable impurities - the rest,

casting, heating slabs to 1150 ÷ 1200 ° C, hot rolling with a temperature of rolling end at 910 ÷ 920 ° C, winding at 740 ÷ 750 ° C, cold rolling with a total compression of at least 70%, heating the strip at a speed of 10 ÷ 20 ° C / s to the annealing temperature, determined depending on the Nb / C ratio by the formulas

at 3.1≤Nb / C≤4.65

T _ref = 7.52 · (Nb / C) ² + 45.55 · Nb / C + 791 ° C,

at 4.65≤Nb / C≤10

T _anne = 1.75 · (Nb / C) ² + 33.81 · Nb / C + 730 ° C,

where Nb and С are the content of niobium and carbon in steel, wt.%, holding at annealing temperature for 50 ÷ 60 s and cooling at a speed of 15 ÷ 25 ° С / s to 340 ÷ 360 ° С.

The method is aimed at stabilizing the complex of mechanical properties while providing a category of very particularly complex hoods while simultaneously obtaining a strengthening effect (BH effect) of at least 40 MPa (RF Patent No. 2212457 IPC C21D 8/04, 09/20/2003 - prototype).

The disadvantage of this method is the possibility of its use only for continuous thermal units. During heat treatment in bell-type furnaces, when the annealing temperature does not exceed 730 ÷ 750 ° C, the required value of the HV effect is not provided. In addition, the corrosion resistance of such steel is low.

The objective of the invention is to optimize the chemical composition and other technological parameters of the production of cold rolled steel with a technical result in the form of increased corrosion resistance and a tendency to the HV effect, including during heat treatment in bell-type furnaces while maintaining high formability.

The technical result is achieved by the fact that in the known method for the production of cold rolled steel for cold stamping, including the smelting of steel containing carbon, manganese, phosphorus, sulfur, aluminum, nitrogen, niobium, iron and inevitable deoxidizers and impurities, casting, hot rolling, winding strips coils, cold rolling and recrystallization annealing, according to the invention, steel is smelted, additionally containing copper and titanium in the following ratio, wt.%:

carbon - 0.002 ÷ 0.015

silicon - 0.005 ÷ 0.050

Manganese - 0.05 ÷ 0.50

phosphorus - 0.005 ÷ 0.09

sulfur - 0.003 ÷ 0.020

copper - 0.1 ÷ 0.6

aluminum - 0.02 ÷ 0.07

nitrogen - 0.002 ÷ 0.007

titanium - not more than 0,040

niobium - no more than 0,060

iron and unavoidable impurities - the rest,

under the following conditions:

With _ef. = [C] -C _Ti -C _Nb ≥0,0006% (1),

where C _eff. - effective carbon content not bound by titanium or niobium;

[C] is the total carbon content in steel,

С _Ti is the carbon content associated with titanium: when the ratio of the titanium content [Ti] to the content [N] [Ti] / [N] <3.43 C _Ti = 0, when [Ti] / [N] ≥3.43 С _Ti = ([Ti] -3.43N) / 4;

C _Nb is the carbon content associated with niobium, C _Nb = Nb / 7.74;

With _ef. +0.05 [P] ≥0.003% (2),

where [P] is the phosphorus content in the steel,

also by the fact that the strip is coiled into rolls at a temperature of not more than 650 ° C, and also by the fact that recrystallization annealing is carried out in a bell furnace at a temperature of at least 700 ° C with regulated heating: heating of the strip to a temperature of 450 ÷ 500 ° C at a speed not less than 50 ° С / hour with subsequent deceleration of heating, at least up to 550 ÷ 600 ° С with a speed of not more than 30 ° С / hour, then with a speed of not more than 50 ° С / hour to the annealing temperature.

The invention is reduced to the following. To ensure high punchability and to ensure a certain value of the HV effect, the content of free carbon in the ferrite is 6–20 ppm. In the case of continuous annealing, high cooling rates prevent the formation of carbon in the form of cementite and it is possible to provide the required carbon content in the solid solution by providing certain ratios between carbon, titanium and niobium (taking into account the nitrogen and sulfur content). During slow cooling during bell annealing, a significant part of the carbon can be released in the form of cementite and the required value of the HH effect will not work. Therefore, one of the ways to ensure the BH effect in the case of bell annealing is to provide at least 30 ppm of carbon content before starting cooling than in the case of continuous annealing. Another way to ensure the required value of the HV effect with a sufficiently low carbon content in the solid solution before accelerated cooling starts from 6 ppm is to alloy steel with phosphorus, which, by reducing the diffusion rate of carbon, contributes to its preservation in the solid solution in an amount sufficient for the manifestation of BH- effect. The fulfillment of condition (1) With _eff = [C] -C _Ti -C _Nb ≥0,0006% is necessary so that before cooling starts carbon in an amount equal to With _eff. was present in solid solution. With slow cooling, part of this carbon may be released as cementite. To prevent this from happening, it is necessary to fulfill the condition (2) With _eff. +0.05 [P] ≥0.003%, the meaning of which is as follows. With an increase in the carbon content in the solid solution before cooling (C _eff. ), The minimum necessary phosphorus content decreases, which ensures the conservation of carbon in the solid solution. When the value of C _eff. ≥0.00275% VN effect can be obtained even with a minimum phosphorus content of 0.005%, although with an increase in phosphorus content, the VN effect increases. When the value of C _eff. <0.00275% to ensure the BH effect, doping with phosphorus is necessarily the more so, the lower the C _eff. (in accordance with equation (2)). C _Ti is the carbon content of titanium bound: when the ratio of the titanium content [Ti] to the content of [N] [Ti] / [N] <3.43 C _Ti = 0, since all titanium will be spent on nitrogen binding, at [Ti ] / [N] ≥3.43 carbon can be bound by the amount of titanium that remains after nitrogen binding With _Ti = ([Ti] -3.43N) / 4 (titanium in the amount of 3.43 N will be used up for nitrogen binding). That is, the fulfillment of condition (2) will provide an increase in the set of properties in the case of annealing both in a continuous unit and in a bell furnace.

Alloying steel with copper provides increased resistance to atmospheric corrosion.

The limitation of the lower limit of the carbon content is due to the fact that with a further decrease in the carbon content, the propensity to the BH effect decreases. The limitation of the minimum nitrogen content is associated with its participation in the release of aluminum nitride during bell annealing, which favorably affects the formability. The lower limit of the content of phosphorus, sulfur, silicon and manganese in steel is determined by the capabilities of existing steelmaking technologies. A further decrease in the content of these elements does not cause a significant improvement in consumer properties, but leads to a significant increase in the cost of metal products.

The increase in the content of carbon, nitrogen, sulfur, silicon and manganese, as well as phosphorus above the upper limits of the claims leads to deterioration of stampability.

The minimum aluminum content in steel is determined by the need for sufficient deoxidation of the steel and the binding of nitrogen to aluminum nitride. The limitation of the upper limit of the aluminum content is associated with its negative effect on the formability due to an increase in the amount of aluminum nitrides and, consequently, structural heterogeneity.

An increase in the content of titanium and niobium above the upper limit, in addition to negatively affecting the formability, lowering the magnitude of the HV effect, leads to an increase in the cost of steel.

The minimum copper content is determined by the need to provide resistance to atmospheric corrosion, the limitation of the upper limit of the copper content is associated with its negative effect on the formability.

The winding temperature is limited to no more than 650 ° C due to the need to preserve nitrogen in the solid solution after hot rolling, which subsequently, upon annealing, released in the form of fine particles of aluminum nitride, favorably affects the structure, texture and formability of steel.

An increase in the heating rate during recrystallization annealing to a temperature of 450–500 ° C of at least 50 ° C / h is associated with the need to suppress the release of ALN particles before recrystallization, and a decrease in the heating rate in the temperature range 450–500 ° C to 550–600 ° C more than 30 ° C / hour - with the need to ensure a more complete separation of ALN particles in the initial stages of recrystallization. The limitation of the rate of subsequent heating to no more than 50 ° C / h, as well as the minimum annealing temperature of 700 ° C, is associated with the need to create conditions for a more complete process of collective recrystallization, which is also required to ensure high stampability.

Examples of specific performance of the method

Six melts of low-carbon steels were smelted in a 300-ton converter at Severstal and cast on a continuous casting plant into slabs with a cross section of 250 × 1290 mm. Hot rolling of slabs into strips with a thickness of 3.2 mm was carried out at the 2000 mill. The temperature of the end of rolling was 850 ÷ 890 ° C. Strips after dosing were wound into rolls at a temperature of 560–700 ° С. After etching and cold rolling on strips with a thickness of 0.9 mm, the strips were subjected to recrystallization annealing in laboratory conditions according to a regimen simulating continuous annealing, or in a bell furnace at a temperature of 700-730 ° C. After training with a compression ratio of 1.0%, complex mechanical tests of rolled products were carried out with the determination of the magnitude of the BH effect.

Option 1 - steel containing 0.005% carbon, 0.009% silicon, 0.20% manganese, 0.035% phosphorus, 0.012% sulfur, 0.25% copper, 0.03% aluminum, 0.004% nitrogen, 0.015% titanium, 0.019% niobium, iron and unavoidable impurities, the rest, while the expression C _eff = [C] -C _Ti —C _Nb = 0.005-0.00032-0.00245 = 0.00223%> 0.0006%, that is, corresponds to the claims; expression C _eff. +0.05 [P] = 0.00223 + 0.00175 = 0.00398%> 0.003%, that is, corresponds to the claims. Annealing was carried out according to the regime: heating to annealing temperature of 850 ° C at a rate of 5 ° C / s, holding time 60 s; cooling to 400 ° C at a rate of 10 ° C / s, holding for 3 minutes, cooling in air (the option corresponds to claim 1).

Option 2 - steel containing 0.009% carbon, 0.020% silicon, 0.15% manganese, 0.040% phosphorus, 0.011% sulfur, 0.25% copper, 0.05% aluminum, 0.005% nitrogen, 0.02% titanium, 0.03% niobium, iron, and the rest are inevitable impurities, with the expression C _eff. = [C] —C _Ti —C _Nb = 0.009-0.0007-0.0039 = 0.0044%> 0.0006%, that is, corresponds to the claims; expression C _eff. +0.05 [P] = 0.0044 + 0.002 = 0.0064%> 0.003% also corresponds to the claims. The temperature of winding hot rolled strips into rolls was 560 ° С, the heating rate during annealing in a bell furnace to 450 ° С was about 60 ° С / h, then up to 550 ° С about 25 ° С / h, then to the annealing temperature of 700 ° С about 35 ° C / hour (option fully consistent with claims 1-3 of the claims).

Option 3 - steel containing 0.007% carbon, 0.010% silicon, 0.22% manganese, 0.050% phosphorus, 0.010% sulfur, 0.30% copper, 0.03% aluminum, 0.003% nitrogen, 0.01% titanium, 0.04% niobium, iron, and the rest are inevitable impurities, with the expression C _eff. = [C] -C _Ti -C _Nb = 0.007-0.0052 = 0.0018%> 0.0006%, i.e., corresponds to the claims

expression C _eff. +0.05 [P] = 0.0018 + 0.0025 = 0.0043%> 0.003% also corresponds to the claims. The temperature of winding hot rolled strips into rolls was 600 ° С, the heating rate during annealing in a bell furnace to 450 ° С was about 60 ° С / h, then up to 550 ° С about 25 ° С / h, then to an annealing temperature of 700 ° С about 35 ° C / hour (option fully consistent with claims 1-3 of the claims).

Option 4 - steel containing 0.008% carbon, 0.013% silicon, 0.18% manganese, 0.040% phosphorus, 0.009% sulfur, 0.05% copper, 0.04% aluminum, 0.045% niobium, 0.002% nitrogen, iron and unavoidable impurities rest, while the expression C _eff. = [C] —C _Ti —C _Nb = 0.008-0.0058 = 0.0022%> 0.0006%, i.e., corresponds to the claims (C _Ti = 0, since the steel does not contain titanium); expression C _eff. +0.05 [P] = 0.0022 + 0.002 = 0.0042%> 0.003%, that is, corresponds to the claims. The temperature of winding hot rolled strips into rolls was 600 ° С, the heating rate during annealing in a bell furnace to 450 ° С was about 60 ° С / h, then up to 550 ° С about 25 ° С / h, then to an annealing temperature of 700 ° С about 35 ° C / hour (option does not match the invention according to the copper content).

Option 5 - steel containing 0.006% carbon, 0.023% silicon, 0.18% manganese, 0.060% phosphorus, 0.007% sulfur, 0.20% copper, 0.05% aluminum, 0.014% titanium, 0.043% niobium, 0.004% nitrogen, iron and unavoidable impurities the rest, while the expression of C _eff. = [C] —C _Ti —C _Nb = 0.006-0.00007-0.00555 = 0.00038% <0.0006%, that is, does not correspond to the claims; expression C _eff. +0.05 [P] = 0.00038 + 0.003 = 0.00338%> 0.003%, that is, corresponds to the claims. The temperature of winding hot rolled strips into rolls was 600 ° С, the heating rate during annealing in a bell furnace to 450 ° С was about 60 ° С / h, then up to 550 ° С about 25 ° С / h, then to an annealing temperature of 700 ° С about 35 ° C / hour (option does not correspond to the claims according to the meaning of expression (1)).

Option 6 - steel containing 0.005% carbon, 0.015% silicon, 0.18% manganese, 0.050% phosphorus, 0.010% sulfur, 0.03% copper, 0.04% aluminum, 0.005% nitrogen, 0.012% titanium, 0.030% niobium, iron and inevitable impurities the rest, while the expression of C _eff. = [C] -C _Ti -C _Nb = 0.005-0.0039 = 0.0011%> 0.0006%, i.e., corresponds to the claims

expression C _eff. +0.05 [P] = 0.0011 + 0.0025 = 0.0036%> 0.003% also corresponds to the claims. The temperature of winding hot-rolled strips into rolls was 680 ° С, the heating rate during annealing in a bell furnace to 450 ° С was about 40 ° С / h, then up to 550 ° С about 25 ° С / h, then to the annealing temperature of 720 ° С at a speed about 35 ° C / hour (the variant does not correspond to the claims in terms of winding temperature, heating rate during annealing to 450 ° C and copper content).

Mechanical testing of samples of cold-rolled steel from the steel of the above-mentioned heats was carried out on an INSTRON-1185 electromechanical testing machine. The dimensions of the sample were 20 × 120 mm.

The tests were carried out in a semi-automatic mode with a longitudinal strain tensometer (strain gauge base 12.5 mm). The tensile rate was 10 mm / min.

In the case of tensile curves without a physical yield strength, the yield strength was determined from the readings of the tensometer taking into account the linear portion of the tensile diagram (in addition, an analysis of the machine tensile diagram was used for control).

The strain hardening coefficient n was determined in the deformation range from 10 to 17%.

The coefficient of normal plastic anisotropy r was determined when the tests were stopped (when reaching 17%) by manually measuring the width of the sample (in three sections).

For samples with a width of 20 mm, the elongation δ _{4 was} determined on the basis of 80 mm (A ₈₀ ).

Tests to determine the hardening of steel during drying of the paintwork (VN effect) were carried out in the following sequence:

1) the samples were stretched to a strain of 2%, which was determined by an extensometer (26 mm base); in this case, σ _{2 was} determined — stress at strain 2%;

2) the samples were placed in an oven heated to a temperature of 170 ± 10 ° C, and kept for 20 minutes;

3) the samples were tensile, determining the magnitude of the BH effect, as the difference between the yield strength σ _t (BH) and σ ₂ .

The results of the mechanical tests are shown in the table. The main mechanical characteristics were determined: yield strength σ _t , tensile strength σ _b , elongation δ ₄ , normal plastic anisotropy coefficient r and strain hardening coefficient n. The criterion for ensuring the required stampability was considered to be obtaining a relative elongation of at least 38%, a normal plastic anisotropy coefficient r of at least 2.0, and a strain hardening coefficient of n of at least 0.20. At the same time, they sought to ensure the magnitude of the BH effect of at least 40 N / mm ² .

As a method of corrosion testing of samples of cold-rolled steel, we used the method of variable immersion of steel sheet samples in a solution of 3.5% NaCl with arrival in it for 10 minutes and then taking it to air (50 minutes) in accordance with ASTM G 44-75.

Corrosion resistance was evaluated by weight gain (gain) per unit surface area of the sample for 30 test cycles. If the mass gain was not more than 5 g / m ² , then the corrosion resistance was considered satisfactory.

For steel according to options 1, 2 and 3, the required indicators of stampability, the magnitude of the HV effect and corrosion resistance were obtained. For option 5, due to the failure of condition (1), even before cooling, the bulk of the carbon is bound to niobium or titanium carbide, which leads to the absence of the HH effect. For option 6, a high winding temperature and a low cooling rate to the temperatures of the onset of recrystallization lead to the release of nitrogen in the form of aluminum nitride even before the onset of recrystallization, which negatively affects the stampability (the value of r and elongation decreases). In addition, for options 4 and 6 due to the low copper content, insufficient corrosion resistance was obtained. Thus, only cold-rolled steel obtained by options 1, 2 and 3, corresponding to the claims, has high stampability, the magnitude of the HV effect and corrosion resistance.

That is, the use of this proposal significantly increases the magnitude of the VN effect of steel even after recrystallization annealing in a bell furnace, as well as the corrosion resistance of steel while maintaining high formability.

Table The results of mechanical tests of steels obtained by the used options. Option No. σ _t N / mm ² σ _B , N / mm ² δ ₄ ,% r n BH effect, N / mm ² The specific increase in scale, g / mm ² one 220 315 38 2.1 0.22 60 2,5 2 200 310 40 2.1 0.22 45 2,8 3 220 345 38 2.0 0.22 fifty 3,1 four 185 300 45 2.2 0.21 40 7.5 5 230 350 38 2.0 0.21 0 2.2 6 210 310 36 1.8 0.21 fifty 7.0

Claims (3)

1. Method for the production of cold rolled steel for cold forming, including the smelting of steel containing carbon, silicon, manganese, phosphorus, sulfur, aluminum, nitrogen, niobium, iron and inevitable deoxidizers and impurities, casting, hot rolling, winding strips into coils, cold rolling and recrystallization annealing, characterized in that the steel is smelted, optionally containing copper and titanium, in the following ratio of components, wt.%: carbon 0.002-0.015 silicon 0.005-0.050 manganese 0.05-0.50 phosphorus 0.005-0.09 sulfur 0.003-0.020 copper 0.1-0.6 aluminum 0.02-0.07 nitrogen 0.002-0.007 titanium no more than 0,040 niobium no more than 0,060 iron and inevitable impurities rest, subject to the conditions With _{eff ..} = [C] -C _Ti -C _Nb > 0.0006% and With _eff +0.05 [P] ≥0.003%, where C _eff. - effective carbon content not bound by titanium or niobium; [C] is the total carbon content in steel; С _Ti is the content of carbon bound by titanium, with С _Ti = 0 at [Ti] / [N] <3.43 and C _Ti = {[Ti] -3.43N} / 4 at [Ti] / [N] ≥ 3.43, With _Nb is the carbon content associated with niobium, With _Nb = Nb / 7,74, [P] - phosphorus content in steel. 2. The method according to claim 1, characterized in that the winding of the strip into rolls is carried out at a temperature of not more than 650 ° C. 3. The method according to claim 2, characterized in that the recrystallization annealing is carried out in a bell furnace at a temperature of at least 700 ° C with regulated heating: first up to 450-500 ° C at a speed of at least 50 ° C / h, then at least at least up to 550-600 ° C with a speed of not more than 30 ° C / h, then with a speed of not more than 50 ° C / h to the annealing temperature.