Packaging sheet metal product

Abstract

A packaging sheet metal product from a cold-rolled steel sheet with a thickness of less than 0.6 mm has a specified composition. The packaging sheet metal product during biaxial deformation in a bulge test has a lower yield strength (Sb.sub.eL) of more than 300 MPa and a corresponding elongation at break (Ab) of more than 10% and in the plastic region between the Lüders elongation (Ab.sub.e) and an upper (plastic) elongation limit of ε.sub.max=0.5.Math.Ab.Math.(Sb.sub.eL/Sb.sub.m) has a biaxial stress/strain diagram σ.sub.B(ε) that can be represented by a function ε.sub.B=b.Math.ε.sup.n, with: σ.sub.B is the true biaxial stress in MPa; ε is the amount of true elongation in the thickness direction in %; Sb.sub.m is the absolute strength; b is a proportionality factor; and n is a strain-hardening exponent. A strengthening of the packaging sheet product in the thickness direction is characterized by a strain-hardening exponent of n≥0.353-5.1.Math.Sb.sub.eL/10.sup.4 MPa.

Claims

1. A packaging sheet metal product comprising a cold-rolled steel sheet having a thickness (d) of less than 0.6 mm and containing components, in terms of weight: C: 0.001-0.06%, Si: <0.03%, Mn: 0.17-0.5%, P: <0.03%, S: 0.001-0.03%, Al: 0.001-0.1%, and N: 0.002-0.12%, wherein the packaging sheet metal product, during biaxial deformation in a bulge test, which bulge test determines mechanical parameters of the packaging sheet metal product such as a true biaxial stress (σ.sub.B), a true elongation (ε) in a thickness direction, an absolute strength (Sb.sub.m), a lower yield strength (Sb.sub.eL), an elongation at break (Ab), and a Lüders elongation (Ab.sub.e), has a lower yield strength (Sb.sub.eL) of more than 300 MPa, a corresponding elongation at break (Ab) of more than 10%, a plastic region between the Lüders elongation (Ab.sub.e) and an upper plastic elongation limit of ε.sub.max=0.5.Math.Ab.Math.(Sb.sub.eL/Sb.sub.m), and a biaxial stress/strain diagram σ.sub.B(ε) represented by a function σ.sub.B=b.Math.ε.sup.n, wherein σ.sub.B is the true biaxial stress in MPa, ε is an amount of true elongation in the thickness direction in %, Sb.sub.eL is the lower yield strength, Sb.sub.m is the absolute strength, Ab.sub.e is the Lüders elongation, b is a proportionality factor, and n is a strain-hardening exponent, and wherein a strengthening of the packaging sheet metal product in the thickness direction is characterized by a strain-hardening exponent of
n≥0.353-5.1.Math.Sb.sub.eL/10.sup.4 MPa.

2. The packaging sheet metal product according to claim 1, wherein a weight fraction of nitrogen of at least 0.002% is incorporated interstitially in steel of the packaging sheet metal product in unbonded form.

3. The packaging sheet metal product according to claim 1, further comprising at least one of, in terms of weight: Cr: <0.1% Ni: <0.1%, Cu: <0.1%, Ti: <0.01%, B: <0.005%, Nb: <0.01%, Mo: <0.02%, Sn: <0.03%, residual iron, and unavoidable impurities.

4. The packaging sheet metal product according to claim 1, wherein the packaging sheet metal product is obtained by: hot rolling a steel slab to produce a hot strip having a thickness in a range of 2 mm to 4 mm, winding the hot strip at a winding temperature below an Ar1 temperature, cold rolling the hot strip at a reduction ratio of at least 80% to produce a cold-rolled steel strip, increasing nitrogen content of the cold-rolled steel strip in an annealing furnace in a presence of a nitrogen donor at a temperature of at least 550° C. and recrystallization annealing the cold-rolled steel strip in the annealing furnace at an annealing temperature of at least 630° C., cooling the recrystallization-annealed steel strip to room temperature, and rerolling the recrystallization-annealed steel strip at a final reduction of 0.2% to 45%.

5. The packaging sheet metal product according to claim 4, wherein a final rolling temperature during hot rolling of the steel slab is greater than an Ar3 temperature.

6. The packaging sheet metal product according to claim 4, wherein a dwell time of the cold-rolled steel strip in the annealing furnace is between 10 seconds and 400 seconds.

7. The packaging sheet metal product according to claim 4, wherein the final reduction is between 0.2% and 20%.

8. The packaging sheet metal product according to claim 4, wherein the nitrogen donor is at least partially dissociated to atomic nitrogen at temperatures in the annealing furnace.

9. The packaging sheet metal product according to claim 4, wherein the nitrogen donor is ammonia gas.

10. The packaging sheet metal product according to claim 4, wherein the hot strip has an initial weight fraction of nitrogen N.sub.0 in a range of 0.001 wt % to 0.016 wt % and wherein the weight fraction of nitrogen in the cold-rolled steel strip is increased by ΔN≥0.002 wt % in the presence of the nitrogen donor during annealing in the annealing furnace.

11. The packaging sheet metal product according to claim 1, wherein the cold-rolled steel strip contains a surface coating on at least one surface.

12. The packaging sheet metal product according to claim 11, wherein the surface coating includes at least one of an electrolytically applied tin coating, a chromium/chromium oxide coating, an organic coating, an organic varnish, and a polymer film.

13. The packaging sheet metal product according to claim 1, wherein the mechanical parameters of the packaging sheet metal product are obtained by carrying out at least one of aging of the packaging sheet metal product and varnishing followed by drying of the packaging sheet metal product.

14. The packaging sheet metal product according to claim 4, wherein a total degree of cold rolling (GKWG) resulting from the thickness (d) of the packaging sheet metal product and a thickness (D) of the hot strip, defined as GKWG [total degree of cold rolling]=1−d/D, is at least 0.90.

15. The packaging sheet metal product according to claim 1, wherein the packaging sheet metal product is a singly or doubly reduced steel sheet having the thickness (d) in a range of 0.10 mm to 0.50 mm.

16. A packaging sheet metal product comprising a cold-rolled steel sheet having a thickness (of less than 0.6 mm, produced from a hot strip by single or double cold rolling of the hot strip at a reduction ratio of at least 80%, wherein the hot strip has a composition containing, in terms of weight: C: 0.001-0.06%, Si: <0.03%, Mn: 0.17-0.5%, P: <0.03%, S: 0.001-0.03%, Al: 0.001-0.1%, N: <0.016%, remainder iron, and unavoidable impurities, and wherein the cold-rolled steel sheet is nitrogenized, in an annealing furnace in a presence of a nitrogen donor at a temperature of at least 550° C., to a nitrogen content of ΔN≥0.002% relative to weight and recrystallization annealed at an annealing temperature of at least 630° C., then cooled to room temperature and finally cold-rolled at a final degree of rolling of 0.2% to 45% and then subjected to a biaxial deformation in a bulge test in a plastic range for characterization of deformation capacity, where the packing sheet metal product exhibits a lower yield stress (Sb.sub.eL) of more than 300 MPa and a corresponding elongation at break (Ab) of more than 10%, as well as in a region between Lüders elongation (Ab.sub.e) and an upper (plastic) elongation limit of ε.sub.max=0.5.Math.Ab.Math.(Sb.sub.eL/Sb.sub.m) exhibits a biaxial stress-strain diagram σ.sub.B(ε) that can be represented by function σ.sub.B=b.Math.ε.sup.n, where σ.sub.B is a true biaxial stress in MPa, ε is an amount of true elongation in a thickness direction in %, Sb.sub.eL is a lower yield strength, Sb.sub.m is an absolute strength, Ab.sub.e is the Lüders elongation, b is a proportionality factor, and n is a strain-hardening exponent that satisfies n≥0.353-5.1.Math.Sb.sub.eL/10.sup.4 MPa.

17. The packaging sheet metal product according to claim 1, wherein a weight fraction of nitrogen of more than 0.004% is incorporated interstitially in steel of the packaging sheet metal product in unbonded form.

18. The packaging sheet metal product according to claim 4, wherein a final rolling temperature, during hot rolling of the steel slab, is in a range of 800° C. to 920° C. and the winding temperature, during winding of the hot strip, is in a range of 500° C. to 750° C.

19. The packaging sheet metal product according to claim 13, wherein the mechanical parameters of the packaging sheet metal product are obtained by carrying out aging of the packaging sheet metal product and wherein the aging is at least one of natural aging by storage over a predefined period of time and artificial aging by heat treatment over 20 to 30 minutes at an aging temperature in a range of 200° C. to 210° C.

20. The packaging sheet metal product according to claim 16, further comprising at least one of: Cr: <0.1%, Ni: <0.1%, Cu: <0.1%, Ti: <0.1%, B: <0.005%, Nb: <0.01%, Mo: <0.02%, and Sn: <0.03%, in terms of weight.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The drawings show:

(2) FIG. 1: Example for a biaxial stress/strain curve σ.sub.B(ε) determined from a bulge test of an aged steel sheet sample in which the recorded mechanical properties according to Table 1 are entered and the region of elastic-plastic deformation is shown enlarged in the insert;

(3) FIG. 2: Detailed view of the plastic region of the biaxial stress/strain curve of FIG. 1 above the Lüders elongation (Ab.sub.e) with a corresponding fit of the function σ.sub.B=b.Math.ε.sup.n;

(4) FIGS. 3A&3B Biaxial stress/strain curves determined from a bulge test of steel sheet samples according to the invention and not according to the invention, each with comparable composition of the hot strip and different nitrogen content and the same final reduction, FIG. 3A showing the stress/strain curve of steel sheet samples according to the invention and not according to the invention with low carbon content (C<0.03 wt %), and FIG. 3B showing the stress/strain curves of steel sheet samples according to the invention and not according to the invention with a higher carbon content (C>0.03 wt %);

(5) FIGS. 4A&4B Depiction of the trend of the lower yield strength (Sb.sub.eL in MPa) determined from the biaxial stress/strain curve of steel sheet samples according to the invention and not according to the invention as a function of final reduction (NWG [degree of rerolling] in %), FIG. 4A showing the values of the samples with low carbon content (C<0.03 wt %), and FIG. 4B showing the values of the samples with higher carbon content (C>0.03 wt %);

(6) FIGS. 5A&5B Depiction of the trend of elongation at break (Ab in MPa) determined from the biaxial stress/strain curve of steel sheet samples according to the invention and not according to the invention as a function of final reduction (NWG in %), FIG. 5A showing the value of the samples with low carbon content (C<0.03 wt %), and FIG. 5B showing the value of the samples with higher carbon content (C>0.03 wt %);

(7) FIGS. 6A&6B Depiction of the trend of strain-hardening exponents n determined from the plastic region of the biaxial stress/strain curve σ.sub.B=b.Math.ε.sup.n of steel sheet samples according to the invention and not according to the invention as a function of final reduction (NWG in %), FIG. 6A showing the values of the samples with low carbon content (C<0.03 wt %), and FIG. 6B showing the values of the samples with higher carbon content (C>0.03 wt %);

(8) FIG. 7: Depiction of the trend of strain-hardening exponents determined from the elastic-plastic region of the biaxial stress/strain curve σ.sub.B=b.Math.ε.sup.n of FIGS. 6A and 6B of steel sheet samples according to the invention and not according to the invention as a function of their lower yield strength (Sb.sub.eL in MPa) according to FIGS. 4A and 4B; and

(9) FIGS. 8-12: Show Table 1, Table 2A, Table 2B, Table 3A, and Table 3B. Note that all Tables presented herein follow the European numerical convention of using a comma as a decimal marker.

DETAILED DESCRIPTION

(10) A slab is cast from a steel melt and hot-rolled to a hot strip for production of packaging sheet metal products according to the invention. The components of the steel from which the packaging sheet metal products according to the invention can be produced are explained in detail below, the data in percent referring to the weight fractions of the steel components.

(11) Composition of the Steel: Carbon, C: at least 0.001% and at most 0.06%:

(12) Carbon has hardness and strength-increasing effect. The steel therefore contains at least 0.001 wt % carbon. Steels with low carbon content exhibit higher total cold reduction optimum, for which reason thinner steel sheets with equivalent earing tendency can be produced from hot strips with low carbon content and ordinary hot strip thicknesses in the range of 2 to 4 mm by cold rolling. In order to ensure rollability of the steel sheet during primary cold rolling and optionally in a second cold rolling step (rerolling or cold finishing) and at the same time low earing tendency and not reduce elongation at break, the carbon content should therefore be no higher than 0.06%. A lower carbon content also prevents pronounced anisotropy during production and processing of the steel sheets in the form of banding, since the carbon is largely present in the form of cementite due to the low solubility in the ferrite lattice of the steel. Moreover, the surface quality deteriorates with increasing carbon content and the risk of slab cracks increases with approach to the peritectic point. Manganese, Mn: at least 0.17% and at most 0.5%:

(13) Manganese also has a hardness- and strength-increasing effect. Manganese also improves the weldability and wear resistance of steel. The tendency toward red shortness is also reduced during hot rolling by the addition of manganese, since sulfur is bonded to the less hazardous MnS. Manganese also leads to grain refining and the solubility of nitrogen in the iron lattice is increased by manganese and diffusion of carbon to the surface of the slab can be prevented. A manganese content of at least 0.17 wt % is therefore preferred. To achieve higher strength, a manganese content of more than 0.2 wt %, especially 0.30 wt % or more is preferred. If the manganese content, however, becomes too high, this will be at the expense of the corrosion resistance of the steel and food compatibility is no longer guaranteed. The strength of the hot strip also becomes too high at unduly high manganese contents, which means that the hot strip can no longer be economically cold-rolled. The upper limit for manganese content is therefore 0.5 wt %. Phosphorus, P: less than 0.03%

(14) Phosphorus is an undesired accompanying element in steels. A high phosphorus content leads especially to embrittlement of the steel, and therefore causes a deterioration in deformation capability of steel sheets, for which reason the upper limit for phosphorus content lies at 0.03 wt %. Sulfur, S: more than 0.001% and at most 0.03%

(15) Sulfur is an undesired accompanying element that causes a deterioration in stretchability and corrosion resistance. No more than 0.03 wt % sulfur should therefore be contained in the steel. On the other hand, demanding and cost-intensive measures must be employed for desulfurization, for which reason a sulfur content of less than 0.001 wt % is no longer tolerable from an economic standpoint. The sulfur content therefore lies in the range of 0.001 wt % to 0.03 wt %, especially between 0.005 wt % and 0.01 wt %. Aluminum, Al: more than 0.001% and less than 0.1%

(16) Aluminum is required during steelmaking as a deoxidizer for killing. Aluminum also increases the scale resistance and deformation capability. The aluminum content therefore lies at more than 0.001 wt %. However, with nitrogen, aluminum forms aluminum nitrides, which are a disadvantage in the steel sheets according to the invention, since they reduce the fraction of free nitrogen. Unduly high aluminum concentrations can also lead to surface defects in the form of aluminum clusters. Aluminum is therefore used in a maximum concentration of 0.1 wt %. Silicon, Si: less than 0.03%:

(17) Silicon increases the scale resistance in steel and is a solid solution hardener. In steelmaking, silicon serves as deoxidizer. A further positive effect of silicon on steel is that it increases the tensile strength and yield strength. A silicon content of 0.002 wt % or more is therefore preferred. If the silicon content, however, becomes too high and especially exceeds 0.03 wt %, the corrosion resistance of the steel can deteriorate and surface treatments, especially by electrolytic coatings, can be hindered. Optionally nitrogen, No: less than 0.007% and preferably more than 0.001%

(18) Nitrogen is an optional component in the steel melt from which the steel is produced for the steel sheets according to the invention. Nitrogen does have a hardness- and strength-increasing effect as a solid solution hardener. However, an unduly high nitrogen content in the steel melt means that the hot strip produced from the steel melt can only be cold rolled with more difficulty. A high nitrogen content in the steel melt also increases the risk of defects in the hot strip, since the hot deformability becomes lower at nitrogen concentrations of 0.007 wt % or more. In the production of packaging sheet metal products according to the invention, it is proposed to increase the nitrogen content of the steel sheet subsequently by increasing the nitrogen content of the cold-rolled steel sheet in an annealing furnace. For this reason, the introduction of nitrogen into the steel melt can also be fully dispensed with. To achieve high solid solution hardening, however, it is preferable that an initial nitrogen content of more than 0.001 wt % be already contained in the steel melt.

(19) To introduce an initial nitrogen content N.sub.0 in the steel sheet before increasing the nitrogen content in the annealing furnace, nitrogen in an appropriate amount can be added to the steel melt, for example, by blowing in nitrogen gas and/or by the addition of a solid nitrogen compound, such as calcium cyanamide or manganese nitride. Optionally: nitride formers, especially niobium, titanium, boron, molybdenum, chromium:

(20) Nitride-forming elements, such as aluminum, titanium, niobium, boron, molybdenum and chromium are a disadvantage in steel of the steel sheets according to the invention because they reduce the fraction of free nitrogen by nitride formation. These elements are also expensive and therefore increase production costs. On the other hand, the elements niobium, titanium and boron have a strength-increasing effect by grain refinement as microalloy components without reducing toughness. The mentioned nitride formers can therefore be advantageously added within certain limits as alloy components of the steel melt. The steel can therefore (optionally) contain the following nitride-forming alloy components, in terms of weight: Titanium, Ti: preferably more than 0.0005% but less than 0.01% for cost reasons, Boron, B: preferably more than 0.0005% 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% in order to permit the use of scrap in the production of the steel melt and to hinder diffusion of carbon to the surface of the slab, but to avoid carbides and nitrides, at most 0.1% and/or Molybdenum Mo: less than 0.02% in order to avoid unduly severe increase in the recrystallization temperature;

(21) To avoid a reduction in the fraction of free unbonded nitrogen N.sub.free by nitride formation, the total weight fraction of the mentioned nitride former in the steel melt is preferably less than 0.1%.

(22) Further Optional Components:

(23) In addition to the residual iron (Fe) and unavoidable impurities, the steel melt can contain further optional components, such as optionally copper, Cu: more than 0.002% in order to permit the use of scrap in production of the steel melt, but less than 0.1% in order to guarantee food compatibility; optionally nickel, Ni: more than 0.01% in order to permit the use of scrap in the production of the steel melt and to improve toughness, but less than 0.1% in order to guarantee food compatibility; optionally tin, Sn: preferably less than 0.03%;

(24) Production Method:

(25) A steel melt that is extruded and divided into slabs after cooling is initially produced with the described composition of the steel to make the packaging sheet metal products according to the invention. The slabs are then again heated to preheating temperatures of more than 1100° C., especially 1200° C. and hot rolled to produce a hot strip with a thickness in the range of 2 to 4 mm.

(26) The final rolling temperature during hot rolling preferably lies above the Ar3 temperature in order to remain austenitic and lies especially between 800 and 920° C.

(27) The hot strip is wound to a coil at a stipulated and expediently constant winding temperature (coiling temperature, HT). The winding temperature then preferably lies below Ar1 in order to remain in the ferritic region, preferably in the range of 500 to 750° C. and especially less than 640° C. in order to avoid precipitation of AlN. For economic reasons, the winding temperature should lie at more than 500° C. After winding, the coil of the hot strip is cooled by natural cooling. Formation of iron nitrides on the surface of the hot strip can be avoided by active cooling of the hot strip after completion of hot rolling up to winding at higher cooling rates.

(28) To produce a packaging steel in the form of a thin steel sheet in the thickness range of less than 0.6 mm (fine sheet thicknesses) and preferably with a final thickness of less than 0.35 mm, the hot strip is initially pickled and then cold rolled, wherein a thickness reduction (cold reduction) of at least 80% and preferably in the range of 85 to 98% expediently occurs. To restore the crystal structure destroyed during cold rolling of the steel, the cold rolled steel strip is then recrystallization annealed in an annealing furnace. This occurs, for example, by passing the steel sheet present in the form of cold-rolled steel strip through a continuous annealing furnace at a strip speed of at least 200 m/min, in which the steel strip is heated to temperatures above the recrystallization temperature of the steel. An increase in the nitrogen content of the cold-rolled steel sheet then occurs before or preferably simultaneously with recrystallization annealing by heating the steel sheet in the annealing furnace in the presence of a nitrogen donor. Increasing the nitrogen content is then preferably conducted simultaneously with recrystallization annealing by introducing a nitrogen donor into the annealing furnace, especially in the form of nitrogen-containing gas and heating the steel sheet to an annealing temperature above the recrystallization temperature of the steel and holding it for an annealing time (holding time) of preferably 10 to 150 seconds at the annealing temperature. The annealing temperature then preferably lies above 630° C. and especially in the range of 640 to 750° C. The nitrogen donor is chosen so that atomic nitrogen is formed at the temperatures in the annealing furnace by dissociation of the nitrogen donor, which can diffuse into the steel sheet. Ammonia has been shown to be a suitable nitrogen donor for this purpose. In order to avoid oxidation of the surface of the steel sheet during annealing, a protective gas atmosphere is expediently used in the annealing furnace. The atmosphere in the annealing furnace preferably consists of a mixture of the nitrogen-containing gas acting as nitrogen donor and a protective gas, such as forming gas or nitrogen gas (N.sub.2 gas), wherein the volume fraction of protective gas during feed is preferably between 95% and 99.98% and the remaining volume fraction of the supplied gas is formed by the nitrogen-containing gas, especially ammonia gas (NH.sub.3 gas). An equilibrium concentration from 0.02 to 2 vol % ammonia is preferably maintained during an increase in the nitrogen content in the annealing furnace, and ammonia gas is simultaneously sprayed onto the surfaces of the steel sheet by means of nozzles. The formation of a hard and brittle nitride layer on the surface of the steel sheet is thereby prevented, and this ensures that the nitrogen diffuses in high concentration into the interior of the steel sheet and is interstitially incorporated there uniformly in the (ferrite) lattice of the steel. An increase in the initial nitrogen concentration No by ΔN≥0.002 wt % preferably occurs by increasing the nitrogen content. The weight fraction of total nitrogen in the recrystallized and nitrogenized steel sheet produced by increasing the nitrogen content in the annealing furnace preferably lies between 0.002 and 0.12%, especially between 0.004 and 0.07%.

EMBODIMENT EXAMPLES

(29) Embodiment examples of the invention and comparative examples are explained below. The steel sheets of the embodiment examples of the invention and the comparative examples were produced from steel melts with the alloy compositions listed in Tables 2A and 2B (FIGS. 9 and 10) by hot rolling and subsequent cold rolling. The cold-rolled steel sheets were then recrystallization annealed in a continuous annealing furnace, in which steel sheets were held during a stipulated annealing time in the range of 10 to 120 seconds at annealing temperatures of 630° C. or more.

(30) The steel sheets according to the invention, which are marked “according to the invention” in Tables 2A and 2B, were nitrogenized before or during the recrystallization annealing in the annealing furnace by setting an ammonia atmosphere with an equilibrium concentration of ammonia of 0.02% to preferably 2 vol % in the annealing furnace and simultaneously directing ammonia gas onto the surfaces of the steel sheets by means of nozzles. The nitrogen content was brought from an initial nitrogen content N.sub.0 of the hot strip in the steel sheets according to the invention to a higher nitrogen content N. Both the initial nitrogen content N.sub.0 and also the nitrogen content N=N.sub.0+ΔN achieved after increasing the nitrogen content in the annealing furnace are shown in Tables 2A and 2B in the steel sheets according to the invention, ΔN being the nitrogen content introduced to the steel sheet on increasing the nitrogen content in the annealing furnace.

(31) During recrystallization annealing of the steel sheets not according to the invention, which are marked in Tables 2A and 2B “not according to the invention,” an inert gas atmosphere without nitrogen donor (i.e., without nitrogenizing components) was present in the annealing furnace so that the steel sheets not according to the invention were not nitrogen-treated in the annealing furnace and the weight fraction of nitrogen is the same before and after heat treatment in the annealing furnace (i.e., N=N.sub.0).

(32) After heat treatment in the annealing furnace, both the steel sheets according to the invention and the steel sheets of the embodiment examples not according to the invention (not nitrogenized in the annealing furnace), which are marked in Tables 2A and 2B “not according to the invention,” were rerolled or finished in a second cold-rolling step.

(33) Subsequently, i.e., after the second cold rolling (rerolling or finishing), artificial aging of the steel sheets was achieved by heating the sample for 20 minutes to 200° C. The mechanical properties of the samples of the steel sheets according to the invention and the practical examples not according to the invention artificially aged in this way are shown in Tables 3A and 3B (FIGS. 11 and 12), in which Thickness is the final thickness of the rerolled steel sheets (in mm), NWG is the final reduction during secondary cold rolling (in %), Sb.sub.eH is the upper yield strength (in MPa), Sb.sub.eL is the lower yield strength (in MPa), Sb.sub.m is the absolute strength (in MPa), Ab is the elongation at break (in %), Ab.sub.e is the Lüders elongation (in %), b is a proportionality factor in MPa and n is a strain-hardening exponent, which is obtained from a description of the biaxial stress/strain curve determined in the bulge test σ.sub.B(ε) in the plastic region above the Lüders elongation (Ab.sub.e) by the function σ.sub.B=b.Math.ε.sup.n, where σ.sub.B is the (true) biaxial stress in MPa determined in the bulge test and ε is the amount of the true elongation (in %) in the thickness direction (the true elongation in the thickness direction is negative due to the thickness reduction in the biaxial tensile test of the bulge test; elongation ε is therefore always understood to mean the amount of negative elongation in the thickness direction of the sheet).

(34) The mechanical characteristics of the samples, such as the upper yield strength (Sb.sub.eH in MPa), the lower yield strength (Sb.sub.eL in MPa), the absolute strength (Sb.sub.m in MPa), the elongation at break (Ab in %) and the Lüders elongation (Ab.sub.e in %) were then determined from the biaxial stress/strain diagram as explained by means of the example of FIG. 1.

(35) Biaxial stress/strain curves are shown in FIGS. 3A and 3B, which were determined from a bulge test on samples of steel sheets according to the invention and not according to the invention, FIG. 3A showing the samples with low carbon content (C<0.03%), and FIG. 3B showing samples with higher carbon content (C>0.03%). The samples according to the invention and not according to the invention are then contrasted with identical composition and the same final reduction (NWG). From comparison of the biaxial stress/strain curves of samples according to the invention and not according to the invention, it is apparent that the biaxial stress in the plastic region above the Lüders elongation (ε>Ab.sub.e) is regularly greater in the samples according to the invention than in the samples not according to the invention. This indicates higher cold hardening of the samples according to the invention in the bulge test. The difference in cold hardening between the samples according to the invention and the samples not according to the invention is particularly high at higher carbon concentrations (C>0.03%) in the composition of the steel (see FIG. 3B).

(36) A further gauge for hardening of a steel sheet sample is the (biaxial) lower yield strength Sb.sub.eL determined in the bulge test. This is dependent, inter alia, upon the final reduction (NWG). For graphic representation of hardening of samples according to the invention and not according to the invention lower yield strengths Sb.sub.eL determined from the bulge test are shown in FIGS. 4A and 4B as a function of the final reduction NWG (in %), where FIG. 4A again shows steel sheet samples with low carbon content (C<0.03%) and FIG. 4B shows samples with higher carbon content (C>0.03%).

(37) It is apparent from a comparison of the samples according to the invention and the samples not according to the invention from the depictions in FIGS. 4A and 4B that the samples according to the invention have a higher lower yield strength (Sb.sub.eL) at the same final reduction (NWG) relative to the samples not according to the invention.

(38) The trend of elongation at break (Ab in %) from the bulge test of samples according to the invention and samples not according to the invention is shown in FIGS. 5A and 5B as a function of final reduction (NWG in %), FIG. 5A showing samples with lower carbon content (C<0.03%) and FIG. 5B showing samples with higher carbon content (C>0.03%). It can be deduced from a comparison of the elongation at break of the samples according to the invention and the samples not according to the invention from FIGS. 5A and 5B that the elongation at break of the samples according to the invention is higher at the same final reduction (NWG).

(39) The proportionality factor b and the strain-hardening exponent n were determined by fit functions σ.sub.B=b.Math.ε.sup.n from the biaxial stress/strain curves determined in the bulge test of the samples according to the invention and the samples not according to the invention in the plastic region between the Lüders elongation Ab.sub.e and an upper (plastic) yield strength of ε.sub.max=0.5.Math.Ab.Math.(Sb.sub.eL/Sb.sub.m), where Ab is the elongation at break, Sb.sub.eL is the lower yield strength, and Sb.sub.m is the absolute strength. The values determined for the investigated samples for the proportionality factor b and the strain-hardening exponent n are stated in Tables 3A and 3B. The strain-hardening exponent n then represents a gauge of cold hardening of a steel sheet sample in the bulge test. Since the strain-hardening exponent n is also dependent on the final reduction (NWG), the strain-hardening exponents n of samples according to the invention and samples not according to the invention determined from the bulge test are shown in FIGS. 6A and 6B as a function of the final reduction (NWG in %), FIG. 6A showing the samples with low carbon content (C<0.03%) and FIG. 6B showing samples with higher carbon content (C>0.03%). It can be deduced from a comparison of the samples according to the invention and the samples not according to the invention that the strain-hardening exponent n of the samples according to the invention is higher at the same final reduction (NWG) than in the samples not according to the invention.

(40) A quantification of cold hardening of steel sheet samples in the bulge test independent of final reduction can be achieved by representing the strain-hardening exponents n determined in the bulge test as a function of the lower yield strength Sb.sub.eL. FIG. 7 therefore shows the strain-hardening exponents n determined in the bulge test as a function of lower yield strength Sb.sub.eL. It can be deduced from FIG. 7 that the strain-hardening exponents n of the samples according to the invention at the same lower yield strength Sb.sub.eL are higher than in the samples not according to the invention. For lower yield strengths of Sb.sub.eL>300 MPa and a lowest elongation at break of Ab>10% a delimitation of the sample according to the invention from the sample not according to the invention can be stated by the following trend of the strain-hardening exponents n as a function of lower yield strength Sb.sub.eL (in MPa):
n≥0.353-5.1.Math.Sb.sub.eL/10.sup.4 MPa.

(41) The samples according to the invention that satisfy the equation above are characterized in comparison with the samples not according to the invention by a higher yield strength and a higher cold hardening, and are therefore better suited for multiaxial deformations in comparison with the samples not according to the invention, as occur, for example, during production of three-dimensional can bodies from the packaging sheet metal products. The samples according to the invention are then characterized in particular by a higher cold hardening after aging (i.e., after natural or artificial aging of the sample). The higher cold hardening in the samples according to the invention can be achieved by incorporating unbonded nitrogen on increasing the nitrogen content of the samples in the annealing furnace and the resulting solid solution hardening.