Cold rolling flat steel product for packaging material

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

[Technology Field]

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

[0002] From the perspective of resource efficiency and cost reduction, efforts are underway to reduce the thickness of flat steel products (steel plates and strip steel) used in the manufacture of packaging materials (hereinafter also referred to as packaging steel). The thickness of cold-rolled packaging steel is usually in the black plate range, i.e., between 0.1 and 0.6 mm. However, since reducing the thickness also leads to a decrease in material rigidity, it is necessary to increase the strength of the packaging steel so that the material can meet the requirements necessary to ensure cold workability in forming operations during the manufacture of packaging materials, such as deep drawing or ironing. However, at the same time, it is necessary to maintain the formability of the steel plate in the cold working process. Therefore, there is a great need for high-strength steel sheets that have a yield strength exceeding 550 MPa and at the same time possess good formability, such as a minimum elongation at break of 5% and / or an Erichsen index (according to DIN standard 50101, also known as the Erichsen cupping index, measured according to the Erichsen cupping test standardized in DIN standard EN ISO20482, with a minimum of 5 mm).

[0003] For example, 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, increased strength due to the formation of a multiphase structure in the steel, or grain boundary hardening. However, many of these measures come with undesirable side effects.

[0004] As strain hardening increases, an increase in length and width differences occurs during the manufacturing of cold-rolled steel sheets, resulting in increased anisotropy and an unbalanced decrease in ductility.

[0005] During solid solution hardening, foreign atoms (e.g., N, C, P, Mn, Si) are incorporated (invaded) into substitutional or interstitial sites of the steel's host lattice. Many potential alloying elements have negative associated effects (for example, P is detrimental to steel, and Mn and Si impair surface quality), which is why adding these alloying elements to increase strength does not always lead to the desired result.

[0006] When steel is alloyed with carbon, the strength of the steel increases with carbon content. However, during processing of the steel sheet, significant anisotropy occurs in a striped morphology because carbon exists mainly in the form of cementite due to the low solubility of the steel in the ferrite lattice. Furthermore, as the carbon content increases, the surface quality deteriorates, and the risk of crack formation in the slab increases as it approaches the peritectic point. Therefore, the carbon content should be reduced to a maximum of 0.1% by weight in order to effectively prevent crack formation within the slab and the resulting point oxidation (diffusion of oxygen into cracks).

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

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

[0009] Normative requirements regarding the alloying composition of steel severely limit the ability to increase the strength of packaging steel by forming multiphase structures in steel. Therefore, conventional multiphase steels, such as those used in the automotive industry, cannot be used for packaging steel because the standard requirements of the packaging steel standard (DIN standard EN10202) limit the use of alloying components such as manganese-silicon to a maximum weight percentage. While it is possible to form multiphase structures in packaging steel using special cooling techniques, the resulting microstructure is characterized by considerable instability, and in most cases, the increase in strength is closely related to a decrease in formability. If the multiphase structure is primarily based on the alloying element carbon, an additional risk is that the anisotropy of cementite may migrate into the multiphase structure, resulting in further strengthening.

[0010] In grain boundary hardening, a fine grain structure can be formed by technically achieving a low reel temperature (winding temperature after hot rolling), high deformation during cold rolling, and annealing of cold-rolled steel sheets using continuous annealing. This allows for increased steel strength while maintaining unchanging formability. Furthermore, it is possible to form a fine grain structure by using microalloys and influencing the precipitation characteristics of the hot strip. However, the alloying elements required for this purpose are expensive and increase the annealing temperature necessary for recrystallization. Moreover, the base strength of the hot strip increases, reducing cold rollability and making the steel sheet more susceptible to defects.

[0011] As a result, the aforementioned possibility of increasing the strength of the steel sheet while maintaining formability presents problems, particularly with respect to isotropy, i.e., with respect to the directional dependence of material properties. Packaging materials such as beverage and food cans are, for example, almost always (rotationally) symmetrical items, so the steel sheets used in the manufacture of packaging materials are frequently available in the form of round blanks (i.e., in the form of flat cylindrical sheet metal blanks), formed using deep drawing and ironing to create cylindrical can bodies or bottoms or lids of cylindrical cans. 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 of the metal surface. In the case of cold-rolled steel sheets, which are available in the form of strip steel due to the nature of the manufacturing process, this is a very complex requirement because, due to the nature of the manufacturing process, the material properties due to the nature of the manufacturing process always depend on the rolling direction, whether hot-rolled or cold-rolled. Therefore, cold-rolled steel sheets always have significant anisotropy due to the nature of the manufacturing process. This anisotropy is definitively due to the high deformation during cold rolling, which is necessary to achieve the thickness of very thin metal sheets. In the manufacture of packaging materials, cold-rolled steel sheets are always processed regardless of the rolling direction, which frequently leads to difficulties during the forming process, such as uneven strength and formability across the entire circumference of a round blank.

[0012] Therefore, there is a great need for packaging steel in the form of cold-rolled flat steel products that exhibit the most isotropic properties possible within the plane of the sheet metal flat steel product. This is a difficult and contradictory goal to achieve, given the continuous decrease in the thickness of flat steel products and the corresponding increase in strength. Furthermore, in addition to the isotropy of flat steel products, other requirements that packaging steel must satisfy, particularly flexibility in forming operations, shape of the packaging material, reduction of scrap, and the realization of the most uniform and homogeneous properties of the packaging material, must be considered in the manufacture of packaging materials. [Prior art documents] [Patent Documents]

[0013] [Patent Document 1] US2011 / 0076177A1 [Overview of the project]

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

[0015] Since packaging steel is processed into finished packaging material after aging, i.e., a relatively long storage period, and, if appropriate, after painting and drying, it is necessary to optimize the material by taking into account the effects of aging that occur after a relatively long storage period and / or after painting and subsequent drying. Accordingly, the technical parameters of packaging steel are determined after the material has been artificially aged, which can be done by heating a test specimen to 200°C for 20 minutes, in accordance with DIN standard EN10202. During the (natural or artificial) aging of steel sheets, the strength and formability are particularly affected, so the effects of aging must be taken into consideration when optimizing the material properties.

[0016] For the reasons mentioned above, improvements in the strength and formability of cold-rolled steel sheets are made at the expense of isotropy in the material properties. Various metallurgical and process engineering techniques can be used in the manufacture of cold-rolled steel sheets to impart isotropy to the steel sheets. For example, one option to specifically improve the isotropy of cold-rolled steel sheets is the addition of boron as an alloying agent. However, boron has adverse effects on the workability of the steel and the final product (steel sheet). After adding boron as an alloying agent, the annealing temperature required to recrystallize the steel sheet after cold rolling increases, reducing the weldability of the material and decreasing its aging potential (i.e., the increase in strength of the steel sheet during aging).

[0017] Therefore, another problem to be solved by the present invention is to make available a cost-effective and producible packaging steel and a method for producing the same, which on the one hand has the highest possible strength while maintaining sufficient formability for deep drawing and ironing, and on the other hand has the highest possible isotropy of the material properties with respect to the strength and formability of the material under aging conditions.

[0018] According to the present invention, the aforementioned problems are solved by a flat steel product having the features described in claim 1. In this context, the flat steel product is intended to refer to a slab or strip-shaped steel plate having a thickness in the range of black plates, particularly in the range of 0.1 to 0.6 mm.

[0019] This invention is based on the discovery that solid solution hardening by alloy components incorporated between the lattice of steel allows for simultaneous improvement of strength, formability, and isotropy, and that solid solution hardening with carbon and nitrogen is particularly effective when the fixation of carbon and nitrogen to carbides and nitrides can be technically suppressed at least significantly. The formation of carbides and nitrides will promote the formation of anisotropic properties.

[0020] Another discovery based on the present invention is that nitrogen incorporation by nitriding cold-rolled flat steel products in an annealing furnace in the presence of a nitrogen donor at the end of the packaging steel manufacturing route is particularly suitable for both achieving effective solid solution hardening by nitrogen and improving material properties, particularly isotropy with respect to yield strength and elongation at break, which are related to further processing of flat steel products in the manufacture of packaging materials. This is because, in contrast to increasing the nitrogen content by introducing nitrogen into molten steel, nitriding in an annealing furnace has been found to essentially result in nitrogen incorporation into the interstitial space without fixing the nitrogen in the nitride.

[0021] Surprisingly, it has been found that nitrogen incorporated into the interstitial space within cold-rolled flat steel products during the nitriding cycle in an annealing furnace (particularly in a continuous annealing furnace before or during recrystallization annealing) has a positive effect on formability and isotropy in terms of material properties. Clearly, due to the uniform distribution of nitrogen into the (ferrite) interstitial space of the steel, the incorporation of nitrogen into the interstitial space results in excellent isotropy with respect to the mechanical properties of the flat steel products.

[0022] This is further reinforced by the fact that, compared to carbon, nitrogen incorporation into the interstitial space shifts the location of peritectic points to considerably higher alloy content, meaning that the incorporation of large amounts of nitrogen at the interstitial sites of the steel grid is less significant than the incorporation of carbon into the interstitial space with respect to surface quality and the risk of crack formation in the slab. To prevent crack formation in the slab, the weight percentage of carbon in the flat steel product of the present invention is limited to 0.10%. On the other hand, with respect to nitrogen incorporation, the nitrogen content is limited only by the solid solubility limit of nitrogen in the ferrite grid of the steel and the economic feasibility of the manufacturing process. Thus, considering the solid solubility limit of nitrogen in the ferrite grid of about 0.1 wt% and the partial fixation of nitrogen to nitride 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 wt%. From a process engineering point of view, the nitrogen content of the nitrided flat steel product according to the present invention is preferably limited to a maximum of 0.070 wt%. This is because nitriding cold-rolled flat steel products in (continuous) annealing furnaces exceeding this amount requires a level of technical complexity that is not yet economically viable, at least not currently. Therefore, for technical and economic reasons, the nitrogen weight content is most preferably 0.050% or less.

[0023] In this regard, in order to prevent the material properties occurring in the rolling direction and the transverse direction with respect to the rolling direction from changing as a result of passing through cold rolling, particularly after nitriding, it is particularly useful to add nitrogen to the flat steel product as late as possible during the manufacturing process. The flat steel product according to the present invention can be nitrided, for example, after the (primary) cold rolling process, before or during the annealing process in a continuous annealing furnace.

[0024] Since nitriding is carried out only after the (primary) cold rolling process, nitrogen is not part of the processing steps of hot rolling and (primary) cold rolling that bring about significant anisotropy in material properties. The interstitially incorporated nitrogen into the iron lattice (ferrite lattice) during or after recrystallization annealing further enhances the homogeneity of the packaging steel according to the present invention. More specifically, the risk of nitride precipitation that would increase the direction dependence of material properties is avoided during temper rolling.

[0025] In the case of a cold-rolled steel sheet rolled twice, as a result of solid solution hardening, the base strength of the steel is increased by the nitrogen interstitially incorporated, so that the temper rolling rate in the second cold rolling can be reduced, thereby minimizing the resulting anisotropy. Therefore, the temper rolling rate during the second cold rolling can preferably be limited to 18% or less.

[0026] Therefore, the present invention is a (one-time or two-time) 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 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%, 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 remainder is iron and unavoidable impurities. Includes, Aged flat steel products have a yield strength of at least 450 MPa at a 0.5% offset (Rp0.5), a minimum elongation at break of 5% (A), and a deformation energy W(α) defined as the product of the elongation at break (A) and the yield strength at a 0.5% offset (Rp0.5) as a function of the angle (α) with respect to the rolling direction (0°). The deformation energy W(α) is 60% or more and 140% or less of the deformation energy W(0°) in the rolling direction.

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

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

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

[0030] In the flat steel product according to the present invention, the fracture elongation A(α), which depends on the angle (α) with respect to the rolling direction (0°), is also within the range between an upper limit and a lower limit, the lower limit being a minimum of 60% of the fracture elongation A(0°) in the rolling direction, the upper limit being a maximum of 140% of the fracture elongation A(0°), and the fracture elongation (A) in the rolling direction being a minimum of 5%.

[0031] Therefore, the deformation energy W(α), which depends on the angle (α) with respect to the rolling direction (0°), is preferably in the range of a minimum of 70% to a maximum of 130% of the deformation energy W(0°) in the rolling direction.

[0032] To ensure complete recrystallization, nitrogen diffuses into the cold-rolled flat steel product during the nitriding cycle in the annealing furnace, preferably at an annealing temperature higher than 630°C (temperature of the flat steel product). The diffused nitrogen is uniformly distributed and incorporated into the steel grid. This uniform distribution of nitrogen incorporated into the grid results in high isotropy with respect to the mechanical properties of the nitrided flat steel product affected by the nitriding treatment, particularly with respect to elongation at break and yield strength, and consequently, with respect to deformation energy, which is the product of yield strength (Rp0.5) and elongation at break (A) at a 0.5% offset, relevant as a quality criterion for deep drawing applications. The highest possible uniform distribution of nitrogen incorporated in the annealing furnace is observed with longer residence times of the flat steel product in the annealing furnace, particularly longer annealing times during recrystallization annealing. The residence time of the flat steel product 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. For residence times longer than 400 seconds, the throughput rate of strip-shaped flat steel products passing through a continuous annealing furnace over a typical throughput path length must be so low, for economic reasons, that it can no longer demonstrate process efficiency. This is why annealing times exceeding 400 seconds can only be set in batch annealing processes.

[0033] The isotropy of material properties related to the cold working cycle, such as yield strength (Rp0.5), elongation at break (A), and deformation energy (W) at a 0.5% offset, achieved in the flat steel product according to the present invention by nitriding after the cold rolling cycle, can be achieved despite the unavoidable elongation of the steel grains resulting from (one or two) cold rolling. In the flat steel product according to the present invention, the grains of the steel structure typically have an average fiber length of 3.0 to 6.0 μm, and, 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. Therefore, in the flat steel product according to the present invention, it is possible to achieve isotropic properties with respect to 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 a steel structure is defined as the ratio of the average horizontal fiber length (S_H) to the average vertical fiber length (S_V). In the direction transverse to the rolling direction (α=90°), the grain elongation (S) of a flat steel product, which depends on the angle α, is usually at least 1.2.

[0035] Solid solution hardening resulting from nitriding flat steel products is most effective when added nitrogen is incorporated into the interstitial sites of the steel (especially the ferrite lattice). Therefore, to prevent nitrogen from bonding in the form of nitrides, 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. Accordingly, the alloy composition of the steel preferably has the following upper limits for the weight percentage content of these strong nitride alloy components: 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%. This ensures, in particular, that the weight content of free nitrogen exceeds 0.01%.

[0037] Weight content of free nitrogen in hot-rolled strips 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 present in the steel within the above threshold are completely fixed with nitrogen and 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) Here, N0 is the weight percentage of nitrogen in the molten steel, and the Al coefficient is defined as a function of the reel temperature HT (winding temperature of the hot-rolled strip) and the aluminum content Al (weight %) as follows: If HT < 640℃: Al coefficient = 0, For temperatures of 750°C ≥ HT ≥ 640°C: Al coefficient = N0 - N0 × (-0.682HT + 536) / 100 = N0 × (1 - (-0.682HT + 536) / 100) And it is an addendum. |N0-Ti / 3.4-B / 0.8-Nb / 6.6-Al coefficient| This is defined as the sum of the differences "N0-Ti / 3.4-B / 0.8-Nb / 6.6-Al coefficients". In Equation 1, this sum takes into account that at most, only the total nitrogen actually present in the hot-rolled strip (i.e., molten steel) can be fixed by the nitride-forming elements present in the hot-rolled strip (i.e., molten steel).

[0038] The total weight content of free nitrogen in cold-rolled flat steel products is calculated using the formula 1 above for hot-rolled strips (N 遊離 The free nitrogen content of the (hot-rolled strip) is obtained from the sum of the nitrogen ΔN added by nitriding in a 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 the continuous annealing furnace is incorporated into the interstitial sites. The upper limit of the weight content of free nitrogen in the cold-rolled flat steel product is determined by the solubility limit of nitrogen in the ferrite lattice of the steel and is about 0.1% by weight.

[0040] Free nitrogen in cold-rolled flat steel product (N 遊離 ) The total weight content of free nitrogen is preferably more than 0.01%. In order to introduce the highest possible proportion of unbound nitrogen into the cold-rolled flat steel product, preferably, most of the total weight content of nitrogen is introduced by nitriding in the 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 produced 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. The hot-rolled strip is wound up at a winding temperature (reel temperature HT) below Ar1, particularly in the range of 500°C to 750°C, and after cooling, cold-rolled with a rolling ratio of at least 80% to form a flat steel product (strip steel). 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 annealing is carried out, and then cooled to room temperature, and finally temper rolling or dressing is carried out at a temper rolling ratio of 0.2% to 45%. To ensure that the isotropy is not adversely affected by the temper rolling cycle, the temper rolling ratio is preferably less than 18%.

[0042] Nitriding of flat steel products in a continuous annealing furnace can be performed before, during, or after recrystallization annealing. For example, the nitriding process can be performed in a first upstream zone of the continuous annealing furnace at a first temperature lower than the recrystallization temperature in the presence of a nitrogen donor, and then the flat steel products can be heated in a second downstream zone of the continuous annealing furnace at a second temperature higher than 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 performing them in different zones of the continuous annealing furnace has the advantage of allowing optimal temperatures to be set for each procedure, with the optimal temperature for the nitriding process being lower than the optimal temperature for the recrystallization annealing cycle. However, for economic reasons, it is preferable to perform nitriding and annealing of flat steel products simultaneously in a continuous annealing furnace at a temperature above the recrystallization temperature in the presence of a nitrogen donor.

[0043] The properties of flat steel products manufactured in this manner evolve after the tempered rolled strip steel has been aged, which is achieved artificially by heating to 200°C for 20 minutes, or by coating the flat steel products with varnish and then drying the varnish.

[0044] The hot-rolled strips preferably already have an initial nitrogen content N0 in the range of 0.001% to 0.016% by weight in order to maximize the total nitrogen content of the cold-rolled flat steel product and the solid solution hardening caused by nitriding the cold-rolled strips. To prevent crack formation in the slab during casting and hot rolling of the slab and to ensure that the strength of the hot-rolled strips does not increase to such an extent that they can no longer be cold-rolled by commonly used cold-rolling equipment, the weight nitrogen content in the molten steel for producing the hot-rolled strips shall not exceed 0.016%. The total nitrogen content of the flat steel product according to the present invention, which consists of the sum of the initial nitrogen content N0 and the nitrogen content ΔN incorporated 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, and at the annealing temperature, the dissociated atomic nitrogen of the nitrogen donor diffuses into the cold-rolled flat steel product, thereby increasing the nitrogen content by 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 steel product to more than 0.014 wt% when the initial nitrogen content N0 in the molten steel is lower than 0.014 wt%. Cold-rolled flat steel 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 steel product nitrides in the continuous annealing furnace can be in the range of about 0.1%, which is (at least theoretically) the solid solubility limit of nitrogen in the (ferrite) lattice of steel.

[0045] The nitrogen donor involved may 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 flat steel product before the product is heated in the annealing furnace. The nitrogen donor used must be of a type that dissociates in the annealing furnace to diffuse nitrogen into the flat steel product, thereby making atomic nitrogen available. The nitrogen donor involved may be, in particular, 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 above 400°C during the nitriding of the cold-rolled flat steel product.

[0046] In the presence of a nitrogen donor, the increase in strength due to solid solution hardening caused by nitriding the flat steel product during annealing in a (continuous) annealing furnace eliminates the need to perform temper rolling of the flat steel product according to the present invention at a high temper rolling rate and further increase its strength through strain hardening. Therefore, the temper rolling rate can preferably be limited to a maximum of 18%, thereby preventing the deterioration of the isotropy of the material properties caused by a second cold rolling cycle at a high temper rolling rate.

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

[0048] The excellent isotropic mechanical properties of the steel sheet according to the present invention enable the manufacture of pull tab lids for cans (so-called "easy-open end," EOE) or aerosol cans or components of aerosol cans (e.g., the bottom or lid of an aerosol can), for example, pull tab lids or aerosol lids or aerosol cans and their components having isotropy across their entire surface. The isotropic properties of the steel sheet according to the present invention are particularly advantageous for circular or elliptical pull tab lids and circular bottoms of aerosol cans, as the associated products have substantially uniform mechanical properties around their entire circumference. The isotropic mechanical properties of the steel sheet according to the present invention are also advantageous for deep drawing applications where round steel sheet parts (circular blanks) are formed to manufacture can bodies for two-part cans, for example, because uniform mechanical properties are achieved around the formed sheet metal part, and no thinning regions of the material are generated during the forming process where the thickness of the sheet metal decreases.

[0049] These and other properties, features, and advantages of the flat steel product according to the present invention are derived from the embodiments described below with reference to the attached drawings and tables. The drawings show the following: [Brief explanation of the drawing]

[0050] [Figure 1] This is an example of a schematic stress-strain diagram obtained in a tensile test of a flat steel product according to the present invention. [Figure 2] Figure 2a shows the angle dependence of the fracture elongation (A) in the plane of flat steel products tested in tensile tests, and Figure 2b shows the results for test specimens 14 to 26. [Figure 3] Figure 3a shows the angle dependence of the yield strength (Rp0.5) at a 0.5% offset in the plane of flat steel products tested in tensile tests. Figure 3b shows the results for test specimens 1-13 and test specimens 14-26. [Figure 4] Figure 4a shows the angle dependence of the deformation energy W = A·Rp0.5 in the plane of flat steel products tested in tensile tests, and Figure 4b shows the results for test specimens 14-26. [Figure 5] This is a schematic diagram of the test used to identify the grain structure of the tested flat steel product. [Figure 6] This diagram shows the effect of aging on isotropy in cold-rolled flat steel products regarding elongation at fracture. [Modes for carrying out the invention]

[0051] To manufacture the flat steel products according to the present invention, slabs are cast from molten steel and hot-rolled to form hot-rolled strips. The alloy composition of the molten steel is preferably guided by thresholds specified by standards for packaging steel (for example, as defined in ASTM A623-11 "Standard specifications for tin mill products" or "European standard EN10202"). The steel compositions that can be used to manufacture the flat steel products according to the present invention are described in detail below.

[0052] Composition of steel

[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 should contain at least 0.02% by weight of carbon. In the primary cold rolling process, and where appropriate in the secondary cold rolling process (temper rolling or dressing), the carbon content should not be too high in order to ensure the rollability of flat steel products and to avoid a decrease in elongation at break. Furthermore, as the carbon content increases, significant anisotropy is formed in the form of stripes during the manufacturing and processing of flat steel products, because carbon has low solubility in the ferrite lattice of steel and therefore exists mainly in the form of cementite. In addition, as the carbon content increases, the surface quality deteriorates and the risk of crack formation in the slab increases as it approaches the peritectic point. Therefore, the carbon content should be limited to a maximum of 0.1% by weight to effectively prevent crack formation in the slab and the resulting point oxidation (diffusion of oxygen into cracks).

[0054] Manganese (Mn): Minimum 0.17%, maximum 0.5%. Manganese also has the effect of increasing hardness and strength. Furthermore, manganese improves the weldability and wear resistance of steel. In addition, the addition of manganese causes sulfur to bind to form less harmful MnS, thus reducing the tendency of red-hot brittleness during hot rolling. Moreover, manganese causes grain refinement, increases the solubility of nitrogen in the iron grid, and can prevent the diffusion of carbon to the slab surface. Therefore, a minimum manganese content of 0.17 wt% is preferable. 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 is too high, the corrosion resistance of the steel decreases, and food-grade quality cannot be guaranteed. Furthermore, if the manganese content is too high, the strength of the hot-rolled strip becomes too high, making it impossible to cold-roll the hot-rolled strip. Therefore, the upper limit of the manganese content is 0.5 wt%.

[0055] • Phosphorus P: Less than 0.03% Phosphorus is an undesirable residual element in steel. In particular, a high phosphorus content makes the steel brittle, negatively affecting the formability of flat steel products. Therefore, the upper limit for phosphorus content is 0.03% by weight.

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

[0057] Aluminum (Al): greater than 0.002%, less than 0.1% Aluminum is necessary in steel production as a deoxidizing agent for killing (deoxidizing) 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 undesirable aluminum nitride in the flat steel products according to the present invention. The reason why the formation of aluminum nitride is undesirable is that it reduces the free nitrogen content. Furthermore, if the aluminum concentration is too high, surface defects may occur in the form of aluminum clusters. Therefore, aluminum can be used at a maximum concentration of 0.1% by weight.

[0058] • Silicon Si: Less than 0.03% Silicon enhances the scale resistance of steel and acts as a precipitation hardening agent. In steelmaking, Si functions as a deoxidizer. Another beneficial effect of Si on steel is that it increases tensile strength and yield stress. Therefore, a silicon content of 0.003% by weight or more is preferable. However, if the silicon content is excessively high, more specifically exceeding 0.03% by weight, the corrosion resistance of the steel decreases, and surface treatment, particularly by electrolytic coating, may be hindered.

[0059] Optionally, nitrogen (N0) content should be less than 0.016%, preferably greater than 0.001%. Nitrogen is an optional component in molten steel, and steel for flat steel products according to the present invention is produced from molten steel. Nitrogen as a precipitation hardening agent has the effect of improving hardness and strength, but if the nitrogen content in the molten steel is excessively high, exceeding 0.016% by weight, it becomes more difficult to cold roll the hot-rolled strip produced from the molten steel. Furthermore, since hot formability decreases at nitrogen concentrations of 0.016% by weight or higher, a high nitrogen content in the molten steel increases the risk of defects in the hot-rolled strip. According to the present invention, the nitrogen content of the flat steel product is intended to be 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, in order to achieve high strength by 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 N0 into flat steel products before nitriding in the annealing furnace, an appropriate amount of nitrogen can be added to the molten steel, for example, by blowing in nitrogen gas and / or by adding solid nitrogen compounds such as calcium nitrogen (calcium cyanamide) or manganese nitride.

[0061] Optionally, nitride-forming elements, especially niobium, titanium, boron, molybdenum, and 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, thus increasing manufacturing costs. However, on the other hand, elements such as niobium, titanium, and boron, as microalloy components, increase strength by refining the crystal grain without reducing toughness. Therefore, adding the above nitride-forming elements to the molten steel in a limited amount as alloy components may be useful. Thus, the steel may (optionally) contain the following nitride-forming alloy components by weight %: Titanium Ti: Preferably more than 0.002%, but less than 0.01% for cost reasons. Boron B: Preferably more than 0.001%, but less than 0.005% for cost reasons, and / or Niobium Nb: Preferably more than 0.001%, but less than 0.01% for cost reasons, and / or Chromium Cr: Preferably more than 0.01% to enable the use of scrap in the production of molten steel and to prevent carbon diffusion on the slab surface, but up to 0.08% and / or to prevent the formation of carbides and nitrides. 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 To avoid a decrease in the proportion of the above nitride-forming elements, the total weight content of the above nitride-forming elements in the molten steel is preferably less than 0.1%.

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

[0064] Manufacturing method for flat steel products: Using the steel composition described above, molten steel is produced, which is first continuously cast, cooled, and then divided into slabs. The slabs are then reheated to a preheating temperature higher than 1100°C, particularly 1200°C, and hot-rolled to produce hot-rolled strips with a thickness ranging from 1 to 4 mm.

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

[0066] The hot-rolled strip is wound to form a coil at a predetermined, preferably constant, winding temperature (reel temperature, HT). The winding temperature is preferably lower than Ar1 and preferably in the range of 500°C to 750°C, and most preferably below 640°C, in order to maintain the ferrite range. For economic reasons, the winding temperature needs to be higher than 500°C. The formation of iron nitride 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 winding.

[0067] To produce packaging steel in the form of thin flat steel products with a thickness range of less than 0.6 mm (thickness of black plate), preferably less than 0.4 mm, hot-rolled strips are cold-rolled to a thickness reduction rate (degree of reduction or deformation during cold rolling) of at least 80%, preferably in the range of 85% to 98%. To restore the crystalline structure of the steel that was destroyed during cold rolling, the cold-rolled steel strips are then recrystallized annealed in an annealing furnace. This is achieved, for example, by passing the flat steel products in the form of cold-rolled steel strips through a continuous annealing furnace where the steel strips are heated to a temperature above the recrystallization temperature of the steel. Prior to, or preferably concurrently with, recrystallized annealing, the cold-rolled flat steel products are nitrided by heating the flat steel products in an annealing furnace in the presence of a nitrogen donor. Nitriding is carried out simultaneously with recrystallization annealing in the annealing furnace by introducing a nitrogen donor, particularly in the form of a nitrogen-containing gas, preferably in the form of ammonia (NH3), into the annealing furnace, heating the flat steel product to an annealing temperature above the recrystallization temperature of the steel, and maintaining the flat steel product at the annealing temperature for an annealing time (holding time) of preferably 10 to 150 seconds. The annealing temperature is preferably higher than 630°C, particularly 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 that can 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. The atmosphere inside 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, with the volume content of the protective gas preferably in the range of 90% to 99.5%, and the remainder of the volume content of the gas atmosphere being formed by the nitrogen-containing gas, particularly ammonia gas (NH3 gas).

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

[0069] Next, the cold-rolled flat steel products were recrystallized 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) represents the nitrogen content after nitriding in an annealing furnace. D is the thickness of the steel plate (mm). NWG represents the percentage of temper rolling during secondary cold rolling. NH3 represents the ammonia content (by volume) in the annealing furnace. Rp0.5 represents the yield strength (MPa) at a 0.5% offset in the rolling direction. A is the elongation at break in the rolling direction (%). Rm is the tensile strength (MPa) in the rolling direction.

[0071] In the examples of the present 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 product 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 present invention by nitriding in the ammonia-containing gas atmosphere of the continuous annealing furnace is shown as N (after nitriding) [weight %]. The total nitrogen content N was determined according to DIN standard EN ISO 14284 (especially subparagraph 4.4.1) after removing the surface iron nitride layer formed during nitriding from the surface of the test specimen.

[0072] The total nitrogen weight content consists of the initial nitrogen content (N0, see Table 1) in the molten steel and the nitrogen content ΔN incorporated by nitridation in the continuous annealing furnace, and the total nitrogen content N 遊離 A substantial portion of which is obtained in the unbound state, and the remainder is obtained in the bound state as nitrides. See Equation (1). Using Equation (1), the weight content of free nitrogen N 遊離 can be estimated based on the weight content of nitride-forming elements contained in the steel.

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

[0074] Figure 6 shows the effect of aging on the angular dependence of the elongation at break in Comparative Example 5, comparing the non-aged state with the aged state, and the aged state further compares artificial aging and natural aging. This indicates that significant anisotropy occurs only after aging. However, in the actual processing of packaging steel, aging is virtually unavoidable, so it is particularly important that the isotropy is determined and optimized in the aged state, which is the object of the present invention.

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

[0076] The stress-strain diagram of aged flat steel products exhibits a discontinuous pattern. In principle, the upper yield strength limit or lower yield strength is used as a reference value characterizing the strength, and in some cases, the tensile strength. The upper yield strength measured in a tensile test is highly dependent on the measurement conditions, the testing machine used, and its orientation. Furthermore, the range of values ​​is particularly large for certain testing machines. In the case of 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 beyond the Lüders region, determining the lower yield strength is difficult or impossible. Moreover, in this case, the tensile strength is undefined. 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 a 0.5% offset (Rp0.5), because this value can ultimately be determined. The yield strength at a 0.2% offset (Rp0.2), a parameter frequently determined to characterize unaged flat steel products, is unreliable in aged specimens because it is too close to the upper yield strength and lies in a region where the strain has not yet stabilized. For these reasons, the yield strength at a 0.5% offset (Rp0.5) is determined here as the measure of strength relevant to the sample. Furthermore, the elongation at break (A) of the specimen was determined by tensile testing. 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 (Rp0.5) and elongation at break (A) at a 0.5% offset were performed along the rolling direction (0°) and in 10° increments within the angular range of 10° to 170° relative to the rolling direction. The determined dependence of the fracture elongation A(α) at ​​angle α with respect to the rolling direction (0°) is shown in the pie chart in 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 angle α with respect to the rolling direction (0°) is shown in the pie chart in 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 measured values ​​determined regarding the dependence of yield strength (Rp0.5) and fracture elongation (A) at a 0.5% offset at an angle α with respect to the rolling direction, the deformation energy W(α), a parameter defined as the product of fracture elongation A(α) and yield strength Rp0.5(α) at ​​a 0.5% offset, was calculated. The results of the deformation energy W(α), determined as a function of angle α with respect to the rolling direction (0°), are shown in a pie chart in Figure 4. Figure 4a shows the results for specimens 1 to 13, and Figure 4b shows the results for specimens 14 to 26.

[0078] As shown in Figures 2 to 4, compared to comparative examples (Examples 4 to 9, 13, 14, 17, 20 and 24) (not nitrided in a continuous annealing furnace), the specimens according to the present invention exhibit improved isotropy with respect to the yield strength (Rp0.5), elongation at break (A), and the deformation energy W(α) obtained as their product at a 0.5% offset. As shown in Figure 2, the specimens according to the present invention have an elongation at break A(α) such that the elongation at break A(0°) in the rolling direction is in the range of 60% to 140% in the plane of the sheet metal. Figure 3 shows that the specimens according to the present invention have a yield strength (Rp0.5) at a 0.5% offset that depends on the angle α with respect to the rolling direction (0°), and the yield strength at a 0.5% offset (Rp0.5) such that the yield strength in the rolling direction Rp0.5(0°) in the plane of the sheet metal is in the range of 90% to 110%. As shown in Figure 4, the specimen according to the present invention has a deformation energy W(α) in the plane of the sheet metal that depends on the angle α with respect to the rolling direction (0°), and the deformation energy W(α) in the rolling direction is in the range of 60% to 140%. In contrast, as shown in Figures 2 to 4, the comparative example exhibits considerable anisotropy with respect to the elongation at break (A), yield strength at a 0.5% offset (Rp0.5), and 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 a plane of the steel sheet. The cross-sections are shown in Figure 5. Using the microsections, grain size and grain elongation were determined by microscopic examination of the cross-sections of the specimens. In these microstructure photographs, the number of intersections occurring between the grid lines and grain boundaries is counted. The average grain size is obtained from the average value (average fiber length) of the linear intersection segments. In Figure 5, the grain elongation or fiber length elongation of the grains in the steel structure is illustrated using directions x (horizontal, along the rolling direction) and y (perpendicular, in the thickness direction of the flat steel product). In the x-direction of the grain, the horizontal fiber length S_H is determined. In the y-direction perpendicular to it, the vertical fiber length S_V is determined. This is done in both microsections obtained along the rolling direction and in the direction transverse to the rolling direction. When determining fiber length, instead of measuring each grain individually, a uniform grid pattern is placed on a micrograph of the structure, and the fiber length is determined by the length of the grid and the number of intersections, which can be used instead of grain size. The average horizontal and vertical fiber lengths correspond to the average values ​​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] Table 3 lists the grain size (determined using comparative photographs according to ASTM E112 and DIN standard EN ISO643), grain elongation S (determined by the linear section method), and average fiber length of the test specimens. All test specimens had an average fiber length in the range of 3.3 to 5.4 μm. In the longitudinal section of the flat steel product, the direction-dependent grain elongation (S) in the rolling direction (0°) was a minimum of 1.4, and in the planar section of the flat steel product, it was a minimum of 1.1. The grain elongation (S) in the direction transverse to the rolling direction (90°) had a minimum value of 1.2. In this respect, no significant difference was observed between the test specimens according to the present invention and the comparative test specimens.

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

[0082]

[0083]

[0084]

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 to 0.12%, preferably less than 0.07%; optionally Cr: <0.1%, preferably 0.01 to 0.08%, Optionally, Ni: <0.1%, preferably 0.01 to 0.05%, optionally Cu: <0.1%, preferably 0.002-0.05%, 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 greater than or equal to 60% and greater than or equal to 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 have a mean segment length of 3.0 to 6.0 μm.

7. 7. A flat steel product according to claim 1, wherein the grains of the steel structure have a mean horizontal line segment length (S_H) and a mean vertical line segment length (S_V) and a grain elongation (S), the grain elongation (S) being defined as the ratio of the mean horizontal line segment length (S_H) to the mean vertical line segment length (S_V), and the direction-dependent grain elongation (S) in the rolling direction (0°) has a minimum value of 1.4 in the longitudinal section of the flat steel product and a minimum value of 1.1 in the planar section of the flat steel plate.

8. 8. Flat steel product according to claim 7, characterized in that the grain elongation (S) in the direction transverse to the rolling direction (90°) has a minimum value of 1.

2.

9. The flat steel product is 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 said cold rolled steel strip in an annealing furnace, in particular a continuous annealing furnace, in the presence of a nitrogen donor at a minimum temperature of 550°C and recrystallization-annealing said 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 9. A flat steel product according to any one of claims 1 to 8, wherein the properties of the flat steel product are obtained after ageing the temper rolled steel strip.

10. 10. A flat steel product according to claim 9, characterized in that the final rolling temperature during the hot rolling of the slab is higher than the Ar3 temperature.

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

12. 12. A flat steel product according to any one of claims 9 to 11, characterized in that the temper rolling ratio is 18% or less.

13. 13. A flat steel product according to 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. Flat steel product according to 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. A flat steel product 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 ΔN ≥ 0.002 wt. % due to the presence of said nitrogen donors.

16. 16. A flat steel product according to any one of the preceding claims, characterized in that the flat steel product has a surface coating, in particular a tin or chromium / chromium oxide coating applied by electrolysis, and / or an organic coating, in particular in the form of a varnish or a polymer sheet.

17. 17. A flat steel product according to any one of claims 1 to 16, wherein the aged state of the flat steel product is achieved 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. 18. Use of a flat steel product according to any one of claims 1 to 17 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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