COLD-ROLLED STEEL FLAT PRODUCT FOR PACKAGING

Description

[0001] The invention relates to a cold-rolled flat steel product for packaging.

[0002] For reasons of resource efficiency and cost reduction, there are efforts to reduce the thickness of flat steel products (steel sheets and strips) used in the production of packaging (hereinafter also referred to as packaging steel). Typical thicknesses of cold-rolled packaging steels are in the thin sheet metal range, i.e., between 0.1 and 0.6 mm. However, since a reduction in thickness results in a decrease in the material's stiffness, the strength of the packaging steel must be increased so that the material can meet the requirements for cold formability in forming processes during packaging production, such as deep drawing or stretch drawing. At the same time, the formability of the steel sheet during cold forming must also be ensured. Therefore, there is a need for high-strength steel sheets with a yield strength of more than 550 MPa, which simultaneously exhibit good characteristics for formability, such as...exhibit an elongation at break of at least 5% and / or an Erichsen index (also referred to as Erichsen indentation according to DIN 50101, measured according to the Erichsen indentation test standardized in DIN EN ISO 20482) of at least 5 mm.

[0003] Numerous methods exist for increasing the strength of steel sheets, such as cold working, solid solution strengthening (by adding carbon, nitrogen, phosphorus, manganese, and / or silicon), precipitation strengthening, increasing strength by achieving a multiphase steel microstructure, or fine grain hardening. However, many of these methods for increasing the strength of steels have undesirable side effects.

[0004] With increasing work hardening, for example, longitudinal and transverse differences in the production of cold-rolled steel sheets and consequently the anisotropy increase, while at the same time the ductility decreases disproportionately.

[0005] Solid solution strengthening involves the interstitial or substitutional incorporation of foreign atoms (e.g., N, C, P, Mn, Si) into the host lattice of the steel. However, many of the possible alloying elements have negative side effects (e.g., P is detrimental to steel, and Mn and Si impair surface quality), which is why increasing strength by adding these alloying elements is not advisable.

[0006] While adding carbon increases the strength of steel with increasing carbon content, it simultaneously creates a pronounced anisotropy in the form of a linear pattern during processing of the steel sheet. This is because the carbon, due to its low solubility in the steel's ferrite lattice, is predominantly present in the form of cementite. Furthermore, surface quality deteriorates with increasing carbon content, and the risk of slab cracking increases as the peritectic point is approached. Therefore, limiting the carbon content to 0.1 wt.% is necessary, as this is the only way to effectively prevent the formation of slab cracks and the resulting pitting oxidation (diffusion of oxygen into cracks).

[0007] Steel sheets for packaging and processes for their production are known from the prior art in which a sufficient amount of carbon and nitrogen for solid solution strengthening is added to the molten steel to achieve tensile strengths exceeding 500 MPa. For example, US 2011 / 0076177 A1 discloses a high-strength steel sheet for can manufacturing with a carbon content of 0.02% to 0.10% by weight and a nitrogen content of 0.012% to 0.0250%, which exhibits tensile strengths exceeding 500 MPa, where the unbound, i.e., interstitially incorporated, nitrogen by weight is at least 0.0100%. It was found that the unbound nitrogen, in particular, contributes to an increase in the steel's strength through solid solution strengthening and aging hardening.The increase in strength achievable through interstitial nitrogen incorporation is limited, on the one hand, by the partial binding of the nitrogen to nitrides, particularly AlN, and on the other hand, by the fact that the risk of slab breakage during hot rolling increases considerably at a nitrogen content of more than 0.025%. A steel sheet for packaging is known from WO2005 / 068667.

[0008] In precipitation hardening, for example by adding titanium or nitrogen, a problem arises in that precipitates form as early as hot rolling due to the high temperature. Consequently, these precipitates are present in all subsequent manufacturing stages, such as cold rolling, annealing, and possibly re-rolling or tempering, and develop a pronounced anisotropy, similar to cementite, especially when precipitation preferentially occurs at grain boundaries. Furthermore, the precipitation-forming agents titanium and nitrogen contribute to an increase in the recrystallization temperature.

[0009] The increase in strength achieved by forming a multiphase microstructure in steel is inherently very limited in packaging steel due to the standardized requirements regarding the steel's alloying elements. Conventional multiphase steels, such as those used in the automotive industry, cannot be used for packaging steel because, for example, the alloying elements used to form a multiphase microstructure, such as manganese and silicon, can only be used up to a maximum weight percentage in packaging steel due to the standardized packaging steel standard (DIN EN 10202). While it is possible to achieve a multiphase microstructure in packaging steel using a special cooling technique, the resulting microstructures are characterized by high instability, and the increase in strength is usually accompanied by a reduction in formability.Furthermore, if the multiphase microstructure is largely based on the alloying element carbon, there is a risk that the anisotropy of the cementite will be passed on to the multiphase microstructure and thereby intensified.

[0010] While fine-grain hardening can increase the strength of steel by achieving a fine-grained microstructure while maintaining the same formability, this fine-grained structure can be achieved process-wise through a low coiling temperature (winding temperature after hot rolling), high cold rolling degrees, and continuous annealing of the cold-rolled steel sheet. Furthermore, the formation of a fine-grained microstructure can be achieved through microalloying and influencing the precipitation behavior in the hot-rolled strip. However, the necessary alloying elements are expensive and increase the annealing temperature required for recrystallization. Moreover, the increased inherent strength of the hot-rolled strip impairs cold rollability, and the surface of the steel sheet becomes more susceptible to defects.

[0011] The aforementioned methods for increasing the strength of steel sheets while maintaining their formability therefore lead to problems, particularly with regard to isotropy, i.e., the directional dependence of the material properties. Since packaging such as beverage or food cans are mostly rotationally symmetrical components, the steel sheets used for their production are often in the form of blanks (i.e., a flat, circular sheet blank) which are formed into a cylindrical can body or a circular can bottom or lid using deep-drawing and stretch-drawing processes. Due to the symmetry of the final product, the sheets must therefore have the most isotropic material properties possible; that is, the properties of the packaging sheets should be as uniform as possible in all directions of the sheet plane.For cold-rolled steel sheets, which are produced in the form of a steel strip, this is very challenging because the rolling direction during hot and cold rolling always results in a directional dependence of the material properties. Cold-rolled steel sheets therefore always exhibit pronounced anisotropy due to the manufacturing process. This is primarily due to the high degree of cold rolling, which in turn is necessary to achieve the extremely thin sheet thicknesses. Since the processing of cold-rolled steel sheets in the production of packaging is generally independent of the rolling direction, difficulties often arise during the forming process because, for example, the strength and formability are not homogeneous across the circumference of the blank.

[0012] There is therefore a need for packaging steel in the form of a cold-rolled flat steel product, characterized by the most isotropic properties possible within the sheet plane of the steel product. In the context of the continuous reduction in thickness of the steel flat product and the necessary increase in strength, this represents a conflicting objective that is difficult to achieve. Furthermore, in addition to the isotropic properties of the steel flat product, other requirements for packaging steels must be considered in the production of packaging, particularly with regard to the flexibility of the forming processes and the shapes of the packaging, the reduction of material waste, and the necessary achievement of the most uniform and homogeneous properties possible of the packaging.

[0013] One object of the invention is therefore to provide a high-strength steel flat product for the production of packaging with the most isotropic material properties possible in the sheet plane, from which packaging with superior isotropic properties and with a wide variety of geometries can be produced using various forming processes with the least possible material waste.

[0014] Since packaging steel is processed into finished packaging in an aged state, i.e., after prolonged storage and possibly after painting and drying, material optimization must take into account the effects of material aging that occur after extended storage and / or painting followed by drying. The technological properties of packaging steels are therefore determined after artificial aging of the material, which, according to DIN EN 10202, can be achieved by heating the sample to 200°C for 20 minutes. Because the (natural or artificial) aging of steel sheets particularly affects strength and formability, these aging effects must be considered when optimizing the material properties.

[0015] Improving the material properties of cold-rolled steel sheets with regard to strength and formability comes at the expense of the isotropy of these properties for the reasons listed above. Various metallurgical and process engineering methods exist for achieving isotropic properties in the production of cold-rolled steel sheets. One option for specifically improving the isotropy of cold-rolled steel sheets is, for example, the addition of boron. However, boron has adverse effects on the workability of the steel and the final product (steel sheet). For instance, the addition of boron increases the annealing temperature required for recrystallization of the steel sheet after cold rolling, impairs the weldability of the material, and reduces the aging potential (i.e., the increase in strength upon aging of the steel sheet).

[0016] A further object of the invention is therefore to identify cost-effective packaging steels and methods for their production which, on the one hand, have the highest possible strength while maintaining a sufficiently good formability for deep drawing and stretch drawing, and on the other hand, have the highest possible isotropy of the material properties with regard to strength and formability in the aged state of the material.

[0017] The aforementioned problems are solved according to the invention with a steel flat product having the features of claim 1. A steel flat product is understood to be a sheet or strip-shaped steel sheet with a thickness in the thin sheet metal range, in particular in the thickness range of 0.1 mm to 0.6 mm.

[0018] The invention is based on the understanding that solid solution strengthening by means of interstitially embedded alloying elements in steel enables a simultaneous improvement in strength, formability, and isotropy, and that solid solution strengthening by carbon and nitrogen proves to be particularly effective, provided that the binding of carbon and nitrogen to carbides and nitrides can be largely suppressed by process engineering. The formation of carbides and nitrides would promote the development of anisotropic properties.

[0019] A further insight underlying the invention is that the introduction of nitrogen by nitrogen enrichment of a cold-rolled steel flat product in an annealing furnace in the presence of a nitrogen donor, at the end of the packaging steel production process, is particularly suitable for achieving both effective solid solution strengthening by nitrogen and for improving the isotropy of the material properties relevant for further processing of the steel flat product in the manufacture of packaging, in particular the yield strength and elongation at break. It has been shown that nitrogen enrichment in an annealing furnace, unlike increasing the nitrogen content by introducing nitrogen into the molten steel, essentially leads to an interstitial incorporation of nitrogen without the nitrogen binding to nitrides.

[0020] It was surprisingly discovered that the nitrogen interstitially incorporated into the cold-rolled steel flat product during annealing in the furnace (especially in a continuous annealing furnace before or during recrystallization annealing) has a positive effect on the formability and isotropy of the material properties. The interstitial incorporation of nitrogen apparently results in outstanding isotropy of the mechanical properties of the steel flat product due to a uniform distribution of the nitrogen in the (ferrite) lattice of the steel.

[0021] Furthermore, the fact that the interstitial incorporation of nitrogen, compared to carbon, shifts the position of the peritectic point to higher alloy contents contributes to this effect. Therefore, the incorporation of large quantities of nitrogen at interstitial sites of the steel lattice is significantly less critical with regard to surface quality and the risk of slab cracking than with carbon. To avoid slab cracking, the weight fraction of carbon in the steel flat product according to the invention is limited to 0.10%.

[0022] For the incorporation of nitrogen, however, the nitrogen content is limited only by the solubility limit of nitrogen in the ferrite lattice of the steel and the economic viability of the manufacturing process. Considering the solubility limit of nitrogen in the ferrite lattice of approximately 0.1 wt.% and the partial binding of nitrogen into nitrides in the presence of strong nitride formers such as Al, Ti, Nb, and / or B, the maximum nitrogen content in the steel is 0.120 wt.%. From a process technology perspective, the nitrogen content of the nitrogen-impregnated flat steel product according to the invention is preferably at most 0.070 wt.%, since further nitrogen impregnation of the cold-rolled flat steel product in a (continuous) annealing furnace is only feasible with very high technological effort, which is currently not economically viable.For process-related and economic reasons, the proportion of nitrogen by weight is therefore particularly preferred to be 0.050% or less.

[0023] It is particularly advantageous to add the nitrogen to the steel flat product as late as possible in the manufacturing process in order to prevent the development of different material properties in and across the rolling direction after nitrogen impregnation, especially during cold rolling operations. Nitrogen impregnation of the steel flat product according to the invention can, for example, take place after (primary) cold rolling, before or during annealing in a continuous annealing furnace.

[0024] Because the nitrogen injection takes place only after (primary) cold rolling, the nitrogen is not part of the hot rolling and (primary) cold rolling processes, which cause significant anisotropy in the material properties. Interstitial incorporation of nitrogen into the iron lattice (ferrite lattice) during or after recrystallization annealing further promotes the homogeneity of the packaging steel according to the invention. In particular, there is no risk of nitride precipitation, which would exacerbate directional dependence of the material properties during subsequent rolling.

[0025] In the case of double cold-rolled steel sheets, the higher intrinsic strength of the steel achieved through solid solution strengthening by the interstitially incorporated nitrogen allows for a reduction in the degree of re-rolling during the second cold rolling, thus minimizing the resulting anisotropy. The degree of re-rolling during a second cold rolling can therefore preferably be limited to 18% or less.

[0026] The invention therefore relates to a (single or double) cold-rolled steel flat product for packaging with a thickness of less than 0.6 mm, which has been cold-rolled from a steel along one rolling direction (0°) and has the following composition with respect to weight: C: 0.02 to 0.1%, Si: less than 0.03%, Mn: 0.17 to 0.5%, P: less than 0.03%, S: 0.001 to 0.03%, Al: 0.002 to 0.1%, N: 0.014 to 0.12%, preferably less than 0.07%, optional Cr: less than 0.1%, preferably 0.01 to 0.08%, optional Ni: less than 0.1%, preferably 0.01 to 0.05%, optional Cu: less than 0.1%, preferably 0.002 to 0.05%, optional Ti: less than 0.01%, optional B: less than 0.005%, optional Nb: less than 0.01%, optional Mo: less than 0.02%, optional Sn: less than 0.03%, balance iron and unavoidable impurities. wherein the steel flat product in an aged condition has a 0.5% yield strength (Rp0.5) of at least 450 MPa and an elongation at break (A) of at least 5% and the work capacity W(α) defined by the product of elongation at break (A) and 0.5% yield strength (Rp 0.5) as a function of the angle (α) to the rolling direction (0°) is not less than 60% and not more than 140% of the work capacity in the rolling direction W(0°).

[0027] Of the nitrogen contained in the flat steel product, at least a weight fraction of 0.010% is stored interstitially in the steel in unbound form.

[0028] The steel flat product according to the invention is characterized by a high 0.5% yield strength (Rp0.5) of at least 450 MPa and a good elongation at break (A) of at least 5% in the rolling direction (0°), as well as by a homogeneous and only slight directional dependence of the work capacity W(α) in the plane of the steel flat product spanned by the angle α. The work capacity W(α) (which is still direction-dependent due to the rolling processes along the rolling direction) represents a suitable measure for evaluating the suitability of cold-rolled steel sheets for the production of packaging by forming processes such as deep drawing and stretch drawing, because the work capacity W, calculated from the product of the elongation at break (A) and the 0.5% yield strength (Rp0.5), represents both a measure of the strength of the steel sheet and of its formability.For reasons explained in more detail below, the 0.5% yield strength (Rp0.5) has proven to be a suitable measure for assessing the strength of an aged steel flat product.

[0029] Preferably, the 0.5% proof strength (Rp0.5) of the steel flat product according to the invention lies, depending on the angle (α) to the rolling direction (0°), in a range between an upper limit and a lower limit, wherein the lower limit is at least 90% and the upper limit is at most 110% of the 0.5% proof strength in the rolling direction Rp0.5 (0°), wherein the 0.5% proof strength (Rp0.5) in the rolling direction is at least 450 MPa.

[0030] The elongation at break A(α), which depends on the angle (α) to the rolling direction (0°), is preferably also between an upper limit and a lower limit in the steel flat product according to the invention, wherein the lower limit is at least 60% and the upper limit is at most 140% of the elongation at break in the rolling direction A(0°), wherein the elongation at break (A) in the rolling direction is at least 5%.

[0031] The working capacity W(α), which depends on the angle (α) to the rolling direction (0°), is accordingly particularly preferably between at least 70% and at most 130% of the working capacity in the rolling direction W(0°).

[0032] Due to the diffusion of nitrogen into the cold-rolled steel flat product during nitriding in the annealing furnace at annealing temperatures preferably above 630°C (temperature of the steel flat product) to ensure complete recrystallization, the diffused nitrogen is uniformly distributed and incorporated into the steel lattice. This uniform distribution of interstitially incorporated nitrogen results in a high degree of isotropy in the mechanical properties of the nitrided steel flat product, particularly with respect to elongation at break and yield strength, and thus with respect to the work capacity relevant for deep-drawing applications as a quality measure, calculated as the product of the 0.5% yield strength (Rp 0.5) and the elongation at break (A).A more homogeneous distribution of the nitrogen stored in the annealing furnace is achieved with longer residence times of the steel flat product in the furnace, and especially with longer annealing times for recrystallization annealing. Preferably, the residence time of the steel flat product in the annealing furnace is more than 10 seconds, particularly preferably more than 30 seconds, and especially in the range of 100 to 250 seconds. With residence times exceeding 400 seconds, the throughput speed of the strip-shaped steel flat product in a continuous annealing furnace would have to be so low for a typical throughput length that the efficiency of the process would no longer be economically viable. Therefore, annealing times longer than 400 seconds can only be achieved in a bell annealing process.

[0033] The isotropy of material properties relevant for cold forming, such as the 0.5% yield strength (Rp0.5), elongation at break (A), and work capacity (W), achieved in the steel flat product according to the invention by smearing after cold rolling, can be achieved despite the grain elongation of the steel grains, which is unavoidable due to the (single or double) cold rolling. The grains of the steel structure in the steel flat product according to the invention typically exhibit a mean chord length of 3.0 to 6.0 µm and a direction-dependent grain elongation (S), which, for example, is at least 1.4 in longitudinal sections of the steel flat product in the rolling direction (0°) and at least 1.1 in planar sections.It follows that, in the steel flat product according to the invention, isotropic properties with respect to the yield strength, the elongation at break and the resulting work capacity in the sheet plane can be achieved despite a grain elongation present due to manufacturing.

[0034] The grain elongation (S) of the grains in the steel structure is defined as the ratio of the mean horizontal chord length (S_H) to the mean vertical chord length (S_V). Transverse to the rolling direction (α = 90°), the grain elongation (S), which depends on the angle α, is regularly at least 1.2 for the flat steel products according to the invention.

[0035] Since the solid solution strengthening produced by nitrogen treatment of the steel flat product is most efficient when the introduced nitrogen is interstitially incorporated into the interstitial sites of the steel (especially the ferrite lattice), it is advantageous for the alloy composition of the steel to contain as few strong nitride formers as possible, such as Al, Ti, B, and / or Nb, in order to prevent the nitrogen from being bound in the form of nitrides. Therefore, the alloy composition of the steel preferably exhibits the following upper limits for the weight fraction of these strong nitride-forming alloying elements: Al: < 0.1%, preferably less than 0.05%; Ti: < 0.01%, preferably less than 0.002%; B: < 0.005%, preferably less than 0.001%; ​​Nb: < 0.01%, preferably less than 0.002%;

[0036] Preferably, the total weight fraction of the nitride-forming agents is less than 0.1%. This ensures, in particular, a weight fraction of unbound nitrogen of more than 0.01%.

[0037] The weight fraction of unbound nitrogen in hot-rolled strip N free (hot-rolled strip) can be determined via the following: formula 1 described, assuming that the nitride-forming elements Al, Ti, B, and Nb present in the steel within the above limits are completely bound to nitrides with nitrogen: where N 0 is the weight fraction of nitrogen in the melt of the steel, the Al factor is defined as a function of the coiling temperature HT (winding temperature of the hot strip) and the aluminum content Al (in wt.%) as follows: if HT < 640 °C: Al factor = 0; if 750 ≥ HT ≥ 640 °C: Al factor = N 0 - N 0 x (-0.682 HT + 536) / 100 = N 0 x (1 - (-0.682 HT + 536) / 100); and the summand N0−Ti / 3.4−B / 0.8−Nb / 6.6−Al factor as the amount of the difference "N 0 - Ti / 3.4 - B / 0.8 - Nb / 6.6 - Al factor" is defined. In Formula 1, this absolute value term takes into account that at most only the entire The nitrogen actually present in the hot strip (i.e., in the molten steel) can be bound by the nitride formers present in the hot strip (i.e., in the molten steel).

[0038] The total weight fraction of free nitrogen in the cold-rolled steel flat product results from the sum of the free nitrogen content in the hot-rolled strip (Nfree (hot-rolled strip) according to formula 1 above) and the nitrogen ΔN added by nitriding in the continuous annealing furnace: N frei = N frei Warmband + ΔN

[0039] It is assumed that the nitrogen fraction ΔN introduced during nitriding in the continuous annealing furnace is at least substantially incorporated interstitially into interstitial sites. The upper limit for the weight fraction of free nitrogen in the cold-rolled steel flat product is determined by the solubility limit of nitrogen in the ferrite lattice of the steel, which is approximately 0.1 wt.%.

[0040] The total weight fraction of free nitrogen in the cold-rolled steel flat product (Nfree) is preferably greater than 0.01%. To introduce as high a proportion of the nitrogen as possible in unbound form into the cold-rolled steel flat product, the majority of the total weight fraction of nitrogen is preferably introduced by nitrogen impregnation in a continuous annealing furnace, wherein the weight fraction of ΔN is preferably at least 0.002 wt.% and particularly preferably greater than 0.008 wt.%.

[0041] The steel flat product according to the invention is obtainable by a manufacturing process in which a hot-rolled strip is first produced from a slab made from the aforementioned molten steel by hot rolling at a preferred final rolling temperature of > Ar3 and in particular between 800°C and 900°C, which is wound up at a winding temperature (coiling temperature HT) of < Ar1 and in particular in the range of 500°C to 750°C and, after cooling, is cold-rolled to a steel flat product (steel strip) at a reduction ratio of at least 80% and subsequently recrystallized in an annealing furnace, in particular a continuous annealing furnace, at an annealing temperature of at least 630°C under the at least temporary presence of a nitrogen donor, and then cooled to room temperature and finally re-rolled or skin-finished with a re-rolling degree of 0.2% to 45%.Preferably, the degree of re-rolling is less than 18% in order to avoid impairing the isotropic properties through re-rolling.

[0042] Nitrogen etching of the steel flat product in the continuous annealing furnace can take place before, during, or after recrystallization annealing. For example, it is possible to perform nitrogen etching in an upstream first zone of the furnace at a temperature below the recrystallization temperature in the presence of a nitrogen donor, and then heat the steel flat product to a second temperature above the recrystallization temperature in a downstream second zone of the furnace for recrystallization annealing. This sequence of nitrogen etching and recrystallization annealing can also be reversed.Such a decoupling of nitriding and recrystallizing annealing in different zones of the continuous annealing furnace has the advantage that the optimal temperature can be set for each process, with the optimal temperature for nitriding being lower than for recrystallizing annealing. However, for economic reasons, simultaneous nitriding and annealing of the flat steel product in the continuous annealing furnace at a temperature above the recrystallization temperature in the presence of a nitrogen donor is preferable.

[0043] The properties of the steel flat product produced in this way are established after the re-rolled steel strip has aged, whereby the aging can be brought about either artificially by heating to 200°C for 20 minutes or by painting followed by drying the steel flat product.

[0044] Preferably, the hot-rolled strip already has an initial nitrogen content (N₂O) in the range of 0.001 wt.% to 0.016 wt.% in order to maximize the total nitrogen content in the cold-rolled steel flat product and thus the solid solution strengthening caused by the nitrogen treatment of the cold-rolled strip. To prevent the formation of slab cracks during slab casting and hot rolling, and to avoid increasing the strength of the hot-rolled strip to such an extent that it can no longer be cold-rolled with conventional cold-rolling equipment, the weight fraction of nitrogen in the steel melt from which the hot-rolled strip is produced should not exceed 0.016%.The total nitrogen content of the steel flat product according to the invention, which is composed of the sum of the initial nitrogen content N₀ and the nitrogen content ΔN introduced during annealing in the annealing furnace, is adjusted during the annealing of the cold-rolled steel flat product by the presence of the nitrogen donor in the annealing furnace. At the annealing temperatures, dissociated atomic nitrogen from the nitrogen donor diffuses into the cold-rolled steel flat product, thereby increasing the nitrogen content by ΔN. The nitrogen content ΔN introduced during annealing in the annealing furnace is preferably at least 0.002 wt.%, thereby increasing the total nitrogen content of the steel flat product to more than 0.014 wt.% if the initial nitrogen content N₀ in the steel melt was lower than this value. Particularly preferably, the cold-rolled steel flat product is annealed in a continuous annealing furnace to a nitrogen content of more than 0.020 wt.%.The total nitrogen content of the flat steel product annealed in the continuous furnace can (at least theoretically) reach up to the solubility limit of nitrogen in the (ferrite) lattice of the steel of approximately 0.1 wt.%.

[0045] The nitrogen donor can be, for example, a nitrogen-containing gas atmosphere in the annealing furnace, particularly an ammonia-containing atmosphere, or a nitrogen-containing liquid applied to the surface of the cold-rolled steel flat product before it is heated in the annealing furnace. The nitrogen donor should be designed such that atomic nitrogen is provided in the annealing furnace through dissociation, allowing it to diffuse into the steel flat product. In particular, the nitrogen donor can be ammonia gas. To ensure that this dissociates in the annealing furnace to form atomic nitrogen, furnace temperatures of more than 400°C are preferably set when the cold-rolled steel flat product is nitrogen-treated.

[0046] Due to the increase in strength achieved through solid solution strengthening by nitrogen enrichment of the steel flat product during annealing in the (continuous) annealing furnace in the presence of the nitrogen donor, no further rolling with a high degree of re-rolling is required for the steel flat product according to the invention to further increase strength through work hardening. The degree of re-rolling can therefore preferably be limited to a maximum of 18%, thus avoiding a deterioration of the isotropy of the material properties caused by a second cold rolling with high degrees of re-rolling.

[0047] After the second cold rolling or skin rolling, a coating can be applied to the surface of the steel flat product to improve corrosion resistance, e.g. by electrolytic deposition of a tin or chromium / chromium oxide coating and / or by painting with a lacquer or by laminating a polymer film made of a thermoplastic material, in particular a film made of a polyester such as PET or a polyolefin such as PP or PE.

[0048] The outstanding isotropic mechanical properties of the steel sheets according to the invention enable the production of easy-open-end (EOE) lids for cans or of aerosol cans or aerosol can components, such as aerosol can bottoms or lids, with isotropic properties across the entire surface of the lids or aerosol cans and their components. The isotropic properties of the steel sheets according to the invention prove particularly advantageous for round or oval lids as well as round aerosol can bottoms or lids, since virtually uniform mechanical properties are present across the entire circumference of the respective product. This also applies to deep-drawing applications in which a round sheet metal part (blank) is formed, for example, to create a...To produce a can body for a two-part can, the isotropic mechanical properties of the steel sheets according to the invention prove to be advantageous, since uniform mechanical properties of the formed sheet part can be achieved around its circumference and no thinned areas with reduced sheet thickness are created during forming.

[0049] These and other properties, features, and advantages of the steel flat product according to the invention will become apparent from the exemplary embodiments described in more detail below with reference to the accompanying drawings and tables. The drawings show: Fig. 1: Example of a schematic stress-strain diagram obtained in a tensile test on a flat steel product according to the invention; Fig. 2 Pie charts of the angular dependence of the elongation at break (A) in the plane of steel flat products tested in tensile tests, wherein Figure 2athe results of the examined samples 1 to 13 and Figure 2b the results of the examined samples 14 to 26 show; Fig. 3 Pie charts of the angular dependence of the 0.5% yield strength (Rp0.5) in the plane of steel flat products tested in tensile tests, wherein Figure 3a the results of the examined samples 1 to 13 and Figure 3b the results of the examined samples 14 to 26 show; Fig. 4 Pie charts of the angular dependence of the work capacity W = A· Rp0.5 in the plane of steel flat products tested in tensile tests, wherein Figure 4a the results of the examined samples 1 to 13 and Figure 4b the results of the examined samples 14 to 26 show; Fig. 5 schematic representation of the investigations to determine the grain structure of the investigated flat steel products; Fig. 6 Illustration of the influence of aging of cold-rolled steel flat products on the isotropy of elongation at break.

[0050] For the production of the steel flat products according to the invention, a slab is cast from molten steel and hot-rolled into a strip. The alloy composition of the molten steel is expediently based on the limit values ​​specified by standards for packaging steel (such as those defined in ASTM A623-11 "Standard Specification for Tin Mill Products" or in the "European Standard EN 10202"). The components of the steel from which the steel flat products according to the invention can be produced are explained in detail below: Steel composition: • Carbon, C: at least 0.02% and at most 0.1%, preferably less than 0.085%;

[0051] Carbon increases hardness and strength. Therefore, the steel contains at least 0.02 wt.% carbon. To ensure the rollability of the steel flat product during primary cold rolling and, if necessary, in a second cold rolling step (re-rolling or skin burnishing), and to avoid reducing elongation at break, the carbon content should not be too high. Furthermore, with increasing carbon content, a pronounced anisotropy in the form of a row pattern develops during the production and processing of the steel flat product, since the carbon, due to its low solubility in the ferrite lattice of the steel, is predominantly present in the form of cementite. In addition, surface quality deteriorates with increasing carbon content, and the risk of slab cracking increases as the peritectic point is approached. Therefore, limiting the carbon content to a maximum of 0.1 wt.% is recommended.% necessary, as this is the only way to effectively prevent the formation of slab cracks and the resulting point-like oxidation (diffusion of oxygen into cracks). • Manganese, Mn: minimum 0.17% and maximum 0.5%;

[0052] Manganese also increases hardness and strength. Furthermore, manganese improves the weldability and wear resistance of steel. Adding manganese also reduces the tendency for red fracture during hot rolling by binding sulfur to less harmful MnS. Manganese also leads to grain refinement and can increase the solubility of nitrogen in the iron lattice, preventing carbon diffusion to the surface of the slab. Therefore, a manganese content of at least 0.17 wt.% is necessary. To achieve high strength, a manganese content of more than 0.2 wt.%, especially 0.30 wt.% or more, is preferable. However, if the manganese content becomes too high, the corrosion resistance of the steel is compromised, and its suitability for food contact is no longer guaranteed. Moreover, excessively high manganese contents result in excessively high strength of the hot-rolled strip, rendering it unsuitable for cold rolling.Therefore, the upper limit for the manganese content is 0.5 wt.%. • Phosphorus, P: less than 0.03%

[0053] Phosphorus is an undesirable accompanying element in steels. A high phosphorus content leads in particular to embrittlement of the steel and therefore impairs the formability of flat steel products, which is why the upper limit for the phosphorus content is 0.03 wt.%. • Sulfur, S: more than 0.001% and at most 0.03%

[0054] Sulfur is an undesirable impurity that impairs ductility and corrosion resistance. Therefore, the sulfur content in steel must not exceed 0.03 wt.%. On the other hand, desulfurization of steel requires complex and costly measures, which is why a sulfur content of less than 0.001 wt.% is no longer economically viable. The sulfur content is therefore in the range of 0.001 wt.% to 0.03 wt.%, preferably between 0.005 wt.% and 0.01 wt.%. • Aluminium, Al: more than 0.002% and less than 0.1%

[0055] Aluminum is required in steel production as a deoxidizing agent for stabilizing the steel. Aluminum also increases scale resistance and formability. Therefore, the aluminum content is greater than 0.002 wt.%. However, aluminum forms aluminum nitrides with nitrogen, which are disadvantageous in the flat steel products according to the invention because they reduce the proportion of free nitrogen. Furthermore, excessively high aluminum concentrations can lead to surface defects in the form of aluminum clusters. Therefore, aluminum is used in a concentration of no more than 0.1 wt.%. • Silicon, Si: less than 0.03%;

[0056] Silicon increases the scaling resistance of steel and acts as a solid solution hardener. In steel production, silicon serves as a deoxidizing agent. Another positive effect of silicon on steel is that it increases tensile strength and yield strength. Therefore, a silicon content of 0.003 wt.% or more is preferable. However, if the silicon content becomes too high, and especially if it exceeds 0.03 wt.%, the corrosion resistance of the steel can be impaired, and surface treatments, particularly electrolytic coatings, can become more difficult. • optional nitrogen, N₀: less than 0.016%, and preferably more than 0.001%

[0057] Nitrogen is an optional component in the molten steel from which the steel for the flat steel products according to the invention is produced. While nitrogen acts as a solid solution strengthener, increasing hardness and strength, an excessively high nitrogen content in the molten steel (more than 0.016 wt.%) makes the hot-rolled strip produced from the molten steel more difficult to cold-roll. Furthermore, a high nitrogen content in the molten steel increases the risk of defects in the hot-rolled strip, as hot-formability decreases at nitrogen concentrations of 0.016 wt.% or higher. According to the invention, the nitrogen content of the flat steel product can be subsequently increased by nitrogenizing the cold-rolled flat steel product in an annealing furnace. Therefore, the introduction of nitrogen into the molten steel can be completely omitted.However, to achieve a high degree of solid solution strengthening, it is preferable if the steel melt already contains an initial nitrogen content of more than 0.001 wt. %, particularly preferably 0.010 wt. %, or more.

[0058] To introduce an initial nitrogen content N 0 into the flat steel product before nitrogen annealing in the furnace, nitrogen can be added to the molten steel in a corresponding quantity, e.g. by blowing in nitrogen gas and / or by adding a solid nitrogen compound such as calcium cyanamide or manganese nitride. • Optional: Nitride formers, especially niobium, titanium, boron, molybdenum, chromium:

[0059] Nitride-forming elements such as aluminum, titanium, niobium, boron, molybdenum, and chromium are disadvantageous in the steel of the flat steel products according to the invention because they reduce the proportion of free nitrogen through nitride formation. Furthermore, these elements are expensive and therefore increase manufacturing costs. On the other hand, elements such as niobium, titanium, and boron, acting as microalloying components, increase strength by refining the grain structure without reducing toughness. Therefore, the aforementioned nitride formers can be advantageously added to the molten steel within certain limits as alloying elements. The steel can therefore (optionally) contain the following nitride-forming alloying elements by weight: Titanium, Ti: preferably more than 0.002%, but for cost reasons less than 0.01%; Boron, B: preferably more than 0.001%, but for cost reasons less than 0.005%; and / or Niobium, Nb: preferably more than 0.001%, but for cost reasons less than 0.01%; and / or Chromium, Cr: preferably more than 0.01% to allow the use of scrap in the production of the steel melt and to make it more difficult for carbon to diffuse onto the surface of the slab, but to avoid carbides and nitrides, at most 0.08%; and / or Molybdenum, Mo: less than 0.02%, to avoid an excessive increase in the recrystallization temperature;

[0060] To avoid a reduction in the proportion of free, unbound nitrogen Nfree through nitride formation, the total weight fraction of the aforementioned nitride formers in the steel melt is preferably less than 0.1%. Other optional components:

[0061] Besides the residual material iron (Fe) and unavoidable impurities, the steel melt can contain other optional components, such as... Optional copper, Cu: more than 0.002% to allow the use of scrap in the steelmaking process, but less than 0.1% to ensure food safety; optional nickel, Ni: more than 0.01% to allow the use of scrap in the steelmaking process and to improve toughness, but less than 0.1% to ensure food safety; optional tin, Sn: preferably less than 0.03%; Manufacturing process of the steel flat product:

[0062] Using the described steel composition, a steel melt is produced, which is first continuously cast and, after cooling, cut into slabs. The slabs are then reheated to preheating temperatures of more than 1100°C, in particular 1200°C, and hot-rolled to produce a hot-rolled strip with a thickness in the range of 1 to 4 mm.

[0063] The final rolling temperature during hot rolling is preferably above the Ar3 temperature in order to remain austenitic, and is particularly between 800°C and 900°C.

[0064] The hot-rolled strip is wound into a coil at a predetermined and suitably constant winding temperature (coiling temperature, HT). The winding temperature is preferably below Ar1 to remain in the ferritic region, preferably in the range of 500°C to 7500°C, and particularly preferably below 640°C to prevent the precipitation of AlN. For economic reasons, the winding temperature should be above 500°C. The formation of iron nitrides on the surface of the hot-rolled strip can be prevented by cooling the strip after hot rolling until winding at higher cooling rates.

[0065] To produce packaging steel in the form of a thin steel flat product with a thickness of less than 0.6 mm (thin sheet thicknesses) and preferably less than 0.4 mm, the hot-rolled strip is cold-rolled, advantageously with a thickness reduction (degree of reduction or cold rolling) of at least 80% and preferably in the range of 85% to 98%. To restore the crystalline structure of the steel, which is destroyed during cold rolling, the cold-rolled steel strip is subsequently annealed in a recrystallizing furnace. This is done, for example, by passing the steel flat product, which is in the form of a cold-rolled steel strip, through a continuous annealing furnace in which the steel strip is heated to temperatures above the recrystallization temperature of the steel.In this process, the cold-rolled steel flat product is nitrided before or preferably simultaneously with the recrystallization annealing by heating it in an annealing furnace in the presence of a nitrogen donor. This nitriding is preferably carried out concurrently with the recrystallization annealing in the annealing furnace by introducing a nitrogen donor, particularly in the form of a nitrogen-containing gas, preferably ammonia (NH₃), into the furnace and heating the steel flat product to an annealing temperature above the recrystallization temperature of the steel, holding it at this temperature for a holding time of preferably 10 to 150 seconds. The annealing temperature is preferably above 630°C and particularly in the range of 650°C to 750°C.The nitrogen donor is selected such that, at the temperatures in the annealing furnace, atomic nitrogen is formed through dissociation of the nitrogen donor, which can then diffuse into the flat steel product. Ammonia has proven suitable for this purpose. To prevent oxidation of the surface of the flat steel product during annealing, a protective gas atmosphere is advantageously used in the annealing furnace. Preferably, the atmosphere in the annealing furnace consists of a mixture of the nitrogen-containing gas acting as the nitrogen donor and a protective gas such as HNx, wherein the volume fraction of the protective gas is preferably between 90% and 99.5%, and the remainder of the volume fraction of the gas atmosphere is formed by the nitrogen-containing gas, in particular ammonia gas (NH₃ gas). Examples of implementation:

[0066] Exemplary embodiments of the invention and comparative examples are explained below. From molten steel with the in Table 1The listed alloy compositions were used to produce flat steel products (steel sheets) by hot rolling followed by cold rolling.

[0067] The cold-rolled steel flat products were then recrystallised in a continuous annealing furnace by holding the steel flat products at annealing temperatures of 640 °C for an annealing period of 45 seconds.

[0068] The process and material parameters of the thermally treated steel sheets from Table 1 are listed in Table 2 cited, whereby N (after nitriding) is the nitrogen content after nitriding in the annealing furnace, D is the thickness of the steel sheet (in mm), NWG is the degree of rerolling during secondary cold rolling (in %), NH3 is the ammonia content in the annealing furnace (in vol%), Rp0.5 is the 0.5% proof strength (in MPa) in the rolling direction, A is the elongation at break (in %) in the rolling direction and Rm is the tensile strength (in MPa) in the rolling direction.

[0069] In the examples according to the invention (Examples 1 to 3, 10 to 12, 15, 16, 18, 19, 21 to 23, and 25 and 26 of Tables 1 and 2), ammonia was introduced into the continuous annealing furnace during the thermal treatment of the flat steel products, so that a gas atmosphere consisting of ammonia and HNx protective gas was present in the continuous annealing furnace. The volume fraction of ammonia in the gas atmosphere is given in Table 2 as NH3 (vol. %). In the comparative examples (Examples 4 to 9, 13, 14, 17, 20, and 24 of Tables 1 and 2), a 100% HNx protective gas atmosphere was present in the continuous annealing furnace during annealing. The total nitrogen content resulting from the smearing of the samples according to the invention in the ammonia-containing gas atmosphere of the continuous annealing furnace is given in Table 2 as N (after smearing) [wt.%]. The total nitrogen content N was determined in accordance with the standard DIN EN ISO 14284 (in particular section 4.4).1) detected after the removal of a superficial iron nitride layer that formed on the surface of the samples during embroidery.

[0070] The total weight fraction of nitrogen is composed of the initial nitrogen content of the steel melt (N0, see Table 1) and the nitrogen content ΔN introduced by nitrogen enrichment in the continuous annealing furnace, whereby a significant portion of the total nitrogen content N is present in unbound form, and the remainder is present in bound form as nitride, see formula (1). The weight fraction of free nitrogen Nfree can be estimated from the weight fraction of the nitride formers present in the steel using formula (1).

[0071] After thermal treatment in the continuous annealing furnace, the cold-rolled and recrystallised annealed steel flat products underwent a second rolling or skin-dressing process. The degree of second cold rolling or skin-dressing, as well as the thickness of the second-rolled steel flat products, are listed in Table 2. Finally, artificial aging of the steel flat products was induced by heating the sample to 200°C for 20 minutes.

[0072] Figure 6Figure 5 illustrates the influence of aging on the angular dependence of the elongation at break and compares the unaged state to the aged state, further differentiating between artificial and natural aging in the latter case. It becomes apparent that significant anisotropy only develops through aging. Since aging is virtually unavoidable in the practical processing of packaging steel, it is of particular importance to capture and optimize the isotropy in the aged state, which is the subject of the invention.

[0073] Tensile tests and microstructure analyses were performed on the aged samples of Examples 1 to 26. In particular, the 0.5% yield strength (Rp0.5, measured according to DIN EN ISO 6892-1) and the elongation at break (A) were determined in the tensile tests, and the mean grain size and grain elongation were determined in the microstructure analyses. Figure 1An example of a schematic stress-strain diagram from the tensile tests is shown.

[0074] Aged flat steel products exhibit a discontinuous stress-strain diagram. The upper or lower yield strength is typically used as a reference value to characterize strength, and sometimes the tensile strength is also used. The upper yield strength determined in a tensile test is highly dependent on the measurement conditions, the testing machine used, and its orientation. Furthermore, the value varies considerably for a given testing machine. For cold forming, the lower yield strength is a relevant parameter for determining the formability of a flat steel product. However, this is difficult or impossible to determine if the material does not work-harden after the Lüders region. In this case, the tensile strength is also undefined. Therefore, instead of the lower yield strength, the plateau height is estimated, for which the 0.5% proof stress (Rp0.5) is used as a measure because this value can be determined unambiguously. Figure 1The 0.2% proof strength (Rp0.2), a parameter frequently used to characterize unaged steel flat products, is unreliable for aged specimens because it lies too close to the upper yield strength and in a region where the stress has not yet stabilized. For these reasons, the 0.5% proof strength (Rp0.5) is determined here as the relevant measure of the specimens' strength. Furthermore, the elongation at break (A) of the specimens was determined from the tensile tests. All measurements of the 0.5% proof strength (Rp0.5) and the elongation at break (A) were performed along the rolling direction (0°) and in the plane of the steel flat product in 10° increments within an angular range of 10° to 170° to the rolling direction, in order to capture the (an)isotropy of the proof strength (Rp0.5) and the elongation at break (A) in the plane of the sheet. The determined dependence of the elongation at break A(α) on the angle α to the rolling direction (0°) is shown in a pie chart in Figure 2 depicted, whereby Figure 2athe results of the examined samples 1 to 13 and Figure 2b The results of the investigated samples 14 to 26 are shown. The determined dependence of the 0.5% yield strength Rp0.5(α) on the angle α to the rolling direction (0°) is shown in the pie chart of the Figure 3 depicted, whereby Figure 3a the results of the examined samples 1 to 13 and Figure 3b The results of the examined samples 14 to 26 are shown.

[0075] From the measured values ​​for the dependence of the 0.5% proof strength (Rp0.5) and the elongation at break (A) on the angle α to the rolling direction, the work capacity parameter W(α), relevant for cold forming, was calculated. This parameter is defined as the product of the elongation at break A(α) and the 0.5% proof strength Rp0.5(α). The result of the work capacity W(α) thus determined as a function of the angle α to the rolling direction (0°) is shown in the pie charts of the Figure 4 depicted, whereby Figure 4a the results of the samples according to examples 1 to 13 and Figure 4b the results of the samples according to examples 14 to 26 are shown.

[0076] It is from the Figures 2 to 4 It can be seen that the samples according to the invention, compared to the (not annealed in the continuous furnace) comparison examples (Examples 4 to 9, 13, 14, 17, 20 and 24), exhibit significantly better isotropy with respect to the 0.5% yield strength (Rp0.5), the elongation at break (A) and the work capacity W(α) formed therefrom as a product. Figure 2 It can be seen that the samples according to the invention have an elongation at break A(α) which, in the plane of the sheet, lies in a range of 60% to 140% of the elongation at break in the rolling direction A(0°). Figure 3It can be seen that the samples according to the invention have a 0.5% yield strength (Rp0.5) that depends on the angle α to the rolling direction (0°) and lies in the plane of the sheet in a range of 90% to 110% of the yield strength in the rolling direction Rp0.5(0°). The working capacity W(α) of the samples according to the invention lies in the plane of the sheet, depending on the angle α to the rolling direction (0°), in a range of 60% to 140% of the working capacity in the rolling direction W(0°), as shown from Figure 4 As is evident. In contrast, the comparative examples show a significantly higher anisotropy of the elongation at break (A), the 0.5% yield strength (Rp0.5), and the work capacity, as can be seen from the Figures 2 to 4 to recognize.

[0077] To determine the grain structure of the flat steel products, cross-sections of the samples were prepared in planes longitudinal and transverse to the rolling direction, as well as in planar planes of the sheets. The section planes are in Figure 5This was illustrated. The grain size and grain elongation were determined from the polished sections by microphotographic examination of the sample's cross-sectional surfaces. In these microstructure images, the number of intersection points between a grid and the grain boundaries is counted. The mean grain size is derived from the mean value of the line intersection segment (mean chord length). Figure 5The grain elongation, or the elongation of the chord length of the grains in the steel microstructure, is explained using the directions x (horizontal, along the rolling direction) and y (vertical, in the thickness direction of the flat steel product). In the x-direction of the grain, the horizontal chord length S_H is determined. Perpendicular to this, in the y-direction, the vertical chord length S_V is determined. This is done using cross-sections taken both along and across the rolling direction. When determining the chord lengths, not each grain is measured individually, but a uniform grid is superimposed on the microstructure. Using the grid length and the number of intersection points, a chord length is calculated that can be used as a proxy for the grain size. The mean horizontal or vertical chord length corresponds to the mean value of the evaluation of all measured microstructure areas. The grain elongation S is defined as: S = S_H / S_V or S = x / y.

[0078] The grain size (determined according to ASTM E 112 and DIN EN ISO 643 as well as by means of comparison images) and the grain elongation S (determined by the line section method) as well as the mean chord length of the samples are in Table 3 The samples all exhibit a mean chord length in the range of 3.3 to 5.4 µm. The direction-dependent grain elongation (S) in the rolling direction (0°) is at least 1.4 in the longitudinal sections of the steel flat product and at least 1.1 in the planar sections. The grain elongation (S) transverse to the rolling direction (90°) has a value of at least 1.2. No significant difference could be observed between the samples according to the invention and the comparison samples.

[0079] It can be concluded from this that the higher strength of the samples according to the invention is not achieved through grain refinement, but rather primarily through the solid solution strengthening produced by smearing in the continuous annealing furnace. Furthermore, it is evident that the improved isotropy of the mechanical properties of the samples according to the invention can be achieved despite an anisotropy in the microstructure (caused by cold rolling). The anisotropy of the microstructure, also present in the samples according to the invention, results from the grain elongation S, which in the samples according to the invention is comparable to the grain elongation of the reference samples.The solid solution strengthening produced by embossing in a continuous annealing furnace therefore leads not only to an increase in strength (tensile strength Rm) but also to an improvement in the homogeneity of the mechanical properties such as elongation at break A and 0.5% proof strength Rp0.5 as well as the resulting work capacity W = A · Rp0.5.

Claims

1. Cold-rolled flat steel product for packaging with a thickness of less than 0.6 mm, which has been cold-rolled from a steel along a rolling direction of 0° and has the following composition in terms of weight: - C: 0.02 - 0.1 %, - Si: < 0.03 %, - Mn: 0.17 - 0.5 %, - P: < 0.03 %, - S: 0.001 - 0.03 %, - Al: 0.002 - 0.1 %, - N: 0.014 - 0.12 %, preferably less than 0.07 %, - optional Cr: < 0.1 %, preferably 0.01 - 0.08 %, - optional Ni: < 0.1 %, preferably 0.01 - 0.05 %, - optional Cu: < 0.1 %, preferably 0.002 - 0.05 %, - optional Ti: < 0.01 %, - optional B: < 0.005 %, - optional Nb: < 0.01 %, - optional Mo: < 0.02 %, - optional Sn: < 0.03 %, - Residual iron and unavoidable impurities, - wherein at least 0.01 % by weight of the nitrogen is interstitially incorporated in the steel in unbound form and the flat steel product has in an aged state, measured in accordance with the specifications from the description, in the rolling direction of 0° a 0.5% yield strength (Rp0,5) of at least 450 MPa and an elongation at break (A) of at least 5 %, and the working capacity W(α) defined by the product of the elongation at break (A) and the 0.5% yield strength yield strength (Rp 0.5) is not less than 60 % and not more than 140 % of the working capacity in the rolling direction W(0°) as a function of the angle α to the rolling direction of 0° .

2. Flat steel product according to claim 1, characterised in that the 0.5% yield strength (Rp0.5) as a function of the angle α to the rolling direction of 0° is not less than 90% and not more than 110% of the 0.5% yield strength in the rolling direction Rp0.5 (0°).

3. Flat steel product according to any one of the preceding claims, characterised in that the elongation at break A(α) as a function of the angle α to the rolling direction of 0° is not less than 60 % and not more than 140 % of the elongation at break in the rolling direction A(0°).

4. Flat steel product according to any one of the preceding claims, characterised in that the working capacity W(α) as a function of the angle α to the rolling direction of 0° is at least 70 % and at most 130 % of the working capacity in the rolling direction W(0°).

5. Flat steel product according to any one of the preceding claims, characterised in that the grains of the steel structure have an average chord length of 3.0 to 6.0 µm.

6. Flat steel product according to any one of the preceding claims, characterised in that the grains of the steel structure have a mean horizontal chord length (S_H) and a mean vertical chord length (S_V) as well as a grain elongation (S) defined as the ratio of the mean horizontal chord length (S_H) and the mean vertical chord length (S_V), wherein the direction-dependent grain elongation (S) in the rolling direction of 0° has a value of at least 1.4 in longitudinal sections of the flat steel product and a value of at least 1.1 in planar sections of the flat steel product, and / or that the grain elongation (S) transverse to the rolling direction at an angle of 90° has a value of at least 1.2.

7. Flat steel product according to any one of the preceding claims, wherein the flat steel product is obtainable by - Hot rolling of a slab from the steel into a hot strip, - Winding of the hot strip at a winding temperature of 500°C to 750°C, - Cold rolling of the hot-rolled strip with a reduction ratio of at least 80 % to a cold-rolled steel strip, - Nitriding the cold-rolled steel strip in an annealing furnace, in particular a continuous annealing furnace, in the presence of a nitrogen donor at a temperature of at least 550°C, and recrystallisation annealing of the cold-rolled steel strip in an annealing furnace at an annealing temperature of at least 630°C, - Cooling the recrystallising annealed flat steel product to room temperature, - Rerolling of the recrystallised steel strip at a rerolling degree of 0.2 % to 45 %, - whereby the properties of the flat steel product are obtained after ageing of the rerolled steel strip.

8. Flat steel product according to the preceding claim, characterised in that the final rolling temperature during hot rolling of the slab is greater than the Ar3 temperature.

9. Flat steel product according to claims 7 or 8, characterised in that the dwell time of the flat steel product in the annealing furnace is between 10 seconds and 400 seconds.

10. Flat steel product according to any one of claims 7 to 9, characterised in that the degree of rerolling is 18 % or less.

11. Flat steel product according to any one of claims 7 to 10, wherein the nitrogen donor is ammonia gas and / or wherein the nitrogen donor is at least partially dissociated to atomic nitrogen at the temperatures in the annealing furnace.

12. Flat steel product according to any one of claims 7 to 11, characterised in that the hot strip has an initial nitrogen content N0 in the range from 0.001% to 0.016% by weight and that the weight proportion of nitrogen in the flat steel product is increased by ΔN ≥ 0.002 % by weight during annealing due to the presence of the nitrogen donor.

13. Flat steel product according to any one of the preceding claims, wherein the aged state of the flat steel product is obtained either naturally by prolonged storage and / or by lacquering with subsequent drying, or artificially by heating the flat steel product for 20 minutes to temperatures in the range from 200°C to 210°C.

14. Use of a flat steel product according to any one of the preceding claims for the manufacture of tear-off lids of cans or for the manufacture of aerosol cans or aerosol can components, wherein the flat steel product is preferably prior to forming provided with a surface coating, in particular with an electrolytically applied tin or chromium / chromium oxide coating and / or an organic coating, in particular in the form of a lacquer or a polymer film.