Cold-rolled flat steel products for packaging materials

Abstract

To provide a high strength flat steel product for packaging material having isotropic material characteristics.SOLUTION: Cold rolled flat steel products for packaging materials having a thickness of less than 0.6 mm include, by wt.%, C: 0.02 to 0.1%, Si:<0.03%, Mn: 0.17 to 0.5%, P:<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%, optionally, Cr, Ni, Cu, Ti, B, Nb, Mo, Sn, residual iron and inevitable impurities, in which the cold rolling flat steel product has, as a function of yielding strength at least 0.5% offset of minimum 450 MPa in a an aging state, minimum 5% of breaking elongation, and an angle to the rolling direction (α), a deformation energy W(α) defined by the product of rupture elongation and yield strength at 0.5% offset, the deformation energy W (α) is 60% or larger and 140% or smaller of the deformation energy W (0°) in the rolling direction.SELECTED DRAWING: Figure 2a

Description

[Technical Field]

[0001] The present invention relates to cold rolled flat steel products for packaging materials. [Background technology]

[0002] In order to improve resource efficiency and reduce costs, efforts are underway to reduce the thickness of flat steel products (steel plates and strip steel) used in the production of packaging materials (hereinafter also referred to as packaging steel). Cold-rolled packaging steel typically has a thickness in the black plate range, i.e., between 0.1 and 0.6 mm. However, reducing the thickness also reduces the rigidity of the material. Therefore, it is necessary to increase the strength of packaging steel so that the material can meet the requirements for cold workability in forming operations during the production of packaging materials, such as deep drawing or ironing. At the same time, however, it is necessary to maintain the formability of the steel plate during the cold working process. There is therefore a great need for high strength steel sheets which have a yield strength of more than 550 MPa and at the same time good forming properties, such as an elongation at break of at least 5% and / or an Erichsen index (according to DIN standard 50101, also denoted Erichsen cupping index) of at least 5 mm measured according to the cupping test according to Erichsen, which is standardized in DIN standard EN ISO 20482.

[0003] There are many ways to increase the strength of steel sheets, such as strain hardening, solid solution hardening (by adding carbon, nitrogen, phosphorus, manganese and / or silicon as alloying elements), precipitation hardening, increasing the strength by inducing the formation of a multiphase structure in the steel, or grain boundary hardening, etc. However, many of these measures have undesirable associated effects.

[0004] Increased strain hardening causes an increase in the difference in length and width during the production of cold rolled steel sheets, resulting in an increase in anisotropy and a disproportionate decrease in ductility.

[0005] During solution hardening, foreign atoms (e.g., N, C, P, Mn, Si) are incorporated (interstitially inserted) into substitutional or interstitial sites of the steel host lattice. Many of the potential alloying elements have negative concomitant effects (thus, for example, P is detrimental to the steel, and Mn and Si impair surface quality), which is why adding these alloying elements with the aim of increasing strength does not lead to the desired results.

[0006] When steel is alloyed with carbon, its strength increases with the carbon content. However, during processing, significant anisotropy occurs in the morphology of bands, since carbon exists mainly in the form of cementite due to its low solubility in the ferrite lattice of the steel. Furthermore, increasing the carbon content leads to a decrease in surface quality and an increased risk of crack formation in the slab as the peritectic point is approached. Therefore, the carbon content must be reduced to a maximum of 0.1% by weight, as this effectively prevents the formation of cracks in the slab and the resulting oxidation (oxygen diffusion into the cracks).

[0007] The prior art discloses steel sheets for use as packaging materials and methods for their manufacture, in which sufficient carbon and nitrogen are added to the molten steel for solution hardening to achieve strengths exceeding 500 MPa. For example, Patent Document 1 discloses high-strength steel sheets for can manufacturing, having a carbon content of 0.02% to 0.10% by weight and a nitrogen content of 0.012% to 0.0250% by weight, with an ultimate tensile strength exceeding 500 MPa, and a weight percentage of free nitrogen, i.e., the weight percentage of nitrogen incorporated between the steel's lattices, of at least 0.0100%. It has been found that the increase in strength of the steel is decisively due to the free nitrogen during solution hardening and age hardening. However, the strength increase obtained by the interstitial incorporation of nitrogen is limited, on the one hand, by partial fixation of nitrogen in nitrides, particularly AlN, and, on the other hand, by the fact that at nitrogen contents above 0.025%, the risk of crack formation in the slab during hot rolling increases significantly.

[0008] For example, in the case of precipitation hardening by adding alloying elements Ti or Nb, the problem is that precipitates are formed already during hot rolling due to the high temperatures. These precipitates are therefore involved in all subsequent manufacturing steps, such as cold rolling, annealing, and, if necessary, temper rolling or dressing. They are comparable to cementite and exhibit significant anisotropy, especially if the precipitation occurs preferentially at grain boundaries. Furthermore, the precipitating agents Ti and Nb contribute to an increase in the recrystallization temperature.

[0009] Due to prescriptive requirements regarding the alloying elements of steel, increasing the strength of packaging steel by forming a multiphase structure is initially very limited. Therefore, conventional multiphase steels, such as those used in the automotive industry, cannot be used for packaging steel because the standard requirements set out in the packaging steel standard (DIN standard EN10202) only allow alloying elements such as manganese and silicon to be used up to a maximum weight percentage. While it is possible to form a multiphase structure in packaging steel using special cooling techniques, the resulting microstructural state is characterized by considerable instability, and in most cases, increased strength is closely associated with reduced formability. When the multiphase structure is primarily based on the alloying element carbon, an additional risk is that the anisotropy of cementite will be transferred to the multiphase structure, resulting in further strengthening.

[0010] Grain boundary hardening can increase the strength of steel while maintaining unchanged formability by forming a fine grain structure, which can be technically achieved by annealing cold-rolled steel sheets using low reeling temperatures (coiling temperatures after hot rolling), high deformation during cold rolling, and continuous annealing. Furthermore, it is possible to form a fine grain structure by using microalloys to influence the precipitation characteristics of the hot strip. However, the alloying elements required for this purpose are expensive and increase the annealing temperatures required for recrystallization. Furthermore, the increased base strength of the hot strip reduces cold rollability and makes the steel more susceptible to surface defects.

[0011] As a result, the aforementioned possibilities for increasing the strength of steel sheets while maintaining formability pose problems, particularly with regard to isotropy, i.e., the directional dependence of material properties. Packaging materials, such as beverage and food cans, are often rotationally symmetrical items. Therefore, steel sheets used in the manufacture of packaging materials are frequently available in the form of round blanks (i.e., flat cylindrical sheet metal blanks) that are formed into cylindrical can bodies or cylindrical can bottoms or lids using deep drawing and ironing processes. Due to the symmetry of the final product, the material properties of the sheet metal (flat steel) must be as isotropic as possible. That is, the properties of the packaging metal must be as uniform as possible in all directions along the metal surface. For cold-rolled steel sheets, which are available in strip steel due to the nature of the manufacturing process, this is a very complex requirement because the rolling direction of both hot and cold rolling always determines the material properties. Therefore, cold-rolled steel sheets always have significant anisotropy due to the nature of the manufacturing process. This anisotropy is crucially due to the high deformations during cold rolling that are necessary to achieve very thin metal sheet thicknesses. In the production of packaging materials, cold-rolled steel sheets are always processed without regard to the rolling direction, which frequently causes difficulties during forming operations, for example because the strength and formability are not uniform over the entire circumference of the round blank.

[0012] Therefore, there is a great need for packaging steel in the form of cold-rolled flat steel products that are characterized by the most isotropic properties possible within the plane of the flat steel sheet. This is a contradictory goal that is difficult to achieve given the continuous decrease in thickness of flat steel products and the resulting increase in strength. Furthermore, in addition to the isotropy of the flat steel products, other requirements that packaging steel must meet must be taken into account in the production of packaging materials, particularly with regard to flexibility in forming operations and the shape of the packaging material, reduction of scrap, and the realization of the most uniform and homogeneous properties possible for the packaging material. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] US2011 / 0076177A1 Summary of the Invention

[0014] Therefore, one problem to be solved by the present invention is to make available high-strength flat steel products for use in the production of packaging materials having material properties as isotropic as possible in the sheet metal surface, and from the high-strength flat steel products, flat steel product packaging materials having excellent isotropy and the most diverse shapes can be produced in various forming processes with as little scrap as possible.

[0015] Since packaging steel is processed into finished packaging materials in the aged state, i.e. after a relatively long storage period and, if appropriate, after painting and drying, it is necessary to optimize the material taking into account the effects of aging that occur after a relatively long storage period and / or after painting and subsequent drying. The technical parameters of packaging steel are therefore determined after the material has been artificially aged, which can be done by heating test specimens to 200°C for 20 minutes in accordance with DIN standard EN 10202. During aging (natural or artificial) of steel sheets, strength and formability in particular are affected, so the effects of aging must be taken into account when optimizing the material properties.

[0016] For the reasons mentioned above, improvements in the strength and formability properties of cold-rolled steel sheets are made at the expense of their isotropy. To impart isotropy to steel sheets, various metallurgical and process engineering techniques can be used in the production of cold-rolled steel sheets. For example, one option for specifically improving the isotropy of cold-rolled steel sheets is the addition of boron as an alloying agent. However, boron has a negative effect on the steel's workability and the final product (steel sheet). Adding boron as an alloying agent increases the annealing temperature required to recrystallize the steel sheet after cold rolling, reducing the material's weldability and aging potential (i.e., the increase in strength during aging of the steel sheet).

[0017] Another problem to be solved by the present invention is therefore to make available a cost-effective and producible packaging steel and a method for its production, which on the one hand has as high a strength as possible while maintaining adequate formability sufficient for deep drawing and ironing, and on the other hand has as high an isotropy as possible of the material properties with regard to strength and formability in the aged state of the material.

[0018] According to the present invention, the aforementioned problem is solved by a flat steel product having the features of claim 1. In this context, flat steel product is intended to refer to steel sheets in slab or strip form having a thickness in the black plate range, in particular in the thickness range of 0.1 to 0.6 mm.

[0019] The present invention is based on the discovery that solid solution hardening by alloying elements incorporated into the steel interstitial lattice allows for simultaneous improvements in strength, formability, and isotropy, and that solid solution hardening by carbon and nitrogen is particularly effective when the fixation of carbon and nitrogen in carbides and nitrides can be at least significantly suppressed technologically, as the formation of carbides and nitrides will promote the formation of anisotropic properties.

[0020] Another discovery according to the present invention is that the incorporation of nitrogen by nitriding cold-rolled flat steel products in an annealing furnace in the presence of a nitrogen donor at the end of the production route of packaging steel is particularly suitable both for achieving effective solution hardening with nitrogen and for improving the isotropy of the material properties, in particular the yield strength and elongation at break, relevant for the further processing of the flat steel products in the manufacture of packaging materials. This is because, in contrast to increasing the nitrogen content by introducing nitrogen into the liquid steel, it has been found that nitriding in an annealing furnace essentially results in the incorporation of nitrogen into interstitial spaces without fixing the nitrogen in nitrides.

[0021] Surprisingly, it has been found that the nitrogen incorporated into the interstitials in cold rolled flat steel products during the nitriding cycle in the annealing furnace (especially in continuous annealing furnaces before or during the recrystallization anneal) has a positive effect on formability and isotropy with respect to the material properties. Apparently, due to the uniform distribution of nitrogen in the (ferritic) interstitials of the steel, the incorporation of nitrogen into the interstitials leads to very good isotropy with respect to the mechanical properties of the flat steel products.

[0022] This is further reinforced by the fact that, compared to carbon, interstitial nitrogen incorporation shifts the position of the peritectic point to significantly higher alloy contents, meaning that the incorporation of large amounts of nitrogen in interstitial sites of the steel lattice is less critical in terms of surface quality and the risk of crack formation within the slab than the incorporation of carbon interstitially. To prevent crack formation in the slab, the weight percent of carbon in the flat steel product of the present invention is limited to 0.10%. On the other hand, with regard to nitrogen incorporation, the nitrogen content is limited only by the solid solubility limit of nitrogen in the ferrite lattice of the steel and the economic feasibility of the manufacturing process. Therefore, considering the solid solubility limit of nitrogen in the ferrite lattice of approximately 0.1% by weight and the partial fixation of nitrogen into nitrides in the presence of strong nitride-forming elements such as Al, Ti, Nb, and / or B, the nitrogen content of the steel is limited to a maximum of 0.120% by weight. From a process engineering perspective, the nitrogen content of nitrided flat steel products according to the present invention is preferably limited to a maximum of 0.070% by weight. Because nitriding cold-rolled flat steel products in (continuous) annealing furnaces in excess of this amount requires such a high degree of technical complexity that it is not yet economically viable, at least not yet. For technical and economic reasons, the nitrogen content by weight is therefore most preferably not more than 0.050%.

[0023] In this connection, it is particularly useful to add nitrogen to the flat steel product as late as possible in the manufacturing process in order to prevent changes in the material properties that occur after nitriding, in particular in the rolling direction and transverse to the rolling direction as a result of the cold rolling passes along the rolling direction. The flat steel product according to the invention can, for example, be nitrided after the (primary) cold rolling step, before or during the annealing process in a continuous annealing furnace.

[0024] Since nitriding is carried out only after the (first) cold rolling step, nitrogen is not part of the hot rolling and (first) cold rolling process steps, which would result in a large anisotropy in the material properties. The interstitial incorporation of nitrogen into the ferrite lattice during or after recrystallization annealing further enhances the homogeneity of the packaging steel according to the invention. More specifically, the risk of nitride precipitation during temper rolling, which would increase the directional dependency of the material properties, is avoided.

[0025] In the case of cold-rolled steel sheets that are rolled twice, the base strength of the steel is increased by the nitrogen incorporated between the lattices as a result of solid solution hardening, so the temper reduction in the second cold rolling pass can be reduced, thereby minimizing the resulting anisotropy. Therefore, the temper reduction in the second cold rolling pass can be preferentially limited to 18% or less.

[0026] The invention therefore relates to a (single or double) cold-rolled flat steel product for packaging materials, cold-rolled from steel along the rolling direction (0°) and having a thickness of less than 0.6 mm, having the following composition in weight percent: C: 0.02~0.1%, 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%; Optionally, Cr: less than 0.1%, preferably 0.01 to 0.08%; Optionally, Ni: less than 0.1%, preferably 0.01 to 0.05%; Optionally, Cu: less than 0.1%, preferably 0.002 to 0.05%; Optionally, Ti: less than 0.01% Optionally, B: less than 0.005% Optionally, Nb: less than 0.01% Optionally, Mo: less than 0.02% Optionally, Sn: less than 0.03% The balance is iron and unavoidable impurities, Including, The flat steel product in the aged condition has a yield strength at 0.5% offset (Rp0.5) of at least 450 MPa, a breaking elongation (A) of at least 5%, and a deformation energy W(α) defined as the product of the breaking elongation (A) and the yield strength at 0.5% offset (Rp0.5) as a function of angle (α) relative to the rolling direction (0°), wherein the deformation energy W(α) is at least 60% and at most 140% of the deformation energy W(0°) in the rolling direction.

[0027] Preferably, a minimum weight percentage of 0.010% of the nitrogen contained in the flat steel product is incorporated interstitially in the steel in an unbonded state.

[0028] The flat steel product according to the invention is characterized by a high yield strength (Rp0.5) at 0.5% offset of at least 450 MPa in the rolling direction (0°), a good elongation at break (A) of at least 5%, and a uniform and negligible directional dependence of the deformation energy W(α) in the plane of the flat steel product offset by an angle α. The deformation energy W(α) (which remains direction-dependent because the rolls pass along the rolling direction) is a suitable measure for assessing whether a cold-rolled steel sheet can be used for the manufacture of packaging materials by deep drawing and ironing processes, since the deformation energy W, calculated from the product of the elongation at break (A) and the yield strength at 0.5% offset (Rp0.5), is a measure of both the strength of the steel sheet and its formability. For reasons that will be explained further below, the yield strength at 0.5% offset (Rp0.5) has proven to be a suitable measure for assessing the strength of aged flat steel products.

[0029] In the flat steel product according to the invention, the yield strength (Rp0.5) at 5% offset depending on the angle (α) relative to the rolling direction (0°) lies in a range between an upper limit and a lower limit, the lower limit being a minimum of 90% of the yield strength (Rp0.5) at 0.5% offset in the rolling direction (0°), and the upper limit being a maximum of 110%, with the yield strength (Rp0.5) at 0.5% offset in the rolling direction being a minimum of 450 MPa.

[0030] In the flat steel product according to the invention, the breaking elongation A(α) depending on the angle (α) relative to the rolling direction (0°) also lies in a range between an upper and a lower limit, the lower limit being a minimum of 60% of the breaking elongation A(0°) in the rolling direction, and the upper limit being a maximum of 140% of the breaking elongation A(0°), the breaking elongation A(A) in the rolling direction being a minimum of 5%.

[0031] Therefore, the deformation energy W(α) depending on the angle (α) relative to the rolling direction (0°) preferably ranges between a minimum of 70% and a maximum of 130% of the deformation energy W(0°) in the rolling direction.

[0032] Nitrogen diffuses into the cold-rolled steel flat during the nitriding cycle in the annealing furnace, preferably at annealing temperatures above 630°C (the temperature of the steel flat) to ensure complete recrystallization. The diffused nitrogen is uniformly distributed and incorporated into the steel lattice. The uniform distribution of interstitially incorporated nitrogen results in high isotropy of the mechanical properties of the nitrided steel flat affected by the nitriding process, particularly with regard to the elongation at break and yield strength, and consequently, the deformation energy, which is the product of the yield strength at 0.5% offset (Rp0.5) and the elongation at break (A), a relevant quality criterion for deep drawing applications. The most uniform distribution possible of the nitrogen incorporated in the annealing furnace is observed with longer residence times of the steel flat in the annealing furnace, especially during the recrystallization annealing. The residence time of the steel flat in the annealing furnace is preferably longer than 10 seconds, more preferably longer than 30 seconds, and particularly in the range of 100 to 250 seconds. At residence times longer than 400 seconds, the throughput rate of flat steel products in strip form through a continuous annealing furnace over a typical throughput path length must, for economic reasons, be so low that the efficiency of the process can no longer be demonstrated, which is why annealing times greater than 400 seconds can only be established in batch annealing processes.

[0033] The isotropy with respect to material properties related to the cold working cycle, such as yield strength at 0.5% offset (Rp0.5), elongation at break (A) and deformation energy (W), achieved in the flat steel product according to the invention by nitriding after the cold rolling cycle, can be achieved despite the unavoidable elongation of the steel grains due to the (single or double) cold rolling. In the flat steel product according to the invention, the grains of the steel structure typically have an average diameter of 3.0-6.0 μm. Line lengthand for example in the rolling direction (0°) the direction-dependent grain elongation (S) is at least 1.4 in the longitudinal section of the flat steel product and at least 1.1 in the planar section of the flat steel product. It is therefore possible with the flat steel product according to the invention to achieve isotropic properties in terms of yield strength, elongation at break and the resulting deformation energy of the metal surface, despite the grain elongation inherent in the manufacturing process.

[0034] The grain elongation (S) of the grains in a steel structure is Vertical (Y) direction Average line segment length (S_V) Vertical (X) direction It is defined as the ratio of the mean line segment length (S_H). In the direction transverse to the rolling direction (α=90°), the grain elongation (S) of flat steel products, depending on the angle α, is typically a minimum of 1.2.

[0035] Since the solution hardening produced by nitriding flat steel products is most effective when the added nitrogen is incorporated into the interstitial sites of the steel (especially the ferritic lattice), it is useful if the alloy composition of the steel contains as few strong nitride-forming elements as possible, such as Al, Ti, B, and / or Nb, to prevent the nitrogen from bonding in the form of nitrides. Therefore, the alloy composition of the steel preferably has the following upper limits for the weight percent contents of these strong nitriding 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] The total weight content of nitride-forming elements is preferably less than 0.1%, which in particular ensures that the weight content of free nitrogen is greater than 0.01%.

[0037] Weight content of free nitrogen in hot rolled strip, N 遊離(hot rolled strip) can be expressed by Equation 1, based on the assumption that the nitride-forming elements Al, Ti, B, and Nb that may be contained in the steel within the above thresholds are completely fixed with nitrogen to form nitrides: N 遊離 (hot-rolled strip) = 1 / 2 (N0-Ti / 3.4-B / 0.8-Nb / 6.6-Al coefficient + |N0-Ti / 3.4-B / 0.8-Nb / 6.6-Al coefficient|) (Equation 1), where N0 is the weight content of nitrogen in the molten steel, and the Al coefficient is defined as a function of the reel temperature HT (the coiling temperature of the hot rolled strip) and the aluminum content Al (wt%) as follows: For HT<640℃: Al coefficient = 0, For 750 ≥ HT ≥ 640°C: Al coefficient = N0 - N0 × (-0.682HT + 536) / 100 = N0 × (1 - (-0.682HT + 536) / 100) And the addend |N0-Ti / 3.4-B / 0.8-Nb / 6.6-Al coefficient| is defined as the sum of the differences "N0-Ti / 3.4-B / 0.8-Nb / 6.6-Al coefficients." In Equation 1, this summand takes into account that at most only the total nitrogen actually present in the hot rolled strip (i.e., the molten steel) can be fixed by the nitride-forming elements present in the hot rolled strip (i.e., the molten steel).

[0038] The total weight content of free nitrogen in cold rolled flat steel products is calculated by adding the total weight of free nitrogen in hot rolled strip (N according to formula 1 above). 遊離 It is obtained from the sum of the free nitrogen content of the hot-rolled strip and the nitrogen ΔN added by nitriding in the continuous annealing furnace: N 遊離 =N 遊離 (hot rolled strip) + ΔN (Equation 2)

[0039] This is based on the assumption that at least most of the nitrogen content ΔN introduced during the nitriding process in a continuous annealing furnace is incorporated into the interstitial sites. The upper limit value of the weight content of free nitrogen in cold-rolled flat steel products is determined by the solid solubility limit of nitrogen in the ferrite lattice of the steel and is approximately 0.1% by weight.

[0040] The total weight content of free nitrogen in cold-rolled flat steel products (N 遊離 ) is preferably more than 0.01%. In order to introduce as high a proportion of uncombined nitrogen as possible into the cold-rolled flat steel products, preferably, most of the total weight content of nitrogen is introduced by nitriding in a continuous annealing furnace, and the weight content of ΔN is preferably at least 0.002% by weight, and most preferably, higher than 0.008% by weight.

[0041] The flat steel product according to the present invention is first produced from a slab manufactured from the above molten steel by hot rolling at a final rolling temperature preferably above Ar3, particularly between 800°C and 900°C, to produce a hot-rolled strip, and the hot-rolled strip is wound up at a coiling temperature (reel temperature HT) below Ar1, particularly in the temperature range of 500°C to 750°C. After cooling, it is cold-rolled at a rolling rate of at least 80% to form a flat steel product (strip steel), and then, in an annealing furnace, particularly in a continuous annealing furnace, at an annealing temperature of at least 630°C, in the presence of a nitrogen donor, at least intermittently, recrystallization annealed, then cooled to room temperature, and finally, it is obtained by a manufacturing process of being quenched and tempered or dressed at a quenching and tempering rolling rate of 0.2% to 45%. In order to ensure that the isotropy is not adversely affected by the quenching and tempering rolling cycle, the quenching and tempering rolling rate is preferably less than 18%.

[0042] Nitriding of steel flat products in a continuous annealing furnace can be carried out before, during, or after recrystallization annealing. For example, the nitriding step can be carried out in a first upstream zone of the continuous annealing furnace in the presence of a nitrogen donor at a first temperature below the recrystallization temperature, and then the steel flat product can be heated in a second downstream zone of the continuous annealing furnace at a second temperature above the recrystallization temperature for recrystallization annealing. The order of nitriding and recrystallization annealing can also be reversed. Separating the nitriding cycle from the recrystallization annealing cycle and carrying them out in different zones of the continuous annealing furnace has the advantage of allowing optimal temperatures to be set for each step, with the optimal temperature for the nitriding step being lower than that for the recrystallization annealing cycle. However, for economic reasons, it is preferred to simultaneously nitride and anneal steel flat products in a continuous annealing furnace at temperatures above the recrystallization temperature in the presence of a nitrogen donor.

[0043] The properties of the flat steel products produced in this way develop after the temper rolled strip steel has been aged, which can be achieved either artificially by heating to 200°C for 20 minutes, or by coating the flat steel product with varnish and then drying the varnish.

[0044] The hot-rolled strip preferably already has an initial nitrogen content NO in the range of 0.001% to 0.016% by weight to maximize the total nitrogen content of the cold-rolled flat steel product and the solid-solution hardening caused by nitriding the cold-rolled strip. The weight content of nitrogen in the molten steel used to produce the hot-rolled strip must not exceed 0.016% to prevent cracks from forming in the slab during casting and hot rolling and to ensure that the strength of the hot-rolled strip has not increased to the point where it can no longer be cold-rolled using commonly used cold-rolling equipment. The total nitrogen content of the flat steel product according to the present invention, consisting of the sum of the initial nitrogen content NO and the nitrogen content ΔN introduced during nitriding in the annealing furnace, is set during annealing of the cold-rolled flat steel product by the presence of a nitrogen donor in the annealing furnace. At the annealing temperature, the dissociated atomic nitrogen from the nitrogen donor diffuses into the cold-rolled flat steel product, thereby increasing the nitrogen content ΔN. The nitrogen content ΔN introduced during nitriding in the annealing furnace is preferably a minimum of 0.002 wt.%, which increases the total nitrogen content of the flat product to more than 0.014 wt.% if the initial nitrogen content NO in the liquid steel is lower than 0.014 wt.%. Cold rolled flat products are most preferably nitrided in a continuous annealing furnace to a nitrogen content of more than 0.020 wt.%. The total nitrogen content of the flat product nitrides in a continuous annealing furnace can (at least theoretically) range up to about 0.1%, which is the solid solubility limit of nitrogen in the (ferritic) lattice of steel.

[0045] The nitrogen donor involved can be, for example, a nitrogen-containing gas atmosphere in the annealing furnace, in particular an ammonia-containing atmosphere, or a nitrogen-containing liquid applied to the surface of the cold-rolled steel flat product before the product is heated in the annealing furnace. The nitrogen donor used must be of a type such that it dissociates in the annealing furnace, thereby making atomic nitrogen available, in order to diffuse the nitrogen into the steel flat product. The nitrogen donor involved can, in particular, be ammonia gas. To ensure that this ammonia gas dissociates in the annealing furnace to form atomic nitrogen, the annealing furnace is preferably set to a furnace temperature of more than 400°C during the nitriding of the cold-rolled steel flat product.

[0046] Due to the increase in strength resulting from solution hardening by nitriding the flat steel product during annealing in a (continuous) annealing furnace in the presence of nitrogen donors, it is not necessary to temper roll the flat steel product according to the invention with a high temper reduction in order to strain harden it and further increase its strength. The temper reduction can therefore preferably be limited to a maximum of 18%, which makes it possible to avoid a deterioration in the isotropy of the material properties caused by a second cold rolling cycle with a high temper reduction.

[0047] After the second cold rolling or dressing cycle, a coating can be applied to the surface of the flat steel product to improve its corrosion resistance, for example by electrodeposition of a tin or chromium / chromium oxide coating and / or by applying a varnish to the surface, or by laminating to the surface a polymer sheet made from a thermoplastic material, in particular a sheet made from polyester such as PET or a polyolefin such as PP or PE.

[0048] The excellent isotropic mechanical properties of the steel sheet according to the present invention make it possible to produce pull-tab lids for cans (so-called "easy-open-end", EOE) or aerosol cans or components of aerosol cans (e.g., aerosol can bottoms or lids), where the pull-tab lids or aerosol lids or aerosol cans and components thereof have isotropy over their entire surface. The isotropic properties of the steel sheet according to the present invention are particularly advantageous for circular or oval pull-tab lids and circular bottoms of aerosol cans, since the associated products have substantially uniform mechanical properties over their entire circumference. The isotropic mechanical properties of the steel sheet according to the present invention also provide advantages in deep drawing applications in which round steel sheet parts (circular blanks) are formed, for example, to produce can bodies for two-part cans, since uniform mechanical properties are likewise achieved over the circumference of the formed sheet metal part, and no thinned areas of material are generated during the forming process, resulting in a reduction in the thickness of the sheet metal.

[0049] These and other properties, characteristics and advantages of the flat steel product according to the invention will be apparent from the examples further described below with reference to the accompanying drawings and tables. [Brief explanation of the drawings]

[0050] [Figure 1] 1 is an example of a schematic stress-strain diagram obtained in a tensile test of a flat steel product according to the invention. [Figure 2] 2A and 2B are pie charts of the angular dependence of the fracture elongation (A) in the plane of the flat steel products tested in the tensile test, where FIG. 2A shows the results for specimens 1 to 13 and FIG. 2B shows the results for specimens 14 to 26. [Figure 3] Figure 3a shows the results for specimens 1 to 13, and Figure 3b shows the results for specimens 14 to 26. [Figure 4]Fig. 4a shows the results of specimens 1–13, and Fig. 4b shows the results of specimens 14–26. [Figure 5] FIG. 1 is a schematic diagram of the test to identify the grain structure of the tested flat steel products. [Figure 6] FIG. 1 shows the effect of ageing a cold rolled flat steel product on isotropy with respect to elongation at break. DETAILED DESCRIPTION OF THE INVENTION

[0051] To produce the flat steel products according to the invention, slabs are cast from molten steel and hot-rolled to form hot-rolled strip. The alloy composition of the molten steel is preferably guided by thresholds specified by the standards for packaging steel (e.g., as defined in standard ASTM A623-11 "Standard Specification for Tin Mill Products" or "European Standard EN 10202"). Below, the compositions of steels from which the flat steel products according to the invention can be produced are described in detail.

[0052] Steel Composition

[0053] Carbon C: minimum 0.02%, maximum 0.1%, preferably less than 0.085%. Carbon has the effect of increasing hardness and strength. Therefore, steel must contain a minimum of 0.02% carbon by weight. The carbon content must not be too high to ensure the rollability of flat steel products during the primary cold rolling process and, if appropriate, the secondary cold rolling process (temper rolling or dressing), without reducing the breaking elongation. Furthermore, as the carbon content increases, the low solubility of carbon in the ferrite lattice of steel means that carbon exists mainly in the form of cementite, leading to the formation of significant anisotropy in the form of bands during the production and processing of flat steel products. Furthermore, an increase in carbon content leads to a deterioration in surface quality and an increased risk of crack formation in the slab as the peritectic point is approached. Therefore, the carbon content must be limited to a maximum of 0.1% by weight to effectively prevent crack formation in the slab and the resulting oxidation pitting (oxygen diffusion into the cracks).

[0054] Manganese (Mn): min 0.17%, max 0.5%. Manganese also increases hardness and strength. Furthermore, manganese improves the weldability and wear resistance of steel. Furthermore, the addition of manganese binds sulfur to the less harmful MnS, reducing the tendency for red shortness during hot rolling. Furthermore, manganese causes grain refinement and increases the solubility of nitrogen in the steel grid, preventing carbon diffusion to the slab surface. Therefore, a manganese content of at least 0.17 wt.% is preferred. To achieve high strength, a manganese content of more than 0.2 wt.%, particularly 0.30 wt.% or more, is preferred. However, if the manganese content is too high, the corrosion resistance of the steel will decrease, and food-grade quality will no longer be guaranteed. Furthermore, if the manganese content is too high, the strength of the hot-rolled strip will be so high that it can no longer be cold-rolled. Therefore, the upper limit for the manganese content is 0.5 wt.%.

[0055] Phosphorus: Less than 0.03% Phosphorus is an undesirable residual element in steel. In particular, high phosphorus contents embrittle the steel, adversely affecting the formability of flat steel products. Therefore, the upper limit for phosphorus content is 0.03 wt.%.

[0056] ·Sulfur S: More than 0.001%, maximum 0.03% Sulfur is an undesirable residual element that adversely affects ductility and corrosion resistance. Therefore, more than 0.03% by weight of sulfur should not be present in the steel. However, on the other hand, the measures that must be taken to desulfurize the steel are technically complex and expensive, so that for economic reasons, sulfur contents below 0.001% by weight are no longer acceptable. Therefore, the sulfur content is in the range of 0.001 to 0.03% by weight, and most preferably in the range of 0.005 to 0.01% by weight.

[0057] Aluminum (Al): over 0.002% and less than 0.1% Aluminum is necessary in steel production as a deoxidizer for killing (deoxidizing) the steel. Aluminum also improves scale resistance and formability. Therefore, the aluminum content is higher than 0.002% by weight. However, aluminum, together with nitrogen, forms aluminum nitride, which is undesirable in the flat steel product according to the present invention. The formation of aluminum nitride is undesirable because it reduces the content of free nitrogen. Furthermore, if the aluminum concentration is too high, surface defects in the form of aluminum clusters may occur. Therefore, aluminum can be used in a concentration of up to 0.1% by weight.

[0058] Silicon (Si): Less than 0.03% Silicon increases the scale resistance of steel and is a precipitation hardener. In steel production, Si functions as a deoxidizer. Another positive effect of Si on steel is that it increases tensile strength and yield stress. Therefore, a silicon content of 0.003 wt.% or more is preferred. However, if the silicon content is too high, more specifically above 0.03 wt.%, the corrosion resistance of the steel may decrease and surface treatment, especially by electrolytic coating, may be hindered.

[0059] Optionally, nitrogen NO: less than 0.016%, preferably more than 0.001% Nitrogen is an optional component in the molten steel from which the steel for the flat steel products of the present invention is produced. Nitrogen, as a precipitation hardener, has the effect of improving hardness and strength. However, excessively high nitrogen contents in the molten steel, exceeding 0.016% by weight, make it more difficult to cold-roll the hot-rolled strip produced from the molten steel. Furthermore, because hot formability decreases at nitrogen concentrations of 0.016% by weight or more, high nitrogen contents in the molten steel increase the risk of defects in the hot-rolled strip. According to the present invention, the nitrogen content of the flat steel product is increased by nitriding 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 high strength through solid solution hardening, it is preferable that an initial nitrogen content of more than 0.001% by weight, most preferably 0.010% by weight or more, is already present in the molten steel.

[0060] To introduce an initial nitrogen content of NO into the flat steel product before nitriding in the annealing furnace, a suitable amount of nitrogen can be added to the molten steel, for example, by blowing nitrogen gas and / or by adding solid nitrogen compounds such as calcium nitrogen (calcium cyanamide) or manganese nitride.

[0061] optionally nitride-forming elements, in particular niobium, titanium, boron, molybdenum, chromium In the steel of the flat steel product according to the present invention, nitride-forming elements such as aluminum, titanium, niobium, boron, molybdenum, and chromium are undesirable because they reduce the proportion of free nitrogen by forming nitrides. Furthermore, these elements are expensive, and therefore increase the production costs. However, on the other hand, elements such as niobium, titanium, and boron, as micro-alloying elements, increase strength by grain refinement without reducing toughness. Therefore, it may be useful to add the above-mentioned nitride-forming elements as alloying elements in limited amounts to the molten steel. Thus, the steel may (optionally) contain the following nitride-forming alloying elements in weight percent: Titanium Ti: preferably more than 0.002%, but less than 0.01% for cost reasons; 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 molten steel and to prevent the diffusion of carbon at the slab surface, but at most 0.08% to prevent the formation of carbides and nitrides, and / or Molybdenum (Mo): Less than 0.02% to prevent excessive rise in recrystallization temperature.

[0062] Free unbound nitrogen N as a result of nitride formation 遊離 In order to avoid a decrease in the proportion of the nitride-forming elements, the total weight content of the nitride-forming elements mentioned above in the molten steel is preferably less than 0.1%.

[0063] Other optional ingredients In addition to the balance iron (Fe) and unavoidable impurities, the molten steel may also contain other optional components, such as, for example: Optionally, Copper Cu: more than 0.002% to allow for the use of scrap in the production of molten steel, but less than 0.1% to ensure food-grade quality; Optionally, Nickel Ni: more than 0.01% to allow for the use of scrap in the production of molten steel and to improve toughness, but less than 0.1% to ensure food-grade quality; Optionally, tin (Sn): preferably less than 0.03%.

[0064] Flat steel product manufacturing method: Using the above steel composition, molten steel is produced, which is first continuously cast, cooled, and then divided into slabs, which are then reheated to a preheat temperature above 1100°C, typically 1200°C, and hot rolled to produce hot-rolled strip with thicknesses ranging from 1 to 4 mm.

[0065] The final rolling temperature during hot rolling is preferably higher than the Ar3 temperature, in particular in the range of 800°C to 900°C, in order to maintain austenite.

[0066] The hot-rolled strip is coiled to form a coil at a predetermined, preferably constant, coiling temperature (reel temperature, HT). The coiling temperature is preferably lower than Ar1 to maintain the ferritic range, preferably in the range of 500°C to 750°C, and most preferably below 640°C to prevent AlN precipitation. For economic reasons, the coiling temperature should be higher than 500°C. The formation of iron nitrides on the surface of the hot-rolled strip can be prevented by cooling the hot-rolled strip at a higher cooling rate from the end of the hot-rolling cycle until coiling.

[0067] To produce steel for packaging in the form of thin flat steel products in the thickness range of less than 0.6 mm (black plate thickness), preferably less than 0.4 mm, the hot-rolled strip is cold-rolled to a minimum thickness reduction (degree of reduction or deformation during cold rolling) of 80%, preferably in the range of 85% to 98%. To restore the crystalline structure of the steel destroyed during cold rolling, the cold-rolled steel strip is then recrystallized in an annealing furnace. This is achieved, for example, by passing the flat steel product in the form of cold-rolled steel strip through a continuous annealing furnace, where the steel strip is heated to a temperature above the recrystallization temperature of the steel. Prior to, or preferably simultaneously with, the recrystallization annealing, the cold-rolled flat steel product is nitrided by heating the flat steel product in the annealing furnace in the presence of a nitrogen donor. Nitriding is carried out simultaneously with the recrystallization annealing in the annealing furnace by introducing a nitrogen donor, particularly in the form of a nitrogen-containing gas, preferably ammonia (NH), into the annealing furnace, heating the steel flat product to an annealing temperature above the recrystallization temperature of the steel, and maintaining the steel flat product at the annealing temperature for an annealing time (holding time) preferably between 10 and 150 seconds. The annealing temperature is preferably higher than 630°C, in particular in the range of 650°C to 750°C. The nitrogen donor is selected so as to ensure that, at the temperature in the annealing furnace, the nitrogen donor dissociates to form atomic nitrogen, which can diffuse into the steel flat product. Ammonia has proven suitable for this purpose. To prevent oxidation of the surface of the steel flat product during annealing, a protective gas atmosphere is advantageously used in the annealing furnace. The atmosphere in the annealing furnace preferably consists of a mixture of a nitrogen-containing gas acting as a nitrogen donor and a protective gas such as HNx, the volume content of the protective gas preferably being in the range of 90% to 99.5%, the remainder of the volume content of the gas atmosphere being formed by the nitrogen-containing gas, in particular ammonia gas (NH3 gas).

[0068] Working Example: Examples of the present invention and comparative examples are described below. Flat steel products (strip steel) were produced from molten steel having the alloy composition listed in Table 1 by hot rolling and subsequent cold rolling.

[0069] The cold rolled flat steel products were subsequently recrystallization annealed in a continuous annealing furnace by maintaining the flat steel products at an annealing temperature of 640°C for an annealing time of 45 seconds.

[0070] The process and material parameters of the heat-treated steel sheets in Table 1 are shown in Table 2. N(after nitriding) is the nitrogen content after nitriding in the annealing furnace. D is the thickness of the steel plate (mm). NWG represents the temper rolling rate (%) during the second cold rolling. NH3 is the ammonia content (vol %) in the annealing furnace. Rp0.5 is the yield strength (MPa) at 0.5% offset in the rolling direction. A is the elongation at break (%) in the rolling direction. Rm is the tensile strength in the rolling direction (MPa).

[0071] In the examples according to the invention (Examples 1-3, 10-12, 15, 16, 18, 19, 21-23, 25 and 26 in Tables 1 and 2), ammonia was introduced into the continuous annealing furnace during the heat 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. In Table 2, the volume content of ammonia in the gas atmosphere is shown as NH3 (vol.%). In the comparative examples (Examples 4-9, 13, 14, 17, 20 and 24 in Tables 1 and 2), a 100% HNx protective gas atmosphere was present in the continuous annealing furnace during annealing. In Table 2, the total nitrogen content obtained in the test specimens according to the invention by nitriding in an ammonia-containing gas atmosphere in a continuous annealing furnace is shown as N (after nitriding) [wt.%]. The total nitrogen content N was determined according to DIN standard EN ISO 14284 (in particular subparagraph 4.4.1) after removing the surface iron nitride layer formed on the surface of the test specimens during nitriding.

[0072] The total nitrogen content by weight is the sum of the initial nitrogen content in the molten steel (NO, see Table 1) and the nitrogen content ΔN introduced by nitriding in the continuous annealing furnace. 遊離 A significant portion of the free nitrogen, N, is obtained in the unbound state, and the remainder is obtained in the bound state as nitrides. See equation (1). Using equation (1), the free nitrogen, N, is calculated based on the weight content of nitride-forming elements in the steel. 遊離 The weight content of can be estimated.

[0073] After heat treatment in a continuous annealing furnace, the cold-rolled and recrystallization-annealed flat steel products were subjected to temper rolling or dressing. The temper rolling reduction (NWG) of the second cold rolling or dressing and the thickness of the temper-rolled flat steel products are shown in Table 2. Finally, the flat steel products were aged by heating the specimens to 200°C for 20 minutes.

[0074] Figure 6 shows the effect of aging on the angle dependence of fracture elongation for Comparative Example 5, comparing the unaged and aged states. The aged state is further compared to artificial and natural aging. This shows that significant anisotropy only develops after aging. However, since aging is virtually unavoidable in the practical processing of packaging steel, it is particularly important to determine and optimize isotropy in the aged state, which is the objective of this invention.

[0075] Tensile tests and structural examinations were carried out on the aged specimens of Examples 1 to 26. More specifically, the yield strength at 0.5% offset (Rp0.5, measured according to DIN standard EN ISO6892-1) and the elongation at break (A) were determined in the tensile tests, and the average grain size and grain elongation were determined in the structural examinations. Figure 1 shows an example of a stress-strain diagram from a tensile test.

[0076] The stress-strain curve of aged flat steel products exhibits a discontinuous pattern. As a rule, the upper yield limit or lower yield strength is used as a reference value to characterize strength, and sometimes tensile strength. The upper yield strength measured in tensile tests is highly dependent on the measurement conditions, the testing machine used, and its orientation. Furthermore, the spread in values ​​is particularly large for certain testing machines. In cold-working processes, the lower yield strength is a relevant parameter for determining the formability of flat steel products. However, if the material does not strain harden following the Lüders region, it is difficult or impossible to determine the lower yield strength. Furthermore, in this case, the tensile strength is not defined. Therefore, instead of the lower yield strength, the plateau height is calculated (Figure 1), which is used as a measure of the yield strength at 0.5% offset (Rp0.5), because this value can be determined conclusively. The yield strength at 0.2% offset (Rp0.2), a parameter frequently determined to characterize unaged flat steel products, cannot be relied upon for aged specimens because it is too close to the upper yield strength and is in a region where the strain has not yet stabilized. For these reasons, the yield strength at 0.5% offset (Rp0.5) was determined here as a measure related to the strength of the sample. Additionally, the elongation at break (A) of the specimens was determined in tensile tests. To determine the anisotropy / isotropy of the yield strength (Rp0.5) and elongation at break (A) in the plane of the sheet metal, measurements of the yield strength at 0.5% offset (Rp0.5) and elongation at break (A) were performed along the rolling direction (0°) and in the plane of the flat steel product at angles ranging from 10° to 170° relative to the rolling direction in 10° increments. The determined dependence of the elongation at break A(α) at ​​an angle α relative to the rolling direction (0°) is shown in the circular graph of Figure 2, where Figure 2a shows the results for specimens 1 to 13 and Figure 2b shows the results for specimens 14 to 26. The determined dependence of the yield strength Rp0.5(α) at ​​a 0.5% offset at an angle α relative to the rolling direction (0°) is shown in the circular graph of Figure 3, where Figure 3a shows the results for specimens 1 to 13 and Figure 3b shows the results for specimens 14 to 26.

[0077] From the measurements determined for the dependence of the yield strength (Rp0.5) and the elongation at break (A) at an angle α relative to the rolling direction on the yield strength at 0.5% offset, the deformation energy W(α) was calculated, which is a parameter defined as the product of the elongation at break A(α) and the yield strength at 0.5% offset Rp0.5(α). The results of the determined deformation energy W(α) as a function of the angle α relative to the rolling direction (0°) are shown in the pie charts in Figure 4, where Figure 4a shows the results for the specimens of Examples 1 to 13 and Figure 4b shows the results for the specimens of Examples 14 to 26.

[0078] As shown in Figures 2 to 4, compared to the comparative examples (Examples 4 to 9, 13, 14, 17, 20, and 24) (not nitrided in a continuous annealing furnace), the test specimens according to the present invention have improved isotropy with respect to the yield strength at 0.5% offset (Rp0.5), the fracture elongation (A), and the deformation energy W(α) obtained as their product. As shown in Figure 2, the test specimens according to the present invention have a fracture elongation A(α) in the rolling direction (0°) in the plane of the sheet metal, which is in the range of 60% to 140%. Figure 3 shows that the yield strength at 0.5% offset (Rp0.5) of the test specimens according to the present invention depends on the angle α relative to the rolling direction (0°), and that the yield strength Rp0.5(0°) in the rolling direction in the plane of the sheet metal is in the range of 90% to 110%. As shown in Figure 4, the specimens according to the present invention have a deformation energy W(α) that depends on the angle α relative to the rolling direction (0°) in the plane of the sheet metal, and the deformation energy W(α) in the rolling direction (0°) is in the range of 60% to 140%. In contrast, as shown in Figures 2 to 4, the comparative examples have significant anisotropy with respect to the elongation at break (A), the yield strength at 0.5% offset (Rp0.5), and the deformation energy.

[0079] To determine the grain structure of flat steel products, microsections of the specimens were prepared in a plane along the rolling direction, a cross section transverse to the rolling direction, and in the plane of the steel plate. The cross sections are shown in Figure 5. Using the microsections, the grain size and grain elongation were determined by photomicrographic examination of the cross sections of the specimens. In these microstructural photographs, the number of intersections occurring between the grid lines and the grain boundaries are counted. The average grain size is obtained from the average value of the linear intercept segments (average line length). In Figure 5, the direction X (horizontal, along the rolling direction) and Y (perpendicular, through the thickness of the flat steel product) is used to describe the grain elongation or linear length elongation of the grains in the steel structure. X In the direction, X direction The line segment length S_H is determined. Y In the direction, Y direction The line segment length S_V is determined. This is done on microsections taken both along the rolling direction and transverse to the rolling direction. When determining the line segment length, not every grain is measured individually; instead, a uniform grid pattern is placed on the micrograph of the structure, and the length of the grid and the number of intersections determine the line segment length, which can be used as a proxy for grain size. 。 Horizontal and Vertical Directional Average The line length corresponds to the average value of the analysis of all acquired areas of the microstructure. The grain elongation S is defined as S = S_H / S_V or S = x / y.

[0080] Grain size (determined according to ASTM E112 and DIN standard EN ISO 643 using comparative photographs) and grain elongation S (determined by the linear intercept method), and the average of the specimens Line length are listed in Table 3. All specimens had an average particle size ranging from 3.3 to 5.4 μm. Line lengthIn the longitudinal section of the flat steel product, the direction-dependent grain elongation (S) in the rolling direction (0°) is a minimum of 1.4, and in the planar section of the flat steel product it is a minimum of 1.1. The grain elongation (S) in the direction transverse to the rolling direction (90°) has a minimum value of 1.2. In this respect, no significant differences were found between the test specimens according to the invention and the comparative test specimens.

[0081] This leads to the conclusion that the high strength of the specimens according to the invention is not achieved by grain refinement, but rather by the solution hardening caused by nitriding in a continuous annealing furnace. Furthermore, it shows that the improved isotropy of the mechanical properties of the specimens according to the invention can be achieved despite the anisotropy in their structure (caused by cold rolling). The anisotropy in their structure, which is also present in the specimens according to the invention, is due to the grain elongation S of the specimens according to the invention, which is comparable to that of the comparative specimens. Therefore, the solution hardening caused by nitriding in a continuous annealing furnace not only leads to an increase in strength (tensile strength Rm), but also to an improvement in the uniformity of mechanical parameters such as the elongation at break A, the yield strength at 0.5% offset Rp0.5, and the resulting deformation energy W=A·Rp0.5.

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Claims

1. A cold-rolled flat steel product for packaging materials, cold-rolled from steel along the rolling direction (0°) and having a thickness of less than 0.6 mm, having the following composition in weight percent: 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%, optionally Cr: <0.1%, optionally Ni: <0.1%, optionally Cu: <0.1%, optionally Ti: <0.01%, Optionally, B: <0.005%, optionally Nb: <0.01%, optionally Mo: <0.02%, optionally Sn: <0.03%, The balance is iron and unavoidable impurities, and The flat steel product in the aged state has a minimum yield strength at 0.5% offset (Rp0.5) of 450 MPa, a minimum elongation at break (A) of 5%, and a deformation energy W(α) defined as the product of the elongation at break (A) and the yield strength at 0.5% offset (Rp0.5) as a function of angle (α) relative to the rolling direction (0°), the deformation energy W(α) being equal to or greater than 60% and equal to or less than 140% of the deformation energy W(0°) in the rolling direction. Cold rolled flat steel products.

2. 2. A flat steel product according to claim 1, characterized in that a minimum weight percentage of 0.01% of said nitrogen is incorporated interstitially in said steel in an unbonded state.

3. 3. Flat steel product according to claim 1 or 2, characterized in that the yield strength at 0.5% offset (Rp0.5) as a function of the angle (α) relative to the rolling direction (0°) is ≧90% and ≦110% of the yield strength at 0.5% offset Rp0.5(0°) relative to the rolling direction.

4. 4. Flat steel product according to any one of claims 1 to 3, characterized in that the breaking elongation A(α) as a function of the angle (α) relative to the rolling direction (0°) is ≧60% and ≦140% of the breaking elongation A(0°) in the rolling direction.

5. 5. Flat steel product according to any one of claims 1 to 4, characterized in that the deformation energy W(α) as a function of the angle (α) relative to the rolling direction (0°) is a minimum of 70% and a maximum of 130% of the deformation energy W(0°) in the rolling direction.

6. A flat steel product according to any one of claims 1 to 5, characterized in that the grains of the steel structure of the flat steel product have a mean segment length of 3.0 to 6.0 μm.

7. The crystal grains of the steel structure of the flat steel product have, in a plane along the rolling direction (0°) of the flat steel product, an average line segment length (S_H) in the X direction along the rolling direction (0°), an average line segment length (S_V) in the Y direction perpendicular to the X direction, and a crystal grain elongation (S); The grain elongation (S) is defined as the ratio of the average line segment length (S_H) in the X direction to the average line segment length (S_V) in the Y direction; 7. A flat steel product according to claim 1, wherein the grain elongation (S) has a minimum value of 1.4 in a longitudinal section perpendicular to the planar section of the flat steel product and along the rolling direction, and a minimum value of 1.1 in the planar section of the flat steel product.

8. The grains of the steel structure of the flat steel product have, in a cross section perpendicular to the rolling direction (0°) of the flat steel product, a second average line segment length (S_V) in the thickness direction of the flat steel product, a second average line segment length (S_H) in the X direction perpendicular to the Y direction, and a second grain elongation (S); The second grain elongation (S) is defined as the ratio of the average line segment length (S_H) in the second X direction to the average line segment length (S_V) in the second Y direction; 8. Flat steel product according to claim 7, characterized in that the second grain elongation (S) has a minimum value of 1.

2.

9. The method for producing the flat steel product comprises the following steps: hot rolling the slab of steel to form a hot rolled strip; coiling the hot rolled strip at a coiling temperature of 500°C to 750°C; cold rolling the hot rolled strip at a minimum reduction of 80% to form a cold rolled steel strip; nitriding the cold rolled steel strip in an annealing furnace in the presence of a nitrogen donor at a minimum temperature of 550°C and recrystallization-annealing the cold rolled steel strip in an annealing furnace at a minimum annealing temperature of 630°C; cooling the recrystallized-annealed flat steel product to room temperature; temper rolling the recrystallized steel strip at a temper reduction of 0.2% to 45%; It is manufactured by A method for producing a flat steel product according to any one of claims 1 to 8, wherein the properties of the flat steel product are obtained in the aged state of the flat steel product.

10. 10. The method according to claim 9, characterized in that the final rolling temperature during hot rolling of the slab is higher than the Ar3 temperature.

11. A method according to claim 9 or 10, characterized in that the residence time of the flat steel products in the annealing furnace is in the range of 10 seconds to 400 seconds.

12. 12. The method according to claim 9, wherein the temper rolling ratio is 18% or less.

13. 13. The method of any one of claims 9 to 12, wherein at the temperature in the annealing furnace, the nitrogen donor is at least partially dissociated into atomic nitrogen.

14. 14. The method of any one of claims 9 to 13, wherein the nitrogen donor involved is ammonia gas.

15. The hot rolled strip has an initial nitrogen content N in the range of 0.001% to 0.016% by weight. ０ 15. The method according to any one of claims 9 to 14, characterized in that during annealing the nitrogen content by weight of the flat steel product increases by ΔN≧0.002 wt.% due to the presence of the nitrogen donors.

16. A surface coating is applied to the surface of a temper rolled steel plate, 16. The method according to any one of claims 9 to 15, characterized in that the surface coating is selected from at least one of a coating containing tin, chromium or chromium oxide, and an organic coating.

17. 17. The method according to any one of claims 9 to 16, wherein the aged state of the flat steel product is achieved either naturally by long-term storage and / or by applying a paint and subsequent drying, or artificially by heating the flat steel product to a temperature in the range of 200°C to 210°C for 20 minutes.

18. 9. Use of a flat steel product according to any one of claims 1 to 8 in the manufacture of pull-tab lids for cans, or in the manufacture of aerosol cans or components for aerosol cans.

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