EP3610049A1 - Cold-rolled flat steel product annealed in a bell-type furnace, and method for the production of said product - Google Patents

Abstract

The invention relates to a cold-rolled flat steel product which consists of a steel belonging to the high-manganese steels and, after annealing in a bell-type furnace, has a combination of properties which makes it suitable for use in particular in manufacturing car bodies. The cold-rolled flat steel product annealed in a bell-type furnace according to the invention has for this purpose an elastic limit Rp0.2 > 350 MPa, an elongation at break A80 of ≥ 35% and a tensile strength Rm of ≥ 800 MPa, a stacking fault energy of 7-15 mJ/m² and a structure which has a grain size of ≥ ASTM 13 and a carbide density per unit area of ≤ 250 carbide particles per 1000 µm² and consists of a steel containing (in wt%) C: 0.1-0.8%, Mn: 10-25%, Al: 0.3-2%, one or more elements from the group "V, Nb, Ti" with the proviso that the sum of "V, Nb, Ti" is 0.01-0.5%, Si: up to 0.5%, Cr: < 1.5%, S: < 0.03%, P: < 0.08%, N: < 0.1%, Mo: < 2%, Co: ≤ 0.5%, B: < 0.01%, Ni: < 8%, Cu: < 5%, Ca:≤ 0.015%, Mg: ≤ 0.0015%, Sb: ≤ 0.2%, Sn: up to 0.2%, one or more elements from the group "Zr, Ta, W" with the proviso that the sum of the contents of Zr, Ta and W is ≤ 2%, rare earth metals: ≤ 0.2%, remainder iron and unavoidable impurities. The invention also relates to a method for producing such a cold-rolled flat steel product.

Description

 Cold-rolled, bell-annealed flat steel product and process for its production

The invention relates to a cold-rolled, bell annealed flat steel product which has a high manganese content and is in particular

Dimensions suitable for cold forming.

Likewise, the invention relates to a process for the preparation of

Steel flat products according to the invention.

If contents of a steel alloy are mentioned in the present text, the content data always refer to the weight (stated in% by weight), unless otherwise stated. If information on the composition of atmospheres or gas mixtures is given in the present text, the content information given for the individual components always refers to the volume (stated in% by volume), unless otherwise stated.

The term "flat steel product" in the present text in a rolling process produced steel strips or steel sheets and blanks derived therefrom, blanks and similar products understood, the thickness of each is substantially smaller than their width and length.

High-strength, cold-formable steels and steel flat products produced therefrom are used in particular for the production of body components for

Motor vehicles needed. Just there there is a requirement that the sheets from which the components are made, with an optimally low weight are not only well deformed, but also sufficient strength have to make an effective contribution to the stability of the body at low sheet thicknesses.

In addition, when for body parts and similar needs

Certain steels and flat products can be assured of good weldability and no tendency for cracking

Show area of the respective weld. This phenomenon, also known in the trade as "solder rupture", is the result of a weakening of grain boundaries by a medium which infiltrates the grain boundaries during the welding process (eg zinc out of coating, Cu out

Welding filler). The medium which has penetrated into the microstructure can lead to cracks due to the cooling voltages which occur as a result of its presence. For example, when welding galvanized sheet metal, it may happen that it is used as a corrosion protection coating on the

Steel sheet substrate melts zinc due to the high welding temperatures and penetrates at grain boundaries in the steel sheet. In the

subsequent cooling occurs at these grain boundaries stresses that can cause intergranular cracks. The risk of cracking is particularly pronounced for coarser structures, while fine-grained microstructures counteract the solder cracking tendency.

Also, in the case of flat steel products used for body parts, no hydrogen-induced cracks may occur even after a long period of use under the loads occurring in practical use, despite multiple cold forming required for shaping the respective component. This so-called "delayed cracking" can have dangerous consequences for the strength and stability of the formed from the flat steel product component and the body produced with it. The

Hydrogen-induced "delayed cracking" is caused by hydrogen entering from outside the steel material or by the hydrogen present in the material due to the production process. From WO 2012/077150 A1 a method for producing a

austenitic steel with high mechanical resistance and moldability known. The proposed steel has the following chemical

Composition (in% by weight): C: 0.2-1.5%, Mn: 10-25%, Ni: <2%, Si: 0.05-2.00%. AI: 0.01 - 2.0%, N: <0.1, P + Sn + Sb + As: <0.2, S + Se + Te: <0.5, Nb + Co: <1 and / or Re + W: <1, the remainder being iron and

are inevitable impurities. From the thus composed steel, a cold-rolled strip is produced by hot and cold rolling. This is subjected to a final annealing after cold rolling, in which it is recrystallized in a continuous pass at a temperature of 900-1100 ° C. for 60-120 s. Alternatively, the final annealing may also be carried out as a bell annealing in which the cold rolled strip is held at a temperature of 700-800 ° C for 30-400 minutes. The

Annealing atmosphere is adjusted so that their carbon activity is between 0.1 and 1, 0. During the continuous annealing, the nitrogen content is 90-100% and the hydrogen content of the annealing atmosphere is up to 10%, whereas the nitrogen content of the annealing atmosphere during the annealing

Dome annealing 0-100% and their hydrogen content may be between 0% and 100% at a dew point lower than 0 ° C and preferably between -10 ° C and -50 ° C.

The expertise includes in Bleck, Wolfgang (ed.) "Materials science steel for study and practice" - Verlag Mainz, Aachen, 2001, ISBN 3-89653-820-9, explained relationships between the parameters of a recrystallizing annealing and the expression of the structure in recrystallizing annealed steel.

It is known that in suitably assembled steels, the recrystallization or grain growth can be controlled by microalloying during the heat treatment in a continuous continuous furnace. However, such concepts are usually not for a Glühung means

Hood annealing suitable because the necessary there long annealing times too lead to undesirable carbide formation. A strong carbide precipitation leads to a depletion of dissolved carbon, whereby the stacking fault energy changes and the material in addition to the TWIP effect can additionally have a TRIP effect, which has a detrimental effect in a forming process. Furthermore, coarser particles or clusters of fine particles in the microstructure can act as defects in a forming process and, for example, cause surface defects. Furthermore, a significant drop in

Elongation at break through z. As chromium carbides (Cr23C6, Cr7C3) to observe. A coarse-grained structure has a low yield strength and tensile strength and should therefore also be avoided.

These disadvantages are contrasted with the fact that bell heaters often can be used more economically than continuous furnaces, for example when processing steel strips with smaller bandwidths.

Against this background, the task has arisen to provide a cold-rolled steel flat product, which from a genus of

high-manganese steels belonging steel and thereby has a combination of properties even in the glow-annealed condition, which makes it particularly suitable for use in particular in automotive body construction.

In addition, a process for producing such

Flat steel products can be specified.

The invention has achieved this object on the one hand by a flat steel product having the features specified in claim 1.

On the other hand, the solution according to the invention of the above object in the method specified in claim 5. Advantageous embodiments of the invention are specified in the dependent claims and are explained below as the general inventive concept in detail.

An inventive cold-rolled, bell-annealed flat steel product accordingly has a yield strength Rp0.2 of more than 350 MPa, a

Breaking elongation A80 of at least 35% and a tensile strength Rm of at least 800 MPa, the flat steel product consisting of (in% by weight)

C: 0.1 - 0.8%,

 Mn: 10 - 25%,

 AI: 0.3-2%,

 one or more elements from the group "V, Nb, Ti" with the proviso that the sum of the contents of the elements "V, Nb, Ti" is 0.01-0.5%,

 Si: up to 0.5%,

 Cr: less than 1.5%,

 S: less than 0.03%,

 P: less than 0.08%,

 N: less than 0, 1%,

 Mo: less than 2%,

 Co: up to 0.5%,

 B: less than 0.01%,

 Ni: less than 8%,

 Cu: less than 5%,

 Ca: up to 0.015%,

 Mg: up to 0.0015%,

 Sb: up to 0.2%,

 Sn: up to 0.2%,

one or more elements from the group "Zr, Ta, W" with the proviso that the sum of the contents of Zr, Ta and W is at most 2%, Rare earth metals: up to 0.2%,

 Remaining iron and unavoidable impurities

 is composed,

 and

- a stacking fault energy of 7 - 5 mJ / m ²

 such as

- has a microstructure characterized by an ASTM-determined particle size of at least ASTM 13 and having a carbide surface density of not more than 250 carbide particles per 1 000 pm ² .

A method according to the invention for producing flat steel products according to the invention comprises the following steps: a) providing a precursor which consists of a steel which consists of (in

 % By weight) C: 0.1 - 0.8%, Mn: 10 - 25%, Al: 0.3 - 2%, one element or several elements from the group "V, Nb, Ti" with the proviso in that the sum of the contents of the elements "V, Nb, Ti" is 0.01-0.5%, Si: up to 0.5%, Cr: less than 1.5%, S: less than 0, 03%, P: less than 0.08%, N: less than 0.1%, Mo: less than 2%, Co: up to 0.5%, B: less than 0.01%, Ni: less than 8 %, Cu: less than 5%, Ca: up to 0.015%, Mg: up to 0.0015%, Sb: up to 0.2%, Sn: up to 0.2%, one element or several elements from the group "Zr, Ta, W" with the proviso that the sum of the contents of Zr, Ta and W is at most 2%, rare earth metals: up to 0.2% and the balance of iron and unavoidable impurities

 is composed; b) heating or holding the precursor at a holding temperature of 1100-1300 ° C; c) hot rolling the reheated pre-product into a hot-rolled one

Strip with a hot rolling end temperature of at least 800 ° C; d) winding the resulting hot strip into a coil at a reel temperature of at most 750 ° C; e) descaling the hot strip; f) cold rolling the hot strip to a cold strip; g) final annealing of the cold strip, wherein the final annealing is carried out as a bell annealing in which the cold strip is heated at a heating rate of at least 0.1 K / min to a target annealing temperature of 600 - 1200 ° C, at which the cold strip is in the temperature range above 600 ° C is annealed under a protective gas atmosphere containing at least 50% hydrogen with a dew point of less than -50 ° C, in which the cold strip is maintained at the target annealing temperature for a holding time of 0.5-60 h and at which the cold strip after the holding time one

 Cooling rate of at least 0.05 K / min is cooled below the cooling hood to a target cooling temperature of less than 500 ° C, wherein the total residence time under the heating and cooling hood is at most 150 h.

Inventive flat steel products have an optimum combination of weldability and low tendency to delayed formation of cracks with good strength and elongation at break, and a good warm and

Cold workability even if they in their production in

according to the invention have undergone final annealing in the hood furnace (step g) of the method according to the invention).

The alloy of a flat steel product according to the invention and the parameters of those carried out in the production of such a flat steel product

Final annealing (step g) of the method according to the invention) are designed so that the formation of carbides, which do not serve to set a fine-grained structure during the final annealing,

be avoided as far as possible. The fine grain of the structure of a flat steel product according to the invention certainly meets ASTM 13, with overlying fine grains are regularly achieved. The term "microstructure of the microstructure is at least equivalent to ASTM 13" refers to the ASTM guideline series developed by the American Society for Testing Materials ASTM for the evaluation of the grain size of a microstructure.

The particularly fine microstructure achieved on account of the alloy specifications according to the invention in combination with the final bell annealing results in a flat steel product according to the invention an optimum combination of good strength and toughness, which ensures good hot and cold workability. Particularly noteworthy is the soldering-reducing

Effect, as well as the low tendency to delayed cracking of the fine structure, which can be reproduced as a result of the inventive combination of chemical composition and Haubenglühparametern with optimal reliability.

This ensures that the volume fraction of carbides in the microstructure and areal density of carbides (i.e., the number of carbides per grinded surface considered) are low. The areal density of carbides is determined by

Light microscopy and visual evaluation of a Gefügeschliffs at 1000-fold magnification determined.

The invention prescribes an upper limit of a maximum of 250 particles per 1000 μιτι ² for the surface density. This upper limit is safely adhered to when observing the specifications according to the invention in the production of flat steel products according to the invention. By the surface density in the structure of a flat steel product according to the invention is limited to at most 250 particles per 1000 m ² , it is ensured that the mechanical

Properties of a flat steel product according to the invention, namely a yield strength Rp0.2 of more than 350 MPa, in particular more than 400 MPa, an elongation at break A80 of at least 35%, typically 35-45% or 35-40%, and a tensile strength Rm of at least 800 MPa, can be achieved safely and cold working of the steel flat product thus obtained can be carried out without restrictions.

For the good cold workability despite its high strength

The flat steel product according to the invention also stands for the fact that the product Rm x A80 formed from its tensile strength Rm and its elongation at break A80 is regularly greater than 32,000 [MPa%], preferably greater than 35,000 [MPa%]. Tensile strength Rm and breaking elongation A80 are determined according to ISO 6892-1: 2017-02.

A carbide surface density of more than 250 carbides per 1000 pm ² would drastically degrade the mechanical properties and the

Rm ^* A80 product. Carbide area densities of at most 200 per 1,000 pm carbide particles ^2, in particular of at most 170 per 1,000 carbide pm ² or carbide of at most 150 particles per 1,000 pm ^2, therefore, prove to be particularly advantageous.

Steel flat products according to the invention have an austenitic structure and have TWIP properties ("TWIP" = Twinning Induced Plasticity). In the structure of the flat steel product according to the invention, portions of austenite may also be present which brings with it a suitability for the TRIP effect (TRIP = induced plasticity).

The dominant deformation mechanisms arise from the chemical composition and the resulting

Stacking fault energy. A lower limit of the stacking fault energy of 7 mJ / m ² ensures the predominant TWIP deformation mechanism. To use this effect particularly safely, the minimum value of the

Stacking fault energy can be set to 8 mJ / m ² . Values above 15 mJ / m ² increase the alloying agent costs and degrade the processing and service properties (eg weldability, corrosion resistance, etc.). To avoid these negative effects, the value of the

Stacking fault energy to be limited to a maximum of 13 mJ / m ² .

According to Oliver Gräßel, "The development and characterization of new TRIP / TWIP lightweight steels based on Fe-Mn-Al-Si, Clausthal-Zellerfeld: Paper Airplane, 2000, ISBN 3-89720-404-5, p. 46 et seq. to calculate.

To stabilize the Austen itgefüges contributes in the inventive

Flat steel product C content of at least 0.1 wt .-%, in particular at least 0.2 wt .-%, at. The TWIP and TRIP properties of the flat steel product according to the invention can also be specifically influenced via the respective C content, since carbon increases the stacking fault energy. Furthermore, the presence of C according to the invention increases the strength without loss of ductility. At C contents of more than 0.8% by weight, however, it may be in accordance with the invention

envisaged final annealing in the hood furnace to a decrease in

Deformability come. Therefore, C content of an invention

Flat steel product limited to 0.1 to 0.8 wt .-%. The desired effect of the carbon content can be achieved particularly reliably with flat steel products whose C contents are reduced to max. 0.5 wt .-% are limited, with C-contents of 0.2 to 0.5 wt .-% result in particularly good combinations of properties.

Manganese causes in steels according to the invention the required high strength and a higher stacking fault energy. The contents of Mn can be used to adjust the TRIP or TWIP properties of the steels according to the invention. In addition, the presence of high levels of Mn ensures that flat steel products according to the invention have the desired austenitic structure. By the Mn content is at least 10 wt .-%, this effect is certainly achieved. At more than 25 wt .-% Mn levels does not occur in view of the properties of interest here essential Improve more. Instead, there is a risk that at over

25 wt .-% lying manganese contents, the maximum tensile strength decreases. Limiting the Mn content to at most 22% by weight may be advantageous in view of minimizing susceptibility to delayed cracking in combination with the Al contents of the present invention. A limitation of the Mn content to at most 22% by weight,

in particular less than 22 wt .-%, causes a significant reduction in the corrosion potential and counteracts the hydrogen uptake. The

Lower limit of 10 wt .-% ensures ease of manufacture and good processability of the steel, from which an inventive

Flat steel product exists. Mn contents of at least 17% by weight prove particularly favorable in this regard. Optimum effects of the presence of Mn in the steel according to the invention are achieved at Mn contents of 17-22% by weight.

AI increases the inventively predetermined content, the corrosion resistance and reduces the tendency for delayed cracking. Welding tests have furthermore proven that with steels according to the invention, the risk of solder and hot cracking compared to known alloy concepts is lowered by the fact that the Al content is kept within the ranges prescribed according to the invention. Thus, according to the invention, the content of aluminum is limited to 0.3-2% by weight, ensuring weldability of the steels according to the invention, which is superior to that of high-manganese steels having a higher Al content. The provisos for the content of AI according to the invention are chosen so that the otherwise high risk of AI risk to small work areas

Resistance spot welding and the tendency to solder and hot cracking is encountered. The effects achieved by the presence of Al according to the invention can then be used particularly reliably if the Al content is 0.3-1.5% by weight, in particular 0.5-1.3% by weight. Si increases the resistance to delayed cracking in steels according to the invention, but at the same time significantly increases the propensity for solder and hot cracking and reduces the working area during machining

Resistance spot welding. Therefore, the maximum Si content is limited to 0.5% by weight, preferably 0.3% by weight. In order to be able to use the positive influences of Si safely, a minimum content of Si of 0.1% by weight can be provided in the flat steel product according to the invention.

Furthermore, in the case of high manganese steels according to the invention, the system Fe-Mn-C is relevant for achieving properties according to the invention. In particular, in the absence of strong carbide formers such as Cr, coarse iron manganese carbides (e.g., (FeMn) ₃ C, (FeMn) ₇ C ₃ , Mn ₄ C, Mn ₂ 3C ₆ )) can be formed resulting in carbon and manganese depletion of the

austenitic matrix, which in turn affects the properties of the

Steel flat product would worsen.

Particular attention should therefore be paid to the flat steel products according to the invention the simultaneous presence of C and carbide-forming elements, in particular Cr. With increased carbide formation would a

Carbon depletion take place, which may involve a potentially undesirable shift of the stacking fault energy and thus cause or enhance the TRIP effect.

The alloy of a flat steel product according to the invention is therefore adjusted so that despite the very long annealing times in one for the

annealing according to the invention (step g))

Hood furnace an excessive formation of coarser carbides, for example

Chromium carbides (eg Cr23C6) is prevented. For these reasons, the Cr content is limited to at most 1.5% by weight, preferably to at most 0.7% by weight, in particular to at most 0.5% by weight. The inventive limitation of the Cr content also causes a significant improvement in the treatability of steels of the invention. This can be done at a be advantageous flat steel product to limit the Cr content so strong that it is completely ineffective in the technical sense. This can be achieved that in an inventive

Steel flat product at most 0.01 wt .-% Cr are allowed

Steels of the invention contain at least one of

Micro-alloying elements V, Nb and Ti, wherein the sum of the contents of these micro-alloying elements is 0.01-0.5% by weight. The positive effects of V and Nb as well as, with limitations, Ti on the fine grain of the structure of a composite steel according to the invention can be used when titanium, vanadium or niobium each alone or in

Combination with each other in the steel according to the invention are present. That is, in cases where only V, Nb or Ti is present as the micro-alloying element alone, the content of the each alone element may also be 0.01-0.5% by weight. V and Nb have been found to be particularly effective in terms of setting an optimal microstructure for the reasons set forth in detail below.

In this case, prove contents of V, Nb and Ti, the sum of which is at least 0.03 wt .-%, as particularly favorable. It may also be advantageous in terms of safety, with the favorable influences of

Presence of V, Nb and / or Ti, be advantageous if the sum of the contents of these elements max. 0.3 wt .-%, in particular max. 0.2 wt .-%, is.

By virtue of the presence of one or more of the micro-alloying elements V, Nb, Ti provided according to the invention, the prerequisites for optimum fine-grainedness of the microstructure of the invention are met

Created flat steel products, which are final annealed by bell annealing. V, Nb and Ti allow the formation of a fine crystalline microstructure with a high density of precipitates (e.g., VC, VN, VCN, NbC, NbN, NbCN, VNbC, VNbN, VNbCN, TiC, TiN, TiCN) and also contribute a big one Resistance to solder cracking at. Nb, V and Ti also have an influence on the delayed cracking. The precipitate formed by these elements is "trapped" (ie. Trapped) in the steel flat product or penetrating it during its processing

detained) and rendered harmless.

The size of the grains obtained in this way in a flat steel product according to the invention in this way is comparable to the grain sizes which have continuously annealed austenitic high manganese steels in a continuous furnace. So, as mentioned above, for one

According to the invention, according to the invention, the annealed, cold-rolled steel flat product is guaranteed to have a microstructural fineness which corresponds to at least ASTM 13, which is generally finer than ASTM 14. It could be shown by practical experiments that regular structures are obtained, which the

Meet the requirements of ASTM 15.

Titanium forms as a micro-alloying element in steels according to the invention

Precipitants that contribute to the fine grain and can positively influence the mechanical properties of the steel. However, titanium is less effective in terms of setting a fine-grained structure than the invention also provided for this purpose

Alloy elements niobium or vanadium. Therefore, Ti is preferred in

Combination with at least one of the elements V and Nb

Steel added according to the invention. An effect of these elements optimally supporting effect of titanium in the invention

Flat steel product results at Ti contents of at least 0.01 wt .-%. If the contents of Ti are too high, coarse TiN or TiC particles may form, of which cracks may be produced during cold rolling and cold forming of flat products made from steels according to the invention. In addition, the TiN or TiC particles may be destroyed during cold rolling and cold working. Cavities form between the destroyed particles, which in turn can serve as a starting point for cracks. Finally, shallow, coarse TiC particles during cold rolling and cold forming to defects on the

Lead surface. Therefore, the invention provides, if any, to keep the Ti content below an upper limit of 0.5 wt .-%, with Ti contents of up to 0.15 wt .-%, in particular up to 0, 08% by weight, have proven to be particularly favorable, provided that Ti is present in effective amounts.

If steels according to the invention with optimized combinations of properties are to be produced, this can be achieved by reducing the Ti content of the steels according to the invention to values in which Ti has no effect, in favor of higher Nb or V contents. at most, the inevitable impurities is attributable. In this case, as micro alloying elements in a flat steel product according to the invention thus only V and / or Nb present in levels whose sum according to the invention 0.01 - 0.5 wt .-% is, whereas the Ti content in the technical sense the same "0% by weight".

The Nb content of a flat steel product according to the invention can, if Nb alone is present, be 0.01-0.5% by weight, with contents of up to 0.15% by weight, in particular up to 0.08% by weight. -%, as have ausgestallt particularly favorable. The lower upper limits of the Nb content have proven to be particularly advantageous when Nb is present in combination with V and / or Ti.

The Nb and Ti contents optionally present in flat steel products according to the invention already lead to Nb and Ti precipitations during hot rolling and thus increase the rolling resistance in hot and cold

Cold rolling. This may prove to be unfavorable, especially during hot rolling, since even the comparatively high AI and optional Si contents prescribed according to the invention result in increased hot rolling resistance. Therefore, it can with proven problems in the production be advantageous according to the invention flat steel products to reduce the Nb content to a technically ineffective minimum.

In the presence of vanadium vanadium precipitates arise only at the final annealing of the finished rolled sheet and therefore do not hinder the hot and cold rolling. In cases where it proves difficult to roll or cold roll steels according to the invention, it may also be favorable for this reason to increase the vanadium content of the steel in relation to the Nb content or in favor of a high vanadium content to dispense with the addition of niobium and / or titanium. In this case, therefore, the V content of the flat steel product according to the invention can be 0.01-0.5% by weight, whereas Nb and Ti can only be used in technically ineffective, if necessary, the

their contents are in the technical sense equal to "0 wt .-%" are. Here, V contents of max. 0.15 wt .-%, in particular max. 0.08 wt .-%, proved to be particularly favorable. This applies in particular if V in

Combination with Nb is present.

Sulfur and phosphorus inevitably enter the steel intended for the production of flat steel products according to the invention during the course of the melting process. They can lead to embrittlement at the grain boundaries. In particular, with regard to a sufficient hot workability, the S content is therefore limited to less than 0.03 wt .-% and the P content to less than 0.08 wt .-% in flat steel products according to the invention. It goes without saying that the S and P content should preferably be adjusted in each case in such a way that it has no negative effects on the properties of the flat steel product according to the invention, ie it is ineffective in the technical sense.

Nitrogen "N" in amounts of up to 0.1% by weight is used to form

Carbonitrides necessary. For N deficiency, C-rich and N-lean are formed

Carbonitrides. Therefore, the invention preferably provides an N content of at least 0.003 wt .-%, in particular at least 0.005 wt .-%, in a flat steel product according to the invention before. The N content should still be set low. Al and N form precipitates that can significantly degrade the mechanical properties, in particular the elongation values. Even after a subsequent heat treatment, the AIN excretions can no longer be dissolved. For this reason, the maximum content of nitrogen in steels according to the invention is advantageously limited to less than 0.1% by weight, in particular at most 0.025% by weight, in particular at most 0.0170% by weight. Optimal effects of the presence of nitrogen in the flat steel product according to the invention therefore arise when the N content is 0.0030-0.0250% by weight, in particular 0.005-0.0170% by weight.

Optionally in amounts of up to 2% by weight of added Mo may be added to

Improvement of corrosion resistance and concomitantly contribute to a further reduction of the risk of delayed cracking. Mo forms like Cr in addition to the carbon present in the steel and

Nitrogen precipitates, which counteract the addition of hydrogen of the delayed cracking. However, since Mo is also a very strong carbide former, its content is preferably limited to max. 1% by weight,

in particular to max. 0.5% by weight, limited. At the same time, the effect of Mo in the steel of the present invention can be safely utilized by containing Mo in contents of at least 0.1% by weight.

Co may optionally be present in amounts of up to 0.5% by weight, especially up to 0.2% by weight, in the flat steel product of the present invention to inhibit grain growth and thus contribute to the fine grain of the structure. This effect can be achieved at levels of at least 0.01 wt .-%.

Optionally added boron substituted in its effect on the mechanical properties of the alloying element Mn. Thus, it has been found that a steel having an Mn content of 20 wt% and 0.003% boron has a similar property profile to a steel containing 25% Mn but no B. Therefore, the addition of up to 0.01% by weight of boron to a steel alloy of the present invention, while maintaining high strengths, allows for reduced Mn contents which are favorable in terms of prevention of delayed cracking and solder cracking. In addition, small amounts of boron have a positive effect on the strip edge quality of a

Steel alloy according to the invention produced hot strip. Cracks and instabilities in the band edge region, such as those of Al and Si alloys

High manganese steels are known to be this way

suppressed. The effects of B in the flat steel product according to the invention can then be safely used if the B content is at least

0.001 wt .-% is.

Optionally added nickel can contribute to high elongation at break and increase the toughness of the steel of a flat steel product of the invention as well as the resistance to delayed cracking. at

However, according to the invention, this effect is exhausted when the steel contains more than 8% by weight of nickel. Therefore, the upper limit of

According to the invention optionally added nickel contents to 8 wt .-%, preferably 5 wt .-% and in particular 3 wt .-%, limited. The

Effects of Ni in the flat steel product according to the invention can be used safely when the Ni content is at least 0.1% by weight.

Also by the optional addition of copper in levels below 5 wt .-%, the hardness of a steel according to the invention can be increased by the formation of precipitates. However, higher levels of Cu can cause surface defects, for example

Steel flat products made according to the invention could render useless. For this reason, the contents of Cu should preferably be restricted to less than 3% by weight, especially below 0.5% by weight. The effects of Cu in the flat steel product according to the invention can be safely used if the Cu content is at least

0.1 wt .-% is.

Through an optional Ca treatment in steelmaking, the

Castability of the molten steel used for the production of flat steel products according to the invention, especially at high Al-containing

compositions according to the invention can be improved. Ca forms together with alumina (AI203) calcium aluminates, which are taken up in the slag and thus render the clay harmless. In this way, the risk is counteracted that alumina to clogging

(Deposits in the dip tube), which affect the castability. Accordingly, in the steel according to the invention, optionally Ca contents of up to 0.015% by weight, in particular up to 0.01% by weight, are permitted, the advantageous effects of the optional Ca treatment typically being in Ca contents of at least 0 , 0005 wt .-% express.

Mg can optionally be used for deoxidizing during steelmaking and forms with O and S fine oxides, which, when welding a flat steel product according to the invention, have an advantageous effect on the ductility of the steel

Heat affected zone of the weld can affect. At too high levels of Mg, however, coarse precipitates may form in the structure of the flat steel product. Therefore, the optional Mg content is limited to max. 0.0015 wt .-% limited.

Antimony and tin can be embrittling as they too

Grain boundary segregations tend. In addition, they promote the tendency to hydrogen-induced cracking. Therefore, it is advantageous in steel flat products according to the invention to limit the Sb and Sn contents to a maximum of 0.2% in each case. It may be advantageous in flat steel products according to the invention, the Sb and Sn content to a technically ineffective

Minimum to reduce. The likewise optionally present elements Zr, Ta and W can promote the formation of fine particles by carbide formation. However, their effect is behind that provided for this purpose according to the invention

Alloy elements V, Nb or Ti back. In addition, Zr, Ta and W can degrade weldability and cold workability at too high a levels. Therefore, the upper limit of the sum of the contents of Zr, Ta and W in a flat steel product according to the invention to max. 2 wt .-%, in particular max. 1% by weight, limited. The positive effects of Zr, Ta, W can certainly be exploited if at least one of these elements is present in amounts of 0.05% by weight.

The elements belonging to the group of rare earth metals may optionally have contents of up to 0.2% by weight in the invention

Steel flat product be present. They can be used for deoxidizing, if particularly low oxygen contents are to be adjusted, in order to prevent the emergence of unwanted Al oxides. In addition, contents of rare earth metals can have a fine grain and contribute to the formation of non-metallic inclusions. Therefore, the optional content of rare earth metals in a flat steel product according to the invention is preferably limited to at most 0.05 wt .-%. The positive influence of the presence of the rare earth metals can certainly be exploited if the content of rare earth metals is at least 0.02% by weight.

Inventive flat steel products are suitable because of their

Property profile in particular for the production of components by hot or cold deformation. Flat steel products according to the invention are generally characterized by a particularly high energy absorption capacity in the event of sudden loading.

Due to their special property spectrum, flat steel products produced according to the invention are particularly well suited for the production of body components. Because of its exceptionally high Strength and extensibility according to the invention composite and produced material is particularly suitable here for load-bearing and crash-relevant components of vehicle bodies. Thus can be made from inventive

Flat steel products produce structural components where a high

Carrying capacity combined with high protection and low weight.

Because of their high energy absorption capacity are suitable

Steel flat products according to the invention also for the production of

Armor or parts for personal protection. In particular, elements made from flat steel products according to the invention can be used to produce elements which are worn directly on the body and serve to protect against bombardment or comparable impulsive attacks.

Due to their reduced weight and at the same time good deformability and strength, flat steel products according to the invention are beyond that

especially for processing to wheels for vehicles, in particular

Motor vehicles, suitable.

It is also possible to produce components which are used in the field of cryogenic technology from flat steel products produced according to the invention. The favorable property spectrum produced according to the invention

Cold-rolled products remain even at low, in the field of cryogenics temperatures.

It is also conceivable use of steel sheets according to the invention for the production of pipes, in particular for the production of

high-strength engine parts, such as camshafts or piston rods, are determined.

In order to protect the flat steel product according to the invention from surface corrosion, these can at least on their exposed in practice a corrosive attack surface with a metallic Protective coating to be coated. This protective coating can be an Al or Zn-based layer, which can be composed in a manner known per se and in the same known manner

can be applied to the flat product according to the invention, for example by electrolytic galvanizing, by hot-dip galvanizing, by post-annealed or galvanized coating method, as ZnNi coatings or by fire aluminizing. In this case, good coating results can be achieved in particular by electrolytic galvanizing.

For the process according to the invention, in step a) of the

According to the invention provided a precursor which has been prepared from a composite according to the above explanatory steel. The steel may be conventionally produced in a converter steelworks or an electric arc furnace and then cast in a conventional manner into a precursor. This precursor is blocks, slabs or thin slabs produced in a conventional casting process or strip cast by means of the known strip casting process.

Generally speaking, after the production of the precursor becomes

Hot rolling process, which is carried out inline or offline following the casting. The hot strips obtained in these ways are cold rolled into cold strip in a tandem mill, a reversing mill or a Sendzimir mill. For this purpose, the hot strip produced from a steel alloy according to the invention can first be pickled. The resulting cold strip is finally annealed in the annealing furnace and can then optionally surface-coated (Z, ZE, ZF, ZMg, ZN, ZA, AS, S, thin film, tinder-preventing coatings that are suitable for hot forming and press-hardening). A separate heat treatment after application of the surface coating is just as possible. The

Cold strip according to the invention can then be provided with a coating which is used in hot or warm forging processes allows. Particularly in the case of a Zn-coated cold-rolled strip according to the invention, the high resistance of inventive flat steel products against delayed cracking can be further improved by thermal aftertreatment. In this post-treatment zinc-coated material is treated so that alloying of the zinc layer with the base material is used. Due to the thermal post-treatment Zn is alloyed in the

Base material. Material treated in this way will show delayed crack formation only after considerably longer observation periods or even no more cracking.

Specifically, in operation b) of the process according to the invention, the precursor provided in step a) is reheated to a holding temperature of not less than 1100 ° C. or held at this temperature, the holding temperature preferably being at least 1150 ° C. A

Reheating to the holding temperature will be particularly necessary if the precursor is slabs or blocks that are produced in a process separate from hot striping. In those cases in which the precursor is fed directly to the hot rolling in a continuous operation after casting, as is the case, for example, in the known cast roll mills, poured into the thin slabs in continuously successive operations and to

Hot strip can be processed, the step b) also consist of a hold at the respective holding temperature by utilizing the casting heat.

The re-heated or held in step b) to the holding temperature precursor goes through in step c) a hot rolling process in which it is hot rolled to a hot strip at a hot rolling end temperature of at least 800 ° C. Advantageously, the number of passes during hot rolling per pass is at least 10%, in order to produce a hot-rolled under practical production conditions

Inventive flat steel product to obtain optimal texture of its structure. The hot rolling end temperature should expediently not higher than 1050 ° C. As the hot rolling end temperature increases, the tensile strength and yield strength of the hot strip decrease, while the elongation values increase. By varying the Walzendtemperaturen given by the invention frame of 800 - 1050 ° C, especially 950 - 1000 ° C, the cold rollability of the hot strip can be adjusted in a simple manner.

The hot strip obtained in step c) is wound in step d) at a maximum of 750 ° C amounting reel temperature to form a coil. By limiting the reel temperature to a maximum of 750 ° C., in particular less than 700 ° C., in particular 300-600 ° C., the risk of grain boundary oxidation is minimized. A grain boundary oxidation could cause material chipping and, as such, make further processing difficult or even impossible. Furthermore, the premature precipitation of carbonitrides is avoided by reeling at the temperatures predetermined according to the invention. The reel temperature should be at least 300 ° C, since underlying temperatures too

reinforced edge ripples would result.

In the hot strip produced according to the invention, the contents of V, if present, are at least 80%, in particular at least 90%, and Nb, if present, at least 50%, in particular at least 60%, in dissolved form. The remaining contents of V and / or Nb are as

Excretions exist, wherein the amount of bound in the precipitates contents of V and Nb should be minimized by a suitable process control during hot rolling. Due to the high proportion of dissolved microalloying elements in the hot strip, the desired very fine microstructure during subsequent cold rolling and the subsequent bell annealing can be produced reliably. Ti is more than 80% precipitated in the hot strip in the form of precipitates and is therefore less effective for fine grain formation in cold rolled strip. The hot strip is cold rolled after the reeling and an optional surface cleaning by pickling (step e) in a conventional manner to cold strip (step f)). Preferably, the degree of cold rolling achieved in cold rolling is in the range of 30% to 80% to the optimized deformation and strength properties of the finished

Safe to achieve steel flat product according to the invention.

The cold rolling (step f)) is followed by a final annealing in the hood furnace as operating step g) whose annealing temperatures, in order to ensure sufficient recrystallization, are at least 600 ° C. and not more than 1200 ° C., but preferably below 800 ° C., in particular between 650 and 750 ° C, lie.

Before starting the heating process, the atmosphere in the protective hood can first with an inert gas to a non-ignitable

Oxygen content are exchanged (rinsing). This prevents explosive H2-air mixtures. On the other hand, an oxidation of the material can be prevented. The rinsing with hydrogen is then carried out to the desired H2-inert gas mixture in which at least 50% H ₂ are present. The remainder of the annealing atmosphere thus formed is filled by an inert gas, which is typically N ₂ .

The annealing at the target annealing temperature takes place in a reducing atmosphere with a low dew point of below 0 ° C., preferably below -50 ° C., in a protective gas atmosphere with at least 50% hydrogen. An atmosphere of at least 50% H ₂ is necessary in the annealing process in order to be able to achieve the required heating and cooling rates and to be able to comply with them reliably.

Due to the low dew point of the annealing atmosphere, grain boundary oxidation is avoided and the generally high corrosion tendency of Mn steels counteracted, so that the formation of a massive oxide layer on the strip surface is reduced. The dew point must be during the Annealing process can not be kept constant at the respective default.

Rather, it can, as usual in conventional annealing annealing, continuously decrease during the annealing cycle, so for example, starting from a dew point of 0 ° C continuously drops to values below -50 ° C, preferably below -70 ° C. In the high temperature range of the annealing curve (T> 600 ° C), the dew point must be continuously less than -50 ° C, preferably less than -60 ° C, amount.

At the beginning of the annealing process is for recrystallization and thus

In addition, for the fine graininess of the microstructure, the heating rate, with which the cold strip under the heating hood is brought to the target annealing temperature, is particularly relevant. On average, this amounts to at least 0.1 K / min, preferably at least 0.5 K / min. Subsequently, the cold-rolled strip is held for a holding time at the target annealing temperature, which is sufficient for uniform heating of the strip. The holding time depends on the thermal conductivity of the flat steel product, the batch weight, the

Target annealing temperature, the protective gas and the furnace technology used, but it should not fall below 30 minutes and can be up to 60 hours. In any case, it should be so long that a complete recrystallization is ensured even in the middle coil windings of the flat steel product. Typically, the hold time is at least 5 hours, especially at least 7 hours, with maximum hold times of max. 30 hours, in particular max. 20 hours, as proven to have practical.

After the end of the annealing process (heating time and holding time), the heating hood is pulled and a cooling hood is attached. The subsequent cooling to the target cooling temperature, that is, the temperature at which the cooling hood is usually drawn, i. is removed from the annealed coil, in the jargon also called "drawing temperature", consists of a

Slow cooling phase and optionally a rapid cooling phase with additional quick cooling device. During the slow cooling phase, an inert gas purge is performed to allow the batch to be packaged To reach the target cooling temperature (= drawing temperature). The cooling rate during cooling should be as fast as possible, but on average not less than 0.05 K / min and preferably not less than 0.3 K / min. The goal of rapid cooling here is not the recrystallization, since already done completely, but the substantial avoidance of carbide formation, especially when passing through critical temperature ranges during cooling, and a Kornvergröberung. The target cooling temperature (= drawing temperature) of the cooling is below 500 ° C., preferably below 200 ° C.

In this hood annealing are taking into account the

according to the invention the usual for a bell annealing

Completed work steps. Thus, for example, in the usual way, several coils that have been wound from cold strips obtained after cold rolling stacked to form a stack and together the

Be subjected to bell annealing. For hood annealing, a protective hood can also be set, likewise in itself, over the respective coil to be annealed or the stack of coils to be annealed, which serves to adjust the gas atmosphere during the annealing process. About the guard can, as also usual, first put a heating hood over which the heating to the target annealing and holding at the target annealing takes place. For the subsequent cooling, this heating hood, also in a conventional manner, be exchanged for a cooling hood, which is adapted to accelerate a controlled, for example by means of fans cooling gas flow a controlled

To cause cooling.

The total residence time of the cold strip under the heating and cooling hood, in technical language also called "base time" (ie Heizhaubenzeit including cooling time without service time), in the inventive method up to 150 h, preferably up to 80 h. After cooling, the batch obtained can be packaged in accordance with the invention hood-annealed, cold-rolled steel flat product.

After the final annealing, the obtained band has the desired

Fine grain of the structure accordingly safe.

In bell-annealed steels according to the invention, the area fraction of precipitates with micro-alloying elements (eg VC, VN, VCN, NbC, NbN, NbCN, VNbC, VNbN, VNbCN, TiC, TiN or TiCN) is more than 1%, preferably more than 1.5% while steel flat products made from identically assembled steels but annealed in continuous flow exhibit less than 1% areal fraction for these precipitates. The high surface fractions in the heat-treated steels according to the invention stabilize the microstructure in a particular way against grain coarsening. Transmission electron microscopy bright field images (20,000 times magnification) of carbon carrier films were created, binarized, inverted and subsequently analyzed by image analysis to determine the area fractions. The preparation procedure for transmission electron microscopy is described in K. Möldner, Elektronenmikroskopische

Slides and Carrier Films, Carbon Films, pp. 10-11, published in the Carl Zeiss Publication Series, Imprint 34-771, ZS XI / 74 Poo.

As a variant of the method according to the invention, the step g) can be used as open-coil annealing with wrapped wires between the coil turns. Advantages of open-coil annealing are higher heating and cooling speeds of the coil inner turns and, as a result, shorter process times with even more uniform properties over

Strip length.

The inventive choice of the annealing parameters, the formation of the invention sought after fine microstructure is secured, the

Fine Grain regularly meets at least ASTM 13 and finer. The In this case, the invention provides that the contents of V, Nb and / or Ti present in the hot strip in the hot strip form fine precipitates (VCN, NbCN, etc.) during the final annealing, which largely prevent grain growth during the final annealing process.

After the final annealing, the cold strip obtained may optionally be subjected to temper rolling in a manner known per se in order to further improve its dimensional stability and its mechanical properties.

If the flat steel product is to be delivered in the bare state, it can be used instead of a metallic coating for temporary protection

Surface corrosion should be oiled.

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

Table 1 shows the compositions of steels D, G, I, O, P, R, S with composition according to the invention and of steels J, K, L, M, N, Q with compositions which are not according to the invention. In each case so-called full analyzes are shown. In other words, information on the contents is also given for such elements which are present only in technically inactive contents in the respective steel, although the elements in question have no influence on the properties of the respective steel due to their low contents.

In addition, in Table 1, for each of the steels specified there, the stacking fault energy SFE determined according to the above-mentioned publication by Grässel is given.

For various tests 1 to 25, slabs produced from the steels listed in table 1 are at a holding temperature VWT over one for their Warming sufficient duration has been maintained and then hot rolled at a hot rolling end temperature WET to 2 mm thick hot strip. The hot-rolled strips obtained were wound into coils with a coiling temperature HT and, after cooling to room temperature, descaled in a conventional pickling device. Finally, the hot-rolled strip was cold rolled to cold strip with a cold rolling grade KWG.

The resulting cold strips were wound into a coil and placed in a hood annealing furnace.

The typical dimensions of the industrial tests produced

Cold strip coils and the annealing annealing furnace used for their annealing are listed in Table 4. Due to their size and weight, these industrial coils require particularly long annealing and holding times.

In the annealing furnace, the cold strips are at a heating rate HR over a heating time resulting from the heating rate HR to the respective

Target annealing temperature TG have been heated, on which they have been held over a holding time HZ. Then the cold strips with a

Cooling rate HK has been cooled to a drawing temperature (= target cooling temperature) TZ.

It goes without saying that in addition to the steps explained in detail here in the annealing of the cold strips further

Intermediate steps have been completed, as they are usually passed through in a glow hood.

In Table 2, for the experiments 1 to 25 of the steel specified in Table 1 from which the respective tested cold strip existed, the holding temperature VWT, the hot rolling end temperature WET, the coiling temperature HT, the Kaitwaizgrad KWG, the average heating rate HR, the target annealing temperature TG, the base time SZ, the holding time HZ, the average cooling rate HK and the Drawing temperature (= target cooling temperature) TZ indicated, to which the respective coil has been cooled after the end of the holding time HZ.

For the cold tapes obtained from Experiments 1 to 25, the yield strength Rp0.2, the tensile strength Rm, the elongation at break A80, the product Rm x A80 and the fineness of their microstructure were determined in accordance with the relevant ASTM guideline series.

In addition, on a Gefügeschliff for each cold strip the

Carbide surface density determined and classified into classes. Carbide class "1" are cold strips with a carbide surface density of less than 250 particles per 1000 μιτι ² , carbide "2" are cold strips with a carbide surface density of at least 250 particles per 1000 pm ² .

Furthermore, from samples obtained in the experiments 1-25

Cold tapes wells having a blanks / cup diameter ratio β = 2.0 (draw ratio) were drawn. The wells have been subjected to a 28-day corrosion test in which they are without

 Corrosive protective coating of a 5% NaCl solution have been exposed. Cracking did not occur on a well in the test period

Collective of four wells, the respective cold strip sample has been classified as "OK" ("i.O."). Otherwise, it has been rated as "not OK" ("n.i.O").

With further samples of the cold tapes obtained from Experiments 1-25, WPS joining tests according to SEP 1220-2 were carried out, in which they were tested against a conventional, galvanized deep-drawing steel ("heterogeneous

Welding ") were spot-welded to maximum welding current Imax set in accordance with SEP 1220-2, and then each 5 cuts were made and visually inspected for solder and hot cracks, while the cuts were collectively cracks with a maximum length of 20 pm classified as "OK". Otherwise, they were rated "NOK". It was confirmed that due to the very fine microstructure achieved by the addition of V and / or Nb and / or Ti, the cold strips produced according to the invention have a particularly good resistance to

Distinguish solder cracking during welding.

Table 3 summarizes the results of evaluation of the properties of the cold tapes produced in Runs 1-25.

all contents in% by weight, remainder iron and unavoidable impurities

Table 1

Table 2

Table 3

 Table 4

Claims

1. Cold rolled, ductile annealed flat steel product having a yield strength Rp0.2 greater than 350 MPa, an ultimate elongation at break A80 of at least 35% and a tensile strength Rm of at least 800 MPa, the flat steel product consisting of (in% by weight)

 C: 0.1-0.8%,

 Mn: 10-25%,

 AI: 0.3-2%,

 one or more elements from the group "V, Nb, Ti" with the proviso that the sum of the contents of the elements "V, Nb, Ti" is 0.01-0.5%,

 Si: up to 0.5%,

 Cr: less than 1.5%,

 S: less than 0.03%

 P: less than 0.08%

 N: less than 0, 1%,

 Mo: less than 2%,

 Co: up to 0.5%,

 B: less than 0.01%

 Ni: less than 8%,

 Cu: less than 5%,

 Ca: up to 0.05%,

 Mg: up to 0.0015%,

Sb: up to 0.2% Sn: up to 0.2%

 one or more elements from the group "Zr, Ta, W" with the proviso that the sum of the contents of Zr, Ta and W is at most 2%,

 Rare earth metals: up to 0.2%,

 Remaining iron and unavoidable impurities

 is composed,

 and

a stacking fault energy of 7-15 mJ / m ²

 such as

 - has a structure characterized by a ASTM grade of at least ASTM 13 and a

Has carbide surface density of at most 250 carbide particles per 1000 pm ² .

2. flat steel product according to claim 1, characterized in that the sum of its contents of the one or more elements from the group "V, Nb, Ti" 0.03 - 0.3 wt .-% is.

3. Flat steel product according to one of the preceding claims, characterized in that its V content, its Nb content and its Ti content are in each case at most 0.15 wt .-%.

4. Flat steel product according to one of the preceding claims, characterized in that applies to the product formed from its tensile strength Rm and its breaking elongation A80

Rm x A80> 32,000 [MPa%].

5. A method for producing a flat steel product obtained according to any one of claims 1 to 4, comprising the following steps: a) providing a precursor which consists of a steel, which consists of (in wt .-%) C: 0.1 - 0.8 %, Mn: 10 - 25%, Al: 0.3 - 2%, one or more elements of the group "V, Nb, Ti", provided that the sum of the contents of the elements "V, Nb, Ti "0.01-0.5%, Si: up to 0.5%, Cr: less than 1.5%, S: less than 0.03%, P: less than 0.08%, N: less than 0, 1%, Mo: less than 2%, Co: up to 0.5%, B: less than 0.01%, Ni: less than 8%, Cu: less than 5%, Ca: up to 0.015%, Mg: up to 0.0015%, Sb: up to 0.2%, Sn: up to 0.2%, one element or several elements from the group "Zr, Ta, W" with the proviso that the Sum of the contents of Zr, Ta and W is at most 2%, rare earth metals: up to 0.2% and the remainder being composed of iron and unavoidable impurities; b) heating or holding the precursor at a holding temperature of 1100-1300 ° C; c) hot rolling the reheated precursor to a

 hot rolled strip with a hot rolling end temperature of

 at least 800 ° C; d) winding the resulting hot strip into a coil at a

 Reel temperature of at most 750 ° C; e) descaling the hot strip; f) cold rolling the hot strip to a cold strip; g) final annealing of the cold strip, with the final annealing as

Dome annealing is performed, in which the cold-rolled strip with a Heating rate of at least 0.1 K / min is heated to a target annealing temperature of 600 - 1200 ° C, in which the cold strip is annealed in the temperature range above 600 ° C under a protective gas atmosphere containing at least 50% hydrogen with a dew point of less -50 ° C. in which the cold-rolled strip is held at the target annealing temperature for a holding time of 0.5-60 h and at which the cold-rolled strip reaches a target cooling temperature of less than 500 after the holding time has ended at a cooling rate of at least 0.05 K / min under the cooling hood ° C, wherein the total residence time under the heating and cooling hood is at most 150 h.

6. The method according to claim 5, characterized in that the holding temperature is at least 1150 ° C.

7. The method according to any one of claims 5 or 6, d a d u r c h

 characterized in that the stitch decrease during the

 Hot rolling is at least 10% per stitch.

8. The method according to any one of claims 5 to 7, d a d u r c h

 characterized in that the reel temperature is 300-600 ° C.

9. The method according to any one of claims 5 to 8, d a d u r c h

 characterized in that the achieved over the cold rolling

Kaltwalzgrad 30 - 80%.

10. The method according to any one of claims 5 to 9, d a d u r c h

 characterized in that the target annealing temperature is at most 750 ° C.

11. The method according to any one of claims 5 to 10, d a d u r c h

 characterized in that the target annealing temperature is at least 650 ° C. 2. The method according to any one of claims 5 to 11, characterized

 characterized in that the holding time is at most 20 hours.

13. The method according to any one of claims 5 to 12, d a d u r c h

 characterized in that the total residence time is at most 80 hours.

14. The method according to any one of claims 5 to 13, d a d u r c h

 characterized in that the dew point of the atmosphere of the final annealing during the final annealing drops to less than -70 ° C.

15. The method according to any one of claims 5 to 14, d a d u r c h

 characterized in that the target cooling temperature is less than 200 X.