Method of manufacturing a steel sheet having a multilayer crystallization structure and steel sheet with a multilayer crystallization structure

Abstract

A method for producing a steel sheet with a multilayer crystallisation structure, and a steel sheet, in particular for packaging, with a thickness of preferably less than 0.5 mm and a carbon content, relative to the weight, of 10 to 1000 ppm, and a nitrogen content, averaged over the thickness of the steel sheet, of more than 50 ppm, as well as a multilayer crystallisation structure with a core region and a near-surface region surrounding the core region on both sides. The near-surface region is at least substantially recrystallised and the core region is not or at least not entirely recrystallised.

Description

FIELD OF THE INVENTION

[0001] The invention relates to a method of manufacturing a steel sheet having a multilayer crystallization structure and to a steel sheet with a thickness of less than 0.5 mm which has a multilayer crystallization structure.BACKGROUND

[0002] DE 10 2020 112 485 B3 discloses a steel sheet with a multilayer crystallization structure and a process for its production, wherein the steel sheet is produced from a steel with a carbon content (C) of 10 to 1000 ppm by weight and has a thickness of less than 0.5 mm and a multilayer microstructure with at least a first layer and a second layer, wherein the first layer is at least substantially recrystallized and the second layer is not 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 a near-surface region surrounding the core region on both sides, wherein the near-surface region is not or at least not completely recrystallized. To produce this steel sheet with a three-layer microstructure, a cold-rolled steel sheet with a recrystallization temperature (TR) predetermined by the composition of the steel is heated to a predetermined heating temperature (TE) above the recrystallization temperature, the heating taking place at least until the recrystallization temperature (TR) is reached and at least temporarily in the presence of a nitrogen donor, whereby, during heating of the cold-rolled steel sheet, nitrogen diffuses from the nitrogen donor at least into a region of the cold-rolled steel sheet close to the surface and is incorporated in the region close to the surface, as a result of which the recrystallization temperature (TR) of the steel in the region close to the surface is raised by a value ΔT. The heating temperature (TE) is set so that it is lower than the recrystallization temperature (TR+ΔT) of the area near the surface, which is increased by ΔT, so that only the core area of the steel sheet is recrystallized, whereas the area near the surface, which surrounds the core area on both sides, is not or only partially recrystallized. As a result, a three-layer crystallization structure is developed in the steel sheet with a roll-hard, non-recrystallized seam area and the softer, recrystallized core area. The known steel sheet has a high strength with good elongation at break and can be used to manufacture packaging, such as cans for food or beverages, whereby roughening of the surface of the steel sheet can be avoided during forming of the steel sheet, e.g. in deep-drawing processes, due to the roll-hard seam area.

[0003] The steel sheets known from DE 10 2020 112 485 B3 have a multi-layer microstructure, in particular a three-layer crystallization structure, whereby the three-layer microstructure comprises an inner, at least essentially recrystallized core area and an area close to the surface surrounding the core area on both sides, which is not or at least not completely recrystallized. Although this three-layered microstructure achieves high strengths, the non-crystallized (roll-hard) area on the surface of the steel sheets means that formability suffers. Particularly in multi-axis forming and bending operations, the outer roll-hard area dominates, which is particularly noticeable in a higher resistance to bending operations with high bending radii, due to the hard outer fiber of the steel sheet, or a resulting earlier material failure.SUMMARY OF THE INVENTION

[0004] Based on this, the invention is based on the object of improving the formability of the steel sheets, in particular in multi-axis forming processes and in bending forming, without reducing the high strength.

[0005] A steel sheet and a method for manufacturing this steel sheet are disclosed herein.

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

[0007] Compared to the three-layer microstructure known from DE 10 2020 112 485 B3, the order of the areas with different crystallization structures is reversed. This results in improved formability for the steel sheets according to the invention, while maintaining the same tensile strength, because the outer area near the surface is softer and more flowable due to its at least essentially completely or at least predominantly crystallized microstructure. As a result, this microstructure is significantly more flexible and easier to form in forming processes. A high strength of the steel sheet according to the invention is ensured by the stable core area, which is not or at least not completely recrystallized and has at least a lower degree of crystallization than the outer area near the surface (seam area) and is therefore still at least partially roll-hard.

[0008] In the production of the steel sheet according to the invention, the multilayer crystallization structure is produced in the method according to the invention by the following steps:

[0009] Providing a steel sheet cold rolled to a predetermined thickness from a steel having a carbon content (C) by weight of 10 to 1000 ppm and a predetermined original recrystallization temperature (TR0),

[0010] wherein the steel has an initial nitrogen content (N0), based on the weight, of at least 70 ppm and / or nitrogen is incorporated into the cold-rolled steel sheet at least in a near-surface region (1) in a nitriding process by exposing the cold-rolled steel sheet to a nitriding gas atmosphere at a maximum nitriding temperature (TA) which is lower than the original recrystallization temperature (TR0) and the nitrogen deposited in the near-surface region (1) during the nitriding process is homogenized over the thickness of the steel sheet by a temperature treatment for homogenizing the nitrogen deposited during the nitriding process at a homogenization temperature (THo) which is preferably below the original recrystallization temperature (TR0), nitridingnitriding

[0011] Denitriding of at least the near-surface region (1) of the steel sheet by introducing the steel sheet into a hydrogen-containing gas atmosphere during a predetermined denitriding time (tES), whereby a nitrogen gradient is formed during the denitriding over the thickness of the steel sheet with a nitrogen concentration decreasing from an inner core region (2) outwards towards the near-surface region (1), so that the near-surface region (1) has a first recrystallization temperature (TR1) and the core region (2) of the steel sheet has a second recrystallization temperature (TR2) and the first recrystallization temperature (TR1) is lower than the second recrystallization temperature (TR2) due to the denitriding of the near-surface region (1),

[0012] annealing of the steel sheet at an annealing temperature (TG) which is between the first recrystallization temperature (TR1) and the second recrystallization temperature (TR2), wherein the annealing is taking place during the denitriding and / or after the denitriding.

[0013] In the method according to the invention, the reverse order of the areas with different degrees of crystallization compared to the previously known microstructure is produced by the method step of denitriding of the steel sheet in a hydrogen-containing gas atmosphere and annealing during and / or after denitriding. The 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, with the result that different recrystallization temperatures are formed in the near-surface region and in the inner core region, the recrystallization temperature in the region near the surface (first recrystallization temperature TR1) being lower than the recrystallization temperature in the core region (second recrystallization temperature TR2). By forming different recrystallization temperatures in the near-surface region and in the core region, the annealing of the steel sheet, which can take place during and / or after denitriding, can achieve a microstructure with a different crystallization structure in the near-surface region and in the core region, if the annealing temperature (TG) used for the annealing is between the recrystallization temperature in the near-surface-region (first recrystallization temperature) and the recrystallization temperature in the core region (second recrystallization temperature TR2). Therefore, in the method according to the invention, the annealing temperature (TG) is selected such that: TR1<T(G)<T(R)2. This ensures that during annealing of the steel sheet at least essentially only the outer, near-surface region with the lower recrystallization temperature (first recrystallization temperature) is recrystallized, whereas the inner core region with the higher recrystallization temperature (second recrystallization temperature) remains roll-hard, i.e. is not or at least not completely recrystallized.

[0014] In order to achieve sufficient strength, in particular a tensile strength of preferably more than 500 MPa and in particular between 550 MPa and 700 MPa, the steel sheet according to the invention preferably has a weight proportion of nitrogen of at least 50 ppm averaged over the thickness of the steel sheet after denitriding. In order to achieve this, it is expedient if the steel sheet has a nitrogen content by weight (in particular averaged over the thickness of the steel sheet) of at least 70 ppm before the start of denitriding.

[0015] Therefore, the starting material used in the method according to the invention is a steel sheet cold-rolled to a predetermined thickness from a steel with a carbon content (C) of 10 to 1000 ppm by weight and a predetermined initial recrystallization temperature (TR0), wherein the steel either already has an initial nitrogen content (N0) 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 the nitrogen (averaged over the thickness) after the nitriding process is at least 70 ppm.

[0016] In addition to high strength, the steel sheets according to the invention are characterized in particular by improved formability. In particular, the steel sheets according to the invention have a high elongation at break of preferably at least 4% and in particular between 5% and 10%. The steel sheets according to the invention have a soft and flowable seam region (recrystallized, near-surface region) and are therefore easier to form, particularly in bending operations with high bending radii, than known steel sheets with comparable mechanical characteristics for strength and elongation at break, because a higher elongation can be achieved in the soft outer seam region of the steel sheet (near-surface region), particularly on the outside of the bending radius.

[0017] In particular, the steel sheets according to the invention have an outer fiber formed by the soft and flowable seam region (near-surface region) during bending deformations, which has a higher ductility compared to the inner fiber formed by the harder core region. This ensures that the steel sheets according to the invention have a high strength due to the harder core area, while the softer and flowable seam area ensures better formability of the outer fiber during bending deformations. Furthermore, the steel sheets according to the invention also exhibit an acceptable isotropy of the mechanical properties.

[0018] In a first embodiment of the method according to the invention for producing a steel sheet with a multilayer crystallization structure, a steel sheet cold-rolled to a predetermined thickness of preferably less than 0.5 mm and having a carbon content (C) of 10 to 1000 ppm by weight and an original recrystallization temperature (TR0) at least substantially determined by the composition of the steel is subjected to a nitriding process in which nitrogen is incorporated into the cold-rolled steel sheet at least in a region close to the surface, wherein the cold-rolled steel sheet is exposed to a nitriding gas atmosphere at a maximum nitriding temperature (TA) which is lower than the recrystallization temperature (TR), and the nitrogen stored in the region close to the surface during nitriding is subsequently removed by a temperature treatment, in particular at a homogenization temperature (TA), which is lower than the recrystallization temperature (TR), which is homogenized over the thickness of the steel sheet, in particular at a homogenization temperature (THo) which is preferably greater than or equal to the nitriding temperature and below the recrystallization temperature (TR), so that nitrogen is also deposited in a core region of the steel sheet. The temperature treatment to homogenize the nitrogen deposited during the nitriding process is also referred to below as homogenization or the homogenization process.

[0019] 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 means of one or more spray nozzles during the nitriding process. This leads to a faster diffusion of atomic nitrogen into the surface of the cold-rolled steel sheet and produces a homogeneous distribution of the nitrogen thereby incorporated into the steel sheet, both over the cross-section and in the plane of the surface of the steel sheet. Due to the homogeneous incorporation of the nitrogen across the cross-section of the steel sheet, the homogenization times can be selected lower in this embodiment example, as the nitrogen is already homogeneously incorporated when it diffuses into the interior of the steel sheet.

[0020] In the first embodiment of the method according to the invention, after the nitriding process and the homogenization, nitrogen is removed from the area near the surface on at least one side of the steel sheet by introducing the steel sheet into a hydrogen-containing gas atmosphere, a nitrogen gradient with a nitrogen concentration decreasing from the inner core area outwards to the area near the surface being formed during this denitriding over the thickness of the steel sheet, as a result of which the region near the surface attains a first recrystallization temperature (TR1) and the core region of the steel sheet attains a second recrystallization temperature (TR2) which is higher than the first recrystallization temperature (TR1), and the steel sheet is annealed during denitriding or after denitriding at an annealing temperature (TG) which is between the first recrystallization temperature (TR1) and the second recrystallization temperature (TR2).

[0021] In a second embodiment of the method according to the invention, a steel sheet cold-rolled to a predetermined thickness from a steel with a carbon content (C) of 10 to 1000 ppm by weight, which already has a nitrogen content of preferably at least 70 ppm by weight and a predetermined recrystallization temperature (TR), is subjected to denitriding by bringing the cold-rolled steel sheet into a hydrogen-containing gas atmosphere, is subjected to denitriding by placing the cold-rolled steel sheet in a hydrogen-containing gas atmosphere, whereby nitrogen is removed from the steel sheet in an area close to the surface and a nitrogen gradient is formed over the thickness of the steel sheet with a nitrogen concentration decreasing from an inner core area outwards to the area close to the surface, whereby a first recrystallization temperature (TR1) is formed in the region near the surface and a second recrystallization temperature (TR2), which is higher than the first recrystallization temperature (TR1), is formed in the core region of the steel sheet, and wherein the steel sheet is annealed during denitriding or after denitriding at an annealing temperature (TG) which is between the first recrystallization temperature (TR1) and the second recrystallization temperature (TR2).

[0022] To carry out the second embodiment of the method according to the invention, a cold-rolled steel sheet is used, which is produced from a 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.

[0023] In both process variants, the denitriding of the near-surface region of the steel sheet in the hydrogen-containing gas atmosphere takes place during a predetermined denitriding time (tES), which is preferably between 1 second and 600 seconds and particularly preferably between 10 seconds and 300 seconds and in particular in the range from 180 to 300 seconds. Preferably, denitriding is carried out simultaneously with annealing at the annealing temperature (TG), which is between the first recrystallization temperature (TR1) and the second recrystallization temperature (TR2). On the one hand, this is advantageous in terms of process technology and with regard to a shorter process duration and, on the other hand, it has proven to be more effective with regard to the efficiency of denitriding. However, denitriding can also be carried out before annealing, whereby denitriding is then preferably carried out at a denitriding temperature (TES) that is below the original recrystallization temperature (TR0) in order to prevent the steel structure of the steel sheet from already recrystallizing during denitriding

[0024] Furthermore, denitriding can also begin before annealing and continue during annealing. Denitriding can therefore overlap at least temporarily with annealing, especially if both denitriding and annealing are carried out in a continuous annealing furnace. The hydrogen-containing atmosphere in the annealing furnace is crucial for denitriding and maintaining the correct annealing temperature, which is between the first and second recrystallization temperature, and is crucial for annealing in order to remove nitrogen from the near-surface area of the steel sheet during denitriding and (only) recrystallizing the outer, near-surface region during annealing. If denitriding and annealing take place at least temporarily simultaneously or overlapping, the gas atmosphere in the annealing furnace is therefore selected so that it contains a sufficient amount of hydrogen to ensure denitriding, and at the same time the temperature in the annealing furnace is set to a suitable annealing temperature which is between the first and second recrystallization temperature.

[0025] In both embodiments of the method according to the invention, the described process control results in the formation of a three-layer crystallization structure in the steel sheet, which comprises a core region and a near-surface region surrounding the core region on both sides, wherein the near-surface region is at least substantially recrystallized and the core region is not or at least not completely recrystallized.

[0026] In the first embodiment of the method according to the invention, the recrystallization temperature is initially raised to a recrystallization temperature, both in the area of the steel sheet close to the surface and in the core area, during the nitriding process and the subsequent temperature treatment, which is greater than the original recrystallization temperature (TR0) of the steel, and during the subsequent denitriding the recrystallization temperature is lowered to the first recrystallization temperature (TR1) at least essentially (only) in the region near the surface, wherein the first recrystallization temperature (TR1) may be greater than, equal to or less than the original recrystallization temperature (TR0) of the steel, depending on the amount of nitrogen which is removed from the steel sheet from the region near the surface during denitriding. The original recrystallization temperature (TRO) is, depending on the composition of the steel, particularly in the range of 550° C. to 700° C.

[0027] In the second embodiment of the method according to the invention, the recrystallization temperature in the region of the steel sheet close to the surface is lowered to the first recrystallization temperature (TR1) by removing nitrogen, the first recrystallization temperature (TR1) being lower than the original recrystallization temperature (TR0) of the steel and the second recrystallization temperature (TR2) corresponding at least substantially to the original recrystallization temperature (TRO) of the steel, since nitrogen is only removed from the area near the surface during denitriding and no or at least hardly any nitrogen is removed from the core area, so that the recrystallization temperature in the core region hardly changes compared to the original recrystallization temperature (TR0).

[0028] Therefore, in the second embodiment of the method, the first recrystallization temperature (TR1) may be below the original recrystallization temperature (TR0) of the steel, while the second recrystallization temperature (TR2) is at least approximately equal to the original recrystallization temperature (TR0) of the steel. In the first embodiment of the method, the second recrystallization temperature (TR2) may be above the (original) recrystallization temperature (TR0) of the steel, while the first recrystallization temperature (TR1) is above or below the (original) recrystallization temperature (TR0) of the steel or corresponds thereto.

[0029] In both embodiments of the method according to the invention, the weight fraction of nitrogen in the near-surface region is reduced by a predetermined value (ΔN1) by removing nitrogen during denitriding of the near-surface region, whereby the recrystallization temperature in the region near the surface is lowered by a value ΔT1 to the first recrystallization temperature (TR1), the amount of ΔT1 increasing in particular linearly with the reduction in the weight fraction of nitrogen (ΔN1) in the region near the surface. The value ΔT1 by which the recrystallization temperature of the near-surface region (1) is lowered during denitriding is preferably greater than 10° C. and particularly preferably greater than 30° C.

[0030] In the 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 proportion of nitrogen therein by a predetermined value (ΔN2), whereby the recrystallization temperature in the core region is raised by a value ΔT2 to the second recrystallization temperature (TR2=TR+ΔT2), with ΔT2 increasing in particular linearly with the increase in the weight proportion of nitrogen (ΔN2) in the core region (2). The value ΔT2, to which the recrystallization temperature in the core region is raised during the temperature treatment, is preferably greater than 10° C. and particularly preferably greater than 30° C.

[0031] In the first embodiment of the method according to the invention, the annealing temperature (TG) is set so that it is above the recrystallization temperature of the near-surface region, which is lowered by ΔT1 to the first recrystallization temperature TR1 during denitriding, and below the second recrystallization temperature TR2 (where TR2=TR0+ΔT2) in the core region of the steel sheet. The following applies: TR1<TG<TR2.

[0032] To determine a suitable annealing temperature (TG) for this purpose, the original recrystallization temperature (TR0) defined by the composition of the steel sheet is first determined experimentally. This original recrystallization temperature (TR0), which is usually below 700° C. and typically in the range of 650° C. to 700° C., is taken as the lower limit for the suitable annealing temperature, as this temperature is the minimum required during annealing to cause recrystallization of the steel (in the area close to the surface). In iterative tests, the annealing temperature is then gradually increased on samples of steel sheet that have been denitrided in the near-surface area and the microstructure of the steel sheet that forms during annealing is examined metallographically over its thickness. If there is still a non-recrystallized (i.e. roll-hard) microstructure in the (non-denitrified) core area of the steel sheet, it can be assumed that the annealing temperature used is below the recrystallization temperature of the core area (second recrystallization temperature TR2). This results in the upper limit for a suitable annealing temperature, which corresponds to the second recrystallization temperature TR2.

[0033] In the second embodiment of the method according to the invention, the annealing temperature (TG) is set so that it is below the original recrystallization temperature (TR0) and above the first recrystallization temperature (TR1=TR0−ΔT1), which is lowered by ΔT1 during softening, in the area of the steel sheet close to the surface, so that the following applies:TR0>T⁡(G)>T(R)0-Δ⁢T1.

[0034] The upper limit of the annealing temperature (TG) is less than 750° C. for the preferably selected compositions of the steel sheets.

[0035] In the nitriding process of the first embodiment of the method according to the invention, the steel sheet is heated to a nitriding temperature (TA) in an annealing furnace, in particular a continuous annealing furnace, preferably in an ammonia-containing gas atmosphere, and held at the nitriding temperature (TA) for a predetermined holding time (tH), wherein the nitriding temperature (TA) is preferably greater than 300° C. and in particular is between 300° C. and 600° C. and the holding time (tH) is preferably greater than 1 second and in particular is between 2 seconds and 300 seconds. At a temperature of more than 300° C., the nitrogen in the ammonia dissociates to form atomic nitrogen, which can diffuse into the steel sheet. During heating and / or during the holding time, the steel sheet is exposed to the ammonia-containing gas atmosphere in the annealing furnace, whereby atomic nitrogen diffuses through the steel sheet surface into the area near the surface of the steel sheet, in particular due to a catalytic reaction 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 of 1% to 5% In the first embodiment of the method according to the invention, the subsequent temperature treatment for homogenizing the nitrogen deposited in the near-surface area of the steel sheet during the nitriding process is preferably carried out in the same annealing furnace, in particular in a downstream area of the continuous annealing furnace. This enables efficient process control in a single pass. However, the temperature treatment for homogenizing the nitrogen incorporated during the nitriding process can also be carried out in a separate bell annealing furnace (with significantly longer homogenization times in the range of several hours).

[0036] The temperature of the steel sheet during homogenization (homogenization temperature THo) is preferably greater than 300° C. and is in particular between 300° C. and 700° C., in particular between 350° C. and 650° C. Preferably, the homogenization temperature (THo) is below the original recrystallization temperature (TR0) and can, in particular when the method is carried out in a continuous annealing furnace, be equal to or greater than the nitriding temperature (TA). When homogenization is carried out in a bell annealing furnace, the homogenization temperature (THo) can also be lower than the nitriding temperature (TA) at which nitrogen has been introduced into the near-surface region in a separate and preceding nitriding step in a continuous annealing furnace.

[0037] When the process is carried out in a continuous annealing furnace, the temperature of the steel sheet can also change (abruptly or continuously) during homogenization. In particular, the temperature of the steel sheet can be brought to a higher temperature above the original recrystallization temperature (TR0) during homogenization. However, it should be ensured that the steel sheet has already been exposed to a lower homogenization temperature (THo), which is lower than the original recrystallization temperature (TR0), for a sufficiently long time before such a temperature increase in order to prevent the inner core area of the steel sheet from being recrystallized.

[0038] To homogenize the nitrogen introduced in the nitriding process, the steel sheet can be kept at the homogenization temperature (THo) during a homogenization period (tHo) either in a bell-type annealing furnace or in a continuous annealing furnace.

[0039] If 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 a separate bell annealing furnace, the homogenization time (tHo) is preferably between 60 minutes and 6 hours and particularly preferably between 2 hours and 5 hours, with homogenization preferably being carried out in a protective gas atmosphere, in particular an argon or nitrogen gas atmosphere, at the homogenization temperature (THo). This ensures an even distribution of the nitrogen incorporated during the nitriding process across the thickness of the steel sheet. The protective gas atmosphere prevents the steel sheet surface from oxidizing during the temperature treatment and prevents components of the steel from outgassing

[0040] If the temperature treatment for homogenizing the stored nitrogen is carried out in a continuous annealing furnace, the homogenization time (tHo) is preferably between 60 seconds and 600 seconds, in particular between 180 seconds and 300 seconds and particularly preferably between 180 seconds and 400 seconds. These homogenization durations ensure, on the one hand, sufficient uniformity of the nitrogen introduced across the thickness of the steel sheet and, on the other hand, rapid process control with a high belt speed at which the steel sheet is moved through the continuous annealing furnace. During homogenization in a continuous annealing furnace, the homogenization temperature (THo) in the continuous annealing furnace is preferably between 300° C. and 700° C., particularly preferably between 450° C. and 650° C., whereby the steel sheet is kept at the homogenization temperature (THo) in a protective gas atmosphere, in particular an argon or nitrogen gas atmosphere, in the continuous annealing furnace during the homogenization period (tHo).

[0041] Preferably, the homogenization time (tHo) is longer than the holding time (tH) during nitriding in the nitriding process in the continuous annealing furnace and / or longer than the denitriding time (tES).

[0042] The nitriding process of the first embodiment of the method according to the invention can be carried out in a single-stage or multi-stage process. In a single-stage nitriding process, the steel sheet is heated from room temperature to the nitriding temperature over a heating time (tE) and held at the nitriding temperature during a holding time. During heating and / or during the holding time, the steel sheet is exposed to the nitriding gas atmosphere. The heating time (tE) is preferably in the range of 1.0 to 300 seconds and the holding time (tH) is preferably in the range of 1.0 to 300 seconds. Nitriding is particularly preferably carried out in an ammonia-containing gas atmosphere with a volume fraction of ammonia of 0.05 to 10% and an inert gas as the remainder, in particular argon or nitrogen gas.

[0043] During denitriding, the steel sheet is exposed to a hydrogen-containing gas atmosphere for a denitriding time (tES), which is preferably between 1 second and 600 seconds and particularly preferably between 10 seconds and 300 seconds, wherein the gas atmosphere has a hydrogen volume fraction of preferably more than 10%, particularly preferably more than 15% and especially more than 50%. The hydrogen-containing gas atmosphere can also consist largely entirely of hydrogen, i.e. 100% except for unavoidable residual gases. For efficient denitriding, however, hydrogen concentrations of 15% by volume or less are sufficient for the particularly preferred denitriding times (tES) of 100 seconds to 300 seconds.

[0044] Denitriding can be carried out before annealing at a denitriding temperature (TES) which is greater than 300° C. and less than the original recrystallization temperature of the steel and preferably between 300° C. and 600° C. Preferably, however, the denitriding is carried out simultaneously with the annealing at the annealing temperature. In the first embodiment of the method, in which the steel sheet is nitrided in a continuous annealing furnace, the denitriding is preferably carried out in the continuous annealing furnace in which the nitriding and also the subsequent temperature treatment for homogenizing the incorporated nitrogen is carried out. This enables efficient process control.

[0045] During denitriding, the hydrogen-containing gas atmosphere extracts nitrogen from the area near the surface of the steel sheet, whereby the nitrogen diffused from the steel sheet first recombines in an equilibrium reaction to form N2 molecules and then, to a lesser extent, with the hydrogen gas in the gas atmosphere to form ammonia (NH3):N2+3⁢H2→2⁢N⁢H3.

[0046] The hydrogen-containing gas atmosphere therefore contains a low proportion of ammonia from the equilibrium reaction, which is all the higher the more nitrogen has diffused from the steel sheet into the gas atmosphere and is in particular less than 0.1% by volume. For efficient denitriding, the hydrogen-containing gas atmosphere is preferably as free as possible from ammonia gas at the start of denitriding, at least. In order to maintain for long denitriding as possible, the gas atmosphere can, during the course of denitriding, be exchanged with an ammonia-free gas atmosphere temporarily or continuously, for example by feeding hydrogen-containing and ammonia-free gas, particularly in counterflow. By exchanging the ammonia-containing gas atmosphere, which is formed due to a recombination reaction of nitrogen from the steel sheet with hydrogen from the gas atmosphere to form ammonia, for an ammonia-free and hydrogen-containing protective gas atmosphere, the diffusion of nitrogen from the area near the surface of the steel sheet can be promoted.

[0047] The gas atmosphere during denitriding preferably contains a hydrogen content of at least 10% by volume, preferably more than 15% by volume and in particular 30% by volume or more and particularly preferably at least 50% by volume and may contain up to 100% hydrogen gas. Preferably, the gas atmosphere during denitriding is free of ammonia gas or another nitriding gas, formed. except for the low equilibrium concentration of ammonia, which is by the diffusion of nitrogen from the steel sheet and the combination of the diffused nitrogen with the hydrogen from the gas atmosphere to form NH3. In addition to hydrogen, the gas atmosphere during denitriding can also contain inert gases, which are preferably free of nitrogen

[0048] In order to prevent decarburization from taking place at the same time as denitriding occurs, the gas atmosphere during denitriding preferably has a low humidity. The dew point of the gas atmosphere used during denitriding in the annealing furnace is therefore preferably <0° C. and particularly preferably at −30° C. or less. Since by the steel sheet, which is cleaned before entering the annealing furnace and therefore has a residual moisture on the surface, moisture is introduced into the annealing furnace, as well as due to leaks in the annealing furnace, the protective gas, which is pumped into the annealing furnace to maintain the gas atmosphere during denitriding, in particular with a low overpressure of a few mbar, preferably has an even lower dew point of, for example, below −50° C. (and therefore a very low moisture content).

[0049] To avoid impurities on the surface of the steel sheet, which occur and can hinder an efficient diffusion of nitrogen from the steel during annealing, it also helps if the gas atmosphere during denitriding contains little carbon oxides. Preferably, the gas atmosphere during denitriding therefore contains a low volume fraction of CO and CO2, in particular in total (i.e. the sum of CO and CO2) of a maximum of 1000 ppm.

[0050] The annealing of the steel sheet is preferably carried out in the continuous annealing furnace during denitriding and / or after denitriding. If the annealing is carried out separately from the denitriding, i.e. after completion of the denitriding, the annealing is preferably carried out in a protective gas atmosphere, in particular an argon, nitrogen or HNx atmosphere, in order to prevent oxidation of the steel sheet surface. If annealing is carried out at the same time as denitriding, annealing is carried out in the hydrogen-containing gas atmosphere, which preferably contains more than 10% and up to 100% hydrogen gas and preferably an inert gas, e.g. argon (Ar) or nitrogen gas (N2), as any remainder.

[0051] During annealing, the steel sheet is preferably heated to the annealing temperature (TG) within a heating time (tA) of 1.0 second to 300 seconds and held at the annealing temperature (TG) for a predetermined annealing time (tG), with the annealing time (tG) preferably being greater than 1 second and in particular between 1 second and 600 seconds and particularly preferably between 100 seconds and 300 seconds. With these annealing times, complete recrystallization of the area close to the surface can take place on the one hand and, on the other hand, fast and efficient process control in a continuous annealing furnace at a high belt speed is possible. The steel sheet is preferably heated to the annealing temperature within a heating time (tA) of a few seconds by means of an induction heater integrated in the continuous annealing furnace, which ensures a short heating distance and a longer holding time (annealing time tG) at the annealing temperature.

[0052] When the cold-rolled steel sheet is annealed, recrystallization annealing of the cold-rolled steel sheet occurs at least partially (only) in the area near the surface, whereas the core region (due to the higher recrystallization temperature TR2 there) is not or only slightly recrystallized. A sharp distinction between the core region and the surrounding near-surface region can be achieved if the degree of crystallization in the near-surface region is as high as possible, while the core region is not recrystallized. The degree of crystallization of the two regions can be controlled via the process parameters of the manufacturing process. Preferably, the core region has a degree of recrystallization of less than 30% and particularly preferably less than 20% and the degree of recrystallization of the area close to the surface is preferably greater than 70% and particularly preferably greater than 80%

[0053] In order to achieve these degrees of recrystallization, the homogenization time in the first embodiment of the methods according to the invention is preferably greater than the holding time (tH) in which the steel sheet is held at the nitriding temperature (TA) in the nitriding process. Furthermore, in the first embodiment of the method according to the invention, the homogenization time is preferably greater than the denitriding time (tES) in which the steel sheet is exposed to the hydrogen-containing gas atmosphere. Particularly preferably, the homogenization time in the first embodiment of the method according to the invention is at least a factor of two greater than the denitriding time (tES) and / or at least a factor of two greater than the holding time (tH) during nitriding in the nitriding process in the continuous annealing furnace. Due to the long homogenization times, in the first embodiment of the method according to the invention, a uniform distribution of the nitrogen introduced over the thickness of the steel sheet is achieved after nitriding in the continuous annealing furnace. During the denitriding time (tES), which is short compared to the homogenization times, nitrogen is only removed in the region of the steel sheet close to the surface, so that a pronounced nitrogen gradient is formed across the thickness of the steel sheet during denitriding, with a high nitrogen concentration in the core region and a low nitrogen concentration in the region close to the surface. The greater the nitrogen gradient, i.e. the greater the difference between the nitrogen concentration in the core region and in the region close to the surface, the greater the difference between the first recrystallization temperature (in the region close to the surface) and the second recrystallization temperature (in the core region). If the difference between the first recrystallization temperature (in the near-surface region) and the second recrystallization temperature (in the core region) is greater, the difference in the degree of recrystallization that occurs during annealing in the near-surface region and in the core region is also greater. Therefore, a long homogenization time, especially with short denitriding times (tES) during denitriding, leads to a strong difference in the degree of recrystallization of the steel sheet in the area near the surface and in the core area and thus to a stronger development of the multilayer crystallization structure.

[0054] The thickness of the near-surface region and the core region can be controlled via the process parameters of the manufacturing process. Preferably, the near-surface region has a thickness in the range from 5 μm to 200 μm, particularly preferably in the range from 10 μm to 100 μm and especially between 20 μm and 80 μm. The thickness of the core region is preferably in the range from 50 μm to 450 μm, particularly preferably in the range from 90 μm to 400 μm and especially between 150 μm and 300 μm.

[0055] The still roll-hard core region and the at least partially recrystallized near-surface region of the steel sheet according to the invention differ from one another not only in their degree of crystallization 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, with the ratio of the hardness of the core region to the hardness of the near-surface region preferably being greater than 1.2 and particularly preferably greater than 1.4. The core region of the steel sheet has a high microhardness with a Vickers hardness of preferably at least 160 HV0.025 and particularly preferably at least 220 HV0.025 due to solid solution hardening caused by its high nitrogen content.

[0056] The steel of the cold-rolled steel sheet preferably has the following composition by weight:

[0057] C: more than 0.001% and less than 0.1%, preferably less than 0.06%;

[0058] Mn: more than 0.01% and less than 0.6%;

[0059] P: less than 0.04%;

[0060] S: less than 0.04% and preferably more than 0.001%;

[0061] Al: less than 0.08%;

[0062] Si: less than 0.1%;

[0063] optional Cu: less than 0.1%;

[0064] optional Cr: less than 0.1%;

[0065] optional Ni: less than 0.1%;

[0066] optional Ti: less than 0.1%;

[0067] optional Nb: less than 0.08%;

[0068] optional Mo: less than 0.08%;

[0069] optional Sn: less than 0.05%;

[0070] optional B: less than 0.01%, preferably less than 0.005%;

[0071] Residual iron and unavoidable impurities

[0072] wherein in the second embodiment of the method according to the invention, the steel of the cold-rolled steel sheet additionally has a nitrogen content of more than 0.001% and preferably of 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% and preferably of less than 0.016%.

[0073] Values given in % or ppm relating to the content or concentration of an alloy component of the steel or cold-rolled steel sheet refer in each case to the weight of the steel or steel sheet.

[0074] In the first embodiment of the method according to the invention, the nitrogen content averaged over the thickness of the steel sheet is preferably brought to at least 0.005% (50 ppm) and particularly preferably to more than 0.010% (100 ppm) and especially to 0.015% (150 ppm) or more in the nitriding process. In the absence of nitride formers, the vast majority of the nitrogen introduced is present in unbound form, which also applies to the second embodiment of the method, in which the nitrogen is already added to the molten steel.

[0075] 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 (N0), which is preferably greater than 0.001% by weight and is particularly preferably between 0.005% and 0.016% by weight, and the initial nitrogen content of the steel sheet being further increased in a nitriding process by incorporating nitrogen from a nitriding gas atmosphere in an annealing furnace, in particular to a nitrogen content based on weight and averaged over the thickness of the steel sheet of at least 0.005%, in particular in the range between 0.007% and 0.07%.%.

[0076] The nitrogen incorporated into the cold-rolled steel sheet during the nitriding process or the nitrogen already present in the steel of the cold-rolled steel sheet can be present (up to the solubility limit) in dissolved form and / or in bound form as nitride. In the presence of strong nitride formers in the steel, the incorporated nitrogen is at least partially present as nitrogen bound in nitrides, in particular as TiN and / or NbN and / or AlN. If the nitrogen is present in dissolved form and is interstitially embedded in the steel lattice and not bound in nitrides, a more homogeneous distribution of the nitrogen across the thickness of the steel sheet can be achieved during the temperature treatment for homogenization. In addition, the efficiency of denitriding is higher if the nitrogen is present as free nitrogen, as unbound nitrogen atoms that are interstitially embedded in the steel lattice can diffuse better from the steel sheet into the gas atmosphere of the annealing furnace during denitriding, compared to nitrogen atoms that are 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 and particularly preferably less than 50 ppm titanium and / or less than 100 ppm niobium and / or less than 500 ppm and 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, especially preferably less than 500 ppm. This ensures that at least a large proportion of the nitrogen incorporated in 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 and, in particular, is incorporated interstitially in the lattice of the steel, and only a remaining proportion of the nitrogen is bound as nitride, in particular as AlN and / or TiN and / or NbN.

[0077] The method according to the invention can be used to produce cold-rolled steel sheets with a multilayer structure of the crystallization structure, in particular with a three-layer crystallization structure. The invention therefore also relates to a steel sheet, in particular for packaging, with a predetermined thickness of preferably less than 0.5 mm and a carbon content, based on the weight, of 10 to 1000 ppm and a nitrogen content, averaged over the thickness of the steel sheet, of more than 50 ppm, wherein the steel sheet has a multilayer crystallization structure with a core region and a near-surface region surrounding the core region, in particular on both sides, wherein the near-surface region is at least substantially recrystallized and the core region is not or at least not completely recrystallized.

[0078] Features of the steel sheets produced by the method according to the invention therefore also relate to the manufacturing method according to the invention, and vice versa.

[0079] The steel sheets according to the invention preferably have the following composition by weight:

[0080] C: more than 0.001% and less than 0.1%, preferably less than 0.06%;

[0081] Mn: more than 0.01% and less than 0.6%;

[0082] P: less than 0.04%;

[0083] S: less than 0.04% and preferably more than 0.001%;

[0084] Al: less than 0.08%, preferably less than 0.03% and particularly preferably less than 0.01%;

[0085] Si: less than 0.1%;

[0086] optional Cu: less than 0.1%;

[0087] optional Cr: less than 0.1%;

[0088] optional Ni: less than 0.1%;

[0089] optional Ti: less than 0.1% and preferably less than 0.02%;

[0090] optional Nb: less than 0.08% and preferably less than 0.01%;

[0091] optional Mo: less than 0.08%;

[0092] optional Sn: less than 0.05%;

[0093] optional B: less than 0.01%, preferably less than 0.005%;

[0094] 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.

[0095] The near-surface region of the steel sheets according to the invention preferably has a thickness in the range from 5 μm to 200 μm, particularly preferably in the range from 10 μm to 100 μm. The core region preferably has a thickness in the range from 50 μm to 450 μm, particularly preferably in the range from 90 μm to 400 μm and especially between 150 μm and 300 μm.

[0096] The core region has a higher hardness and / or a higher tensile strength than the region close to the surface, whereby the ratio of the hardness of the core region to the hardness of the region close to the surface is preferably greater than 1.2 and particularly preferably greater than 1.4. Preferably, the core region of the steel sheet has a Vickers hardness of at least 160 HV0.025 and particularly preferably of at least 220 HV0.025.

[0097] The degree of crystallization of the core region is preferably less than 30% and is particularly preferably less than 20%. The degree of recrystallization of the near-surface region, on the other hand, is preferably more than 70 and particularly preferably more than 80%. In an ideal crystallization structure, which is a particularly preferred embodiment of the invention, the core region is not crystallized and the near-surface region is 100% crystallized.

[0098] Due to the nitriding and the roll-hard properties of the core region, the steel sheet has a high tensile strength of preferably more than 500 MPa, in particular between 550 MPa and 700 MPa. At the same time, the region near the surface is softer and more flowable than the core region due to the denitriding and recrystallization. This results in good overall forming properties for the steel sheet, in particular a high elongation at break, which is preferably at least 4% and in particular between 5% and 10%,BRIEF DESCRIPTION OF THE DRAWINGS

[0099] These and other advantages of the sheet steel according to the invention and of the manufacturing method are shown in the embodiments described in more detail below with reference to the accompanying drawings. The drawings show:

[0100] FIG. 1: Schematic representation 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;

[0101] FIG. 2: Microscopic cross-sectional image of a sample of a steel sheet according to the invention with a three-layer structure of the crystallization microstructure, which comprises a non-crystallized core region and regions close to the surface surrounding it on both sides, which are at least substantially completely recrystallized;

[0102] FIG. 3: Schematic representation of the course of the recrystallization temperature of steel sheets according to the invention over their cross-section (position x);

[0103] FIG. 4: Representation of stress-strain diagrams of a steel sheet according to the invention and of comparative examples;

[0104] FIG. 5: Illustration of the course of the microhardness over the cross-section of a steel sheet according to the invention and of comparative examples;

[0105] FIG. 6: Microscopic cross-sectional image of two further samples of a steel sheet according to the invention with a three-layer structure of the crystallization microstructure, which comprises a non-crystallized core region and near-surface-regions surrounding it on both sides, which are at least substantially completely recrystallized;

[0106] FIG. 7: Comparative representation of the course of the microhardness over the cross-section of various samples of steel sheets according to the invention;

[0107] FIG. 8: Shows Table 1;

[0108] FIG. 9: Shows Table 2;

[0109] FIG. 10: Shows Table 3; and

[0110] FIG. 11: Shows Table 4.DETAILED DESCRIPTION

[0111] Hot-rolled and subsequently cold-rolled steel sheets with a carbon content of 10 to 1000 ppm by weight are used as the starting product for the production of steel sheets according to the invention using the method according to the invention. The thickness of the cold-rolled steel sheets is preferably 0.5 mm or less, particularly for applications in the packaging sector. For other applications, e.g. the manufacture of bodywork components for automobiles, higher thicknesses in the thin sheet range can also be selected. For applications in the packaging sector, the alloy composition of the steel expediently fulfills the limit values specified by standards for packaging steel (as defined, for example, in the ASTM A623-11 “Standard Specification for Tin Mill Products” standard or in the “European Standard EN 10202”), with the exception of the nitrogen content, which may deviate from the standard in the steel sheets according to the invention, in particular if highly nitrided steel sheets with a very high strength of more than 750 MPa are to be produced. The components of the steel from which steel sheets according to the invention can be produced are explained in detail below:Composition of the Steel:Carbon, C: more than 0.001% and less than 0.1%, preferably less than 0.06%; Carbon increases hardness and strength. The steel therefore preferably contains more than 0.001% carbon by weight. In order to ensure the rollability of the steel sheet during primary cold rolling and possibly in a second cold rolling step (skin pass) and not to reduce the elongation at break, the carbon content should not exceed 0.1% by weight.Manganese, Mn: more than 0.01% and less than 0.6%;

[0113] 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 to red cracking during hot rolling and manganese leads to grain refinement. A manganese content of at least 0.01% by weight is therefore preferable. To achieve high strengths, a manganese content of more than 0.1 wt. %, in particular 0.20 wt. % or more, is preferable. However, if the manganese content is too high, the corrosion resistance of the steel will be impaired. In addition, if the manganese content is too high, the strength becomes too high, which means that the steel can no longer be cold-rolled and formed. Therefore, the preferred upper limit for the manganese content is 0.6% by weight.

[0114] Phosphorus, P: less than 0.04%

[0115] Phosphorus is an undesirable accompanying element in steels. A high phosphorus content leads in particular to embrittlement of the steel and therefore impairs the formability of steel sheets, which is why the upper limit for the phosphorus content is 0.04% by weight.

[0116] Sulphur, S: less than 0.04% and preferably more than 0.001%

[0117] Sulphur is an undesirable by-product that impairs ductility and corrosion resistance. Therefore, steel should not contain more than 0.04% sulphur by weight. On the other hand, the desulphurization of steel requires complex and cost-intensive measures, which is why a sulphur content of less than 0.001% by weight is no longer justifiable from an economic point of view. The sulphur content is therefore preferably in the range from 0.001 wt. % to 0.04 wt. %, particularly preferably between 0.005 wt. % and 0.01 wt. %.

[0118] Aluminum, Al: less than 0.08%

[0119] Aluminium acts as a deoxidizing agent in the casting process during steel production to calm the steel. Aluminum also increases scale resistance and formability. For this reason, aluminum is preferably used in a concentration of 0.005 wt. % or more. On the other hand, aluminum concentrations of more than 0.08% by weight can lead to surface defects in the form of aluminum clusters, which is why this upper limit for the aluminum content should preferably not be exceeded. In addition, aluminium forms nitrides with nitrogen, which are disadvantageous in the process according to the invention, particularly during homogenization and denitriding, which is why the aluminium content is particularly preferably less than 0.005%.

[0120] Silicon, Si: less than 0.1%;

[0121] Silicon increases the resistance to scaling in steel and is a solid solution hardener. In steel production, it has the positive effect of making the melt more fluid and serves as a deoxidizing agent. Another positive effect of silicon on steel is that it increases the tensile strength, yield strength and resistance to scaling. Therefore, a silicon content of 0.003% by weight or more is preferable. However, if the silicon content becomes too high and in particular exceeds 0.10% by weight, the corrosion resistance of the steel may be impaired and surface treatments, in particular by electrolytic coatings, may become more difficult.

[0122] optionally nitrogen, No: more than 0.001% (or 10 ppm) and less than 0.02% (or 200 ppm), in particular less than 0.016% (or 160 ppm),

[0123] Nitrogen is an optional component in the molten steel from which the steel for the steel sheets according to the invention is produced when the method according to the 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 starting material for the method according to the invention may optionally already contain an initial (small) amount of nitrogen (No). Preferably, the molten steel or the 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 defects in the hot-rolled strip, which can occur with a higher nitrogen concentration, are reduced. In order to form a strong solid solution strengthening, it is preferable, in particular if very high strengths are to be achieved, that an initial nitrogen content of more than 0.001% by weight, particularly preferably of 0.010% by weight or more, is already contained in the molten steel even when the method according to the invention is carried out in the first embodiment.

[0124] When carrying out the method according to the invention in the second embodiment, an initial nitrogen content (No) in the molten steel or in the cold-rolled steel sheet of more than 0.007% by weight (or 70 ppm) is to be preferred, since no further addition of nitrogen is provided in the annealing furnace in this process control and the nitrogen content is reduced by the denitriding. In order to ensure a total nitrogen content of the steel sheet of more than 0.005% (or 50 ppm) in the second embodiment of the method according to the invention after the method has been carried out and thus to achieve sufficiently strong solid solution strengthening, the initial nitrogen content (No) in the second embodiment of the process is preferably in the range from 0.007% to 0.016% (or 70 ppm to 160 ppm).

[0125] optional: nitride formers, in particular niobium, titanium, molybdenum, zirconium, vanadium:

[0126] Nitride-forming elements such as aluminum, titanium, niobium, zirconium or vanadium are disadvantageous in the steel of the steel sheets according to the invention because the nitrogen that may already have been originally contained in the molten steel and / or the nitrogen 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 formers, which is disadvantageous for subsequent homogenization and denitriding. Therefore, the weight proportion of the nitride formers titanium, niobium and molybdenum is preferably limited to a maximum of 100 ppm and other nitride formers such as zirconium or vanadium are present at most as unavoidable impurities. On the other hand, nitride formers such as aluminum, titanium and / or niobium can improve the forming behavior of the steel sheets and almost aging-free IF (interstitial free) steel sheets can be produced and grain refinement be achieved. For this reason, the steel optionally and preferably contains

[0127] Titanium, Ti: preferably more than 0.002%, but less than 0.01%, and / or

[0128] Niobium, Nb: preferably more than 0.001% but less than 0.01% and / or

[0129] Aluminum, Al: preferably more than 0.005 wt. % but less than 0.05 wt. %

[0130] and / or molybdenum, Mo: less than 0.08%, preferably less than 0.01%.

[0131] Further optional components:

[0132] In addition to the residual iron (Fe) and unavoidable impurities, the steel may also contain other optional components, such as

[0133] optional copper, Cu: less than 0.1%;

[0134] optional chromium, Cr: less than 0.1%;

[0135] optional nickel, Ni: less than 0.1%;

[0136] optional tin, Sn: less than 0.05%

[0137] optionally boron, B: less than 0.01%, preferably less than 0.005%;

[0138] to give the steel any other advantageous properties that can be achieved with these additional components.Manufacturing Method of the Sheet Steel:

[0139] A molten steel is produced with the described composition of the steel, wherein in preferred embodiments, in particular 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 of the steel sheet by adding nitrogen to the molten steel, for example by blowing in nitrogen gas and / or by adding a solid nitrogen compound such as lime nitrogen (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, so that the hot formability of the steel is maintained and to avoid defects caused by nitrides in the slab produced from the molten steel, it is advantageous if the initial nitrogen content (No) of the steel (i.e. the proportion by weight of nitrogen in the molten steel) is preferably 0.016% by weight or less.

[0140] A slab is first cast from the molten steel, which is then hot-rolled and cooled to room temperature. The hot strip produced in this way has thicknesses in the range of 1 to 4 mm and may be wound into a coil at a predetermined winding temperature (coiling temperature) of 500 to 750° C., preferably in the range of 650° C. to 750° C. To produce a thin steel sheet in the preferred thicknesses of less than 0.5 mm for packaging applications, for example, the hot-rolled strip is cold-rolled, whereby a thickness reduction in the range of 50 to over 90% can be achieved. Such a cold-rolled steel sheet with the above-mentioned preferred composition forms the starting material for carrying out the method according to the invention. The steel sheet has an original recrystallization temperature TR0, which is predetermined by its composition and is typically below 720° C., in particular between 550° C. and 700° C.

[0141] In the following, a preferred embodiment of the method according to the invention in accordance with the first embodiment is explained in more detail using the temperature-time diagram in FIG. 1:

[0142] The starting material selected for this example is a cold-rolled steel sheet with a 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 strip form and is passed through a continuous annealing furnace at a predetermined speed of preferably more than 100 m / min, in which the steel sheet passes through the temperature-time curve shown in FIG. 1.

[0143] In the example shown in FIG. 1, the cold-rolled steel sheet is first heated inductively to a nitriding temperature TA of approx. 500° C. within a short heating time tA of approx. 30 seconds in a first chamber K1 of the continuous annealing furnace and held at this temperature for a holding time tH of approx. 280 seconds. The nitriding temperature TA is below the original recrystallization temperature TR0 of the cold-rolled steel sheet. The first chamber K1 of the continuous annealing furnace contains a nitrogenizing gas atmosphere consisting of a mixture of ammonia gas (NH3) and an inert gas, in particular HNx, for example with a volume concentration of the ammonia gas of 5%. During the heating time (tA) and the holding time (tH), a nitriding process A takes place, whereby nitrogen is deposited at least in the seam regions of the steel sheet due to the nitrogenizing gas atmosphere. In the nitriding process A, the nitriding ammonia gas is preferably additionally directed onto the two surfaces of the steel sheet by means of spray nozzles that are arranged next to each other transversely to the direction of strip travel (direction in which the steel sheet passes through the annealing furnace).

[0144] The steel sheet, which is at the nitriding temperature (TA), is then fed into 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, e.g. 100% HNx by volume. In the second chamber K2, the steel sheet remains at a homogenization temperature THo for a certain time (homogenization time tHo), which in the example in FIG. 1 corresponds to the nitriding temperature TA. However, the temperature of the steel sheet inside the second chamber K2 (homogenization temperature THo), which in the example shown corresponds to the nitriding temperature TA of approx. 500° C., can also deviate from the nitriding temperature (TA) and in particular exceed it. In the example shown in FIG. 1, the steel sheet is at the homogenization temperature THo during a homogenization time tHo of approx. 280 seconds in the second chamber K2 of the continuous annealing furnace. The nitrogen introduced during nitriding in the first chamber is evenly distributed over the cross-section of the steel sheet (homogenization step H). The fact that the temperature of the steel sheet in the second chamber K2 (homogenization temperature THo) corresponds to the nitriding temperature TA, which is lower than the original recrystallization temperature TR0 of the cold-rolled steel sheet, ensures that no recrystallization of the structure of the steel sheet occurs during homogenization step H

[0145] At the end of the second chamber K2, the steel sheet is heated to a temperature TG that is above the homogenization temperature THo, e.g. with an induction heater

[0146] The steel sheet then passes through a third chamber K3 of the continuous annealing furnace, which is separate from the second chamber K2. In the third chamber K3 of the continuous annealing furnace, there is a gas atmosphere containing hydrogen gas. The gas atmosphere in the third chamber K3 can consist of 100% hydrogen gas. For reasons of explosion safety, however, the hydrogen content in the third chamber K3 is lower, e.g. 10% by volume, and the rest of the gas atmosphere is an inert gas such as nitrogen gas. Due to the hydrogen content in the gas atmosphere of the third chamber K3, nitrogen is extracted from the steel sheet in the region near the surface and a nitrogen profile is formed over the cross-section of the steel sheet with a nitrogen content that decreases from the inner core region outwards to the region near the surface. As a result, different recrystallization temperatures are formed in the core region and in the region near the surface of the steel sheet, as described above, with a first recrystallization temperature (TR1) in the region near the surface and a higher second recrystallization temperature (TR2) in the core region. The steel sheet is introduced into the third chamber K3 at the temperature TG which the steel sheet has after heating at the end of the second chamber K2 or at the transition into the third chamber K3 and remains at this temperature in the third chamber K3 for a certain period of time (annealing time tG), which defines an annealing temperature TG. The annealing temperature TG is selected such that it lies between the first recrystallization temperature (TR1) and the second recrystallization temperature (TR2). After passing through the third chamber K3, the steel sheet is guided in an adjoining cooling device K4, which in particular enables multi-stage cooling of the steel sheet, and is preferably cooled in stages from the annealing temperature TG to room temperature. The cooling device K4 can still be partially inside the continuous annealing furnace or completely outside the continuous annealing furnace. Cooling can, for example, initially take place in a downstream cooling zone of the continuous annealing furnace, in which slow cooling initially takes place, for example by means of gas cooling at a cooling rate of 3 to 20 K / s, and then continue in a device for quenching the steel sheet to room temperature at a higher cooling rate, for example with water cooling at a cooling rate of more than 1000 K / s, outside the continuous annealing furnace. The (initially slow) cooling of the steel sheet in the cooling zone of the continuous annealing furnace defines a total annealing time tG in the continuous annealing furnace, which is determined in such a way that the steel sheet is at a temperature above the first recrystallization temperature (TR1) during the annealing time tG, as shown in FIG. 1.

[0147] In the embodiment shown in FIG. 1, the cold-rolled steel sheet is therefore heat-treated in a continuous annealing furnace, whereby the steel sheet is first nitrided to a higher nitrogen content during the heat treatment in an nitriding process A and the nitrogen introduced into the steel sheet is then evenly distributed over the thickness of the steel sheet in a homogenization step H. The nitrogen is then removed from the outer near-surface region 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 take place simultaneously in a joint denitriding and annealing process E / G in the third chamber K3 of the continuous annealing furnace for reasons of better efficiency and faster process control.

[0148] 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 region 1 close to the surface is reduced, while the nitrogen content in the core region 2 of the steel sheet remains largely unchanged. As a result, a nitrogen gradient forms across the cross-section of the steel sheet, with a decreasing nitrogen content from the inner core region 2 outwards to 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 over the thickness of the steel sheet, with an (averaged) first recrystallization temperature TR1 in the near-surface-region 1 and an (averaged) second recrystallization temperature TR2 in the core region 2, whereby the first recrystallization temperature (TR1) of the near-surface-region 1 is lower than the second recrystallization temperature TR2 of the core region 2 due to the lower nitrogen content there.

[0149] The course of the recrystallization temperature over the cross-section (position x) of the steel sheet is shown schematically in FIG. 3. The recrystallization temperature in the core region 2 of the steel sheet has increased by a value of ΔT2 from the original recrystallization temperature TR0 to the second recrystallization temperature TR2 as a result of nitriding the steel sheet and the subsequent homogenization of the nitrogen incorporated in the process: TR2=TR0+ΔT2. In contrast, the recrystallization temperature in the near-surface region 1 during denitriding E decreased by a value of ΔT1 compared to the recrystallization temperature that the steel sheet exhibited uniformly over the entire cross-section after the nitriding process and homogenization (TRHo, FIG. 3). The recrystallization temperature in the near-surface-region 1 a first recrystallization temperature TR1=TRHo−ΔT1 after denitriding, which is slightly higher than the original recrystallization temperature TR0 in the example shown in FIG. 3, but lower than the second recrystallization temperature TR2.

[0150] The value ΔT2, by which the recrystallization temperature in the core region increases compared to the original recrystallization temperature TR0 due to the incorporation of nitrogen during nitriding A and homogenization H, depends on the additional nitrogen content ΔN introduced into the core region of the steel sheet after homogenization has been completed, whereby a linear relationship can be observed, which is determined byΔ⁢T=a·Δ⁢N⁡(ppm)where a is a proportionality constant and ΔN(ppm) is the nitrogen content in ppm (based on the weight of the steel) introduced into the core region during nitriding and homogenization. Tests on samples with different nitrogen contents and otherwise identical alloy compositions have empirically determined a value of a≅1.2 K / ppm. Accordingly, the reduction in the recrystallization temperature in the near-surface region by the value ΔT2 compared to the recrystallization temperature (TRHo, FIG. 3), which is uniform across the cross-section after homogenization in the steel sheet, 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 in turn 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 during denitriding (in particular the denitriding duration) and can be set and calculated accordingly using these process parameters on the basis of empirical values. The recrystallization temperatures in the near-surface region (first recrystallization temperature TR1) and in the core region (second recrystallization temperature TR2) can thus be calculated from the values ΔT1 and ΔT2 for the change in the recrystallization temperature in the near-surface region and in the core region on the basis of the empirically determined correlations of these values with the process parameters of the nitriding process A and the denitriding process E.For recrystallizing annealing G of the steel sheet (only in the near-surface-region 1), the steel sheet is heated in the third chamber K3 (annealing chamber) to an annealing temperature TG that is above the first recrystallization temperature TR1 and below the second recrystallization temperature TR2. In the example shown in FIG. 1, the annealing temperature TG is approx. 600° C. In order to achieve recrystallization of the near-surface region 1 with the highest possible degree of crystallization, the steel sheet in the annealing chamber is kept at the annealing temperature TG or a temperature above the first recrystallization temperature TR1 for a sufficiently long annealing time tG. In the example shown in FIG. 1, the annealing time tG is approx. 200 seconds.

[0152] In the example shown in FIG. 1, denitriding E and annealing G of the steel sheet take place simultaneously in a combined denitriding and annealing process E / G in the third chamber K3 (annealing chamber) of the continuous annealing furnace, which ensures efficient and economical process control. Alternatively, denitriding E and annealing G can also be decoupled by dividing the third chamber K3 of the continuous annealing furnace into a denitriding chamber and an annealing chamber and carrying out denitriding E in the denitriding chamber separate from the annealing chamber, with the denitriding chamber in the continuous annealing furnace upstream of the annealing chamber. Decoupling the denitriding process E and the annealing process G enables a more targeted setting of the preferred parameters for denitriding and annealing (temperature and duration of the respective process and the gas atmosphere in the respective chamber of the continuous annealing furnace). The composition of the gas atmosphere in the denitriding chamber and the annealing chamber differs appropriately in that hydrogen gas is contained in the preferred minimum concentration (or more) for denitriding in the denitriding chamber, whereas annealing in the annealing chamber can take place in an inert gas atmosphere without a significant proportion of hydrogen.

[0153] Due to the setting of the annealing temperature TG according to the invention, which lies between the first recrystallization temperature TR1 and the second recrystallization temperature TR2, recrystallization only takes place in the near-surface-region 1 with the lower recrystallization temperature (TR1), while the inner core region 2, from which no or hardly any nitrogen has been removed during denitriding, is not or hardly recrystallized. The annealing temperature (TG) is selected for this so that it is only above the recrystallization temperature (TR1) in the near-surface region 1 and below the second recrystallization temperature, which is increased to TR2=TR0+ΔT2, in the inner core region 2. Therefore, a three-layer microstructure is formed over the cross-section of the steel sheet in the form of a “sandwich” with an at least essentially, preferably largely completely recrystallized near-surface region 1 and an inner core region 2, whereby the core region 2 is not or at least not completely recrystallized (which is why this three-layer microstructure is also referred to as a “sandwich structure” and is shown in FIG. 3 by the regions marked with the reference signs 1 and 2).

[0154] FIG. 2 shows an example of a steel sheet with such a three-layer microstructure in a microscopic cross-sectional view. FIG. 2 clearly shows the separation of the two outer, near-surface regions 1 from the inner core region 2 due to the different crystallization structure of these regions 1 and 2. The thickness of the recrystallized near-surface regions 1, which symmetrically surround the inner core region on both sides, is 32 μm and 33 μm respectively.Examples

[0155] Examples of embodiments of the steel sheet and the method according to the invention are explained below and compared with comparative examples not according to the invention.

[0156] Steel sheets with a thickness of 0.22±0.01 mm were produced by hot rolling and subsequent cold rolling from steel melts with the alloy composition (melt analysis) listed in Table 1 (the ppm figures refer to the weight proportion of the alloy components in the steel from which the cold-rolled steel sheet was produced). The cold-rolled steel sheets were successively subjected to a thermal treatment in a laboratory furnace for carrying out a nitriding process in an ammonia-containing protective gas atmosphere (nitriding), subsequent homogenization in an argon atmosphere and a final denitriding and annealing (which was carried out in parallel in a hydrogen atmosphere) at different process parameters with respect to temperature and holding time during nitriding A, homogenization H and denitriding / annealing E / G. The resulting examples of the steel sheets according to the invention are labeled “Example A”, “Example B” and “Example C” in Table 4. The atmosphere in the continuous annealing furnace was composed of the ammonia gas with the volume concentration (NH3 content) listed in Table 4 and HNx protective gas as the remainder, whereby the volume proportion of ammonia in the gas atmosphere of the furnace was determined at room temperature and maintained constant by flowing in ammonia during the thermal treatment of the steel sheet. If the experiments are carried out on an industrial scale in a continuous annealing furnace, the ammonia concentrations required for the nitriding process will presumably shift to higher values, since at the high temperatures in a continuously heated continuous annealing furnace, only part of the total ammonia atmosphere is effectively available for nitriding the steel sheet due to dissociation and recombination effects of the ammonia to atomic and molecular nitrogen. In the experiments, the steel sheets were denitrided and annealed in a 100% hydrogen gas atmosphere at the temperature specified in Table 4 (annealing temperature), shown in FIG. 11.

[0157] The microstructure of the steel sheets A, B and C treated in this way was examined microscopically (cold-embedded, ground, polished and etched with Nital (3% nitric acid)). After cooling, the steel sheets treated in the furnace were subjected to a second cold rolling step (skin pass) with a reduction ratio of 1.5%.

[0158] FIG. 2 and FIGS. 6 (a) and 6 (b) show examples of the microstructures of the treated steel sheets of examples A, B and C from Table 4 shown in FIG. 11. In all examples according to the invention (A, B and C), the three-layered microstructure of the crystallization structure with different thicknesses of the crystallized near-surface region 1 and the non-crystallized core region 2 can be seen.

[0159] For comparison purposes, a sample of a steel sheet with the composition according to Table 1 (shown in FIG. 8) was subjected to a thermal treatment (annealing) with an annealing temperature that is above the recrystallization temperature of the steel (whereby the sample was not nitrided in the annealing furnace). This example, which is labelled “recrystallized (comparative example)” in Table 2 (shown in FIG. 9), therefore represents a comparative example of a steel sheet that has been completely recrystallized over its entire cross-section. As a further comparative example, a sample of a cold-rolled steel sheet with the same composition according to Table 1 was cold-rolled and neither nitrided nor annealed after cold rolling. This sample therefore remained in the as-rolled condition after cold rolling (comparative example “as-rolled” in Table 2).

[0160] Hardness measurements (Vickers hardness HV0.025) were carried out over the cross-section of the samples using the examples in Table 2 (example A according to the invention and comparative examples “roll-hard” and “recrystallized”). The results of these hardness measurements are shown in FIG. 5. It can be seen from FIG. 5 that the comparative examples “roll-hard” and “recrystallized” exhibit a uniform course of the microhardness over the cross-section (or the thickness of the samples), whereas embodiment example A according to the invention exhibits a pronounced gradual course of the microhardness with a maximum in the middle of the thickness (core region) and minima at the outer edges (region close to the surface).

[0161] In the same way, hardness measurements were also carried out on the 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 FIG. 7, where it can be seen from FIG. 7 that all three examples A, B and C according to the invention exhibit a pronounced gradual progression of the microhardness over the cross-section with a hard core region and softer outer seam regions, whereby the hardness of the sample of example C is the greatest and the microhardness of example B is the smallest. This is due to the fact that sample C has a thinner recrystallized seam region (near-surface region 1) due to the short annealing time (holding time tG during annealing and denitriding) and therefore has a higher hardness than sample B, which has a thicker seam region than the other two samples due to a longer annealing time or holding time during annealing and denitriding (holding time tG) and a higher annealing temperature TG compared to sample A and is therefore the softest.

[0162] The tensile strength (Rm), the 0.2% yield point (Rp0.2) and the elongation at break (A) were determined in tensile tests (in accordance with EN 10202) on the samples of examples A and B from Table 4 and the comparative examples “roll-hard” and “recrystallized” from Table 2. FIG. 4 shows examples of the stress-strain diagrams of the tensile tests for embodiment example A according to the invention and the comparative examples “roll-hard” and “recrystallized”. FIG. 4 shows the different progression of the stress-strain curves of the compared examples, whereby example A according to the invention is characterized by a high tensile strength of more than 600 MPa with a simultaneously 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 (shown in FIG. 10).

[0163] Accordingly, the method according to the invention can be used to produce (nitrogen-containing or nitrided and denitrided in a near-surface region) steel sheets which have a three-layer crystallization structure with a hard core region and a soft outer seam region (near-surface region) and are characterized by a very high strength of more than 500 MPa, in particular more than 600 MPa, with simultaneously 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 as well as parts thereof such as (tear-off) lids.

[0164] The exact composition of the three-layer microstructure, in particular the thickness of the outer seam region as well as the nitrogen content averaged over the cross-section and the difference in the nitrogen content in the outer seam region and in the core region or the gradient of the nitrogen content over the thickness of the steel sheet, can be influenced by varying the process parameters in the method steps of nitriding and denitriding of the method according to the invention. Therefore, the properties of the steel sheets produced using the method according to the invention can be tailored to different applications, in particular with regard to their mechanical properties such as tensile strength, elongation at break and isotropy (mean r-value), as well as their surface properties. In particular, the formability of the steel sheets can be optimized while maintaining the same or a sufficiently high strength.

Claims

1-28. (canceled)29. A steel sheet having a predetermined thickness and based on a weight of the steel sheet a carbon content of 10 to 1000 ppm and a nitrogen content, averaged over the thickness of the steel sheet, of more than 50 ppm, wherein the steel sheet has a multilayer crystallization structure comprising a core region and a near-surface region surrounding the core region on both sides of the core region, wherein the near-surface region is at least substantially recrystallized and the core region is not recrystallized or is at least not completely recrystallized.

30. The steel sheet according to claim 29, wherein the steel sheet has the following composition by weight:C: more than 0.001% and less than 0.1%,Mn: more than 0.01% and less than 0.6%;P: less than 0.04%;S: less than 0.04%;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%;and a nitrogen content averaged over the thickness of the steel sheet of at least 0.005%,as well as residual iron and unavoidable impurities.

31. The steel sheet according to claim 29, wherein the thickness of the steel sheet is lower than 0.5 mm and the near-surface region has a thickness in the range from 5 μm to 200 μm, and / or the core region has a thickness in the range from 50 μm to 450 μm.

32. The steel sheet according to claim 29, wherein the core region has a higher hardness than the near-surface region, wherein the ratio of the hardness of the core region to the hardness of the near-surface region is greater than 1.2.

33. The steel sheet according to claim 29, wherein the core region has a degree of recrystallization of less than 30% and / or wherein the region near the surface has a degree of recrystallization of more than 70%.

34. The steel sheet according to claim 29, wherein the steel sheet has a tensile strength of more than 500 MPa, and / or an elongation at break of at least 4%.

35. The steel sheet according to claim 29, wherein the steel sheet is manufactured in a method comprising the following steps:providing a cold rolled steel sheet having the predetermined thickness, wherein the steel sheet is made of a steel having a carbon content (C) by weight of 10 to 1000 ppm and a predetermined original recrystallization temperature (TR0),wherein the steel has an initial nitrogen content (N0), based on the weight, of at least 70 ppm and / or nitrogen is incorporated into the cold-rolled steel sheet at least in the near-surface region in a nitriding process by exposing the cold-rolled steel sheet to a nitriding gas atmosphere at a maximum nitriding temperature (TA) which is lower than the original recrystallization temperature (TR0) and the nitrogen deposited in the near-surface region during the nitriding process is homogenized over the thickness of the steel sheet by a temperature treatment for homogenizing the nitrogen deposited during the nitriding process at a homogenization temperature (THo) below the original recrystallization temperature (TR0),denitriding of at least the near-surface region of the steel sheet by introducing the steel sheet into a hydrogen-containing gas atmosphere during a predetermined denitriding time (tES), whereby a nitrogen gradient is formed during the denitriding over the thickness of the steel sheet with a nitrogen concentration decreasing from the inner core region outwards towards the near-surface region, so that the near-surface region has a first recrystallization temperature (TR1) and the core region of the steel sheet has a second recrystallization temperature (TR2), wherein the first recrystallization temperature (TR1) is lower than the second recrystallization temperature (TR2) due to the denitriding of the region (1) close to the surface,annealing of the steel sheet at an annealing temperature (TG) which is between the first recrystallization temperature (TR1) and the second recrystallization temperature (TR2), wherein the annealing is taking place during the denitriding and / or after the denitriding.

36. The steel sheet according to claim 35, wherein the denitriding takes place before the annealing at a denitriding temperature (TES) which is below the original recrystallization temperature (TR0), or in that the denitriding takes place simultaneously with the annealing at the annealing temperature (TG), wherein the first recrystallization temperature (TR1) is at least as high as the original recrystallization temperature (TR0) of the steel and in that the second recrystallization temperature (TR2) is above the original recrystallization temperature (TR0) of the steel.

37. The steel sheet according to claim 35, wherein, during the denitriding of the near-surface region, the proportion by weight of nitrogen in the near-surface region is reduced by a predetermined value (ΔN1) by the removal of nitrogen, whereby the recrystallization temperature in the near-surface region is lowered by a value ΔT1 to the first recrystallization temperature (TR1).

38. The steel sheet according to claim 35, wherein the annealing temperature (TG) is less than 750° C. and wherein the steel sheet in the nitriding process is heated to a nitriding temperature (TA), wherein the nitriding temperature (TA) is greater than 300° C.

39. The steel sheet according to claim 35, wherein the nitriding process takes place in a continuous annealing furnace and wherein the subsequent temperature treatment for homogenizing the nitrogen incorporated during the nitriding process is carried out in the continuous annealing furnace, wherein the temperature of the steel sheet in the continuous annealing furnace is corresponding to a homogenization temperature (THo) which is greater than 300° C., and the steel sheet is kept at the predetermined homogenization temperature (THo) during a homogenization period (tHo) which is between 60 seconds and 600 seconds, in a protective gas atmosphere, or, starting from the predetermined homogenization temperature (THo), the temperature of the steel sheet is steadily increased during the homogenization period (tHo).

40. The steel sheet according to claim 35, wherein during the denitriding, the steel sheet is exposed to a hydrogen-containing gas atmosphere for a denitriding time (tES) of between 1 second and 600 seconds wherein the gas atmosphere has a hydrogen content by volume of 1% to 100%, and the denitriding is taking place at a denitriding temperature (TES) which is greater than 300° C. and lower than the original recrystallization temperature (TR0).

41. The steel sheet according to claim 35, wherein the steel sheet is exposed to a hydrogen-containing gas atmosphere during the denitriding, wherein the gas atmosphere has a volume fraction of hydrogen of more than 10% and an ammonia concentration of less than 0.1% by volume.

42. The steel sheet according to claim 35, wherein the steel sheet is heated to the annealing temperature (TG) during annealing within a heating time (tA) of 1.0 seconds to 300 seconds and is held at the annealing temperature (TG) for a predetermined annealing time (tG), wherein the annealing time (tG) is greater than 1 second.

43. The steel sheet according to claim 35, wherein the steel of the cold-rolled steel sheet has an initial nitrogen content (N0) which is less than 160 ppm, and wherein during the nitriding process the average nitrogen content (N) increases to a value averaged over the thickness of the steel sheet which is between 50 ppm and 1000 ppm above the initial nitrogen content (N0) of the steel, wherein a gradient of the nitrogen content (N(x)) is established during the nitriding process over a cross-section of the cold-rolled steel sheet with a nitrogen content decreasing from the near-surface region to the core region, wherein the gradient of the nitrogen content (N(x)) is homogenized by the temperature treatment at the homogenization temperature (THo).

44. The steel sheet according to claim 35, wherein a gradient of the nitrogen content (N(x)) is established during the nitriding process over a cross-section of the cold-rolled steel sheet with a nitrogen content decreasing from the near-surface region to the core region, wherein the gradient of the nitrogen content (N(x)) is homogenized by the temperature treatment at the homogenization temperature (THo), whereby the steel sheet after the heat treatment has an at least substantially constant nitrogen content over the thickness of the steel sheet, which constant nitrogen content is greater than the initial nitrogen content (N0) of the steel.

45. The steel sheet according to claim 35, wherein at least a major proportion of the nitrogen incorporated in the near-surface region of the cold-rolled steel sheet during the nitriding process is incorporated interstitially in a lattice of the steel, and the remaining proportion of the nitrogen is bound as nitrides.

46. The steel sheet according to claim 35, wherein during annealing of the cold-rolled steel sheet, at least a partial recrystallization annealing of the cold-rolled steel sheet takes place in the near-surface region, whereas the core region is not recrystallized.

47. The steel sheet according to claim 35, wherein the value ΔT1, by which the recrystallization temperature of the near-surface region is lowered during the denitriding, is greater than 10° C., and / or in that the value ΔT2, to which the recrystallization temperature in the core region is raised during the temperature treatment for homogenizing the nitrogen incorporated during the nitriding process, is greater than 10° C.

48. The steel sheet according to claim 35, wherein in the nitriding process a nitriding gas is directed onto at least one surface of the cold-rolled steel sheet by means of one or more spray nozzles.

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