Cold-rolled flat steel product for packaging materials

Abstract

A cold-rolled flat steel product for packaging materials has a thickness of less than 0.6 mm, which has been cold-rolled from steel along a rolling direction (0°) and which has an excellent isotropy with respect to its mechanical properties.

Claims

1. A cold-rolled flat steel product for packaging materials, the cold-rolled flat steel product being cold-rolled from steel along a rolling direction (0°) and having a thickness of less than 0.6 mm and comprising the following components in percent by weight: C: 0.02-0.1%, Si: <0.03%, Mn: 0.17-0.5%, P: <0.03%, S: 0.001-0.03%, Al: 0.002-0.1%, N: 0.014-0.12%, wherein the flat steel product in an aged condition has a 0.5% yield strength (Rp0.5) of at least 450 MPa and an elongation at break (A) of at least 5% and a working capacity W(α) that is defined as the product of the elongation at break (A) and the 0.5% yield strength (Rp0.5), the working capacity being direction dependent and, as a function of the angle (α) relative to the rolling direction (0°), having no less than 60% and no more than 140% of the working capacity in the rolling direction W(0°), and a total weight content of free nitrogen in the flat steel product is at least 0.01% by weight, the free nitrogen being nitrogen that is interstitially incorporated in uncombined form in the flat steel product.

2. The flat steel product as in claim 1, wherein the 0.5% yield strength Rp0.5(α) is direction dependent and, as a function of the angle (α) relative to the rolling direction (0°), is no less than 90% and no more than 110% of the 0.5% yield strength in the rolling direction Rp0.5(0°).

3. The flat steel product as in claim 1, wherein the elongation at break A(α) is direction dependent and, as a function of the angle (α) relative to the rolling direction (0°), is no less than 60% and no more than 140% of the elongation at break in the rolling direction A(0°).

4. The flat steel product as in claim 1, wherein the working capacity W(α) is direction dependent and, as a function of the angle (α) relative to the rolling direction (0°), has is a minimum of 70% and a maximum of 130% of the working capacity in the rolling direction W(0°).

5. The flat steel product as in claim 1, wherein the flat steel product has a steel structure comprising grains with a mean fiber length of 3.0 to 6.0 μm.

6. The flat steel product as in claim 1, wherein the flat steel product has a steel structure comprising grains with a mean horizontal fiber length (S_H) and a mean vertical fiber length (S_V) and a grain elongation (S), which is defined as the ratio of the mean horizontal fiber length (S_H) to the mean vertical fiber length (S_V), the grain elongation (S) in the rolling direction (0°) having a value of at least 1.4 in longitudinal sections of the flat steel product and a value of at least 1.1 in planar sections of the flat steel product.

7. The flat steel product as in claim 6, wherein the grain elongation (S) transverse to the rolling direction (90°) has a value of at least 1.2.

8. The flat steel product as in claim 1, wherein the flat steel product further comprises one or a plurality of the following components in percent by 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%.

9. The flat steel product as in claim 1, wherein the flat steel product bring is produced by: hot rolling a slab of steel to form a hot-rolled steel strip; winding the hot-rolled steel strip at a winding temperature of 500° C. to 750° C., cold rolling the hot-rolled steel strip at a reduction ratio of at least 80% to form a cold-rolled steel strip, nitriding the cold-rolled steel strip in an annealing furnace in the presence of a nitrogen donor comprising ammonia gas in a volume concentration of between 0.04% and 0.4% of a gas atmosphere in the annealing furnace and at a temperature of at least 550° C., after nitriding the cold-rolled steel strip in the annealing furnace, recrystallization annealing the cold-rolled steel strip in the annealing furnace at an annealing temperature of of at least 630° C.; cooling the recrystallization-annealed steel strip to room temperature, and temper rolling the recrystallization-annealed steel strip at a temper rolling degree of 0.2% to 45%, wherein the properties of the 0.5% yield strength (Rp0.5) being at least 450 MPa, the elongation at break (A) being at least 5%, and the working capacity W(α), as a function of the angle (α) relative to the rolling direction (0°), being no less than 60% and no more than 140% of the working capacity in the rolling direction W(0°) of the flat steel product are obtained after aging the temper-rolled steel strip.

10. The flat steel product as in claim 9, wherein a final rolling temperature during the hot rolling of the slab is higher than the Ar3 temperature.

11. The flat steel product as in claim 9, wherein a dwell time of the flat steel product in the annealing furnace during the nitriding and the recrystallization annealing is in a range of 10 seconds to 400 seconds.

12. The flat steel product as in claim 9, wherein the temper rolling degree is 18% or lower.

13. The flat steel product as in claim 9, wherein the nitrogen donor at the temperatures in the annealing furnace during the nitriding is at least partially dissociated to atomic nitrogen.

14. The flat steel product as in claim 9, wherein the hot-rolled steel strip has an initial nitrogen content N.sub.0 in a range of 0.001% by weight to 0.016% by weight, and the nitrogen content in the steel strip is increased by ΔN≥0.002% by weight during recrystallization annealing due to the presence of the nitrogen donor.

15. The flat steel product as in claim 1, wherein the flat steel product has further comprises a surface coating.

16. The flat steel product as in claim 15, wherein the surface coating comprises at least one of the following coatings: an electrolytically applied tin coating, a chromium/chromium oxide coating, an organic coating, an organic paint, or a polymer sheet.

17. The flat steel product as in claim 1, wherein the aged condition of the flat steel product is achieved naturally by prolonged storage and/or by paint aging performed by application of a paint and subsequent drying, or artificially by heating the flat steel product for 20 minutes to temperatures in a range of 200° C. to 210° C.

18. A method for producing a flat steel product having a thickness of less than 0.6 mm, said method comprising: hot rolling a slab of steel to form a hot-rolled steel strip; winding the hot-rolled steel strip at a winding temperature of 500° C. to 750° C.; cold rolling the hot-rolled steel strip along a rolling direction (0°) at a reduction ratio of at least 80% to form a cold-rolled steel strip; nitriding the cold-rolled steel strip in an annealing furnace in the presence of a nitrogen donor comprising ammonia gas in a volume concentration of between 0.04% and 0.4% of a gas atmosphere in the annealing furnace and at a temperature of at least 550° C.; after nitriding the cold-rolled steel strip in the annealing furnace, 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 temper rolling the recrystallization-annealed steel strip at a temper rolling degree of 0.2% to 45%, wherein properties of the flat steel product evolve after the temper-rolled steel strip has been aged.

19. The method as in claim 18, further comprising: after temper rolling the recrystallization-annealed steel strip, aging the temper-rolled steel strip, the aged steel strip having a 0.5% yield strength (Rp0.5) of at least 450 MPa and an elongation at break (A) of at least 5% and a working capacity W(α) that is defined as the product of the elongation at break (A) and the 0.5% yield strength (Rp0.5), the working capacity being direction dependent and, as a function of the angle (α) relative to the rolling direction (0°), having no less than 60% and no more than 140% of the working capacity in the rolling direction W(0°).

20. The method as in claim 18, wherein the properties of the flat steel product after the temper-rolled steel strip has been aged are a 0.5% yield strength (Rp0.5) of at least 450 MPa and an elongation at break (A) of at least 5% and a working capacity W(α) that is defined as the product of the elongation at break (A) and the 0.5% yield strength (Rp0.5), the working capacity being direction dependent, and as a function of the angle (α) relative to the rolling direction (0°), having no less than 60% and no more than 140% of the working capacity in the rolling direction W(0°).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other properties, features, and advantages of the flat steel product according to the present invention follow from the execution examples described further below with reference to the appended drawings and tables. The drawings show:

(2) FIG. 1: An example of a schematic stress-strain diagram obtained in a tensile test on a flat steel product according to the invention;

(3) FIGS. 2A&2B Circular graphs of the angular dependence of the elongation at break (A) in the plane of flat steel products tested in tensile tests, with FIG. 2a showing the results of the tested specimens 1 to 13 and FIG. 2b showing the results of the tested specimens 14 to 26;

(4) FIGS. 3A&3B Circular graphs of the angular dependence of the yield strength at 0.5% offset (Rp0.5) in the plane of flat steel products tested in tensile tests, with FIG. 3a showing the results of the tested specimens 1 to 13 and FIG. 3b showing the results of the tested specimens 14 to 26;

(5) FIGS. 4A&4B Circular graphs of the angular dependence of the energy of deformation W=A-Rp0.5 in the plane of flat steel products tested in tensile tests, with FIG. 4a showing the results of the tested specimens 1 to 13 and FIG. 4b showing the results of the tested specimens 14 to 26;

(6) FIG. 5 A schematic representation of the tests to identify the grain structure of the tested flat steel products;

(7) FIG. 6 Illustration of the influence of aging cold-rolled flat steel products on the isotropy with respect to the elongation at break.

(8) FIGS. 7-10 Tables 1-3. Note that in all Drawings and Tables presented herein, the European numerical convention of using a comma as a decimal marker.

DETAILED DESCRIPTION

(9) To manufacture flat steel products according to the present invention, a slab is cast from a steel melt and hot-rolled to form a hot-rolled strip. The alloy composition of the steel melt is preferably guided by the threshold limit values specified by standards for packaging steel (e.g., as defined in the Standard ASTM A623-11 “Standard Specification for Tin Mill Products” or in the “European Standard EN 10202”). Below, the components of the steel, from which flat steel products according to the invention can be manufactured, will be explained in detail:

(10) Composition of the Steel:

(11) Carbon, C: a minimum of 0.02% and a maximum of 0.1%, preferably less than 0.085%:
carbon has a hardness- and strength-increasing effect. Therefore, the steel contains a minimum of 0.02 wt % of carbon. To ensure the rollability of the flat steel product in the primary cold rolling procedure and, if appropriate, in a second cold rolling step (temper rolling or dressing) and to not decrease the elongation at break, the carbon content should not be too high. Furthermore, as the carbon content increases, a pronounced anisotropy in the form of banding forms during the manufacture and processing of the flat steel product since carbon, because of its low solubility in the ferrite lattice of the steel is present mainly in the form of cementite. In addition, as the carbon content increases, the surface quality deteriorates and the risk of the formation of cracks in the slab increase as the peritectic point is approached. It is therefore necessary to limit the carbon content to a maximum of 0.1 wt %, since only then will it be possible to effectively prevent the formation of cracks in the slab and the resultant point oxidation (diffusion of oxygen into cracks). Manganese, Mn: a minimum of 0.17% and a maximum of 0.5%:

(12) Manganese also has a hardness- and strength-increasing effect. In addition, manganese improves the weldability and wear resistance of steel. Further, the addition of manganese reduces the tendency toward red brittleness during hot rolling in that sulfur is bound to less harmful MnS. Furthermore, manganese leads to grain refining, and manganese can increase the solubility of nitrogen in the iron lattice and prevent diffusion of carbon into the surface of the slab. Therefore, a manganese content of a minimum of 0.17 wt % is to be preferred. To achieve high strengths, a manganese content higher than 0.2 wt %, especially 0.30 wt % or higher is to be preferred. However, if the manganese content is too high, the corrosion resistance of the steel suffers, and the food grade quality is no longer ensured. Furthermore, at excessively high manganese contents, the strength of the hot-rolled strip becomes too high, which has the effect that the hot-rolled strip can no longer be cold-rolled. Therefore, the upper threshold limit for the manganese content is 0.5 wt %. Phosphorus, P: less than 0.03%

(13) Phosphorus is an undesirable residual element in steels. In particular, a high phosphorus content leads to an embrittlement of steel and therefore has a negative effect on the formability of flat steel products, which is the reason that the upper threshold limit for the phosphorus content is 0.03 wt %. Sulfur, S: more than 0.001% and a maximum of 0.03%

(14) Sulfur is an undesirable residual element which has a negative effect on ductility and corrosion resistance. Therefore, no more than 0.03 wt % sulfur should be present in the steel. On the other hand, however, the measures that have to be taken for desulfurizing steel are technically complex and cost-intensive, which is the reason why a sulfur content lower than 0.001 wt % is no longer justifiable for economic reasons. Therefore, the sulfur content is in a range from 0.001 wt % to 0.03 wt %, most preferably from 0.005 wt % to 0.01 wt %. Aluminum, Al: more than 0.002% and less than 0.1%

(15) Aluminum is needed in the production of steel as a deoxidizing agent for killing steel. Aluminum also increases the scale resistance and the formability. Therefore, the aluminum content is higher than 0.002 wt %. However, aluminum in combination with nitrogen forms aluminum nitrides which are undesirable in the flat steel products according to the invention since they reduce the free nitrogen content. Furthermore, excessively high aluminum concentrations can lead to surface defects in the form of aluminum clusters. Therefore, aluminum can be used in a concentration of a maximum of 0.1 wt %. Silicon, Si: less than 0.03%:

(16) Silicon increases the scale resistance in steel and is a precipitation hardening agent. In the production of steel, Si serves as a deoxidizing agent. Another positive influence of Si on steel is that it increases the tensile strength and the yield stress. Therefore, a silicon content of 0.003 wt % or higher is to be preferred. However, if the silicon content is excessively high and, more specifically, exceeds 0.03 wt %, the corrosion resistance of steel can deteriorate, and surface treatments, especially by electrolytical coatings, can be hampered. Optional nitrogen, No: less than 0.016%, and preferably more than 0.001%

(17) Nitrogen is an optional component in the steel melt, from which the steel for the flat steel product according to the invention is produced. Although nitrogen as a precipitation hardening agent has a hardness- and strength-increasing effect, an excessively high nitrogen content in the steel melt greater than 0.016 wt % has the effect that it is more difficult to cold-roll the hot-rolled strip produced from the steel melt. In addition, a high nitrogen content in the steel melt increases the risk of defects in the hot-rolled strip since the hot formability is reduced at nitrogen concentrations of 0.016 wt % or above. According to the invention, the intention is to increase the nitrogen content of the flat steel product afterwards by nitriding the cold-rolled flat steel product in an annealing furnace. Therefore, introducing nitrogen into the steel melt can be completely omitted. However, to achieve high strength by solution hardening, it is preferable if an initial nitrogen content higher than 0.001 wt %, most preferably of 0.010 wt % or higher is already present in the steel melt.

(18) To incorporate an initial nitrogen content N.sub.0 into the flat steel product prior to nitriding in the annealing furnace, nitrogen can be added in the appropriate amount to the steel melt, for example, by blowing in nitrogen gas and/or by adding a solid nitrogen compound such as calcium nitrogen (calcium cyanamide) or manganese nitride. Optional: nitride-forming elements, especially niobium, titanium, boron, molybdenum, chromium:

(19) In the steel of the flat steel products according to the invention, nitride-forming elements such as aluminum, titanium, niobium, boron, molybdenum, and chromium are undesirable since they reduce the portion of free nitrogen due to nitride formation. Furthermore, these elements are expensive and thus increase production costs. However, on the other hand, the elements niobium, titanium, and boron, for example, as microalloying components increase strength by grain refinement without a reduction in toughness. Therefore, it may be useful to add a certain limited amount of the nitride-forming elements mentioned as alloying components to the steel melt. The steel may therefore (optionally) contain the following nitride-forming alloying components relative to the weight: Titanium, Ti: preferably more than 0.002%, but for cost reasons, less than 0.01%, Boron, B: preferably more than 0.001%, but for cost reasons, less than 0.005%, and/or Niobium, Nb: preferably more than 0.001%, but for cost reasons, less than 0.01%, and/or Chromium, Cr: preferably more than 0.01% in order to make it possible to use scrap in the production of the steel melt and to hinder the diffusion of carbon on the surface of the slab, but a maximum of 0.08% so as to prevent the formation of carbides and nitrides, and/or Molybdenum, Mo: less than 0.02% in order to prevent an excessively high increase in the recrystallization temperature;

(20) To avoid a reduction of the portion of the free, uncombined nitrogen N.sub.free as a result of the formation of nitrides, the total weight content of the nitride-forming elements mentioned in the steel melt is preferably lower than 0.1%.

(21) Other Optional Components:

(22) In addition to the residual iron (Fe) and unavoidable impurities, the steel melt may also contain other optional components, such as, e.g. optional copper, Cu: more than 0.002 in order to make it possible to use scrap in the production of the steel melt, but less than 0.1% in order to ensure the food grade quality; optional nickel, Ni: more than 0.01 in order to make it possible to use scrap in the production of the steel melt and to improve the toughness, but less than 0.1% in order to ensure the food grade quality; optional tin, Sn: preferably lower than 0.03%;
Method of Producing the Flat Steel Product:

(23) With the use of the composition of the steel described, a steel melt is produced, which is first continuously cast and, after cooling, divided into slabs. The slabs are then reheated to preheating temperatures higher than 1100° C., especially 1200° C., and hot-rolled to produce a hot-rolled strip with a thickness ranging from 1 to 4 mm.

(24) The final rolling temperature during hot rolling is preferably higher than the Ar3 temperature in order to maintain the austenitic properties and ranges especially between 800° C. and 900° C.

(25) The hot-rolled strip is wound up to form a coil at a specified and preferably constant winding temperature (reel temperature, HT). The winding temperature is preferably lower than Ar1 in order to remain in the ferritic range, preferably in a range from 500° C. to 7500° C., and most preferably at less than 640° C. in order to prevent the precipitation of AlN. For economic reasons, the winding temperature should be higher than 500° C. A formation of iron nitrides on the surface of the hot-rolled strip can be prevented by cooling the hot-rolled strip at the end of the hot rolling cycle until winding it up at higher cooling rates.

(26) To produce a packaging steel in the form of a thin flat steel product in a thickness range of less than 0.6 mm (blackplate thicknesses) and preferably with a thickness of less than 0.4 mm, the hot-rolled strip is cold-rolled, during the course of which a thickness reduction (degree of reduction or degree of deformation during cold rolling) by a minimum of 80% and preferably ranging from 85% to 98% takes place. To restore the crystal structure of the steel, which was destroyed during cold rolling, the cold-rolled steel strip is subsequently recrystallization-annealed in an annealing furnace. This is accomplished, e.g., by passing the flat steel product in the form of a cold-rolled steel strip through a continuous annealing furnace in which the steel strip is heated to temperatures above the recrystallization temperature of steel. Prior to, or preferably simultaneously with, recrystallization annealing, the cold-rolled flat steel product is nitrided by heating the flat steel product in the annealing furnace in the presence of a nitrogen donor. Nitriding is preferably carried out simultaneously with recrystallization annealing in the annealing furnace by introducing a nitrogen donor, in particular in the form of a nitrogen-containing gas, preferably ammonia (NH.sub.3), into the annealing furnace and by heating the flat steel product to an annealing temperature above the recrystallization temperature of steel and maintaining it at the annealing temperature for an annealing time (holding time) of preferably 10 to 150 seconds. The annealing temperature is preferably above 630° C. and especially in a range from 650° C. to 750° C. The nitrogen donor is selected to ensure that at the temperatures in the annealing furnace, the nitrogen donor dissociates to form atomic nitrogen which can diffuse into the flat steel product. Ammonia has proven suitable for this purpose. To prevent an oxidation of the surface of the flat steel product during annealing, a protective gas atmosphere is favorably used in the annealing furnace. The atmosphere in the annealing furnace preferably consists of a mixture of the nitrogen-containing gas, which acts as the nitrogen donor, and a protective gas such as HNx, with the volume content of the protective gas preferably ranging from 90% to 99.5% and the remaining portion of the volume content of the gas atmosphere being formed by the nitrogen-containing gas, especially ammonia gas (NH.sub.3 gas).

(27) Execution Examples:

(28) Execution examples of the invention and comparison examples will be described below. Flat steel products (strip steel) were produced from steel melts having the alloy compositions listed in Table 1 (FIG. 7) by hot rolling and subsequent cold rolling.

(29) The cold-rolled flat steel products were subsequently recrystallization-annealed in a continuous annealing furnace by maintaining the flat steel products over an annealing time of 45 seconds at annealing temperatures of 640° C.

(30) The process and material parameters of the thermally treated sheet steel of Table 1 are listed in Table 2 (FIG. 8), where N (after nitriding) is the nitrogen content after nitriding in the annealing furnace, D is the thickness of the sheet steel (in mm), NWG stands for the temper rolling degree during secondary cold rolling (in %), NH3 is the ammonia content in the annealing furnace (in vol %), Rp0.5 is the yield strength at 0.5% offset (in MPa) in the rolling direction, A is the elongation at break (in %) in the rolling direction, and Rm is the tensile strength (in MPa) in the rolling direction.

(31) In the examples according to the invention (Examples 1 to 3, 10 to 12, 15, 16, 18, 19, 21 to 23 and 25 and 26 in Tables 1 and 2), ammonia was introduced into the continuous annealing furnace during the thermal treatment of the flat steel products, so that a gas atmosphere consisting of ammonia and HNx protective gas was present in the continuous annealing furnace.

(32) In Table 2, the volume content of ammonia in the gas atmosphere is given as NH3 (vol %). In the comparison examples (Examples 4 to 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 resulting in the specimens according to the invention by nitriding in the ammonia-containing gas atmosphere of the continuous annealing furnace is given as N (after nitriding) [wt %]. The total nitrogen content N was determined according to DIN standard EN ISO 14284 (particularly subparagraph 4.4.1) after removal of a superficial iron nitride layer which had formed during nitriding on the surface of the specimens.

(33) The total weight content of the nitrogen is composed of an initial nitrogen content in the steel melt (NO, see Table 1) and the nitrogen content ΔN incorporated by nitriding in the continuous annealing furnace, with a considerable portion of the total nitrogen content N.sub.free being available in uncombined form and the remainder in combined form as nitride, see Equation (1). Using Equation (1), the weight content of the free nitrogen N.sub.free can be estimated based on the weight content of the nitride-forming elements contained in the steel.

(34) After the thermal treatment in the continuous annealing furnace, the cold-rolled and recrystallization-annealed flat steel products were subjected to temper rolling or dressing. The temper rolling degrees (NWG) of the second cold rolling or dressing and the thickness of the temper-rolled flat steel products are listed in Table 2. Finally, the flat steel products were aged by heating the specimen for 20 minutes to 200° C.

(35) FIG. 6 illustrates the effect of aging on the angular dependence of the elongation at break for Comparison Example 5 and compares the unaged condition to the aged condition, with the latter once more differentiating between artificial aging and natural aging. This shows that a significant anisotropy develops only after aging. However, since aging is virtually unavoidable in the practical processing of packaging steel, it is especially important that the isotropy be determined and optimized in the aged condition, which is the object of the present invention.

(36) Tensile tests and examinations of the structure were carried out on the aged specimens of Examples 1 to 26. More specifically, in the tensile tests, the yield strength at 0.5% offset (Rp0.5, measured according to DIN standard EN ISO 6892-1) and the elongation at break (A), and in the examinations of the structure, the mean grain size and the grain elongation, were determined. In FIG. 1, an example of a schematic stress-strain diagram from the tensile tests is shown.

(37) The stress-strain diagram of aged flat steel products has a discontinuous pattern. As a rule, the upper or lower yield strength is used as a reference value for characterizing the strength, sometimes also the tensile strength. The upper yield strength measured in the tensile test is highly dependent on the measuring conditions, the testing machine used, and the orientation thereof. In addition, for a specific testing machine, the spread of the value is especially high. For cold working processes, the lower yield strength is a relevant parameter for determining the formability of a flat steel product. However, it is difficult or impossible to determine the lower yield strength if, following the Luders region, the material does not strain-harden. Furthermore, in this case, the tensile strength is not defined. Therefore, instead of the lower yield strength, the plateau height, which is used as a measure of the yield strength at 0.5% offset (Rp0.5), is calculated since this value can be conclusively determined (FIG. 1). The parameter yield strength at 0.2% offset (Rp0.2), which is frequently determined to characterize unaged flat steel products, is unreliable for aged specimens because it is located too close to the upper yield strength and in a region in which the strain has not yet stabilized. For these reasons, the yield strength at 0.5% offset (Rp0.5) is here determined as a relevant measure of the strength of the samples. In addition, in the tensile tests, the elongation at break (A) of the specimens was determined. To determine the anisotropy/isotropy with respect to the yield strength (Rp0.5) and of the elongation at break (A) in the plane of the sheet metal, all measurements of the yield strength at 0.5% offset (Rp0.5) and of the elongation at break (A) were carried out along the rolling direction (0°) and in the plane of the flat steel product in increments of 100 in an angular range of 10° to 170° relative to the rolling direction. The determined dependence of the elongation at break A(α) on the angle α relative to the rolling direction (0°) is illustrated in a circular graph in FIG. 2, with FIG. 2a showing the results of the tested specimens 1 to 13, and FIG. 2b showing the results of the tested specimens 14 to 26. The determined dependence of the yield strength at 0.5% offset Rp0.5(α) on the angle α relative to the rolling direction (0°) is shown in the circular graph of FIG. 3, with FIG. 3a showing the results of the tested specimens 1 to 13, and FIG. 3b showing the results of the tested specimens 14 to 26.

(38) From the measured values determined for the dependence of the yield strength at 0.5% offset (Rp0.5) and the elongation at break (A) on the angle α relative to the rolling direction, the parameter energy of deformation W(α), defined as the product of elongation at break A(α) and yield strength at 0.5% offset Rp0.5(α), was computed. The result of the energy of deformation W(α) thereby determined as a function of the angle α relative to the rolling direction (0°) is shown in the circular graphs of FIG. 4, with FIG. 4a showing the results for the specimens of Examples 1 to 13, and FIG. 4b showing the results for the specimens of Examples 14 to 26.

(39) As FIGS. 2 to 4 indicate, compared to the comparison examples (Examples 4 to 9, 13, 14, 17, 20 and 24) (which were not nitrided in the continuous annealing furnace), the specimens according to the invention have an improved isotropy with respect to the yield strength at 0.5% offset (Rp0.5), the elongation at break (A) and the energy of deformation W(α) resulting as a product thereof. As FIG. 2 indicates, the specimens according to the invention have an elongation at break A(α), which in the plane of the sheet metal is within a range of 60% to 140% of the elongation at break in the rolling direction A(0°). FIG. 3 shows that the specimens according to the invention have a yield strength at 0.5% offset (Rp0.5) which is dependent on the angle α relative to the rolling direction (0°), and which in the plane of the sheet metal is within a range of 90% to 110% of the yield strength in the rolling direction Rp0.5(0°). As FIG. 4 shows, the specimens according to the invention have an energy of deformation W(α), which in the plane of the sheet metal is dependent on the angle α relative to the rolling direction (0°) and which is within a range of 60% to 140% of the energy of deformation in the rolling direction W(0°). In contrast, as FIGS. 2 to 4 indicate, the comparison examples have a considerably anisotropy with respect to the elongation at break (A), the yield strength at 0.5% offset (Rp0.5), and the energy of deformation.

(40) To determine the grain structure of the flat steel products, microsections of the specimens were prepared in planes along and transverse to the rolling direction as well as in flat planes of the metal sheets. The sectional planes are illustrated in FIG. 5. Using the microsections, the grain size and the grain elongation were determined by means photomicrographic examination of the sectional planes of the specimen. On these microstructure photographs, the number of intersections occurring between the grid lines and the grain boundaries are counted. The mean grain size is obtained from the mean value of the linear intercept segment (mean fiber length). In FIG. 5, the grain elongation or the elongation of the fiber length of the grains of the steel structure is explained, using the directions x (horizontal, along the rolling direction) and y (vertical, in the direction of the thickness of the flat steel product). In the x-direction of the grain, the horizontal fiber length S_H is determined. Perpendicular thereto in the y-direction, the vertical fiber length S_V is determined. This is done in microsections which were acquired both along and transverse to the rolling direction. When determining the fiber lengths, not every grain is individually measured, but instead a uniform grid pattern is placed on the photomicrograph of the structure, and by means of the grid length and the number of the intersections, a fiber length is determined, which can be used as a substitute for the grain size. The mean horizontal and vertical fiber length corresponds to the mean value of the analysis of all acquired areas of the microstructure. The grain elongation S is defined as: S=S_H/S_V or S=x/y.

(41) The grain size ( ) (determined according to ASTM E 112 and DIN standard EN ISO 643 and using comparison photos) and the grain elongation S (determined by means of the linear intercept method) as well as the mean fiber length of the specimens are listed in Table 3 (FIGS. 9 and 10). All specimens have a mean fiber length in the range from 3.3 to 5.4 μm. In the longitudinal sections of the flat steel product, the direction-dependent grain elongation (S) in the rolling direction (0°) is a minimum of 1.4, in the planar sections of the flat steel product, it is a minimum of 1.1. The grain elongation (S) transverse to the rolling direction (90°) has a value of a minimum of 1.2. In this respect, no significant difference between the specimens according to the invention and the comparison specimens was determined.

(42) This leads to the conclusion that the higher strength of the specimens according to the present invention is not realized by means of grain refinement but is decisively achieved by means of solution hardening generated by nitriding in the continuous annealing furnace. Furthermore, it shows that the improved isotropy with respect to the mechanical properties of the specimens according to the present invention can be achieved in spite of an anisotropy (caused by cold rolling) in the structure. The anisotropy of the structure, which is also present in the specimens according to the invention, results from the grain elongation S, which in the specimens according to the invention is comparable to the grain elongation of the comparison samples. The solution hardening, which was generated by nitriding in the continuous annealing furnace, therefore not only led to an increase in the strength (tensile strength Rm), but also to an improvement of the homogeneity of the mechanical parameters such as elongation at break A and yield strength at 0.5% offset Rp0.5, as well as the energy of deformation W=A.Math.Rp0.5 resulting therefrom.