Manufacturing method for packaging steel plate having multilayer crystallized structure and steel plate having multilayer crystallized structure

Abstract

The present invention relates to a method for producing a steel sheet having a multilayer crystallized structure, particularly for packaging, preferably having a thickness of less than 0.5 mm and a carbon content by weight of 10 to 1000 ppm, and a nitrogen content averaged over the thickness of the steel sheet of more than 50 ppm, and to a multilayer crystallized structure having a core region and near-surface regions surrounding the core region on both sides, wherein the near-surface region is at least substantially recrystallized and the core region is not recrystallized, or at least not completely recrystallized.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a steel sheet having a multilayer crystallized structure, and to a steel sheet having a thickness of less than 0.5 mm and having a multilayer crystallized structure. [Background technology]

[0002] German Patent Application Publication No. 10 2020 112 485 B3 discloses a steel sheet having a multilayer crystallized structure and a manufacturing process thereof. The steel sheet is manufactured from a steel having a carbon content (C) of 10 to 1000 ppm by weight, has a thickness of less than 0.5 mm, and has a multilayer microstructure having at least a first layer and a second layer, where the first layer is at least substantially recrystallized and the second layer is not recrystallized or at least not completely recrystallized. In one embodiment, the known steel sheet has a three-layer microstructure with an inner, at least substantially recrystallized core region and near-surface regions surrounding the core region on both sides, where the near-surface regions are not recrystallized or at least not completely recrystallized. To manufacture this steel sheet with a three-layer microstructure, a recrystallization temperature (T) is set at a predetermined temperature depending on the composition of the steel. R ) is heated to at least the recrystallization temperature (T R ) and at least temporarily in the presence of a nitrogen donor, E ), whereby, during heating of the cold rolled steel sheet, nitrogen diffuses from the nitrogen donor to at least the region of the cold rolled steel sheet near the surface and is incorporated into the region near the surface, resulting in the recrystallization temperature (T R ) increases by a value ΔT. E ) is the recrystallization temperature near the surface (T R+ΔT), so that only the core region of the steel sheet is recrystallized, while the surfaces surrounding the core region on both sides are not or only partially recrystallized. As a result, a three-layer crystallization structure develops in the steel sheet, with a roll-hard, non-recrystallized seam region and a softer, recrystallized core region. The known steel sheet has high strength with good elongation at break and can be used to manufacture packaging such as food or beverage cans, thereby avoiding surface roughening of the steel sheet due to the roll-hardened seam region during forming of the steel sheet, for example in a deep drawing process.

[0003] German Patent No. 10 2020 112 485 B3 discloses a steel sheet having a multilayer microstructure, particularly a three-layer crystallized structure, which includes an inner, at least essentially recrystallized core region and two adjacent surface regions surrounding the core region, which are not recrystallized, or at least not completely recrystallized. This three-layer microstructure achieves high strength, but the non-crystallized (roll-hardened) regions on the surface of the steel sheet mean that formability is reduced. In particular, during multiaxial forming and bending, the outer roll-hardened regions dominate. This is particularly evident in the higher resistance to bending with large bend radii due to the hard outer fibers of the steel sheet or the resulting premature material failure. Summary of the Invention

[0004] On this basis, the invention is based on the object of improving the formability of steel sheets, in particular in multiaxial forming processes and bending processes, without compromising their high strength.

[0005] According to the present invention, this object is solved by a steel sheet having the features of claim 23 and by a method for producing this steel sheet having the features of claim 1.

[0006] The steel sheet according to the invention, which is particularly suitable for the production of packaging but can also be used in other fields of application, such as in automotive engineering or the production of housings for machines, has a predetermined thickness, preferably less than 0.5 mm, a carbon content by weight of 10 to 1000 ppm and a nitrogen content by weight of more than 50 ppm averaged over the thickness of the steel sheet, and the steel sheet has a multilayer crystallized structure having a core region and near-surface regions surrounding the core region on both sides, the near-surface regions being at least substantially recrystallized and the core region not being recrystallized, or at least not being completely recrystallized.

[0007] Compared to the three-layer microstructure known from DE 10 2020 112 485 B3, the order of the regions with different crystallization structures is reversed. This improves the formability of the steel sheet according to the invention while maintaining the same tensile strength, since the outer region near the surface is softer and more fluid due to an at least essentially fully or at least mainly crystallized microstructure. As a result, this microstructure is significantly more flexible and easier to form during the forming process. The high strength of the steel sheet according to the invention is ensured by the stable core region, which is not recrystallized, or at least not completely recrystallized, and has at least a lower crystallinity than the outer region near the surface (seam region), and is therefore still at least partially roll-hard.

[0008] In the production of steel sheets according to the invention, the multilayer crystallized structure is produced by the method according to the invention by the following steps: Carbon weight content (C) of 10 to 1000 ppm and a predetermined original recrystallization temperature (T R 0 ), to provide a steel plate cold rolled to a predetermined thickness, The steel has an initial nitrogen content (NO) of at least 70 ppm by weight and / or the cold-rolled steel sheet is heated to the original recrystallization temperature (T R 0) lower than the maximum nitriding temperature (T A ) in the nitriding process, nitrogen is incorporated into the cold rolled steel sheet at least in the near-surface region (1) by exposing it to a nitriding gas atmosphere at a temperature of 1000 K. The nitrogen precipitated in the near-surface region (1) during the nitriding process is preferably at the original recrystallization temperature (T R 0 ) below the homogenization temperature (T Ho nitriding, in which the nitrogen precipitated during the nitriding process is homogenized throughout the thickness of the steel sheet by a temperature treatment to homogenize the nitrogen, -Predetermined denitrification time (t ES ), a step of denitrifying at least the near-surface region (1) of the steel sheet by introducing the steel sheet into a hydrogen-containing gas atmosphere during which the near-surface region (1) reaches a first recrystallization temperature (T R 1 ), and the core region (2) of the steel plate has a second recrystallization temperature (T R 2 ), and the denitrification of the near-surface region (1) leads to a first recrystallization temperature (T R 1 ) is the second recrystallization temperature (T R 2 a nitrogen gradient is formed during denitrification across the thickness of the steel plate, where the nitrogen concentration decreases outward from the inner core region (2) to the near-surface region (1) so that the nitrogen concentration is lower than the First recrystallization temperature (T R 1 ) and the second recrystallization temperature (T R 2 ) between the annealing temperature (T G ) annealing the steel sheet, wherein the annealing is performed during and / or after denitrification.

[0009] In the method according to the invention, the method steps of denitriding the steel sheet in a hydrogen-containing gas atmosphere and annealing during and / or after denitriding produce an inverse order of regions with different crystallinity compared to previously known microstructures. Denitriding removes nitrogen (only) from the outer near-surface region of the steel sheet, while the nitrogen content in the inner core region remains at least essentially the same, resulting in different recrystallization temperatures in the near-surface region and the inner core region, with the recrystallization temperature of the near-surface region (first recrystallization temperature T R 1 ) is the recrystallization temperature of the core region (second recrystallization temperature T R 2 ) is lower than the annealing temperature (T G ) is the recrystallization temperature of the near-surface region (first recrystallization temperature) and the recrystallization temperature of the core region (second recrystallization temperature T R 2 ), the annealing of the steel sheet, which may occur during and / or after denitrification, can achieve a microstructure with different crystal structures in the near-surface and core regions. Therefore, in the method according to the present invention, the annealing temperature (T G ) is T R 1 <T(G)<T(R) 2 This ensures that during annealing of the steel sheet, at least essentially only the outer near-surface region having the lower recrystallization temperature (first recrystallization temperature) is recrystallized, while the inner core region having the higher recrystallization temperature (second recrystallization temperature) remains roll-hard, i.e., is not recrystallized, or at least is not completely recrystallized.

[0010] In order to achieve sufficient strength, particularly preferably a tensile strength of more than 500 MPa, in particular between 550 MPa and 700 MPa, the steel sheet according to the invention preferably has a nitrogen weight fraction of at least 50 ppm averaged over the thickness of the steel sheet after denitrification. To achieve this, it is advantageous if the steel sheet has a nitrogen weight content (in particular averaged over the thickness of the steel sheet) of at least 70 ppm before the start of denitrification.

[0011] Therefore, the starting material used in the method according to the invention has a carbon content (C) of 10 to 1000 ppm by weight and a predetermined initial recrystallization temperature (T R 0 ), wherein the steel already has an initial nitrogen content (NO) of at least 70 ppm by weight and / or so much nitrogen is incorporated into the cold-rolled steel sheet, at least in the region close to the surface, in a nitriding process that the proportion by weight of nitrogen (averaged over the thickness) after the nitriding process is at least 70 ppm.

[0012] In addition to high strength, the steel sheet according to the invention is characterized by particularly improved formability. In particular, the steel sheet according to the invention has a high elongation at break, preferably of at least 4%, in particular 5% to 10%. The steel sheet according to the invention has a soft and flowable seam area (recrystallized near-surface area) and is therefore easier to form than known steel sheets with comparable mechanical properties in terms of strength and elongation at break, especially in bending processes with large bending radii, because higher elongations can be achieved in the soft outer seam area (near-surface area) of the steel sheet, especially outside the bending radius.

[0013] In particular, the steel sheet according to the invention has outer fibers formed by soft, flowable seam regions (near-surface regions) during bending deformation, which have a higher ductility compared to the inner fibers formed by the harder core region. This ensures that the steel sheet according to the invention has a high strength due to the harder core region, while the softer, flowable seam regions ensure better formability of the outer fibers during bending deformation. Furthermore, the steel sheet according to the invention also exhibits acceptable isotropy of the mechanical properties.

[0014] In a first embodiment of the method according to the invention for producing a steel sheet having a multilayer crystallization structure, the steel sheet is cold rolled to a predetermined thickness, preferably less than 0.5 mm, and has a carbon content (C) of 10 to 1000 ppm by weight and an original recrystallization temperature (T) which is at least substantially determined by the composition of the steel. R 0 ) is subjected to a nitriding process in which nitrogen is incorporated into the cold rolled steel sheet at least in the region close to the surface, and the cold rolled steel sheet is heated to a recrystallization temperature (T R ) lower than the maximum nitriding temperature (T A ) to a nitriding gas atmosphere, and subsequently, the nitrogen accumulated in the near-surface region during nitriding is removed by temperature treatment, especially the homogenization temperature (T A ), which is particularly preferably above the nitriding temperature and above the recrystallization temperature (T R ) is less than the homogenization temperature (T Ho ) homogenized at the recrystallization temperature (T R ), resulting in the precipitation of nitrogen also in the core region of the steel sheet. The temperature treatment for homogenizing the nitrogen precipitated during the nitriding process is hereinafter also referred to as homogenization or homogenization process.

[0015] In a preferred variant of the first embodiment of the process according to the invention, a nitriding gas, in particular ammonia, is directed onto the surface of the cold-rolled steel sheet by one or more spray nozzles during the nitriding process. This results in faster diffusion of atomic nitrogen onto the surface of the cold-rolled steel sheet, creating a uniform distribution of nitrogen, which is then incorporated into the steel sheet across both the cross-section and the plane of the surface of the steel sheet. Due to the uniform incorporation of nitrogen across the cross-section of the steel sheet, a shorter homogenization time can be selected in this example, since the nitrogen is already uniformly incorporated as it diffuses into the interior of the steel sheet.

[0016] In a first embodiment of the method according to the invention, after the nitriding process and homogenization, nitrogen is removed from the near-surface region of at least one side of the steel sheet by introducing the steel sheet into a hydrogen-containing gas atmosphere, and a nitrogen gradient is formed through the thickness of the steel sheet, with the nitrogen concentration decreasing from the inner core region outward relative to the near-surface region formed during this denitriding, so that the near-surface region is heated to a first recrystallization temperature (T R 1 ), and the core region of the steel plate reaches the first recrystallization temperature (T R 1 ) higher than the second recrystallization temperature (T R 2 ), and the steel sheet reaches the first recrystallization temperature (T R 1 ) and the second recrystallization temperature (T R 2 ) between the annealing temperature (T G ) and is annealed during or after denitrification.

[0017] In a second embodiment of the method according to the invention, the nitrogen content is preferably at least 70 ppm by weight and a predetermined recrystallization temperature (T R A steel sheet that has been cold-rolled to a predetermined thickness from a steel having a carbon content (C) of 10 to 1000 ppm by weight, which already has a first recrystallization temperature (T), is subjected to denitrification by placing the cold-rolled steel sheet in a hydrogen-containing gas atmosphere, whereby nitrogen is removed from the steel sheet in a region near the surface, and a nitrogen gradient is formed across the thickness of the steel sheet, with the nitrogen concentration decreasing outward from the inner core region to the region near the surface, whereby a first recrystallization temperature (T R 1 ) is formed in the core region of the steel sheet, and the first recrystallization temperature (T R 1 ) higher than the second recrystallization temperature (T R 2 ) is formed, and the steel sheet is heated to a first recrystallization temperature (T R 1 ) and the second recrystallization temperature (T R 2 ) between the annealing temperature (T G ) and is annealed during or after denitrification.

[0018] To carry out the second embodiment of the method according to the invention, cold-rolled steel sheets are used, which are produced from steel to which nitrogen has already been added during steel production in the steel melt, for example in the form of nitrogen gas, manganese nitrogen or calcium cyanamide, preferably in a proportion by weight of 100 ppm to 160 ppm.

[0019] In both process variants, denitrification of the near-surface region of the steel sheet in a hydrogen-containing gas atmosphere is preferably carried out for a predetermined denitrification time (t ) in the range of 1 to 600 seconds, particularly preferably 10 to 300 seconds, in particular 180 to 300 seconds. ES ) is preferably carried out at a first recrystallization temperature (T R 1 ) and the second recrystallization temperature (T R 2 ) between the annealing temperature (T G ) is carried out simultaneously with the annealing. On the one hand, this is advantageous in terms of process technology and shorter process durations, and on the other hand, it has proven to be more effective in terms of the efficiency of denitrification. However, denitrification can also be carried out before annealing, whereby denitrification is then preferably carried out at a temperature lower than the original recrystallization temperature (T R 0 ) below the denitrification temperature (T ES ), which prevents the steel structure of the steel plate from recrystallizing during denitrification.

[0020] Furthermore, denitrification can also begin before annealing and continue during annealing. Therefore, denitrification can overlap with annealing, especially when both denitrification and annealing are performed in a continuous annealing furnace. The hydrogen-containing atmosphere in the annealing furnace is crucial for maintaining the correct annealing temperature between the first and second recrystallization temperatures and is crucial for annealing to remove nitrogen from the near-surface region of the steel sheet during denitrification and to recrystallize (only) the outer near-surface region during annealing. Therefore, when denitrification and annealing are performed simultaneously or overlapping at least temporarily, the gas atmosphere in the annealing furnace is selected to contain a sufficient amount of hydrogen to ensure denitrification, while the temperature in the annealing furnace is set to the appropriate annealing temperature between the first and second recrystallization temperatures.

[0021] In both embodiments of the method according to the invention, the described process control results in the formation of a three-layer crystallized structure in the steel sheet, which comprises a core region and a near-surface region surrounding the core region on both sides, the near-surface region being at least substantially recrystallized and the core region not being recrystallized, or at least not being completely recrystallized.

[0022] In a first embodiment of the method according to the invention, the recrystallization temperature is initially increased to a recrystallization temperature during the nitriding process and during the subsequent temperature treatment, both in the region of the steel sheet close to the surface and in the core region, which is higher than the original recrystallization temperature (T R 0 ), and during the subsequent denitrification, the recrystallization temperature is at least essentially (only) above the first recrystallization temperature (T R 1 ) and the first recrystallization temperature (T R 1 ) is the original recrystallization temperature (T R 0 ) or below the original recrystallization temperature (T R 0 ) is in particular in the range of 550°C to 700°C, depending on the composition of the steel.

[0023] In a second embodiment of the method according to the invention, the recrystallization temperature in the region close to the surface of the steel sheet is increased to the first recrystallization temperature (T R 1 ) to the first recrystallization temperature (T R 1 ) is the original recrystallization temperature (T R 0 ) and the second recrystallization temperature (T R 2 ) is at least substantially the original recrystallization temperature (T R 0 ), which corresponds to the original recrystallization temperature (T R 0 ) is hardly changed compared to

[0024] Therefore, in a second embodiment of the method, the first recrystallization temperature (T R 1 ) is the original recrystallization temperature (T R 0 ) and the second recrystallization temperature (T R 2 ) is the original recrystallization temperature (T R 0 In a first embodiment of the method, the second recrystallization temperature (T R 2 ) is the (original) recrystallization temperature (T R 0 ), and the first recrystallization temperature (T R 1 ) is the (original) recrystallization temperature (T R 0 ) above, below, or corresponding to.

[0025] In both embodiments of the method according to the invention, by removing nitrogen during denitrification of the near-surface region, the weight fraction of nitrogen in the near-surface region is reduced by a predetermined value (ΔN1), thereby lowering the recrystallization temperature of the near-surface region to a first recrystallization temperature (T R 1 ) by a value ΔT1, and the amount of ΔT1 increases linearly with the decrease in the weight fraction of nitrogen (ΔN1), particularly in the near-surface region. Furthermore, ΔT1, which reduces the recrystallization temperature of the near-surface region (1) during denitrification, is preferably greater than 10°C, and particularly preferably greater than 30°C.

[0026] In a first embodiment of the method according to the invention, nitrogen atoms can also diffuse into the core region of the steel sheet as a result of the nitriding process, and the subsequent temperature treatment increases the weight fraction of nitrogen therein by a predetermined value (ΔN2), whereby the recrystallization temperature of the core region increases by a value ΔT2 to a second recrystallization temperature (T R 2 =T R +ΔT2), and ΔT2 increases linearly with an increase in the weight fraction (ΔN2) of nitrogen in the core region (2). ΔT2, at which the recrystallization temperature of the core region increases during the temperature treatment, preferably exceeds 10°C, and particularly preferably exceeds 30°C.

[0027] In a first embodiment of the method according to the invention, the annealing temperature (T G ) is the temperature at which the first recrystallization temperature T R 1 The recrystallization temperature of the near-surface region, which has been lowered by ΔT1, exceeds the second recrystallization temperature T R 2 (where T R 2 =T R 0 +ΔT2). The following applies:T R 1 <T G <T R 2 .

[0028] The annealing temperature (T G), the original recrystallization temperature (T R 0 ) is first determined experimentally. R 0 ) is usually below 700°C, typically in the range of 650°C to 700°C, and is taken as the lower limit of a suitable annealing temperature because it is the minimum temperature required during annealing to induce recrystallization of the steel (in the near-surface region). In a repeat test, the annealing temperature is then gradually increased on a specimen of steel plate that has been denitrified in the near-surface region, and the microstructure of the steel plate that forms during annealing is metallographically examined through its thickness. If the (non-denitrified) core region of the steel plate still has a non-recrystallized (i.e., roll-hard) microstructure, the annealing temperature used should be below the recrystallization temperature of the core region (second recrystallization temperature T R 2 ) which can be assumed to be lower than the second recrystallization temperature T R 2 This gives an upper limit to the appropriate annealing temperature.

[0029] In a second embodiment of the method according to the invention, the annealing temperature (T G ) is the original recrystallization temperature (T R 0 ) and decreases by ΔT1 during softening. R 1 =T R 0 -ΔT1), whereby the following applies: T R 0 > T(G) >T( R) 0 -ΔT1.

[0030] Annealing temperature (T G ) is less than 750°C for the preferred selected compositions of the steel sheet.

[0031] In the nitriding process of the first embodiment of the method according to the invention, the steel sheet is annealed in an annealing furnace, in particular a continuous annealing furnace, preferably in an ammonia-containing gas atmosphere at a nitriding temperature (T A ) and heated to the nitriding temperature (T A ) at a given retention time (t H ) is maintained at the nitriding temperature (T A ) is preferably above 300°C, particularly 300°C to 600°C, and the holding time (t H ) is preferably more than 1 second, particularly 2 to 300 seconds. At temperatures above 300°C, nitrogen in ammonia dissociates to form atomic nitrogen, which can diffuse into the steel sheet. During heating and / or the holding time, the steel sheet is exposed to an ammonia-containing gas atmosphere in the annealing furnace, whereby atomic nitrogen diffuses through the steel sheet surface to the near-surface region of the steel sheet, particularly due to catalytic reactions on the heated surface of the steel sheet. The gas atmosphere preferably contains a volume fraction of ammonia of 0.05% to 10%, particularly preferably 1% to 5%.

[0032] In a first embodiment of the method according to the invention, the subsequent temperature treatment to homogenize the nitrogen precipitated in the near-surface region of the steel sheet during the nitriding process is preferably carried out in the downstream region of the same annealing furnace, in particular a continuous annealing furnace, which allows efficient process control in a single pass. However, the temperature treatment to homogenize the nitrogen incorporated during the nitriding process can also be carried out in a separate bell-type annealing furnace (with significantly longer homogenization times in the range of several hours).

[0033] The temperature of the steel plate during homogenization (homogenization temperature T Ho ) is preferably above 300°C, in particular 300°C to 700°C, in particular 350°C to 650°C. Preferably, the homogenization temperature (T Ho ) is the original recrystallization temperature (T R 0 ), especially when the method is carried out in a continuous annealing furnace, is below the nitriding temperature (T A ) or higher. When homogenization is performed in a bell annealing furnace, the homogenization temperature (T Ho ) is the nitriding temperature (TA ) can also be lower.

[0034] If this process is carried out in a continuous annealing furnace, the temperature of the steel sheet may also change (suddenly or continuously) during homogenization. In particular, the temperature of the steel sheet may change from its original recrystallization temperature (T R 0 However, to prevent the inner core region of the steel from recrystallizing, such a temperature increase must be allowed to continue until the steel has reached its original recrystallization temperature (T R 0 ) lower than the homogenization temperature (T Ho ) for a long enough time.

[0035] To homogenize the nitrogen introduced in the nitriding process, a homogenization period (t Ho ) while the steel plate is heated to the homogenization temperature (T Ho ) can be maintained.

[0036] If the nitriding process is carried out in a continuous annealing furnace and the subsequent temperature treatment to homogenize the nitrogen incorporated during the nitriding process is carried out in a separate bell annealing furnace, the homogenization time (t Ho ) is preferably 60 minutes to 6 hours, particularly preferably 2 hours to 5 hours, and the homogenization is preferably carried out in a protective gas atmosphere, particularly an argon or nitrogen gas atmosphere, at a homogenization temperature (T Ho ) This ensures that the nitrogen incorporated during the nitriding process is uniformly distributed throughout the thickness of the steel. The protective gas atmosphere prevents oxidation of the steel surface during the temperature treatment and prevents outgassing of steel constituents.

[0037] When the temperature treatment for homogenizing the stored nitrogen is carried out in a continuous annealing furnace, the homogenization time (t Ho) is preferably between 60 seconds and 600 seconds, particularly between 180 seconds and 300 seconds, and particularly between 180 seconds and 400 seconds. These homogenization durations ensure, on the one hand, sufficient uniformity of the nitrogen introduced throughout the thickness of the steel sheet, and, on the other hand, rapid process control using a high belt speed at which the steel sheet passes through the continuous annealing furnace. During homogenization in the continuous annealing furnace, the homogenization temperature (T Ho ) is preferably 300°C to 700°C, particularly preferably 450°C to 650°C, and the steel sheet is thereby Ho ) in a continuous annealing furnace in a protective gas atmosphere, especially in an argon or nitrogen gas atmosphere, at a homogenization temperature (T Ho ) is maintained.

[0038] Preferably, the homogenization time (t Ho ) is the holding time during nitriding in the nitriding process in a continuous annealing furnace (t H ) and / or denitrification time (t ES ) longer than

[0039] The nitriding process of the first embodiment of the method according to the invention can be carried out in a single stage or a multi-stage process. In a single stage nitriding process, the steel sheet is heated for a heating time (t E ) from room temperature to the nitriding temperature and held at the nitriding temperature for a holding time. During the heating and / or holding time, the steel sheet is exposed to a nitriding gas atmosphere. E ) is preferably in the range of 1.0 to 300 seconds, and the retention time (t H ) is preferably in the range of 1.0 to 300 seconds. It is particularly preferred that the nitriding is carried out in an ammonia-containing gas atmosphere having a volume fraction of 0.05 to 10% of ammonia and the remainder being an inert gas, particularly argon or nitrogen gas.

[0040] During denitrification, the steel sheet is heated for a denitrification time (t ES), the gas atmosphere preferably has a hydrogen volume fraction of more than 10%, particularly preferably more than 15%, and in particular more than 50%. The hydrogen-containing gas atmosphere can also consist mostly of hydrogen (i.e., 100% excluding unavoidable residual gases). However, for efficient denitrification, a hydrogen concentration of 15% by volume or less is preferred for denitrification times (t) of 100 to 300 seconds, which are particularly preferred. ES ) is sufficient.

[0041] Denitriding is carried out at a denitriding temperature (T) above 300°C and below the original recrystallization temperature of the steel, preferably between 300°C and 600°C, before annealing. ES ), however, denitriding is preferably carried out simultaneously with annealing at the annealing temperature. In a first embodiment of the method for nitriding steel sheet in a continuous annealing furnace, denitriding is preferably carried out in the continuous annealing furnace where a subsequent temperature treatment for nitriding and homogenizing the incorporated nitrogen is carried out. This allows for efficient process control.

[0042] During denitrification, the hydrogen-containing gas atmosphere extracts nitrogen from the near-surface region of the steel sheet, whereby the nitrogen that diffuses out of the steel sheet first recombines in an equilibrium reaction to form N2 molecules and then, to a lesser extent, with hydrogen gas in the gas atmosphere to form ammonia (NH3). N2+3H2→ 2NH 3。

[0043] Therefore, the hydrogen-containing gas atmosphere has a low proportion of ammonia from the equilibrium reaction, which is even higher the more nitrogen diffuses from the steel sheet into the gas atmosphere, particularly less than 0.1% by volume. For efficient denitrification, it is preferable that the hydrogen-containing gas atmosphere contains as little ammonia gas as possible, at least at the beginning of denitrification. To maintain denitrification for as long as possible, the gas atmosphere can be temporarily or continuously replaced with an ammonia-free gas atmosphere during the denitrification process, for example, by supplying hydrogen-containing and ammonia-free gases, particularly in countercurrent flow. Replacing the ammonia-containing gas atmosphere, which forms ammonia by the recombination reaction of nitrogen from the steel sheet with hydrogen from the gas atmosphere, with an ammonia-free hydrogen-containing protective gas atmosphere can promote the diffusion of nitrogen from the near-surface region of the steel sheet.

[0044] The gas atmosphere during denitrification preferably contains a hydrogen content of at least 10% by volume, preferably more than 15% by volume, in particular 30% by volume or more, particularly preferably at least 50% by volume, and may contain up to 100% hydrogen gas. Preferably, the gas atmosphere during denitrification does not contain formed ammonia gas or other nitriding gases. However, the equilibrium concentration of ammonia is low, which is due to the diffusion of nitrogen from the steel sheet and the combination of the diffused nitrogen with hydrogen from the gas atmosphere to form NH3. In addition to hydrogen, the gas atmosphere during denitrification can also contain an inert gas, preferably not containing nitrogen.

[0045] In order to prevent decarburization from occurring simultaneously with denitrification, the gas atmosphere during denitrification preferably has low humidity. Therefore, the dew point of the gas atmosphere used during denitrification in the annealing furnace is preferably below 0°C, particularly preferably below -30°C. Moisture is introduced into the annealing furnace by steel sheets that have been cleaned before entering the annealing furnace and therefore have residual moisture on their surfaces, and also due to leaks in the annealing furnace, the protective gas pumped into the annealing furnace to maintain the gas atmosphere during denitrification, especially at a low overpressure of a few mbar, preferably has an even lower dew point, for example below -50°C (and therefore has a very low moisture content).

[0046] It is also helpful if the gas atmosphere during denitrification contains small amounts of carbon oxides, in order to avoid impurities on the surface of the steel sheet that may arise and hinder the efficient diffusion of nitrogen from the steel during annealing. Preferably, therefore, the gas atmosphere during denitrification contains a low volume fraction of carbon oxides, in particular a maximum of 1000 ppm in total (i.e., CO and CO combined). の Contains CO and CO2.

[0047] Annealing of the steel sheet is preferably carried out in a continuous annealing furnace during and / or after denitrification. When annealing is carried out separately from denitrification, i.e., after completion of denitrification, annealing is preferably carried out in a protective gas atmosphere, particularly an argon, nitrogen, or HNx atmosphere, to prevent oxidation of the steel sheet surface. When annealing is carried out simultaneously with denitrification, annealing is carried out in a hydrogen-containing gas atmosphere, which preferably contains more than 10% to 100% hydrogen gas, and the remainder is preferably an inert gas, such as argon (Ar) or nitrogen gas (N2).

[0048] During annealing, the steel sheet is preferably heated for a heating time (t A ) indicates the annealing temperature (T G ) and annealed for a given time (t G ) annealing temperature (T G ) and annealing time (t G ) is preferably more than 1 second, in particular 1 second to 600 seconds, particularly preferably 100 seconds to 300 seconds. These annealing times allow complete recrystallization of the region close to the surface on the one hand, and on the other hand allow for fast and efficient process control in a continuous annealing furnace at high belt speeds. The steel sheet is heated for a heating time (t) of a few seconds by an induction heater built into the continuous annealing furnace. A ), which allows for a short heating distance and a longer holding time at the annealing temperature (annealing time t G ) is ensured.

[0049] When cold-rolled steel sheets are annealed, the recrystallization annealing of the cold-rolled steel sheets occurs at least partially (only) in the region near the surface, but not in the core region (where the higher recrystallization temperature TR 2 (due to the above) is not recrystallized or is only slightly recrystallized. If the crystallinity of the near-surface region is as high as possible while the core region is not recrystallized, a clear distinction between the core region and the surrounding near-surface region can be achieved. The crystallinity of the two regions can be controlled by the process parameters of the manufacturing process. Preferably, the core region has a recrystallization degree of less than 30%, particularly preferably less than 20%, and the recrystallization degree of the near-surface region is preferably more than 70%, particularly preferably more than 80%.

[0050] In order to achieve these degrees of recrystallization, the homogenization time in the first embodiment of the method according to the invention is preferably set so that the steel sheet does not exceed the nitriding temperature (T A ) retention time (t H Furthermore, in the first embodiment of the method according to the present invention, the homogenization time is preferably longer than the denitrification time (t ES ) is particularly preferably longer than the denitrification time (t ES ) and / or the holding time during nitriding in the nitriding process in a continuous annealing furnace (t H ) is at least two times longer. Due to the long homogenization time, in the first embodiment of the method according to the invention, a uniform distribution of the nitrogen introduced throughout the thickness of the steel sheet after nitriding in the continuous annealing furnace is achieved. The short denitridation time (t) compared to the homogenization time ES During denitrification, nitrogen is removed only in the region of the steel sheet near the surface, resulting in the formation of a significant nitrogen gradient across the thickness of the steel sheet during denitrification, with a high nitrogen concentration in the core region and a low nitrogen concentration in the region near the surface. The larger the nitrogen gradient, i.e., the greater the difference in nitrogen concentration between the core region and the region near the surface, the greater the difference between the first recrystallization temperature (region near the surface) and the second recrystallization temperature (core region). The greater the difference between the first recrystallization temperature (region near the surface) and the second recrystallization temperature (core region), the greater the difference in the degree of recrystallization that occurs during annealing between the region near the surface and the core region. Therefore, long homogenization times, especially short denitrification times (tES ) leads to a strong difference in the degree of recrystallization of the steel sheet in the near-surface region and the core region, and therefore leads to a stronger development of multilayer crystallized structures.

[0051] The thicknesses of the near-surface region and the core region can be controlled by the process parameters of the manufacturing process. Preferably, the near-surface region has a thickness in the range of 5 μm to 200 μm, particularly preferably in the range of 10 μm to 100 μm, and particularly preferably in the range of 20 μm to 80 μm. The thickness of the core region is preferably in the range of 50 μm to 450 μm, particularly preferably in the range of 90 μm to 400 μm, and particularly preferably in the range of 150 μm to 300 μm.

[0052] The steel roll core region and the at least partially recrystallized near-surface region of the steel sheet according to the present invention differ from each other not only in their crystallinity but also in their strength and hardness. The core region has a higher hardness and / or a higher tensile strength than the near-surface region, and the ratio of the hardness of the core region to the hardness of the near-surface region is preferably greater than 1.2, particularly preferably greater than 1.4. The core region of the steel sheet preferably has a hardness of at least 160 HV due to solid solution hardening caused by its high nitrogen content. 0.025 , particularly preferably at least 220 HV 0.025 It has high microhardness with a Vickers hardness of 1000 MPa.

[0053] The steel of the cold-rolled steel sheet preferably has the following composition by weight: C: more than 0.001% and less than 0.1%, preferably less than 0.06%; · Mn: more than 0.01% and less than 0.6%; ·P: less than 0.04%; · S: less than 0.04%, preferably more than 0.001%; ·Al: less than 0.08%; ·Si: less than 0.1%; ·Optional Cu: less than 0.1%; ·Optional Cr: less than 0.1%; ·Optional Ni: less than 0.1%; ·Optional Ti: less than 0.1%; ·Optional Nb: less than 0.08%; ·Optional Mo: less than 0.08%; ·Optional Sn: less than 0.05%; · optional B: less than 0.01%, preferably less than 0.005%; Residual iron and unavoidable impurities In a second embodiment of the method according to the invention, the steel of the cold-rolled steel sheet further has a nitrogen content of more than 0.001%, preferably more than 0.010%, and in the first embodiment of the method according to the invention the steel can optionally have an (initial) nitrogen content (NO) of more than 0.001%, preferably less than 0.016%.

[0054] Values ​​given in % or ppm for the content or concentration of alloying elements in steel or cold-rolled steel sheet refer in each case to the weight of the steel or steel sheet.

[0055] In a first embodiment of the method according to the invention, the nitrogen content averaged over the thickness of the steel sheet is preferably set in the nitriding process to at least 0.005% (50 ppm), particularly preferably more than 0.010% (100 ppm), in particular 0.015% (150 ppm) or more. In the absence of nitride formers, the majority of the introduced nitrogen is present in unbound form, which also applies to the second embodiment of the method, in which nitrogen is already added to the molten steel.

[0056] The two embodiments of the method according to the invention can also be combined, i.e. the steel of the cold-rolled steel sheet can already have an initial nitrogen content (NO) of preferably more than 0.001% by weight, particularly preferably 0.005% to 0.016% by weight, which initial nitrogen content of the steel sheet is further increased in the nitriding process by taking up nitrogen from the nitriding gas atmosphere in the annealing furnace to a nitrogen content in the range of at least 0.005%, in particular 0.007% to 0.07%, averaged over the thickness of the steel sheet, in particular by weight.

[0057] Nitrogen introduced into cold-rolled steel sheets during the nitriding process or already present in the steel of the cold-rolled steel sheets can exist in dissolved and / or bound forms (up to the solubility limit) as nitrides. If strong nitride formers are present in the steel, the introduced nitrogen will exist at least partially as bound nitrogen in nitrides, particularly TiN and / or NbN and / or AlN. If nitrogen exists in dissolved form, interstitially embedded in the steel lattice and not bound to nitrides, a more uniform distribution of nitrogen throughout the thickness of the steel sheet can be achieved during the homogenization temperature treatment. Furthermore, the efficiency of denitrification is higher when nitrogen exists as free nitrogen, since unbound nitrogen atoms embedded interstitially in the steel lattice can diffuse from the steel sheet into the gas atmosphere of the annealing furnace during denitrification compared to nitrogen atoms bound in nitrogen compounds (as nitrides) in the steel. Therefore, the steel of the cold-rolled steel sheet preferably contains less than 100 ppm by weight, particularly preferably less than 50 ppm titanium and / or less than 100 ppm niobium and / or less than 500 ppm, particularly preferably less than 300 ppm aluminum. Particularly preferably, the total weight proportion of the strong nitride formers Ti, Nb, and Al is less than 700 ppm, particularly preferably less than 500 ppm. This ensures that at least the majority of the nitrogen incorporated into the near-surface region of the cold-rolled steel sheet during the nitriding process or the nitrogen already present in the steel of the cold-rolled steel sheet is present in dissolved form, particularly interstitially incorporated in the steel, with only the remaining nitrogen being bound as nitrides, particularly AlN and / or TiN and / or NbN.

[0058] The method according to the invention can be used to produce cold-rolled steel sheets having a multi-layer structure of crystallized structures, in particular a three-layer structure. The invention therefore also relates to a steel sheet, particularly for packaging, having a predetermined thickness, preferably less than 0.5 mm, a carbon content by weight of 10 to 1000 ppm, and a nitrogen content averaged over the thickness of the steel sheet of more than 50 ppm, in which the steel sheet has a multi-layer crystallized structure having a core region and a near-surface region surrounding the core region, in particular on both sides, in which the near-surface region is at least substantially recrystallized, and the core region is not recrystallized, or at least not completely recrystallized.

[0059] Therefore, the characteristics of the steel sheet produced by the method according to the invention are also relevant for the production method according to the invention, and vice versa.

[0060] The steel sheet according to the invention preferably has the following composition by weight: C: more than 0.001% and less than 0.1%, preferably less than 0.06%; Mn: more than 0.01% and less than 0.6%; P: less than 0.04%; S: less than 0.04%, preferably more than 0.001%; Al: less than 0.08%, preferably less than 0.03%, particularly preferably less than 0.01%; Si: less than 0.1%; Any Cu: less than 0.1%; Any Cr: less than 0.1%; Any Ni: less than 0.1%; Optional Ti: less than 0.1%, preferably less than 0.02%; Optional Nb: less than 0.08%, preferably less than 0.01%; Any Mo: less than 0.08%; Any Sn: less than 0.05%; Optional B: less than 0.01%, preferably less than 0.005%; and a nitrogen content averaged over the thickness of the steel sheet of at least 0.005%, preferably more than 0.010%, particularly preferably more than 0.015%; as well as residual iron and unavoidable impurities.

[0061] The near-surface region of the steel sheet according to the present invention preferably has a thickness in the range of 5 μm to 200 μm, particularly preferably in the range of 10 μm to 100 μm, and the core region preferably has a thickness in the range of 50 μm to 450 μm, particularly preferably in the range of 90 μm to 400 μm, and in particular in the range of 150 μm to 300 μm.

[0062] The core region has a higher hardness and / or a higher tensile strength than the region closer to the surface, so that the ratio of the hardness of the core region to the hardness of the region closer to the surface is preferably greater than 1.2, particularly preferably greater than 1.4. Preferably, the core region of the steel sheet has a Vickers hardness of at least 160 HV0.025, particularly preferably at least 220 HV0.025.

[0063] The degree of crystallinity of the core region is preferably less than 30%, and more preferably less than 20%, while the degree of crystallinity of the near-surface region is preferably greater than 70%, and more preferably greater than 80%. In an ideal crystallized structure, which is a particularly preferred embodiment of the present invention, the core region is not crystallized and the near-surface region is 100% crystallized.

[0064] Due to the nitriding and roll hardening properties of the core region, the steel sheet preferably has a high tensile strength of more than 500 MPa, especially 550 MPa to 700 MPa. At the same time, the region near the surface is softer and more fluid than the core region due to denitridation and recrystallization. This results in good overall formability of the steel sheet, especially a high elongation to break of preferably at least 4%, especially 5% to 10%. These and other advantages of the steel sheet and manufacturing method according to the invention are illustrated in the embodiments described in more detail below with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0065] [Figure 1]1 is a schematic diagram of the steps of an embodiment of the method according to the invention carried out in a continuous annealing furnace in the form of a temperature-time diagram. FIG. [Figure 2] 1 is a microscopic cross-sectional image of a sample of steel sheet according to the present invention having a three-layer structure of crystallized microstructure, the crystallized microstructure including an amorphous core region and near-surface regions surrounding it on both sides that are at least substantially fully recrystallized. [Figure 3] 2 is a schematic illustration of the course of the recrystallization temperature of the steel sheet according to the invention across the cross section (position x). FIG. [Figure 4] 1 is a representation of stress-strain diagrams for steel sheets according to the invention and comparative examples. [Figure 5] FIG. 2 is an illustration of the course of microhardness across a cross section of a steel sheet according to the invention and a comparative example. [Figure 6] 10A-10C are microscopic cross-sectional images of two further samples of steel sheet according to the present invention having a three-layer structure of crystallized microstructure comprising an amorphous core region and near-surface regions surrounding it on both sides that are at least substantially fully recrystallized. [Figure 7] FIG. 2 is a comparative diagram of the microhardness profile across the cross section of various samples of steel sheet according to the invention.

[0066] The starting product for the production of steel sheets according to the present invention using the method according to the present invention is a hot-rolled, subsequently cold-rolled steel sheet with a carbon content of 10 to 1000 ppm by weight. The thickness of the cold-rolled steel sheet is preferably 0.5 mm or less, particularly for applications in the packaging sector. For other applications, such as the production of bodywork parts for automobiles, greater thicknesses in the thin sheet range can also be selected. For applications in the packaging sector, the alloy composition of the steel advantageously meets the limits specified by the standards for packaging steels (e.g., as defined in ASTM A623-11 "Specifications for Tin Mill Products" or European Standard EN 10202), with the exception of the nitrogen content, which may deviate from the specifications for steel sheets according to the present invention, especially when highly nitrided steel sheets with very high strengths exceeding 750 MPa are produced. The components of the steel from which the steel sheets according to the present invention can be produced are described in detail below.

[0067] Steel composition: Carbon, C: more than 0.001% but less than 0.1%, preferably less than 0.06%; Carbon increases hardness and strength. Therefore, the steel preferably contains more than 0.001% by weight of carbon. In order to ensure the rollability of the steel sheet during the first cold rolling and possibly the second cold rolling step (skin pass) and not to reduce the elongation at break, the carbon content should not exceed 0.1% by weight.

[0068] · Manganese, Mn: more than 0.01% and less than 0.6%; Manganese also increases hardness and strength. Manganese also improves the forgeability, weldability, and wear resistance of steel. Furthermore, the addition of manganese reduces the tendency for red cracking during hot rolling, and manganese leads to grain refinement. Therefore, a manganese content of at least 0.01% by weight is preferred. To achieve high strength, a manganese content of more than 0.1% by weight, especially 0.20% by weight or more, is preferred. However, if the manganese content is too high, the corrosion resistance of the steel will be impaired. Furthermore, if the manganese content is too high, the strength will be too high and the steel will not be able to be cold-rolled. Therefore, the preferred upper limit of the manganese content is 0.6% by weight. Phosphorus, P: Less than 0.04%

[0069] Phosphorus is an undesirable accompanying element in steel. High phosphorus contents impair the formability of the steel sheet, in particular by embrittlement of the steel, so the upper limit for phosphorus content is 0.04% by weight. Sulfur, S: less than 0.04%, preferably more than 0.001%

[0070] Sulfur is an undesirable by-product that impairs ductility and corrosion resistance. Therefore, steel should not contain more than 0.04% by weight of sulfur. On the other hand, desulfurization of steel requires complex and cost-intensive measures, so sulfur contents below 0.001% by weight are no longer justified from an economic point of view. Therefore, the sulfur content is preferably in the range of 0.001% to 0.04% by weight, particularly preferably 0.005% to 0.01% by weight. Aluminum, Al: Less than 0.08%

[0071] Aluminum acts as a deoxidizer during the casting process in steel production, calming the steel. Aluminum also improves scale resistance and formability. For this reason, aluminum is preferably used in concentrations of 0.005% by weight or more. On the other hand, aluminum concentrations above 0.08% by weight can lead to surface defects in the form of aluminum clusters, which is why it is preferable not to exceed this upper limit of the aluminum content. Furthermore, aluminum forms nitrides with nitrogen, which is disadvantageous in the process according to the invention, especially during homogenization and denitrification, which is why the aluminum content is particularly preferably less than 0.005%.

[0072] ·Silicon, Si: less than 0.1%; Silicon increases the resistance of steel to scaling and is a solid-solution hardener. In steelmaking, it has the positive effect of making the melt more fluid and acts as a deoxidizer. Another positive effect of silicon on steel is that it increases tensile strength, yield strength, and scaling resistance. Therefore, a silicon content of 0.003% by weight or more is preferred. However, if the silicon content is too high, especially above 0.1% by weight, the corrosion resistance of the steel may be impaired and surface treatment, especially by electrolytic coating, may become more difficult.

[0073] optionally nitrogen, N0: more than 0.001% (or 10 ppm) and less than 0.02% (or 200 ppm), in particular less than 0.016% (or 160 ppm); Nitrogen is an optional component in the molten steel from which the steel for the steel sheet according to the present invention is produced when the method according to the present invention is carried out in the first embodiment, in which the steel sheet is nitrided in an annealing furnace. In this embodiment, the cold-rolled steel sheet used as the starting material for the method according to the present invention may optionally already contain an initial (small) amount of nitrogen (NO). Preferably, the molten steel or cold-rolled steel sheet contains an initial nitrogen content (NO) of less than 0.016%. This ensures that the hot-rolled strip produced from the molten steel can be cold-rolled using conventional rolling equipment and reduces defects in the hot-rolled strip that may occur with higher nitrogen concentrations. In order to form a strong solid-solution strengthening, especially when achieving very high strengths, it is preferred that the molten steel already contains an initial nitrogen content of more than 0.001% by weight, particularly preferably 0.010% by weight or more, even when the method according to the present invention is carried out in the first embodiment.

[0074] When carrying out the method according to the present invention in the second embodiment, if the initial nitrogen content (NO) in the molten steel or in the cold-rolled steel sheet exceeds 0.007% by weight (or 70 ppm), no further nitrogen is added into the annealing furnace in this process control, as the nitrogen content is preferably reduced by denitrification. After the method is carried out, in order to ensure a total nitrogen content of the steel sheet in the second embodiment of the method according to the present invention of more than 0.005% (or 50 ppm) and thus achieve sufficiently strong solid solution strengthening, the initial nitrogen content (NO) in the second embodiment of the process is preferably in the range of 0.007% to 0.016% (or 70 ppm to 160 ppm).

[0075] Optional: nitride formers, especially niobium, titanium, molybdenum, zirconium, vanadium: Nitride-forming elements such as aluminum, titanium, niobium, zirconium, or vanadium are disadvantageous in the steel of the steel sheet according to the present invention because the nitrogen that may have originally been contained in the molten steel and / or that was subsequently introduced by the nitriding process in the annealing furnace is at least partially bound in the form of nitrides in the presence of nitride-forming agents, which is disadvantageous for subsequent homogenization and denitrification. Therefore, the weight proportion of the nitride-forming agents titanium, niobium, and molybdenum is preferably limited to a maximum of 100 ppm, and other nitride-forming agents such as zirconium or vanadium are present at most as unavoidable impurities. On the other hand, nitride-forming agents such as aluminum, titanium, and / or niobium can improve the forming behavior of the steel sheet, enable the production of almost aging-free IF (interstitial-free) steel sheets, and achieve grain refinement. For this reason, the steel optionally, but preferably, contains: Titanium, Ti: preferably more than 0.002% and less than 0.01%, and / or Niobium, Nb: preferably more than 0.001% and less than 0.01%, and / or Aluminum, Al: preferably greater than 0.005% by weight but less than 0.05% by weight and / or molybdenum, Mo: less than 0.08%, preferably less than 0.01%.

[0076] Further optional components: In addition to residual iron (Fe) and unavoidable impurities, steel contains: ·Optional copper, Cu: less than 0.1%; ·Optional chromium, Cr: less than 0.1%; · Optional nickel, Ni: less than 0.1%; Optional tin, Sn: Less than 0.05% optionally boron, B: less than 0.01%, preferably less than 0.005%; and any other advantageous properties that can be achieved with these additional constituents.

[0077] Steel plate manufacturing method: The molten steel is produced with the stated composition of the steel, and in preferred embodiments, particularly in the second embodiment of the method according to the invention, the steel can already have an initial nitrogen content NO to achieve a high (average) nitrogen content in the steel sheet by adding nitrogen to the molten steel, for example by blowing in nitrogen gas and / or by adding solid nitrogen compounds such as calcium cyanamide or manganese nitride. In order to prevent the strength of the steel sheet produced from the molten steel from becoming too high due to nitrogen solid solution solidification, to maintain the hot formability of the steel and to avoid defects caused by nitrides in the slabs produced from the molten steel, it is advantageous for the initial nitrogen content (NO) of the steel (i.e. the proportion by weight of nitrogen in the molten steel) to be preferably less than or equal to 0.016% by weight.

[0078] First, a slab is cast from molten steel, then hot-rolled and cooled to room temperature. The hot strip thus produced has a thickness in the range of 1 to 4 mm and can be wound into a coil at a predetermined winding temperature (coiling temperature) in the range of 500 to 750°C, preferably in the range of 650 to 750°C. To produce thin steel sheets with a preferred thickness of less than 0.5 mm for packaging applications, for example, the hot-rolled strip is cold-rolled, thereby achieving a thickness reduction in the range of 50 to over 90%. Such cold-rolled steel sheets with the preferred composition described above form the starting material for carrying out the method according to the present invention. The steel sheet has an original recrystallization temperature T , which is predetermined by its composition and is typically less than 720°C, particularly 550 to 700°C. R 0 It has.

[0079] In the following, a preferred embodiment of the method according to the invention according to a first embodiment will be explained in more detail using the temperature-time diagram of FIG. The starting material selected for this example is a cold-rolled steel sheet having the composition of the molten steel according to Table 1 (melt analysis), whereby the cold-rolled steel sheet has a carbon content (C) of 29 ppm by weight and an initial nitrogen content (N0) of 19 ppm. The steel sheet is in the form of a strip and passes through a continuous annealing furnace at a predetermined speed, preferably above 100 m / min, and the steel sheet passes through the temperature-time curve shown in Figure 1.

[0080] In the example shown in FIG. 1, the cold-rolled steel sheet is first heated in the first chamber K1 of the continuous annealing furnace for a short heating time t of approximately 30 seconds. A Nitriding temperature T is approximately 500°C A and held at this temperature for approximately 280 seconds, t H The nitriding temperature T A is the original recrystallization temperature T of the cold-rolled steel sheet. R 0 The first chamber K1 of the continuous annealing furnace contains a nitrogenated gas atmosphere consisting of a mixture of ammonia gas (NH3) and an inert gas, in particular HNx, with a volume concentration of, for example, 5% ammonia gas. The heating time (t A ) and retention time (t H During the nitriding process A, nitrogen is precipitated at least in the seam region of the steel sheet due to the nitrogen gas atmosphere. In the nitriding process A, the nitriding ammonia gas is preferably further directed onto the two surfaces of the steel sheet by spray nozzles arranged adjacent to each other transversely to the strip movement direction (the direction in which the steel sheet passes through the annealing furnace).

[0081] Then, the nitriding temperature (T A The steel sheet in the first chamber K1 is fed to a second chamber K2 of the continuous annealing furnace, which is separate from the first chamber K1. The second chamber K2 of the continuous annealing furnace contains an inert gas atmosphere, for example, 100% by volume of HNx. In the second chamber K2, the steel sheet is homogenized for a certain time (homogenization time t Ho ) homogenization temperature T Ho In the example shown in Figure 1, the nitriding temperature T A However, in the illustrated example, the nitriding temperature T AThe temperature of the steel plate in the second chamber K2 (homogenization temperature T Ho ) is also determined by the nitriding temperature (T A ) can deviate from, and in particular exceed, the homogenization time t . In the example shown in FIG. 1, the steel sheet is subjected to a homogenization time t of approximately 280 seconds in the second chamber K2 of the continuous annealing furnace. Ho During this time, the homogenization temperature T Ho The nitrogen introduced during nitriding in the first chamber is distributed uniformly over the cross section of the steel sheet (homogenization step H). The temperature of the steel sheet in the second chamber K2 (homogenization temperature T Ho ) is the original recrystallization temperature T R 0 Nitriding temperature T A This fact ensures that no recrystallization of the steel sheet structure occurs during the homogenization step H. At the end of the second chamber K2, the steel plate is heated to a homogenization temperature T Ho Temperature T G is heated to

[0082] The steel sheet then passes through a third chamber K3 of the continuous annealing furnace, which is separate from the second chamber K2. A gas atmosphere containing hydrogen gas is present in the third chamber K3 of the continuous annealing furnace. The gas atmosphere in the third chamber K3 can consist of 100% hydrogen gas. However, for explosion safety reasons, the hydrogen content in the third chamber K3 is lower, for example 10% by volume, with the remainder of the gas atmosphere being an inert gas such as nitrogen gas. The hydrogen content in the gas atmosphere in the third chamber K3 extracts nitrogen from the steel sheet in the near-surface region, forming a nitrogen profile across the cross section of the steel sheet, in which the nitrogen content decreases from the inner core region outward to the near-surface region. As a result, as described above, different recrystallization temperatures are formed in the core region and the near-surface region of the steel sheet, with the first recrystallization temperature (T R 1 ) is formed in the core region, and the second recrystallization temperature (T R 2 ) is formed. The steel sheet is heated to a temperature TG and introduced into the third chamber K3 at an annealing temperature T G The annealing time t G ) in the third chamber K3 at this temperature. G is the first recrystallization temperature (T R 1 ) and the second recrystallization temperature (T R 2 After passing through the third chamber K3, the steel sheet is guided into an adjacent cooling device K4, which allows in particular a multi-stage cooling of the steel sheet, and the annealing temperature T G Preferably, the steel sheet is cooled stepwise from 1000 K / s to room temperature. The cooling device K4 may still be partly inside the continuous annealing furnace or may be completely outside the continuous annealing furnace. The cooling can, for example, first be carried out in a cooling zone downstream of the continuous annealing furnace, where slow cooling is first carried out, for example by gas cooling at a cooling rate of 3 to 20 K / s, and then continued outside the continuous annealing furnace in a device for quenching the steel sheet to room temperature at a higher cooling rate, for example by water cooling at a cooling rate of more than 1000 K / s. The (initially slow) cooling of the steel sheet in the cooling zone of the continuous annealing furnace is carried out within the total annealing time t G This is defined as the annealing time t G During this time, the steel sheet reaches the first recrystallization temperature (T R 1 ) is determined to be at a temperature above

[0083] Thus, in the embodiment shown in FIG. 1 , a cold-rolled steel sheet is heat treated in a continuous annealing furnace, whereby the steel sheet is first nitrided to a higher nitrogen content during heat treatment in a nitriding process A, and then the nitrogen introduced into the steel sheet is uniformly distributed throughout the thickness of the steel sheet in a homogenization step H. Nitrogen is then removed from the region near the outer surface of the steel sheet in a denitriding process E, and the steel sheet is finally annealed in an annealing process G at a predetermined annealing temperature T(G). In the example shown in FIG. 1 , the denitriding process E and the annealing process G are carried out simultaneously in a joint denitriding and annealing process E / G in a third chamber K3 of the continuous annealing furnace for better efficiency and faster process control.

[0084] The hydrogen-containing gas atmosphere in the third chamber K3 of the continuous annealing furnace removes nitrogen from the near-surface region 1 of the steel sheet, so that the nitrogen content in the near-surface region 1 decreases, while the nitrogen content in the core region 2 of the steel sheet remains almost unchanged. As a result, a nitrogen gradient is formed across the cross section of the steel sheet, with the nitrogen content decreasing from the inner core region 2 towards the near-surface region 1. Due to this cross-sectional profile of the nitrogen content and the dependence of the recrystallization temperature of the steel on the nitrogen content, a corresponding profile of the recrystallization temperature of the steel is formed across the thickness of the steel sheet, resulting in the (averaged) first recrystallization temperature T R 1 and the (averaged) second recrystallization temperature T of the core region 2 R 2 whereby the first recrystallization temperature (T R 1 ) is the second recrystallization temperature T of the core region 2 due to the lower nitrogen content. R 2 Lower than.

[0085] The progression of the recrystallization temperature across the cross section (position x) of the steel plate is shown diagrammatically in Figure 3. The recrystallization temperature in the core region 2 of the steel plate increases as a result of the nitriding of the steel plate and the subsequent homogenization of the nitrogen incorporated in the process, compared to the original recrystallization temperature T R 0 to the second recrystallization temperature T R 2 The value of ΔT2 increased to T R 2 =T R 0 +ΔT2. In contrast, the recrystallization temperature E of the near-surface region 1 during denitriding was reduced by the value of ΔT1 (T R Ho , Fig. 3). The recrystallization temperature of the near-surface region 1 is the original recrystallization temperature T R 0 The second recrystallization temperature T R 2the first recrystallization temperature T after denitrification is lower than R 1 =T R Ho -ΔT1.

[0086] The incorporation of nitrogen during nitriding A and homogenization H causes the recrystallization temperature of the core region to rise above the original recrystallization temperature T R 0 The value of the increase ΔT2 compared to ΔT2 depends on the additional nitrogen content ΔN introduced into the core area of ​​the steel sheet after homogenization is completed, whereby a linear relationship can be observed, determined by: ΔT=a·ΔN(ppm)

[0087] where a is the proportionality constant and ΔN (ppm) is the nitrogen content (ppm) (based on the weight of the steel) introduced into the core region during nitriding and homogenization. Tests on specimens with different nitrogen contents and otherwise identical alloy compositions have empirically determined a value of approximately 1.2 K / ppm. Therefore, the recrystallization temperature (T) in the steel sheet is uniform across the cross section after homogenization. R Ho The reduction in the recrystallization temperature of the near-surface region by a value ΔT2 compared to the first recrystallization temperature T (Fig. 3) is linearly dependent on the amount of nitrogen removed from the near-surface region during denitriding E. The amount of nitrogen introduced in the nitriding process and the amount of nitrogen removed from the near-surface region during denitriding then depend on the process parameters during nitriding (in particular the volume fraction of the nitrogen donor (especially ammonia) in the annealing furnace and the holding time of the nitriding process) and the process parameters during denitriding (in particular the denitriding duration), and can be appropriately set and calculated using these process parameters based on empirical values. Therefore, the first recrystallization temperature T R 1 ) and the core region (second recrystallization temperature T R 2 The recrystallization temperature of ) can be calculated from the values ​​of ΔT1 and ΔT2 for the change in recrystallization temperature of the near-surface region and the core region based on the empirical correlation between these values ​​and the process parameters of the nitriding process A and the denitriding process E.

[0088] For the recrystallization annealing G of the steel sheet (only in the near-surface region 1), the steel sheet is heated in a third chamber K3 (annealing chamber) to a first recrystallization temperature T R 1 Higher second recrystallization temperature T R 2 Annealing temperature T G In the example shown in Figure 1, the annealing temperature T G is approximately 600°C. In order to achieve the recrystallization of the near-surface region 1 with the highest possible crystallinity, the steel sheet in the annealing chamber is annealed for a sufficiently long time t G During this time, the annealing temperature T G or the first recrystallization temperature T R 1 In the example shown in Figure 1, the annealing time t G is approximately 200 seconds.

[0089] In the example shown in Figure 1, denitrification E and annealing G of the steel sheet are carried out simultaneously in a combined denitrification and annealing process E / G in the third chamber K3 (annealing chamber) of the continuous annealing furnace, ensuring efficient and economical process control. Alternatively, denitrification E and annealing G can be decoupled by dividing the third chamber K3 of the continuous annealing furnace into a denitrification chamber and an annealing chamber, with the denitrification chamber in the continuous annealing furnace upstream of the annealing chamber, and performing denitrification E in a separate denitrification chamber. Separating the denitrification process E and annealing process G allows for more targeted setting of the preferred parameters for denitrification and annealing (temperature and duration of the gas atmosphere in each process and in each chamber of the continuous annealing furnace). The composition of the gas atmosphere in the denitrification chamber and the annealing chamber suitably differs in that hydrogen gas is contained at (or above) the minimum concentration preferred for denitrification treatment in the denitrification chamber, while annealing in the annealing chamber can be carried out under an inert gas atmosphere containing almost no hydrogen.

[0090] First recrystallization temperature T R 1 and the second recrystallization temperature T R 2The annealing temperature T according to the present invention is between G By setting the recrystallization temperature (T R 1 ), whereas the inner core region 2, where little or no nitrogen has been removed during denitrification, is not recrystallized or is barely recrystallized. G ) is the recrystallization temperature of the near-surface region 1 (T R 1 ) and T of the inner core region 2 R 2 =T R 0 +ΔT2. Thus, a three-layer microstructure is formed on the cross section of the steel sheet in the form of a "sandwich" with a near-surface region 1 that is preferably nearly completely recrystallized and an inner core region 2, whereby the core region 2 is not recrystallized, or at least not completely recrystallized (this is why this three-layer microstructure is also called a "sandwich structure" and is illustrated in FIG. 3 by the regions marked with reference numerals 1 and 2).

[0091] Figure 2 shows a microscopic cross-section of an example of such a three-layered steel sheet. Figure 2 clearly shows the separation of the two outer near-surface regions 1 from the inner core region 2 due to the different crystallization structures of these regions 1 and 2. The thicknesses of the recrystallized near-surface regions 1, which symmetrically surround the inner core region on both sides, are 32 μm and 33 μm, respectively.

[0092] example Examples of embodiments of steel sheets and methods according to the invention are described below and compared with comparative examples not according to the invention.

[0093] Steel sheets with a thickness of 0.22±0.01 mm were produced by hot rolling and subsequent cold rolling from molten steel having the alloy composition (melt analysis) listed in Table 1 (the ppm values ​​refer to the weight percentage of the alloying elements in the steel from which the cold-rolled steel sheets were produced). The cold-rolled steel sheets were successively heat treated (nitrided) in a laboratory furnace for carrying out the nitriding process in an ammonia-containing protective gas atmosphere, followed by homogenization in an argon atmosphere and final denitriding and annealing (carried out in parallel in a hydrogen atmosphere) with different process parameters regarding temperature and holding time during nitriding A, homogenization H and denitriding / annealing E / G. The obtained examples of steel sheets according to the invention are designated "Example A", "Example B" and "Example C" in Table 4. The atmosphere in the continuous annealing furnace consisted of ammonia gas with the volume concentration (NH3 content) listed in Table 4, with the remainder being HNx protective gas. The volume fraction of ammonia in the furnace gas atmosphere was determined at room temperature and maintained constant by flowing ammonia during the heat treatment of the steel sheet. When conducting experiments on an industrial scale in a continuous annealing furnace, the ammonia concentration required for the nitriding process will likely shift to higher values ​​because, at high temperatures in a continuous annealing furnace, only a portion of the total ammonia atmosphere is effectively available for nitriding the steel sheet due to the dissociation and recombination of ammonia into atomic and molecular nitrogen. In the experiments, the steel sheet was denitrified and annealed in a 100% hydrogen gas atmosphere at the temperature (annealing temperature) listed in Table 4.

[0094] The microstructures of the steel sheets A, B, and C thus treated were observed under a microscope (cold-embedded, ground, polished, and etched with Nital (3% nitric acid)). After cooling, the furnace-treated steel sheets were subjected to a second cold rolling step (skin pass) with a reduction of 1.5%.

[0095] Figure 2 and Figures 6(a) and 6(b) show examples of the microstructures of the treated steel sheets of Examples A, B, and C in Table 4. In all examples (A, B, and C) according to the present invention, a three-layer microstructure of crystallized structures with different thicknesses of the crystallized near-surface region 1 and the non-crystallized core region 2 is observed.

[0096] For comparison, a sample of steel sheet having the composition of Table 1 was heat treated (annealed) at an annealing temperature higher than the recrystallization temperature of the steel (so that the sample was not nitrided in the annealing furnace). Therefore, this example, labeled "Recrystallized (Comparative Example)" in Table 2, represents a comparative example of a steel sheet that was fully recrystallized throughout its cross section. As a further comparative example, a sample of cold-rolled steel sheet having the same composition according to Table 1 was cold rolled and was neither nitrided nor annealed after cold rolling. Therefore, this sample remained in the as-rolled state after cold rolling (Comparative Example "as-rolled" in Table 2).

[0097] Using the examples in Table 2 (Example A according to the invention and the comparative examples "Roll-hard" and "Recrystallized"), hardness measurements (Vickers hardness HV) were taken across the cross section of the sample. 0.025 ) were performed. The results of these hardness measurements are shown in Figure 5. From Figure 5 it can be seen that the comparative examples "Roll Hard" and "Recrystallized" show a uniform progression of microhardness across the cross section (or thickness of the sample), whereas Example A according to the invention shows a pronounced gradual progression of microhardness with a maximum value in the middle of the thickness (core region) and a minimum value at the outer edge (region close to the surface).

[0098] Similarly, hardness measurements were also taken on other Examples B and C according to the invention from Table 4 and compared with the hardness curve of Example A. This is shown graphically in Figure 7, from which it can be seen that all three Examples A, B and C according to the invention show a significant gradual progression of microhardness across the cross section with a hard core region and a softer outer seam region, whereby the Example C specimen has the greatest hardness and the Example B specimen has the least microhardness. This is because the Sample C exhibits a significant increase in hardness over the annealing time (holding time during annealing and denitriding t G ) is shorter, the recrystallized seam (near-surface region 1) is thinner, and therefore the annealing time or the holding time during annealing and denitriding (holding time t G ) is longer, the seam area is thicker than the other two specimens, and the annealing temperature T G This is due to the fact that sample B has a higher hardness than sample B, and is therefore the softest.

[0099] In tensile tests (according to EN 10202) of samples from Examples A and B in Table 4 and the comparative examples "Roll Hard" and "Recrystallized" in Table 2, the tensile strength (Rm), 0.2% yield point (Rp0.2), and elongation at break (A) were determined. Figure 4 shows examples of stress-strain diagrams from tensile tests of Example A according to the invention and the comparative examples "Roll Hard" and "Recrystallized." Figure 4 shows the different progression of the stress-strain curves of the compared examples, with Example A according to the invention being characterized by a high tensile strength of more than 600 MPa and simultaneously a high elongation at break of more than 8%. The material parameters of Example A according to the invention and the two comparative examples determined from the tensile tests are listed in Table 3.

[0100] The method according to the invention can therefore be used to produce steel sheets (containing nitrogen or nitrided and denitrified in the near-surface region) that have a three-layer crystallized structure with a hard core region and a soft outer seam region (near-surface region) and are characterized by very high strengths of more than 500 MPa, in particular more than 600 MPa, and at the same time good elongation at break of more than 5%, in particular more than 8%. Such steel sheets are excellent for use in forming processes for the production of stable packaging such as cans and beverage cans, and their parts such as (tear-off) lids.

[0101] The exact composition of the three-layer microstructure, particularly the thickness of the outer seam region, as well as the nitrogen content averaged across the cross section and the difference between the nitrogen content of the outer seam region and the core region, or the gradient of the nitrogen content across the thickness of the steel sheet, can be influenced by varying the process parameters in the nitriding and denitriding method steps of the method according to the invention. Thus, the properties of steel sheets produced using the method according to the invention can be tailored to different applications, particularly with regard to mechanical properties such as tensile strength, elongation at break, and isotropy (average r-value), as well as their surface properties. In particular, the formability of the steel sheet can be optimized while maintaining the same or a sufficiently high strength.

[0102] [Table 1] Table 2 Table 3 Table 4

Claims

1. 1. A method for producing a steel sheet having a multilayer crystallized structure, said method comprising: A carbon weight content (C) of 10 to 1000 ppm, and a predetermined original recrystallization temperature (T Ｒ ０ ) to provide a steel plate cold rolled to a predetermined thickness, The steel has an initial nitrogen content (N ０ ) and / or the cold rolled steel sheet has the original recrystallization temperature (T Ｒ ０ ) lower than the maximum nitriding temperature (T Ａ Nitrogen is incorporated into the cold rolled steel sheet at least in the near-surface region (1) during the nitriding process by exposing it to a nitriding gas atmosphere at a temperature of 1000 K. The nitrogen precipitated in the near-surface region (1) during the nitriding process is preferably at the original recrystallization temperature (T Ｒ ０ ) below the homogenization temperature (T Ｈｏ ) homogenizing the nitrogen precipitated during the nitriding process through the thickness of the steel sheet by a temperature treatment to homogenize the nitrogen; - Predetermined denitrification time (t ＥＳ ), a step of denitrifying at least the near-surface region (1) of the steel sheet by introducing the steel sheet into a hydrogen-containing gas atmosphere during which the near-surface region (1) reaches a first recrystallization temperature (T Ｒ １ ), and the core region (2) of the steel plate has a second recrystallization temperature (T Ｒ ２ ), and the first recrystallization temperature (T Ｒ １ ) is the second recrystallization temperature (T Ｒ ２ forming a nitrogen gradient during said denitrification across the thickness of said steel plate, wherein the nitrogen concentration decreases outward from an inner core region (2) to said near-surface region (1) such that the nitrogen concentration is lower than the nitrogen concentration in said inner core region (2) The first recrystallization temperature (T Ｒ １ ) and the second recrystallization temperature (T Ｒ ２ ) between the annealing temperature (T Ｇ annealing the steel sheet at 2000 K, wherein the annealing is performed during and / or after the denitrification; A method comprising:

2. 2. The method of claim 1, wherein the steel of the cold-rolled steel sheet has the following composition by weight: C: more than 0.001% and less than 0.1%, preferably less than 0.06%; Mn: more than 0.01% and less than 0.6%; P: less than 0.04%; S: less than 0.04%, preferably more than 0.001%; Al: less than 0.08%; Si: less than 0.1%; optional Cu: less than 0.1%; Any Cr: less than 0.1%; Optional Ni: less than 0.1%; Optional Ti: less than 0.1%; optional Nb: less than 0.08%; Any Mo: less than 0.08%; optional Sn: less than 0.05%; optional B: less than 0.01%, preferably less than 0.005%; Optionally, N ０ : More than 0.001%, particularly more than 0.007% and less than 0.02%, particularly less than 0.016%; - residual iron and unavoidable impurities, the weight average nitrogen content after heating said cold rolled steel sheet in the presence of a nitrogen donor is at least 0.005%, preferably at least 0.015%;

3. 3. The method according to claim 1 or 2, wherein the steel of the cold rolled steel sheet contains less than 100 ppm by weight, preferably less than 50 ppm, of titanium, and / or less than 100 ppm of niobium, and / or less than 700 ppm, preferably less than 500 ppm of aluminum.

4. The denitrification occurs at the original recrystallization temperature (T Ｒ ０ ) below the denitrification temperature (T ＥＳ ) or the denitrification is carried out before the annealing at the annealing temperature (T Ｇ 4. The method according to claim 1, wherein the annealing is carried out simultaneously with the annealing at 2000 K.

5. The first recrystallization temperature (T Ｒ １ ) is the original recrystallization temperature (T Ｒ ０ ) at least as high as said second recrystallization temperature (T Ｒ ２ ) is the original recrystallization temperature (T Ｒ ０ 5. The method according to claim 1, wherein the .lambda.

6. During the denitrification of the near-surface region (1), the proportion by weight of nitrogen in the near-surface region (1) decreases to a predetermined value (ΔN １ ), thereby reducing the recrystallization temperature of the near-surface region (1) by the first recrystallization temperature (T Ｒ １ ) to the value ΔT １ It decreases by ΔT １ In particular, the amount of nitrogen in the near-surface region (1) is １ 6. The method according to claim 1, wherein the temperature of the heating element increases linearly with the decrease of the temperature of the heating element.

7. During the temperature treatment for homogenizing the nitrogen incorporated in the near-surface region (1) during the nitriding process, nitrogen atoms diffuse through the thickness of the steel sheet to the core region (2) of the steel sheet, increasing the weight fraction of nitrogen therein to a predetermined value (ΔN ２ ), whereby in the core region (2) the recrystallization temperature is increased by a value ΔT ２ rises by ΔT ２ is the weight ratio of nitrogen in the core region (2) (ΔN ２ 7. The method according to claim 1, wherein the saturation temperature increases in a particularly linear manner with increasing saturation temperature.

8. The annealing temperature (T Ｇ 8. The method according to claim 1, wherein the temperature is less than 750°C, preferably between 580°C and 700°C.

9. The steel sheet is heated to a nitriding temperature (T Ａ ) and held for a predetermined time (t Ｈ ) during the nitriding temperature (T Ａ ) and the nitriding temperature (T Ａ ) is preferably above 300°C, preferably 300°C to 600°C, particularly 300°C to 550°C, and the holding time (t Ｈ 9. The method according to claim 1, wherein the time is preferably more than 1 second, in particular between 1 and 400 seconds, particularly preferably between 10 and 300 seconds.

10. 10. The method according to any one of claims 1 to 9, characterized in that in the nitriding process the steel sheet is exposed to an ammonia-containing gas atmosphere, the gas atmosphere preferably having a volume fraction of ammonia of 0.05% to 10%, particularly preferably 1% to 5%.

11. The nitriding process is carried out in a continuous annealing furnace, and a subsequent temperature treatment for homogenizing the nitrogen incorporated during the nitriding process is carried out in a separate top hat annealing furnace, and the homogenization temperature (T Ｈｏ ) is above 300°C, preferably 350°C to 650°C, and the steel sheet is subjected to a homogenization period (t Ｈｏ ) in a protective gas atmosphere, in particular an argon or HNx atmosphere, at the homogenization temperature (T Ｈｏ 11. The method according to claim 1, wherein the temperature is maintained at 1000.degree.

12. The nitriding process is carried out in a continuous annealing furnace, and the subsequent temperature treatment for homogenizing the nitrogen incorporated during the nitriding process is carried out in the continuous annealing furnace, and the temperature of the steel sheet in the continuous annealing furnace is a homogenization temperature (T Ｈｏ ) and the steel sheet is subjected to a homogenization period (t) of 60 to 600 seconds, preferably 180 to 300 seconds, in a protective gas atmosphere, in particular an argon atmosphere or an HNx atmosphere. Ｈｏ ) during the predetermined homogenization temperature (T Ｈｏ ) or the predetermined homogenization temperature (T Ｈｏ ) and the temperature of the steel plate is maintained at the homogenization period (t Ｈｏ 12. The method according to claim 1, wherein the temperature is increased during

13. During the denitrification, the steel sheet is heated for a denitrification time (t ＥＳ ), the gas atmosphere has a hydrogen content by volume of 1% to 100%, preferably more than 10%, in particular more than 15%, and the denitrification is preferably carried out at a temperature above 300°C and above the original recrystallization temperature (T Ｒ ０ ) and the denitrification temperature (T ＥＳ 13. The method according to claim 1, wherein the method is carried out in a step (b) of

14. 14. The method according to any one of claims 1 to 13, characterized in that the steel sheet is exposed to a hydrogen-containing gas atmosphere during the denitrification, the gas atmosphere having a volume fraction of hydrogen of 1% to 100%, preferably more than 10%, in particular more than 15%, and an ammonia concentration of less than 0.1% by volume.

15. The steel sheet is heated for a heating time (t Ａ ) during annealing at the annealing temperature (T Ｇ ) and annealed for a predetermined time (t Ｇ ) during the annealing temperature (T Ｇ ) and the annealing time (t Ｇ 15. The method according to claim 1, wherein the time for which the heating is performed is preferably more than 1 second, in particular between 1 second and 600 seconds, preferably between 100 seconds and 300 seconds.

16. The cold-rolled steel sheet is heated for the heating time (t Ｅ ) in the nitriding process from room temperature to the heating temperature (T Ｅ ) to the heating temperature (T Ｅ ) and then a predetermined holding time (t Ｈ ) over the heating temperature (T Ｅ ) and the heating time (t Ｅ ) is preferably in the range of 1.0 to 300 seconds, and / or the holding time (t Ｈ 16. The method according to any one of claims 1 to 15, characterized in that the time t1 is preferably in the range of 1.0 seconds to 300 seconds.

17. The steel of the cold rolled steel sheet preferably has an initial nitrogen content (N ０ ), and during the nitriding process, the average nitrogen content (N) is increased to the initial nitrogen content (N ０ 17. The method according to claim 1, wherein the concentration of Cr in the steel sheet is increased to a value averaged over the thickness of the steel sheet of 50 ppm to 1000 ppm, preferably 100 ppm to 700 ppm above the reference concentration of Cr in the steel sheet.

18. During the nitriding process across the cross section of the cold rolled steel sheet, a gradient of the nitrogen content (N(x)) is established, with the nitrogen content decreasing from the near-surface region (1) towards the core region (2), and the gradient of the nitrogen content (N(x)) is maintained at the homogenization temperature (T Ｈｏ ), whereby the steel sheet after the heat treatment is homogenized at the initial nitrogen content (N ０ 18. The method of claim 17, wherein the nitrogen content is at least substantially constant across the thickness greater than 1000 nm.

19. 19. The method according to any one of claims 1 to 18, wherein at least a large part of the nitrogen incorporated in the near-surface region (1) of the cold-rolled steel sheet during the nitriding process is present in dissolved form, in particular incorporated interstitially in the steel, and the remaining part of the nitrogen is bound as nitrides, in particular AlN and / or TiN and / or NbN.

20. 20. The method according to any one of claims 1 to 19, characterized in that during annealing of the cold rolled steel sheet, an at least partial recrystallization annealing of the cold rolled steel sheet is carried out in the near-surface region (1), while the core region (2) is not recrystallized.

21. The value ΔT that reduces the recrystallization temperature of the near-surface region (1) during denitrification １ is greater than 10°C, preferably greater than 20°C, and / or a value ΔT ２ 21. The method according to any one of claims 7 to 20, characterized in that the temperature is greater than 10°C, preferably greater than 30°C.

22. 22. A method according to any one of the preceding claims, characterized in that in the nitriding process, a nitriding gas, in particular ammonia, is directed onto the surface of the cold-rolled steel sheet by means of one or more spray nozzles.

23. 1. A steel sheet, particularly for packaging, having a predetermined thickness, preferably less than 0.5 mm, and having a carbon content of 10 to 1000 ppm by weight and a nitrogen content of more than 50 ppm averaged over the thickness of the steel sheet, characterized in that the steel sheet has a multilayer crystallized structure having a core region (2) and near-surface regions (1) surrounding the core region (2) on both sides, the near-surface region (1) being at least substantially recrystallized, and the core region (2) not being recrystallized, or at least not being completely recrystallized.

24. 24. The steel sheet according to claim 1 , wherein the steel sheet has the following composition by weight: C: more than 0.001% and less than 0.1%, preferably less than 0.06%; Mn: more than 0.01% and less than 0.6%; P: less than 0.04%; S: less than 0.04%, preferably more than 0.001%; Al: less than 0.08%, preferably less than 0.03%, particularly preferably less than 0.01%; Si: less than 0.1%; optional Cu: less than 0.1%; Any Cr: less than 0.1%; Optional Ni: less than 0.1%; optional Ti: less than 0.1%, preferably less than 0.02%; optional Nb: less than 0.08%, preferably less than 0.01%; Any Mo: less than 0.08%; optional Sn: less than 0.05%; optional B: less than 0.01%, preferably less than 0.005%; and a nitrogen content averaged over the thickness of the steel sheet of at least 0.005%, preferably more than 0.010%, particularly preferably more than 0.015%; - as well as residual iron and unavoidable impurities.

25. 25. Steel sheet according to claim 23 or 24, characterized in that the near-surface region (1) has a thickness in the range of 5 μm to 200 μm, preferably in the range of 10 μm to 100 μm, and / or the core region (2) has a thickness in the range of 50 μm to 450 μm, preferably in the range of 90 μm to 400 μm, in particular 150 μm to 300 μm.

26. The core region (2) has a higher hardness than the surface-near region (1), the ratio of the hardness of the core region (2) to the hardness of the surface-near region (1) being preferably greater than 1.2, particularly preferably greater than 1.4, and the core region (2) of the steel sheet preferably has a hardness of at least 160 HV. ０．０２５ , particularly preferably at least 220 HV ０．０２５ 26. Steel sheet according to any one of claims 23 to 25, characterized in that it has a Vickers hardness of

27. 27. Steel sheet according to any one of claims 23 to 26, characterized in that the core region (2) has a degree of recrystallization less than 30%, preferably less than 20%, and / or the near-surface region (1) has a degree of recrystallization greater than 70%, preferably greater than 80%.

28. 28. Steel sheet according to any one of claims 23 to 27, characterized in that the steel sheet has a tensile strength of more than 500 MPa, in particular between 550 MPa and 700 MPa, and / or an elongation at break of at least 4%, preferably between 5% and 10%.

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