EP4146839A1

EP4146839A1 - Steel sheet and method for producing a steel sheet for packaging

Info

Publication number: EP4146839A1
Application number: EP21722147.2A
Authority: EP
Inventors: Laura Pörzgen; Manuel Köhl; Philipp Schmalenbach; Burkhard KAUP
Original assignee: ThyssenKrupp Rasselstein GmbH
Current assignee: ThyssenKrupp Rasselstein GmbH
Priority date: 2020-05-08
Filing date: 2021-04-23
Publication date: 2023-03-15
Also published as: WO2021224026A1; DE102020112485B3; US20230175092A1; JP2023524109A; CA3177443A1; JP7530443B2

Abstract

The invention relates to a method for producing a steel sheet for packaging - cold-rolling a hot-rolled steel sheet made from a steel having a carbon content (C) of 10 to 1000 ppm by weight, the steel of the hot-rolled steel sheet having a predetermined recrystallisation temperature (T_R), - heating the cold-rolled steel sheet to a predetermined heating temperature (T_E), where T_R ≤ T_E, - the heating being performed at least partially in the presence of a nitrogen donor at least until the recrystallisation temperature (T_R) is reached, as a result of which, when the cold-rolled steel sheet is heated, nitrogen from the nitrogen donor is diffused at least into a near-surface region (1) of the cold-rolled sheet steel and is incorporated in the near-surface region (1), as a result of which the recrystallisation temperature (T_R) of the steel in the near-surface region (1) is increased by a value ΔT, where additionally for the heating temperature (T_E): T_E < T_R + ΔT. Using this method, high-strength steel sheets having a multi-layer microstructure can be produced.

Description

Steel sheet and method of making a steel sheet for

packagings

The invention relates to a method for producing a steel sheet and a steel sheet, in particular for packaging.

For the production of packaging from sheet steel, such as tinplate or electrolytic chromium coated steel (ECCS), increasingly thinner steel sheets with thicknesses in the range from 0.1 to 0.25 mm are used for reasons of resource efficiency. In order that sufficiently stable packaging can be produced from thinner steel sheets, the strength of packaging steels must be increased. Furthermore, it must be ensured that the steel sheets remain easily deformable despite their lower thickness and higher strength, so that the steel sheet can be subjected to the strong deformations that occur in the manufacture of packaging in deep-drawing and ironing processes.

From the prior art it is known in principle to increase the strength of steels by introducing unbound nitrogen dissolved in the steel. The introduction of unbound nitrogen into the steel is known as embroidery or nitriding or nitriding and is a well-known process for solid solution strengthening of steel and steel products.

In the case of steel sheets, which are intended for the production of packaging (these are also referred to as packaging steel), it is also known to increase the strength by embroidering the steel. In EP 0 216 399 B1, for example, a steel sheet for packaging purposes and a method for its production is described, which was produced from an aluminum-calmed, continuously cast carbon-manganese steel and obtained an amount of unbound nitrogen by embroidery, the minimum amount being unbound nitrogen is defined as a function of a desired hardness category of the steel sheet and (for example for hardness category T61 of the European Standard 145-78) has an amount of unbound nitrogen of at least 5 ppm. The chemical composition of the steel sheet disclosed there corresponds to the usual soft steels for carbon and manganese content Packaging applications and has, for example, a carbon content in the range of 0.03-0.1% by weight and a manganese content of 0.15-0.5% by weight. The steel sheet is characterized by a high yield point in the range of 350-550 N / mm ² . A maximum value of 100 ppm is specified for the amount of unbound nitrogen dissolved in the steel, which explains why the sheet steel with a higher content of unbound nitrogen can no longer be cold-rolled due to the associated increase in strength and therefore not for the intended use as cold-rolled packaging steel suitable is. In the process for producing this known packaging steel, a steel is first continuously cast, then hot-rolled, cold-rolled, annealed in a recrystallizing manner and finally re-rolled. After rerolling, there is a thermal aftertreatment, in which free dislocations that are formed in the steel by rerolling are fixed by the unbound nitrogen introduced by the embroidery in order to increase the hardness and yield strength above the values after rerolling. The thermal aftertreatment can expediently be combined with another thermal treatment of the re-rolled steel, which has to be carried out in any case in the context of the production of packaging steel, such as melting a tin layer electrolytically applied to the surface of the steel sheet or baking a lacquer layer applied to the steel sheet surface .

Because of the upper limit proposed in EP 0 216 399 B1 for the amount of unbound nitrogen dissolved in the steel of a maximum of 100 ppm, the strengths of this known packaging steel are limited. Higher strengths, in particular of more than 600 MPa, can be achieved without impairing the cold rollability of packaging steels by introducing a higher amount of nitrogen if the embroidery is carried out in two stages, namely in a first stage in which the steel melt is in such a large amount Nitrogen is added so that the hot strip produced from the steel melt can still be cold-rolled, and in a second stage, in which the cold-rolled steel sheet is further embroidered in an annealing furnace during recrystallization annealing in a nitrogen-containing gas atmosphere, in particular in an ammonia atmosphere, to reduce the nitrogen content in the cold rolled steel sheet beyond the initial nitrogen content of the molten steel. Such a two-stage embroidery of a packaging steel is described in EP 3 186401 A1. From US Pat. No. 3,219,494 a method for the production of high-strength steel strips for the production of tinplate and other packaging steels by embroidering the steel strip is known, in which the steel strip wound into a coil is embroidered in a hood annealing furnace to initially create a nitrogen-rich outer shell in the To achieve steel strip, the embroidering in the hood annealing furnace is carried out by an ammonia gas atmosphere, and a uniform distribution of the nitrogen introduced near the surface over the thickness of the steel strip by diffusion of the nitrogen when the steel sheet is heated in an inert gas atmosphere to temperatures above the recrystallization temperature , whereby the nitrogen from the nitrogen-rich outer shell can diffuse through the steel belt into its core area. In the case of steel sheets with a thickness of 0.25 mm, strengths in the range from 439 MPa to 527 MPa could be achieved.

The object of the invention is to provide a flat steel product (sheet steel or steel strip) for the production of packaging, which has the highest possible strength with at the same time good elongation at break and good deformation properties. In particular, a packaging steel with strengths of at least 600 MPa with an elongation at break of at least 5% is to be made available. The higher-strength packaging steel must also have sufficient formability for the intended use as packaging steel, for example in deep-drawing or stretch-drawing processes, so that packaging, such as food or beverage cans, can be manufactured from the flat steel product as intended. In particular, strong roughening of the surface should be avoided when deep drawing or when stretching. The packaging steel present as a flat steel product should have the usual thicknesses in the ultra-thin sheet range of 0.5 mm or less and in particular in the range from 0.15 mm to 0.25 mm, this thickness of the packaging steel being produced by cold rolling. Another object of the invention is to provide a method of manufacturing such a steel sheet for packaging.

These objects are achieved with a method with the features of claim 1 and with a steel sheet with the features of claim 21. Preferred Embodiments of the method according to the invention and of the steel sheet are shown in the dependent claims.

When speaking of sheet steel, a flat steel product in the form of a sheet metal sheet or a strip is meant. Information in% or ppm relating to a content or a concentration of an alloy component of the steel or of the cold-rolled steel sheet relate in each case to the weight of the steel or of the steel sheet.

In the method according to the invention for producing a steel sheet for packaging, a hot-rolled steel sheet is first cold-rolled, which is made from a steel with a carbon content (C) based on weight of 10 to 1000 ppm, the steel of the (cold-rolled) steel sheet having a ( _{has a recrystallization temperature (T R} ) which is essentially predetermined by the steel composition. The cold-rolled steel sheet is heated to a predetermined (maximum) heating temperature (T _{E) in the presence of a nitrogen donor.} The heating temperature T _E is preferably the maximum temperature in the thermal treatment of the cold-rolled steel sheet, that is, neither before, during or after the thermal treatment according to the method according to the invention, the cold-rolled steel sheet is raised to temperatures higher than the (maximum) heating temperature T _E warmed. When the cold-rolled steel sheet is heated, nitrogen from the nitrogen donor is stored at least in a near-surface (seam) area of the steel sheet, in that (atomic) nitrogen from the nitrogen donor diffuses into the near-surface (seam) area. As a result, the recrystallization temperature (T _R ) of the steel in the near-surface (seam area is raised by a value ΔT. _{According to the invention, the heating temperature (T E} ) is selected so that T _R <T _E <T _R + ΔT applies (T _E ) is thus set in the method according to the invention in such a way that it changes between the (original) recrystallization temperature (T _R ) of the steel used for the production of the cold-rolled steel sheet and that caused by the near-surface embroidery of the steel sheet in the near-surface (seam) area the value ΔT increased recrystallization temperature (T _R + ΔT).

In order to achieve an at least essentially complete recrystallization of the core area (or of the one layer), the temperature of the steel sheet after reaching the heating temperature is over a predetermined annealing time (t _G ) at the heating temperature (T _E ) held. The annealing time is preferably more than 1 second, in particular in the range from 1 to 80 seconds, preferably in the range from 1 to 10 seconds. In the case of longer annealing periods of, for example, more than 10 seconds, in addition to complete recrystallization of the core area, the distribution of the nitrogen over the thickness of the steel sheet is evened out.

By setting the heating temperature (T _E ) according to the invention, a multi-layer microstructure, in particular a two- or three-layer microstructure, is formed over the cross section of the steel sheet. The two-layer microstructure has an at least substantially recrystallized first layer and a not or at least not completely recrystallized second layer. The three-layer microstructure contains an inner, at least substantially recrystallized core area and a near-surface hem area surrounding the core area on both sides, the hem area not or at least not completely recrystallized and thus still (from cold rolling) as hard as possible. When a seam area is spoken of here, an area of the steel sheet is meant which has a microstructure which has not or at least not completely recrystallized and which is at least partially still as hard as possible. When a core area is spoken of here, an area of the steel sheet is meant which has an at least substantially recrystallized microstructure which, due to the recrystallization, is softer than the hard-rolled edge area.

The three-layer microstructure has the shape of a “sandwich” with an at least essentially, preferably largely completely recrystallized core area and a near-surface edge area surrounding the core area, the edge area not or at least not completely recrystallized. This three-layer structure according to the invention is therefore also referred to below and in the drawings as a “sandwich structure”.

The invention therefore also relates to a steel sheet which can be used in particular for the production of packaging and is made from a steel with a carbon content (C) based on weight of 10 to 1000 ppm and a thickness of less than 0.5 mm has, wherein the steel sheet has an at least substantially recrystallized core area and a core area surrounding or at the Has adjacent core area, near the surface and not or at least not completely recrystallized edge area.

It has surprisingly been shown that steel sheets with a corresponding microstructure (in particular a three-layer “sandwich structure”) have a very high strength of more than 800 MPa with an extensibility (elongation at break) of at least 5%, which is acceptable for deep-drawing applications. The steel sheets according to the invention are therefore very suitable for deep-drawing applications, for example for the production of packaging or body parts in automobile construction.

The value ΔT by which the recrystallization temperature in the edge area increases due to the incorporation of nitrogen when the steel sheet is heated is preferably greater than 50 ° C. and particularly preferably greater than 100 ° C. and is in particular in the range from 100 ° C. to 250 ° C.

The value ΔT by which the recrystallization temperature in the seam area increases as a result of the incorporation of nitrogen when the steel sheet is heated depends on the nitrogen content introduced into the seam area of the steel sheet after the end of the heating process by embroidery, whereby a linear relationship can be observed in particular can be described by ΔT = aAN (ppm), where a is a constant of proportionality and ΔN (ppm) is the nitrogen content in ppm (based on the weight of the steel) introduced into the seam area when the steel sheet is heated. By means of tests on samples embroidered with different nitrogen contents and with otherwise the same alloy composition, a value of a = 1.2 K / ppm could be determined as an example. Accordingly, an increase in the recrystallization temperature in the border area in the range of ΔT = 10 K to 24 K can be achieved even with a low nitrogen concentration of DN in the range from 10 ppm to 20 ppm (corresponding to 0.001 to 0.002% by weight). With a higher nitrogenization of, for example, ΔN = 100 ppm (corresponding to 0.01% by weight), the (theoretically achievable) increase in the recrystallization temperature in the seam area is already ΔT = approx. 120 K.

The steel sheet according to the invention can also be produced in a two-stage thermal treatment, with embroidering in a first stage at an intermediate temperature and in a second stage at a higher temperature than the intermediate temperature Heating temperature annealing takes place (which is why the heating temperature is also referred to below as the annealing temperature). The intermediate temperature is lower than the recrystallization temperature T _R. In the two-stage process, the steel sheet is heated in the first stage from room temperature to the intermediate temperature T _Z <T _R within a first heating time and is kept at least approximately at this temperature during a holding time ΪH. The intermediate _{temperature T Z} is preferably in the range from 300 ° C. to 600 ° C., particularly preferably between 400 ° C. and 550 ° C., because from a temperature of approx. 300 ° C. when ammonia gas is used as the nitrogen donor, the dissociation to atomic nitrogen begins on metallic surfaces. In any case, at temperatures up to approx. 550 ° C., there is still no (complete) recrystallization for most of the alloy compositions according to the invention. With the preferred

Intermediate _{temperatures T Z} in the first stage therefore diffusion of the dissociated nitrogen from the nitrogen donor into the edge area of the steel sheet takes place, but there is still no recrystallization. The recrystallization of the core area only takes place in the second stage, in which the steel sheet is _{heated to the heating temperature (annealing temperature) T E} , which is equal to or greater than the original recrystallization temperature T _R but is below T _R + ΔT. The heating temperature T _E is, for example, in the range from 650.degree. C. to 800.degree. C., depending on the value of the original recrystallization temperature T _{R of} the steel used, and in particular around 750.degree.

As with the single-stage heating of the steel sheet in the first exemplary embodiment, at least essentially only one (partial) recrystallization of the core area of the steel sheet also takes place in the two-stage method. Recrystallization of the hem area embroidered in the first stage is prevented by the heating temperature (annealing temperature) T _E _ being chosen so that it is below the level caused by the embroidering of the

Recrystallization temperature T _R + ΔT increased by .DELTA.T. The following applies: T _R T _E T _R + ΔT.

In the two-stage thermal treatment, the steel sheet is expediently first in a continuous _{annealing furnace in a first stage at a lower intermediate temperature T Z} , which is below the (original) recrystallization temperature T _{R of} the steel, during a holding time, which is preferably in the range from 10 to 150 Seconds, embroidered in the hem area and in a second stage at a heating temperature (annealing temperature T _E ) which is higher than the intermediate temperature and which is above the The (original) recrystallization temperature of the steel is _{at least partially recrystallizing annealed in the core area during an annealing time t G} , which is preferably in the range from 1 to 300 seconds, particularly preferably from 1 to 10 seconds. The first heating time (ΪE ¹ ), in which the steel sheet is heated from room temperature to the intermediate temperature in the two-stage process, is preferably in the range from 1.0 to 120 seconds, particularly preferably between 10 and 90 seconds, and can vary according to the desired material properties of the steel sheet according to the invention can be adapted, as in the first embodiment. The holding time (ΪH) in which the steel sheet is held at the intermediate temperature is also preferably in the range from 1.0 to 90 seconds, particularly preferably between 10 and 60 seconds, and is also selected according to the desired material properties of the steel sheet according to the invention. After the holding time has elapsed, the steel sheet can be heated to the heating temperature T _E (annealing temperature) either after cooling or immediately without cooling in the second stage in a second heating time (t _E ² ) and at least approximately at this heating temperature during an annealing period (t _G) T _{E are} held. During the annealing period, the nitrogen donor can optionally be present in the annealing furnace, which makes dissociated (atomic) nitrogen available so that the steel sheet can be (further) embroidered _{during the annealing period t G.} This leads to an embroidering of the core area and thus to an improvement in the deformability of the steel sheet, in particular to a higher ductility

(Elongation at break). According to the invention, the heating temperature T _E (annealing temperature) lies between the original recrystallization temperature T _R _{and the recrystallization temperature increased to T R} + ΔT as a result of the stitching in the seam area of the steel sheet. The following also applies here in the two-stage process for the heating temperature T _E : T _R <T _E <T _R + ΔT, the intermediate _{temperature (T Z} ) being lower than the original recrystallization temperature T _R.

In both the single-stage and the two-stage process, the steel sheet is exposed at least temporarily to the nitrogen donor during the heating up and before the recrystallization temperature is reached, which provides dissociated (atomic) nitrogen in the annealing furnace, which initially diffuses into the steel sheet close to the surface and thereby in the Edge area increases the recrystallization temperature. The steel of the cold-rolled steel sheet preferably has the following composition based on weight:

• Carbon, C: more than 0.001% and less than 0.1%, preferably less than 0.06%;

Manganese, Mn: more than 0.01% and less than 0.6%; Phosphorus, P: less than 0.04%; • sulfur, S: less than 0.04% and preferably more than 0.001%;

• Aluminum, Al: less than 0.08%;

• silicon, Si: less than 0.1%;

• optional copper, Cu: less than 0.1%;

• optional chromium, Cr: less than 0.1%;

• optional Ni disgust, Ni: less than 0.1%;

• optional titanium, Ti: less than 0.1% and preferably more than 0.02

%; optionally niobium, Nb: less than 0.08% and preferably more than 0.01%;

• optional molybdenum, Mo: less than 0.08%;

• optional tin, Sn: less than 0.05%;

• optional boron, B: less than 0.01%, preferably less than 0.005

% and preferably more than 0.0005%;

• optionally nitrogen, N ₀ : less than 0.02%, in particular less than

0.016%, and preferably more than 0.001%;

• remainder of iron and unavoidable impurities,

• wherein the weight average nitrogen content (N) of the steel sheet after heating the cold-rolled steel sheet in the presence of the nitrogen donor is at least 0.015% and preferably at least 0.02%.

When speaking of an average nitrogen content (N) or an average nitrogen content, the nitrogen concentration averaged over the respective thickness is meant. The mean nitrogen content (N) of the steel sheet accordingly means the nitrogen concentration averaged over the thickness of the steel sheet. The steel of the cold-rolled steel sheet can already have an initial nitrogen content N ₀ of preferably more than 0.001% by weight and less than 0.02% by weight and particularly preferably less than 0.016% by weight. However, a steel can also be used which, apart from unavoidable nitrogen impurities, does not contain any nitrogen. Limiting the original nitrogen content to values of less than 0.02% by weight enables problem-free cold rolling of the hot strip produced from the steel by hot rolling with the usual cold rolling devices (rolling mills for the cold rolling of steel sheets to fine sheets). Furthermore, with low initial nitrogen contents of N ₀ <0.02% by weight in the steel, the formation of defects during the casting of a slab is prevented. In order to achieve the highest possible (average) nitrogen content in the cold-rolled steel sheet, and thereby to achieve a high solid solution strengthening, it is advantageous if the steel used to produce the hot strip already has an (initial) nitrogen content, which is preferably in the range of 0.001 wt .% to 0.02% by weight and particularly preferably between 0.005% by weight and 0.016% by weight.

If the steel of the cold-rolled steel sheet has an initial nitrogen content (N ₀ ), when the cold-rolled steel sheet is heated, particularly in the seam area, the nitrogen content increases to a _{value above the initial nitrogen content (N 0} ). If the weight-related nitrogen content (N _S ) in the hem area increases to a value averaged over the thickness of the hem area, which is preferably more than 50 ppm and particularly preferably between 400 and 800 ppm above the initial nitrogen content (N ₀ ) Steel, an (sometimes considerable) increase in the strength of the steel sheet is observed. Strengths of the steel sheet of more than 800 MPa with elongations at break of at least 5% can be achieved. The (mean) nitrogen content (N _S ) of the nitrogen stored in the hem area as a result of the embroidery can reach up to the solubility limit of nitrogen in iron of approx. 1000 ppm. Depending on the setting of the process parameters, in particular the holding time and the annealing time, as well as the (optionally present) concentration of the nitrogen donor in the annealing furnace during annealing of the steel sheet at the heating temperature (annealing temperature), the core area of the steel sheet is also embroidered to a certain extent. The embroidering of the core area can be (considerably) less. However, it can also be a The core area is embroidered to nitrogen concentrations which at least approximately correspond to the nitrogen concentrations of the hem area. Embroidering the core area results in a nitrogen content (N _K ) in the core area that is at least greater than the initial nitrogen content (N ₀ ) of the steel, the difference between the weight-related nitrogen content (N _K ) in the core area and the initial nitrogen content (N ₀ ) of the steel is preferably greater than 30 ppm. By embroidering the core area to a nitrogen concentration> 30 ppm above the initial nitrogen content (N ₀ ) of the steel (ie to an average nitrogen content in the core of N _K > N ₀ + 30 ppm), the steel sheet can have sufficient ductility, in particular an elongation at break of more than 5%, as a result of which a formability sufficient for deep-drawing applications is achieved. A stitching of the core area, which is preferable with regard to good deformability, is, however, not absolutely necessary in order to form the multilayer structure (in particular the three-layer sandwich structure) according to the invention.

In preferred exemplary embodiments, the steel from which the steel sheet according to the invention is produced by hot rolling and subsequent cold rolling contains, based on the weight, more than 0.001% and less than 0.1% C, more than 0.01% and less than 0.6% Mn, less than 0.04% P, less than 0.04% S, less than 0.08% A1, less than 0.1% Si, and optionally an initial nitrogen content (N ₀ ) of up to 0.020% and preferred 0.016% or less and the balance iron and unavoidable impurities. After embroidering, the steel sheet preferably contains an average nitrogen content N> N ₀ of at least 0.020% or more, particularly preferably 0.025% or more and in particular in the range from 0.040 to 0.080% N. The tensile strength of the steel sheet is at least 800 MPa, especially preferably at least 900 MPa, with a simultaneous elongation at break in the range from 2% to 10%.

The nitrogen stored in the seam area when the cold-rolled steel sheet is heated can be present (up to the solubility limit) in dissolved form and / or in bound form as nitride. If strong nitride formers are present in the steel, the stored nitrogen is at least partially present as nitrogen bound in nitrides, in particular as TiN and / or NbN and / or AlN. Depending on the concentration of nitrogen in the nitrogen donor, a superficial nitride layer, in particular an iron nitride layer, can also form on the free surface of the skirt layer (that is to say the surface facing away from the core). The formation of a superficial nitride layer depends very essentially on the concentration of nitrogen in the nitrogen donor. When using a gas atmosphere with ammonia as nitrogen donor, the formation of a nitride layer on the surface of the border area can be observed, for example on a laboratory scale from an ammonia content of approx. 2-3% by volume. The nitride layer is very thin compared to the thickness of the seam area and has a thickness in the range of approx. 10 μm or less. The volume fraction of ammonia in the gas atmosphere relates to the conditions in a laboratory test in which steel sheets were heated with induction heating, the volume fraction of ammonia in the gas atmosphere of the laboratory furnace being determined at room temperature.

A particularly sharp demarcation of the recrystallized core and the not or at least not completely recrystallized edge area of the steel sheet can be observed if the steel contains a sufficient amount of strong nitride formers such as Ti, Nb, Mo and / or Al. The steel of the cold-rolled steel sheet therefore preferably contains more than 200 ppm titanium and / or more than 100 ppm niobium and / or more than 50 ppm aluminum, based on the weight. The sum of the proportions by weight of the strong nitride formers is particularly preferably more than 300 ppm and preferably more than 500 ppm. Steel sheets with a titanium content (Ti) based on weight of at least 500 ppm are particularly advantageous. The strong nitride formers such as Ti, Nb and / or aluminum bind the nitrogen of the nitrogen donor stored in the seam area and thereby prevent the nitrogen, which is initially only stored near the surface, from diffusing further into the interior of the steel sheet. Thus a very thin seam layer with a high nitrogen content compared to the plate thickness is produced on the surface of the steel sheet, which leads to a strong increase in the recrystallization temperature in this seam area. If, according to the method according to the invention, the cold-rolled steel sheet is heated to temperatures which, on the one hand, are higher than the (original) recrystallization temperature of the steel in the (not or only slightly embroidered) core area of the steel sheet and, on the other hand, lower than the (increased) recrystallization temperature of the embroidered hem area , only the core area is (completely) recrystallized and the edge area remains uncrystallized due to the increased recrystallization temperature. This results in the multilayer structure (sandwich structure) of the steel sheet according to the invention with sharp boundaries between the seam area and the core area. Of the The non-recrystallized seam area remains as hard as rolling and gives the steel sheet a high level of strength. The recrystallized core area, on the other hand, gives the steel sheet good ductility and thus good formability. In the case of steel sheets that do not contain strong nitride formers, a multilayer microstructure can still be created with regard to the degree of crystallization, but the transitions between the hard-rolled, non-crystallized edge area and the at least partially recrystallized core area are less sharp.

The formation of the multilayer structure in relation to the degree of crystallization of the steel structure can be influenced by the heating time and the annealing time. The cold-rolled steel sheet is preferably heated from room temperature (optionally in stages, with holding phases inserted in between, in which the temperature is kept constant during a holding time) to the heating temperature (T _E ) within a heating time of 1.0 to 120 seconds and over an annealing period (t _G ) held between 1.0 and 90 seconds at the heating temperature. A short heating time and a short annealing time contribute to a stronger formation of a nitrogen gradient, since with short heating times and annealing times the nitrogen, which is initially only stored on the surface of the steel sheet, cannot diffuse into the core area. With longer annealing times, a diffusion of nitrogen from the seam area into the core area can be observed, with the result that the core area of the steel sheet is also embroidered on.

The thickness of the non-recrystallized seam area can be controlled by the heating time, it being possible to observe a linear relationship between the thickness of the seam area and the heating time. In this way, an adjustable process parameter of the process according to the invention is available over the heating time, via which the thickness of the non-recrystallized and therefore hard-as-rolled edge area can be set in a targeted manner. Depending on the selected heating time, the thickness of the seam area is in the range from 5 μm to 150 μm and preferably in the range from 10 μm to 100 μm and in particular between 30 μm and 80 μm.

When the cold-rolled steel sheet is heated in the presence of the nitrogen donor, a gradient in the nitrogen content (N _S ) is established in the seam area, the nitrogen content from the surface to a core area of the cold-rolled steel sheet decreases. The amount of nitrogen stored in the border area can be controlled via the nitrogen concentration of the nitrogen donor.

The nitrogen donor can be formed, for example, by a nitrogen-containing gas atmosphere with atomic nitrogen, in which the steel sheet is heated. If a gaseous nitrogen donor, such as ammonia gas, is used, a three-layer structure is formed in the steel sheet with an inner, recrystallized core and two outer, non-crystallized seam areas that surround the core on both sides. The steel sheet is expediently heated in an annealing furnace with a nitrogen-containing gas atmosphere, in particular in a continuous annealing furnace through which the steel sheet in the form of a strip is passed. The nitrogen-containing gas atmosphere can in particular be provided by introducing ammonia gas into the annealing furnace, with the ammonia molecules thermally dissociating into atomic nitrogen during heating, which can diffuse into the surface of the steel sheet. The volume concentration of the ammonia in the nitrogen-containing gas atmosphere is preferably more than 0.1%, in particular between 0.1% and 10% and particularly preferably between 0.1% and 3%, in particular between 0.5% and 2.5% . The cold-rolled steel sheet is particularly preferably heated in an inert protective gas atmosphere, which in particular contains HNx, the volume concentration of the HNx in the nitrogen-containing gas atmosphere preferably being between 90% and 99.5%.

The nitrogen donor can also comprise a nitrogen-containing liquid or be formed by a nitrogen-containing liquid which is applied to one or both sides of the surface of the cold-rolled steel sheet before or during the heating. By applying a nitrogen-containing liquid as a nitrogen donor to a surface of the steel sheet only on one side, a two-layer microstructure with an upper, non-recrystallized (and therefore hard-as-roll) layer facing the liquid nitrogen donor and a recrystallized layer underneath can be formed. The lower, recrystallized layer forms the core area and the overlying, non-recrystallized layer forms a hard-rolled edge area.

With the method according to the invention, steel sheets made of a steel with a carbon content (C) based on weight of 10 to 1000 ppm and a Thickness of less than 0.5 mm can be produced, which contain 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. The multilayer microstructure can include two layers, namely an at least substantially recrystallized first layer and a not or at least not completely recrystallized second layer. The multilayer microstructure can also be designed as a three-layer microstructure with an inner, at least substantially recrystallized core area and a near-surface hem area surrounding the core area on both sides, the hem area not or at least not completely recrystallized and thus still hard (from cold rolling).

A three-layer microstructure with an inner, recrystallized core and two outer, not or at least not completely crystallized seam areas, which surround the core on both sides, has particularly good deformation properties. The outer, hard-rolled seam area prevents larger grains from the recrystallized core area from being able to push themselves through to the surface of the steel sheet during forming, which on the one hand creates undesirable optical effects and on the other hand leads to increased porosity and cracks on the surface paint applied to the steel sheet. A preferred embodiment of the steel sheet according to the invention therefore has a three-layer microstructure (“sandwich structure”) with an inner, recrystallized core and two outer, non-crystallized or at least incompletely crystallized seam areas that surround the core on both sides.

With the method according to the invention it is possible to produce steel sheets which have a tensile strength of more than 800 MPa and preferably of more than 950 MPa and an elongation at break of more than 4% and preferably of more than 5%. After embroidering, the hem area of the steel sheet preferably has a Vickers hardness of at least 220 HVO, O25 and particularly preferably of at least 300 HV _0.025 . The Vickers hardness in the core area is preferably at least 100 HV _0.025 and less than 280 HV _0.025 .

If the average nitrogen content (N _S ) in the seam area of the steel sheet, based on the weight after embroidering, is between 400 and 800 ppm, particularly high strengths and hardness values can be achieved. At the mean nitrogen content (N _S ) in that Edge area is the mean concentration of dissolved nitrogen over the thickness of the edge area. This can reach up to the solubility limit of nitrogen in steel of approx. 1000 ppm. At least in the seam area of the steel sheet, when embroidering, i.e. when the steel sheet is heated in the presence of the nitrogen donor, a gradient in the nitrogen content is formed, the nitrogen content in the seam area decreasing from the surface to the core area. Depending on the concentration of nitrogen in the nitrogen donor and / or the duration of annealing, nitrogen build-up (ie an increase in the nitrogen content above the initial nitrogen content N _{0 of} the steel) can be observed both in the seam area and in the core area. The embroidery in the hem area is usually larger than in the core area, ie in the hem area there is a greater mean nitrogen concentration after embroidery than in the core area, so that overall, over the entire thickness of the steel sheet, for example with a three-layer structure ("sandwich structure") Nitrogen gradient with nitrogen content decreasing from the outside to the inside or, in the case of a two-layer structure (with an upper hem area and a lower core area), a nitrogen gradient with a nitrogen content decreasing from the hem area to the core area. This means that the edge area of the steel sheet has a higher hardness or higher tensile strength than the core area. The ratio of the hardness of the hem area to the hardness of the core area is preferably greater than 1.2 and particularly preferably greater than 1.4.

The individual layers of the two- or three-layer structure differ from one another not only in terms of their hardness or strength but also in terms of their texture. Thus, the at least substantially recrystallized first layer and the hardly or not at all recrystallized second layer can be distinguished from one another, for example, by the ratio of the {001} orientation and the {III} orientation in the e-fiber. The relationship between the {001} -orientation and the {111} -orientation in the e-fiber can be defined as the “deformation index”, which characterizes the deformation behavior of the steel sheet. A {111} orientation enables good formability and has a good Lankford coefficient (r value), whereas the {001} orientation is less easy to form. The e-fiber is defined by the <110> vector lying parallel to the transverse direction (perpendicular to the rolling direction and to the normal direction in the strip plane of the steel strip). In the steel sheets according to the invention, each has recrystallized first layer (which is on the inside in a three-layer "sandwich structure") has a deformation index of less than 0.8 and the second layer (which is on the outside in a three-layer "sandwich structure") has a deformation index of more than 2.0 and in particular im Range from 2.0 to 5.0. Corresponding characterizations of the textures in the first and second layers can also be defined for other fibers of the structure, for example for the a-fiber, which is defined by the <110> vector lying in the rolling direction.

To achieve particularly high tensile strengths of 900 MPa or more, a nitrogen content generated by embroidering the steel is preferably present both in the seam area and in the core area, which is higher than the initial nitrogen content (N ₀ ) of the steel, the (average) nitrogen content ( N _S ) in the hem area can be higher than the (average) nitrogen content (N _K ) in the core area or it can also be the same. With the method according to the invention it was possible to produce steel sheets with tensile strengths of up to 1100 MPa.

With an increase in the nitrogen content both in the seam area and in the core area, the ductility (elongation at break) of the steel sheet increases. Therefore, the ratio of the nitrogen content (N _S ) of the border area to the nitrogen content (N _K ) of the core area is preferably less than 2.8, preferably less than 2.5. The steel sheet is preferably embroidered in the core area to such an extent that the elongation at break of the steel sheet is more than 4%, preferably more than 5%. With the method according to the invention it was possible to produce steel sheets with elongations at break of more than 10% and tensile strengths of over 1000 MPa.

A particularly sharp demarcation of the seam area from the core area is given if the steel in the seam area has a degree of (re) crystallization of less than 30%, preferably less than 20%, and / or if the core area has a degree of (re) crystallization of more than 70%, preferably more than 80%.

A particularly sharp delimitation of the hem area from the core area can be achieved if the following applies to the heating temperature (T _E ): T _E = T _R + ΔT / 2. The heating temperature T _E is preferably in the range from T _R + ΔT / 3 to T _R + 2 ΔT / 3. The area of application of the steel sheets according to the invention is not limited to the area of packaging steels, but extends, for example, to steel sheets for the production of motor vehicle bodies or housings for machines.

These and other advantages of the packaging steel according to the invention and of the production method emerge from the exemplary embodiments described in more detail below with reference to the accompanying drawings. The drawings show:

FIG. 1: a schematic representation of temperature-time diagrams of the method according to the invention in three different exemplary embodiments, the exemplary embodiment in FIG. La showing a single-stage and the exemplary embodiments in FIGS. 1b and 1c each showing a two-stage heating of the steel sheet in an annealing furnace;

Figure 2: Schematic representation of the microstructure of the invention

Sheet steel in a first embodiment with a three-layer structure (Figure 2a) and in a second embodiment with a two-layer structure (Figure 2b);

FIG. 3: Comparison of microscopic cross-sectional recordings from

Structure of a conventional steel sheet (FIG. 3a) and a steel sheet according to the invention with a three-layer structure (FIG. 3b);

Figure 4: Comparison of the microscopic images of the microstructure of a conventional steel sheet (Figure 4a) and a steel sheet according to the invention with a three-layer structure (Figure 4b), the cross-sectional profile of the Vickers hardness (HV 0.05) over the thickness of the steel sheet is shown;

Figure 5: Comparison of the on a comparative example (conventional

Sheet steel) and a stress-strain diagram measured on an exemplary embodiment of the sheet steel according to the invention; FIG. 6: Representation of microscopic cross-sectional recordings of the _{microstructure of exemplary embodiments of steel sheets according to the invention with a three-layer microstructure which have been embroidered at a different heating temperature (T E} , FIG. 6a) or with different annealing durations (tG, FIG. 6b);

FIG. 7: Representation of microscopic cross-sectional images of the _{microstructure of exemplary embodiments of steel sheets according to the invention with a three-layer microstructure which have been embroidered in an annealing furnace in the presence of a nitrogen donor (NH 3} gas atmosphere) with different nitrogen concentrations (volume fraction of NH ₃ in the gas atmosphere);

FIG. 8: Representation of the course of the seam thickness of the seam area, which occurs when embroidering steel sheets with the method according to the invention in

Sheet steel forms, depending on the nitrogen concentration of the nitrogen donor used when embroidering (ammonia content of the gas atmosphere);

FIG. 9: Representation of the course of the microhardness measured on exemplary embodiments according to the invention of 0.25 mm thick steel sheets over the cross section of the steel sheet;

FIG. 10: Representation of the course of the seam thickness of the seam area, which occurs when embroidering steel sheets with the method according to the invention in

Sheet steel forms, depending on the annealing time (t _G ), over which the steel sheet has been kept at the heating temperature (t _E = 750 ° C);

FIG. 11: Representation of the course of the strength versus elongation of steel sheets according to the invention _{which have been embroidered using the method according to the invention at a heating temperature t E} = 750 ° C. and different heating times; FIG. 12: Gradient distribution on the nitrogen content over the cross-sectional profile of a steel sheet according to the invention;

FIG. 13: Representation of the influence of the nitrogen content of steel sheets embroidered at different heights on the hardness and the recrystallization temperature on the basis of hardness diagrams as a function of the heating temperature during recrystallization annealing;

FIG. 14: Representation of the dependence of the microhardness (Vickers hardness HV _0.025 ) and the recrystallization temperature on the nitrogen content of steel sheets embroidered at different heights;

FIG. 15: Representation of the curve of the microhardness over the cross section of exemplary embodiments of steel sheets according to the invention which _{have been embroidered with different levels of ammonia content (NH 3} in% by volume) in the annealing furnace;

FIG. 16: Representation of a microscopic cross-sectional image of the microstructure of an exemplary embodiment of a steel sheet according to the invention with a three-layer microstructure,

FIG. 17: Representation of the course of the microhardness of a steel sheet according to FIG

Invention over the cross section;

FIG. 18: Representation of the strength-elongation diagram of steel sheets according to FIG

Invention with different degrees of skin pass (DG);

FIG. 19: Representation of the orientation density distribution function f (g) along the e-fiber of a steel sheet according to the invention with a three-layer structure (sandwich structure).

The starting product for the production of steel sheets according to the invention using the method according to the invention are hot-rolled and then cold-rolled steel sheets with a carbon content of 10 to 1000 ppm by weight used. The alloy composition of the steel expediently fulfills the limit values specified by standards for packaging steel (such as defined in ASTM A623-11 “Standard Specification for Tin Mill Products” or in “European Standard EN 10202”), but can in particular with regard to the original nitrogen content differ from this if, in particular, highly embroidered steel sheets with a high nitrogen content of more than 0.02% by weight 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. Therefore, the steel preferably contains more than 0.001% by weight of carbon. In order to ensure the rollability of the steel sheet during primary cold rolling and, if necessary, 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%;

Manganese also increases hardness and strength. Manganese also improves the forgeability, weldability and wear resistance of steel. Furthermore, the tendency to red breakage during hot rolling is reduced by adding manganese, and manganese leads to grain refinement. Therefore, a manganese content of at least 0.01% by weight is preferable. To achieve high strengths, a manganese content of more than 0.1% by weight, in particular of 0.20% by weight or more, is preferred. However, if the manganese content becomes too high, the corrosion resistance of the steel is at the expense. 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 or formed. Therefore, the preferred upper limit for the manganese content is 0.6% by weight.

• Phosphorus, P: less than 0.04%

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. • Sulfur, S: less than 0.04% and preferably more than 0.001%

Sulfur is an undesirable accompanying element that deteriorates ductility and corrosion resistance. The steel should therefore not contain more than 0.04% by weight of sulfur. On the other hand, elaborate and cost-intensive measures have to be taken to desulphurize steel, which is why, from an economic point of view, a sulfur content of less than 0.001% by weight is no longer justifiable. The sulfur content is therefore preferably in the range from 0.001% by weight to 0.04% by weight, particularly preferably between 0.005% by weight and 0.01% by weight.

• Aluminum, Al: less than 0.08%

In steel production, aluminum acts as a deoxidizer in the casting process to calm the steel. Aluminum also increases the scaling resistance and formability. In addition, aluminum forms nitrides with nitrogen, which are advantageous in the steel sheets according to the invention. Therefore, aluminum is preferably used in a concentration of 0.005% by weight 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.

• silicon, Si: less than 0.1%;

Silicon increases the scaling resistance in steel and is a solid solution hardener. In steel production, it has the positive effect of making the melt thinner and serves as a deoxidizer. Another positive influence of silicon on steel is that it increases tensile strength, yield strength and resistance to scaling. Therefore, a silicon content of 0.003 wt% or more is preferable. However, if the silicon content becomes too high, and in particular exceeds 0.1% by weight, the corrosion resistance of the steel can be deteriorated and surface treatments, particularly by means of electrolytic coatings, can be made difficult.

• optionally nitrogen, N ₀ : less than 0.02%, in particular less than 0.016%, and preferably more than 0.001%

Nitrogen is an optional component in the steel melt from which the steel for the steel sheets according to the invention is produced. Admittedly, nitrogen acts as a Solid solution hardener increasing hardness and strength. However, if the nitrogen content in the steel melt is too high of more than 0.02% by weight, the hot strip produced from the steel melt can no longer be cold-rolled. Furthermore, a high nitrogen content in the steel melt increases the risk of defects in the hot strip, since the hot formability is lower at nitrogen concentrations of 0.016% by weight or more. According to the invention, provision is made for the nitrogen content of the steel sheet to be subsequently increased by stitching on the cold-rolled steel sheet in an annealing furnace. The introduction of nitrogen into the steel melt can therefore be dispensed with entirely. To achieve strong solid solution strengthening, however, it is preferable if the steel melt already contains an initial nitrogen content of more than 0.001% by weight, particularly preferably 0.010% by weight or more.

• optional: Nitride-forming agents, in particular niobium, titanium, zirconium, vanadium: Nitride-forming elements such as aluminum, titanium, niobium, zirconium or vanadium are optionally advantageous in the steel of the steel sheets according to the invention in order to avoid the nitrogen originally contained in the steel and the later embroidering in the annealing furnace to bind subsequently introduced nitrogen at least partially in the form of nitrides. As a result, the Elm shape behavior can be improved and almost non-aging IF (Interstitial Free) steel sheets can be produced. Aluminum, titanium and / or niobium are particularly preferred as constituents of the steel, since, in addition to their property as strong nitride formers, they also act as micro-alloy constituents by refining the grain without reducing the toughness.

Therefore, based on weight, the steel contains optional and preferred

• Titanium, Ti: preferably more than 0.02%, particularly preferably more than 0.02% but less than 0.1%, and / or

• Niobium, Nb: preferably more than 0.01%, but less than 0.08%, and / or

• Aluminum, Al: preferably more than 0.005% by weight but less than 0.08% by weight, and / or

• Molybdenum, Mo: less than 0.08%;

Further optional components:

In addition to the residual iron (Fe) and unavoidable impurities, the steel can also contain other optional components, such as • optional copper, Cu: less than 0.1%;

• optional chromium, Cr: less than 0.1%;

• optional nickel, Ni: less than 0.1%;

• optional tin, Sn: less than 0.05%;

• optionally boron, B: less than 0.01%, preferably less than 0.005% and preferably more than 0.0005%; in order to give the steel further advantageous properties, if necessary, which can be achieved with these additional components.

Manufacturing process of steel sheet:

With the composition of the steel described, a steel melt is produced, whereby in preferred exemplary embodiments the steel can already have an initial nitrogen content N ₀ in order to achieve a high (medium) nitrogen content in the steel sheet by adding nitrogen to the steel melt, 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 not to let the strength of the steel sheet produced from the molten steel become too high due to nitrogen solid solution strengthening and in order to maintain the hot formability of the steel and to avoid defects in the slab produced from the molten steel caused by nitrides, it is advantageous if the initial The nitrogen content (N ₀ ) of the steel does not exceed 0.02% by weight and is preferably 0.016% by weight or less.

A slab is first poured from the steel melt and this is then hot-rolled and cooled to room temperature. The hot strip produced in this way has thicknesses in the range from 1 to 4 mm and, if necessary, becomes a roll (coil) at a specified winding temperature (coiling temperature) of 500 to 750 ° C, preferably in the range of 650 ° C to 750 ° C wound up. To produce a packaging steel in the form of a thin steel sheet in the usual fine sheet thicknesses of less than 0.5 mm, preferably less than 0.3 mm, the hot strip is cold-rolled, with a thickness reduction in the range from 50 to over 90%. To restore the crystal structure of the steel that was destroyed during cold rolling, the cold-rolled steel strip is annealed in an annealing furnace in a recrystallizing manner. This is done, for example, by passing the steel sheet present in the form of a cold-rolled steel strip through a Continuous annealing furnace in which the steel strip is heated to temperatures above the (original) recrystallization temperature T _{R of} the steel. In the method according to the invention, before or preferably simultaneously with the recrystallization annealing, the cold-rolled steel sheet is embroidered by heating the steel sheet in the presence of a nitrogen donor. The embroidery is preferably carried out simultaneously with the recrystallization annealing in the annealing furnace by introducing a nitrogen donor, in particular in the form of a nitrogen-containing gas, preferably ammonia (NEE) into the annealing furnace and heating the steel sheet to a temperature above the (original) recrystallization temperature T _{R of} the steel will. The nitrogen donor is selected in such a way that at the temperatures in the annealing furnace, atomic nitrogen is formed by dissociation of the nitrogen donor, which can diffuse (superficially) into the steel sheet. In order to avoid oxidation of the steel sheet surface during annealing, a protective gas atmosphere is expediently used in the annealing furnace. The atmosphere in the annealing furnace preferably consists of a mixture of the nitrogen-containing gas acting as a nitrogen donor and a protective gas such as HNx, the volume fraction of the protective gas preferably being between 90% and 99.5% and the remainder of the volume fraction of the gas atmosphere being the nitrogen-containing gas, in particular Ammonia gas (NH ₃ gas) is formed. In FIG. 1 a, a temperature-time profile of the thermal treatment for embroidering and recrystallization annealing of the steel sheet in the annealing furnace is shown schematically in a first exemplary embodiment for the method according to the invention. In this first embodiment, a single-stage heat treatment of the steel sheet takes place in a (continuous) annealing furnace, the steel sheet being simultaneously annealed and embroidered in a recrystallizing manner during the (single-stage) heating. As seen in Figure la, the steel sheet in this embodiment, from room temperature is within a heating time (t _E) with a preferred (average) heating rate of 10 to 15 ° C / heated s to a heating temperature T _E> T _R and during an annealing time ( tc,) held at least approximately at this temperature. The heating temperature T _E corresponds to the annealing temperature at which the steel sheet is annealed to recrystallize (in some areas) and lies between the original recrystallization temperature T _R and the recrystallization temperature increased _{to T R} + ΔT due to the stitching in the seam area of the steel sheet. The following applies here for the heating temperature T _E : T _R <T _E <T _R + ΔT. The heating time (t _E ) is preferably in the range from 1.0 to 120 seconds, particularly preferably between 10 and 90 seconds, and can be adapted according to the desired material properties of the steel sheet according to the invention, as will be explained below. To adapt the heating time, the heating rate at which the steel sheet is heated in the annealing furnace, or the rate of passage of the steel sheet through a continuous annealing furnace, can be set according to the desired heating time. To set the preferred heating-up times (t _E ) in the range from 1.0 to 120 seconds, for example, a heating rate of 10 K / s to 80 K / s can be selected. The annealing time (t _G ) is preferably in the range from 1.0 to 90 seconds, particularly preferably between 10 and 60 seconds, and is also selected according to the desired material properties of the steel sheet according to the invention. After the annealing time (t _G ) has elapsed, the steel sheet leaves the annealing furnace and either cools passively in the environment or is cooled to room temperature by active cooling, for example water cooling or gas flow cooling. Appropriate cooling rates are in

Range from 3 K / s to 20 K / s with gas flow cooling and at more than 1000 K / s when using water cooling.

In the exemplary embodiments shown in FIGS. 1b and 1c on the basis of schematic temperature-time profiles of the thermal treatment, the steel sheet is thermally treated in two stages in the annealing furnace, with embroidering in a first stage and recrystallization annealing of the core area in a second stage. In the exemplary embodiment of FIG. 1b, the steel sheet is heated in the first stage from room temperature within a first heating time (t _E ¹ ) to an intermediate _{temperature T Z} <T _R and at least approximately at this temperature during a holding time (t _H)

Intermediate temperature held. The intermediate _{temperature T Z} is preferably in the range from 300.degree. C. to 600.degree. C., particularly preferably between 500.degree. C. and 600.degree. The first heating time (t _E ¹ ) is also here preferably in the range from 1.0 to 120 seconds, particularly preferably between 10 and 90 seconds, and can be adapted according to the desired material properties of the steel sheet according to the invention, as in the first exemplary embodiment. The holding time (t _H ) here is likewise preferably in the range from 1.0 to 90 seconds, particularly preferably between 10 and 60 seconds, and is also selected according to the desired material properties of the steel sheet according to the invention. After the holding time has elapsed, the sheet steel leaves the annealing furnace and cools either passively in the environment or is actively cooled to room temperature, the cooling rates being expediently in the range from 3 K / s to 20 K / s. The steel sheet is then again placed in an annealing furnace, in particular a continuous annealing furnace, and in a second stage is heated to the heating temperature (annealing temperature T _E _{) in a second heating time (t E} ² ) and at least approximately to the heating temperature for an annealing period (t _G) T _E held. A nitrogen donor can optionally be present in the annealing furnace during the annealing period (t _G). The steel sheet can, however, also be heated in the first stage in a first chamber of the (continuous annealing) furnace, in which there is a nitrogen-containing gas atmosphere with dissociated ("atomic") nitrogen, while the second stage takes place in a second chamber of the furnace, in which, for example, a pure protective gas (such as HNx, consisting for example of 95% N2 and 5% Eb) forms the furnace atmosphere without dissociated (“atomic”) nitrogen being present in it. According to the invention, the heating temperature (annealing temperature T _E ) lies between the original recrystallization temperature T _R and the recrystallization temperature increased _{to T R} + ΔT by the stitching in the seam area of the steel sheet. The following also applies here for the heating temperature T _E : T _R T _E T _R + ΔT, the intermediate _{temperature (T Z} ) being lower than the (original) recrystallization temperature T _R.

FIG. 1c shows a modified exemplary embodiment of a two-stage thermal treatment of the steel sheet on the basis of a schematic temperature-time diagram. As lb in the embodiment of figure, the steel sheet is in this case in the first stage from room temperature within a first heating time (t _E ¹⁾ heated to an intermediate temperature T _Z ≤ T _R and during a holding time (t _H) is at least approximately at this intermediate temperature is maintained, by passing the steel sheet through a continuous annealing furnace. Unlike even in the embodiment of Figure lb, in the embodiment of figure, however, no cooling lc of the steel sheet after the expiry of the hold time, but directly in the second step during a second heating time _(E ² t), a further heating from the intermediate temperature to the heating temperature T _E , which is higher than the intermediate _{temperature T Z} and lies between the original recrystallization temperature T _R and the recrystallization temperature increased _{to T R} + ΔT by the stitching in the seam area of the steel sheet.

The heating to the heating temperature T _E can expediently within a very short second heating time (t _E ² ) of less than 5 seconds by an im downstream area of the continuous annealing furnace arranged induction heating take place. As lb in the second embodiment, the figure of the figure applies lc for the heating temperature T _E in the third embodiment: T _R ≤ T _E <T _R + .DELTA.T, wherein the intermediate temperature (T _Z) is smaller than the (original) recrystallization temperature T _R is. This third exemplary embodiment is distinguished by an improved efficiency compared to the second exemplary embodiment, because the cooling of the steel sheet at the end of the first stage and repeated heating at the beginning of the second stage from room temperature is dispensed with. The efficiency can be improved even further if induction heating is used for heating in the second stage, with which very rapid heating with high heating rates can be achieved. The third embodiment can expediently be carried out in a continuous annealing furnace with two chambers arranged one behind the other, the steel sheet being heated in the first stage in a first chamber of the continuous annealing furnace in which a nitrogen-containing gas atmosphere with dissociated ("atomic") nitrogen prevails, and In the second stage, the steel sheet is heated by means of the induction heating in the second chamber of the continuous annealing furnace, with either a pure protective gas (such as HNx) forming the furnace atmosphere in the second chamber without dissociated ("atomic") nitrogen being present in it, or here, too, another embroidery takes place in a gas atmosphere with atomic nitrogen. The induction heating is expediently arranged in the downstream area of the second chamber.

The (original) recrystallization temperature T _{R of} the steel depends on the composition of the steel and is typically in the range from 550 to 720 ° C. Correspondingly, the heating temperature T _{E is} preferably in the range 630.degree. C. to the Curie temperature of approx. 768.degree. The preferred upper limit value of the Curie temperature of 768 ° C. for the heating temperature T _E results from apparatus-related reasons, since inductive heating can only take place up to this limit temperature. If the heating in the annealing furnace takes place conductively or by thermal radiation, it is also possible to heat to heating temperatures T _E above the Curie temperature.

When the cold-rolled steel sheet is heated in the annealing furnace, nitrogen from the nitrogen donor is initially only stored in a seam area of the steel sheet close to the surface, in that atomic nitrogen from the nitrogen donor diffuses into the seam area. The nitrogen that has diffused into the seam area can either interstitially store itself in the iron lattice of the steel or is bound as nitride, especially if strong nitride formers such as Al, Nb, Ti, or B are present in the steel. As a result of the incorporation of nitrogen, the recrystallization temperature (T _R ) of the steel in the seam area is raised by a value ΔT. This increase in the recrystallization temperature (T _R ) in the border area is shown in Figures la to lc with ΔT.

According to the invention, the heating temperature (T _E ) or the annealing temperature is now selected such that T _R T _E <T _R + ΔT applies. The heating temperature (T _E ) or the annealing temperature is thus set in the method according to the invention in such a way that it is between the (original) recrystallization temperature (T _R ) of the steel used for the production of the cold-rolled steel sheet and that caused by the embroidering of the steel sheet near the surface in the seam area the recrystallization temperature (T _R + ΔT) increased by the value ΔT. This setting of the heating temperature (T _E ) (or the annealing temperature) results in recrystallization only in a core area of the steel sheet adjoining the outer seam area inward, in which at least initially no nitrogen is (yet) stored during annealing and simultaneous embroidery of the steel sheet has been. The heating temperature (T _E ) is only in the core area above the recrystallization temperature (T _R ) and in the border area in which the recrystallization temperature has been increased by ΔT due to the embedded nitrogen, the heating temperature (T _E ) is below that at T _R + ΔT increased recrystallization temperature. Therefore, a three-layer microstructure in the form of a “sandwich” with an at least essentially, preferably largely completely recrystallized core area and a near-surface edge area surrounding the core area is formed over the cross-section of the steel sheet, the edge area not or at least not completely recrystallized (which is why this three-layer structure is also referred to as a "sandwich structure").

The microstructure resulting from the heating of the steel sheet in the presence of the nitrogen donor therefore comprises an at least essentially completely recrystallized core area 2 and a hem area 1 surrounding the core area 2 on both sides, as shown in the schematic sectional illustration of a steel sheet according to the invention in FIG. 2a. The seam area 2 is not or at least not completely recrystallized and therefore remains in the as-rolled state of the cold-rolled steel sheet. The respective The degree of recrystallization of the core area 2 and the border area 2 can be set via the heating temperature (T _E ) and the annealing time (t _G ). A sharp demarcation of the core area 2 and the border area 1 can be achieved, for example, if the glow duration (t _G ) is greater than 10 seconds and the heating temperature (T _E ) is between T _R + ΔT / 3 and T _R + 2ΔT / 3 . Likewise, the thickness of the seam area 1 can be _{adjusted via the process parameters of the heating temperature (T E} ) and the heating time (t _E ), which will be explained in detail below.

The method according to the invention can also be used to produce two-layer microstructures with an at least largely completely recrystallized core area 2 and an overlying, hard-rolled edge area 1 if, instead of a gaseous nitrogen donor, a liquid or solid nitrogen donor is used, which is only applied to one side of the steel sheet . In this embodiment of the method according to the invention, the liquid or solid nitrogen donor is applied to one side of the steel sheet prior to annealing and the steel sheet coated on one side with the nitrogen donor is then annealed in the annealing furnace in the manner described above. The microstructure shown schematically in Figure 2b is formed with a lower area (core area) 2 and an upper area (seam area) 1, the lower area (core area) being recrystallized and the upper area (seam area) remaining uncrystallized and thus hard as rolling. Nitrogen-containing liquids that can be used as nitrogen donors are, for example, nitrogen compounds dissolved in water, such as guanidine or guanidine hydrochloride, urea (urea) or melamine. Aqueous solutions of these nitrogen-containing compounds can be _{applied as a fine spray mist on one or both sides of the surface of the steel sheet using a CO 2} cartridge, for example, which can then be dried before the steel sheet coated with a dry layer of the nitrogen-containing compound is annealed in is brought to the annealing furnace. The annealing furnace can be a hood annealing furnace or also a continuous annealing furnace, with a protective gas atmosphere, for example 100% HNx, preferably being present in the annealing furnace. The nitrogen concentration of the nitrogen donor applied in this way to the surface of the steel sheet can be adjusted via the concentration of the nitrogen-containing compound in the aqueous solution or via the thickness of the dried layer (dry application). The nitrogen donor can also be applied to one or both sides of the steel sheet as a nitrogen-containing powder or granules. Nitrides or nitrates, for example, can be used as nitrogen-containing powder or granules. Melamine resins can also be used as nitrogen donors and can be applied as a viscous mass to the surface of the steel sheet.

After the steel sheets according to the invention have been produced, they can be coated in the usual way with conversion or protective layers on one or both sides, in particular by electrolytic tin-plating or chrome-plating.

Examples:

Exemplary embodiments for the steel sheet and the method according to the invention are explained below.

From steel melts with the alloy compositions A, B and C listed in Table 1 (the ppm data relate to the weight fraction of the alloy components in the steel from which the cold-rolled steel sheet has been produced), hot-rolling and subsequent cold-rolling were used to produce steel sheets with a thickness of 0 , 22 ± 0.01 mm. The cold-rolled steel sheets were subjected to thermal treatment in a laboratory furnace with induction heating in an ammonia-containing protective gas atmosphere with different process _{parameters in relation to the heating temperature T E} , the heating time ΪE, the annealing time tG and the volume concentration of the ammonia in the furnace. The atmosphere in the continuous annealing furnace was composed of the ammonia gas and the remainder of the HNx protective gas, with the volume fraction of ammonia in the gas atmosphere of the furnace being determined at room temperature and kept constant by the inflow of ammonia during the thermal treatment of the steel sheet. If the experiments are carried out on a large-scale in a continuous annealing furnace, the ammonia concentrations required for embroidery will probably shift to higher values, since at the high temperatures in a continuously heated continuous annealing furnace, due to the dissociation and recombination effects of the ammonia into atomic and molecular nitrogen, only a part the entire ammonia atmosphere is effectively available for embroidering the steel sheet. The microstructure of the heat-treated steel sheets was examined microscopically (cold-embedded, ground, polished and etched according to Nital with 3% nitric acid). After cooling, the steel sheets heat-treated in the furnace were subjected to a second cold rolling step (skin pass) with a degree of reduction of 1.5%. Table 2a shows the parameters of the heat treatment for various examples of steel sheets with the alloy composition A from Table 1, such as the heating time t _E , the annealing time tG, the heating temperature T _E and the volume concentration of the ammonia NH ₃ (%) in the gas atmosphere of the furnace. In FIG. 3, the microstructures of heat-treated steel sheets of the examples from Table 2a are compared by way of example, FIG. 3a showing the example “completely recrystallized” and FIG. 3b showing the example according to the invention “sandwich structure”. It becomes clear that in the "sandwich structure" example treated according to the invention, a three-layer structure ("sandwich structure") with a recrystallized core area 2 and border areas 1 surrounding this on both sides, the seam area 1 each having a thickness of approx. 65 μm. In comparison example 2, on the other hand, the steel sheet is largely completely recrystallized over the entire thickness of the steel sheet. The tensile strength Rm, the 0.2% yield strength (Rp0.2) and the 0.5% yield strength (Rp0.5) and the elongation at break A, the uniform elongation Ag and the hardness ( according to Vickers or Rockwell). The course of the hardness was also measured over the profile of the steel sheets (in the direction of the thickness). The properties determined on the measured samples of the examples from Table 2a are listed in Table 2b and exemplary hardness profiles over the thickness of the steel sheets are compared in FIG. It can be seen that in the sample according to the invention (“sandwich structure”) of FIG , 05 is formed in the center of the sheet steel profile. The sample of the comparative example not according to the invention in FIG. 4a (completely recrystallized), on the other hand, has a largely constant hardness value of approx. 100 HV 0.05 over the entire thickness of the steel sheet. It follows from this that the steel sheet heat-treated according to the invention of the example according to the invention in FIG Sample of the comparative example of FIG. 4a as well as a clear hardness gradient with a hardness falling from the outside towards the core area 2. This course of the hardness over the thickness of the steel sheet is due to a nitrogenization of the steel sheet of the example according to the invention both in the seam area 1 and in the core area 2 with a nitrogen content decreasing from the outside towards the center of the steel sheet as well as the (complete) recrystallization of the core area 2 during the thermal To lead back treatment in the annealing furnace. The hardness profile of the sample of the example according to the invention of Figure 4b and the comparison of the absolute hardness values of this sample in the hem and core area with the measured hardness of the sample of the comparative example of Figure Hem area 1 an increase in nitrogen (increase in nitrogen content) and an increase in hardness or strength resulting therefrom due to solid solution strengthening, and that the increase in nitrogen content in hem area 1 was so high that the recrystallization temperature in the hem area increased by a value ΔT to a final value of T. _R + ΔT has been raised, which was above the heating temperature T _E , with the result that no or only incomplete recrystallization has taken place in the edge region 1. The seam area 1 of the sample of Example 1 is therefore still as hard as rolling and has a high hardness with a hardness maximum at the surface of the steel sheet.

Strength and elongation measurements were carried out on the samples from the examples from Table 2a. In FIG. 5, examples of the stress-strain diagrams of the samples are shown. The samples compared in FIG. 5 are cold-rolled steel sheets with a composition according to Example A from Table 1 and, after cold-rolling, were subjected to a thermal treatment in an annealing furnace as follows:

• "Sandwich structure" (sample according to the invention): single-stage heating of the sample using the method according to the invention in an annealing furnace to a heating temperature T _E of 750 ° C., with a heating time t _E of 62 seconds and an annealing time t _G of 45 seconds (depending on the temperature -Time diagram of Figure 1), wherein (at least) a nitrogen donor was present in the annealing furnace during the heating time;

• “as hard as rolling” (comparative example): no annealing;

• “Completely recrystallized” (comparative example): Annealing in a standard annealing process at an annealing temperature of 750 ° C without nitrogenous Gas atmosphere (ie without nitrogen donor, especially without ammonia addition in the annealing furnace and therefore without nitrogenization in the annealing furnace);

• "Completely recrystallized and embroidered" (comparative example): first recrystallizing annealing at an annealing temperature of 750 ° C without a nitrogen-containing gas atmosphere (ie without nitrogen donor) in the annealing furnace, whereby the sample was first completely recrystallized and embroidered in the annealing furnace with an ammonia atmosphere Embroidery in the annealing furnace was carried out in the same way as the "sandwich structure" sample, but in a fully recrystallized state.

It can be seen from FIG. 5 that the sample according to the invention (“sandwich structure”) has a significantly higher tensile strength of> 1000 MPa (in comparison to <800 MPa of the comparative examples) with approximately the same elongation at break Avon has approx. 7.8%.

With the method according to the invention, (embroidered) steel sheets can therefore be produced which are characterized by a very high strength of more than 800 MPa with at the same time good elongation at break of more than 5%, preferably more than 7%. Such steel sheets are excellent in forming processes for producing stable ones

Packaging such as food and beverage cans and parts thereof such as (tear-open) lids can be processed.

The exact composition of the microstructure, in particular the thickness of the seam area as well as the nitrogen content in the

The edge area and in the core area as well as the gradient of the nitrogen content over the thickness of the steel sheet can be influenced by varying the process parameters. Therefore, the properties of the steel sheets produced with the method according to the invention can be tailored to different applications.

To make this clear, FIGS. 6 and 7 show the microstructure of samples of steel sheets according to the invention, each of which has been heat-treated with variation of a process parameter in the single-stage process described above. Here, FIG. 6a shows a sequence of microstructures of (in relation to the Composition same) samples that have been heated to a different heating temperature T _E (with a constant heating time of t _E = 62 seconds and constant annealing time of t _G = 45 seconds) and FIG Heating temperature of T _E = 750 ° C and constant heating time of t _E = 62 seconds have been kept at the heating temperature for different lengths of time (i.e. have had _{a different glow duration t G with the same heating time in each case).}

From the images of the microstructure in FIG. 6a it can be seen that a multilayer microstructure with a hard-rolled edge area 1 and a recrystallized core area 2 is established as soon as the heating temperature of T _{E is} greater than the recrystallization temperature T _{R of} the steel. In the first image from the left of FIG. 6a, the heating temperature T _{E is} still below the recrystallization temperature T _R and therefore no recrystallization takes place over the entire thickness of the steel sheet. In the second image from the left, T _E > T _R and recrystallization only takes place in the area of the core. In the border area 1, the (original) recrystallization temperature T _{R was} increased by ΔT, which is why the heating temperature T _{E there is} below the increased recrystallization temperature (T _R + ΔT) and no recrystallization has taken place. In the second image from the left in FIG. 6a, the thickness of the border area is approx. 67 ± 5 μm. If the heating temperature T _{E is} increased further up to the preferred upper limit of 768 ° C (Curie temperature, inductive heating can be used up to this temperature), the microstructure does not change significantly as long as the heating temperature T _{E is} less than the increased recrystallization temperature (T _R + ΔT) is in the seam area, ie it remains with the formation of a three-layer microstructure with a seam thickness of the hard-rolled, non-recrystallized seam area of approx. 67 ± 5 μm, Figs. 3 to 6 of FIG. 6a).

FIG. 6b shows the influence of the annealing time on the formation of the microstructure. The composition of the three-layer microstructure is largely independent of the annealing time tG, ie in particular the seam thickness of the hard-rolled seam area remains approximately the same. In the examples shown in FIG. 6b with an annealing time between 30 seconds and 90 seconds, the seam thickness is approximately 67 ± 5 μm. However, with longer annealing times it can be observed that nitrogen diffuses from the border area into the core area. On the one hand, this creates the transition between the hem area and the core area less sharp and, on the other hand, due to solid solution strengthening due to the storage of nitrogen in the core area, there is an increasing increase in strength, so that with longer annealing times with t _H > 40 seconds a noticeable increase in strength can also be observed in the core area, as can also be seen from the comparison of the hardness measurements in FIG. 4a and 4b becomes clear.

The nitrogen concentration of the nitrogen donor (ammonia content in the gas atmosphere of the annealing furnace) has a much greater influence on the structure. In Figure 7, images of the microstructure of samples of steel sheets according to the invention are shown, using a different volume concentration of the ammonia gas in the gas atmosphere of a laboratory furnace with induction heating (with otherwise the same parameters, namely T _E = 750 ° C, t _E = 62 seconds and t _H = 45 seconds) have been heat-treated in a one-step process. The volume fraction of ammonia in the gas atmosphere of the laboratory furnace was determined at room temperature. As can be seen from Figure 7a, an ammonia concentration of 0.5% by volume already results in a three-layer structure with a recrystallized core area and a hem area surrounding it on both sides, the hem thickness here being approx. 34 ± 3 mih (top left image in Figure 7a). With increasing ammonia content, the seam thickness initially increases linearly and then saturates with an ammonia content of approx. 2-3% by volume and a seam thickness of approx. 65 mih (see FIG. 8). A further increase in the ammonia content no longer leads to an expansion of the seam area. With an ammonia content of 10% by volume, the seam thickness of approx. 61 gm (FIG. 7a, picture on the far right) is even slightly below the maximum value of approx. 66 gm, which was observed with an ammonia content of 2.5% by volume.

On closer inspection of the microstructure with higher resolution, it can be seen that with ammonia concentrations> 2% by volume, a very thin nitride layer, in particular an iron nitride layer, has formed on the surface of the steel strip on both sides. The nitride layer can be seen from the enlarged representations of the microstructure in FIG. 7b. Particularly in the pictures with the high ammonia content> 5% by volume, it can be seen that the superficial nitride layer is composed of several individual layers on top of each other. The individual layers are probably different phases (γ-, γ'-, ε-phase) of the iron nitrides. The formation of the (iron) nitride layer on the surface of the steel sheet influences the thickness of the seam area. To Formation of a superficial nitride layer, the seam thickness increases with increasing nitrogen content of the nitrogen donor (ammonia gas) and as soon as a nitride layer is formed (from approx. 2 vol.% Ammonia content) the seam area no longer expands towards the center of the steel sheet, as shown in the figure 8 can be seen. Thus, in the method according to the invention, the structure of the multilayer structure and in particular the seam thickness of the seam area can be adjusted via the nitrogen content of the nitrogen donor. As soon as a superficial nitride layer forms, it increases in thickness with increasing ammonia content. However, the hem area is no longer enlarged, ie the hem area is limited to a maximum thickness of approx. 65 gm, which is already achieved with an ammonia content of approx. 2% by volume.

The unwanted surface layer of (iron) nitride is expediently removed by a chemical analysis before the composition of the steel sheets is recorded (based on the normative specification in section 4.4.1 of the DIN EN ISO 14284 standard). The alloy composition listed in Table 1 of the samples examined was determined in a corresponding manner after removing the nitride surface layer in order to avoid falsification of the measured nitrogen content of the sample.

FIG. 9 shows hardness profiles of samples with the alloy composition according to Example A of Table 1, the samples having been thermally treated after cold rolling like the corresponding examples in FIG. 5 (see page 32, last paragraph).

In Figure 9, the curve of the microhardness is shown over the sheet thickness. It has been shown that there is a correlation between the micro-hardness and the nitrogen content in all embroidered samples. In the non-embroidered state, the sample with the alloy composition according to Example A of Table 1 has a hardness of around 250 HV in the as-rolled state and around 100 HV in the fully recrystallized state. The hardness increases in both textures as a result of the embroidery, as can be seen from the hardness curves of the samples “sandwich structure” (sample according to the invention) and “completely recrystallized and embroidered”.

From a comparison of the sample according to the invention (“sandwich structure”) with the sample “completely recrystallized and embroidered” it can be seen from FIG. 9 that the course of the microhardness over the sample cross-section (thickness of the sample) deviates from one another. In particular, it can be seen that the sample according to the invention (“sandwich structure”) has a higher microhardness over the entire sample cross-section. In the core area of the sample in particular, the micro-hardness is significantly higher compared to the “completely recrystallized and embroidered” sample, with the difference in the micro-hardness of these two samples in the core area being even greater than in the outer edge areas (seam area). From this it can be seen that in the sample according to the invention (“sandwich structure”) not only the hem area but also the core area has been embroidered, because an increase in hardness can be observed in the middle of the sample due to the nitrogen apparently also introduced in the core area. In contrast to this, the sample “completely recrystallized and embroidered” (which has gone through the same embroidery cycle as the sample “sandwich structure”, but in the already fully recrystallized state) shows only a slight increase in microhardness in the core area compared to the sample “fully recrystallized” observe, which suggests that the sample “completely recrystallized and embroidered” hardly any nitrogen was stored in the core area of this sample when it was embroidered. It can be seen from this that the diffusion of nitrogen during embroidery in the annealing furnace depends heavily on the structure of the steel and that a non-recrystallized structure leads to faster diffusion of nitrogen into the core area of a steel sheet. In the case of a (completely or partially) recrystallized microstructure, on the other hand, the diffusion process is slowed down, which means that nitrogen is essentially only stored in the outer edge areas of an already recrystallized steel sheet when embroidering in the annealing furnace.

In the method according to the invention, therefore, when embroidering in the annealing furnace, the diffusion of nitrogen into the steel sheet is much more efficient than in the case of completely recrystallized steel sheets, due to an embroidering in the hard-rolled area. In particular, the method according to the invention can be used to produce steel sheets with a three-layer structure ("sandwich structure"), which have an increased nitrogen concentration in the core area as well as in the outer edge areas (seam areas) and thus an overall significantly increased microhardness and tensile strength.

In the method according to the invention, the mean total hardness or the tensile strength of the steel sheet and the hardness profile over the cross section of the steel sheet can therefore also be controlled via the nitrogen content of the nitrogen donor. By comparing hardness profiles of samples of steel sheets according to the invention that have different levels of Ammonia content have been embroidered in the annealing furnace, as shown in FIG. 15, it can be seen that the mean hardness averaged over the thickness of the steel sheet increases with increasing nitrogen content (ammonia content) of the nitrogen donor. In the case of the sample embroidered according to the invention (“sandwich structure”), a hardness profile over the thickness of the steel sheet forms in any case - regardless of the ammonia content - with a hardness maximum on the surface and a minimum hardness value in the center, as shown in FIG. 9 for the sample To recognize "sandwich structure". With a low ammonia content of, for example, 0.5% by volume, the hardness in the center of the steel sheet, i.e. in the middle of the core area, corresponds approximately to the hardness of a completely recrystallized, non-embroidered sample (comparative example “fully recrystallized” Figure 9). This means that with a low ammonia content of 0.5% by volume, noticeable nitrogen build-up occurs only in the seam area close to the surface, but not in the core area. In the case of a higher ammonia content, in particular of 1% by volume or more, nitrogen is deposited both in the seam area and in the core area, with the result that the core area is also hardened by solid solution strengthening. The ratio of the mean hardness values in the seam area and in the core area is about 3: 1 or more for the low ammonia contents (of <1% by volume) and about 1.3 or more for the higher ammonia contents (of ≥ 5% by volume) fewer. A sharp delimitation of the core area from the hem area can be achieved if the ratio of the mean hardness values in the hem area and in the core area is 1.2 or more. The ratio of the mean hardness values in the seam area and in the core area is preferably 1.4 or more.

From the hardness profiles of FIG. 9 it can also be seen that in the sample according to the invention the mean nitrogen content generated by the embroidery in the annealing furnace is _{significantly greater in the seam area (N S} ) than in the core area. It can be assumed that the increase in hardness is solely due to the solidification of the mixed crystals and thus to the increased nitrogen content caused by the embroidery. Proceeding from this, it can be concluded from the hardness profiles in FIG. 9 that the ratio of the mean nitrogen content of the hem area (N _S ) to the mean nitrogen content of the core area (N _K ) at the higher ammonia contents (of ≥ 5% by volume) is also around 1, 3 and with the low ammonia contents (of <1 vol.%) Is about 3: 1 or more. This could be confirmed by measurements of the profile of the nitrogen content of samples of steel sheets according to the invention with successive pickling of the layer near the surface. There has been a clearly pronounced gradient of the nitrogen content over the thickness of the Steel sheet, with nitrogen contents based on weight of approx. 900 ppm on the surface of the steel sheet (the superficial Fe nitride layer being removed before measuring the nitrogen content) and nitrogen contents of approx. 550 ppm in the core area (FIG. 12). The degree of nitrogenization in the seam area and in the core area can thus be adjusted via the nitrogen concentration in the nitrogen donor.

The thickness of the hem area can be controlled _{by the heating time t E.} This becomes clear from the diagram in FIG. 10, which shows the dependence of the seam thickness on the heating time t _E with two different ammonia contents (1% by volume and 5% by volume). In both cases, an approximately linear increase in the seam thickness can be determined with increasing heating time. The straight lines of the linear course have approximately the same slope with different ammonia content and are only shifted by approx. 10 μm in the absolute value of the seam thickness, as can be seen from FIG. As a result, the composition of the structure and in particular the thickness of the outer, hard-rolled edge area can be set in a defined manner via the process parameter of the heating time. The process parameter of the heating time can be controlled very well in terms of apparatus via the heating rate or the speed at which the steel sheet is guided through the continuous annealing furnace, so that a targeted and precise setting of a desired seam thickness can be achieved. The average hardness of the steel sheet (averaged over the entire thickness) or its tensile strength and its deformability (via the thickness of the core area and its average nitrogen content) can in turn be set via the seam thickness.

FIG. 11 shows strength-elongation curves of samples embroidered according to the invention with the composition A of Table 1, the samples being heated to a heating temperature of t _E = 750 ° C. with different heating times ΪE and each with the same annealing time of t _G = 45 Seconds have been held at the heating temperature in a 5% ammonia atmosphere. The material properties achieved are shown in Table 3a and shown graphically in FIG. It can be seen that the strength Rm can be increased by increasing the heating time, but that the elongation at break A is reduced (slightly). By varying the heating time t _E , a value for the tensile strength and the elongation at break or the working capacity resulting therefrom as the product of the tensile strength and the elongation at break can be set for cold forming. It can be done with the Steel sheets according to the invention achieve values for the work capacity W = Rm (in MPa) -A (in%) of more than 5000 MPa •%.

FIG. 12 shows the gradient of the nitrogen concentration over the thickness of a steel sheet embroidered in an annealing furnace (with a composition according to Example A from Table 1) Heating time of 62 s and an annealing time of 45 s over the cross-section of the steel sheet resulting nitrogen profile, the sample was gradually pickled in hydrochloric acid and the nitrogen content was measured over the entire remaining sheet thickness by means of carrier gas hot extraction.

From the diagram in FIG. 13, the influence of the nitrogen content of steel sheets embroidered on at different heights on the hardness and the recrystallization temperature can be seen on the basis of hardness diagrams as a function of the heating temperature during recrystallization annealing. The diagram in FIG. 13 shows the course of the hardness as a function of the heating temperature (annealing temperature) for three different samples with different nitrogen contents with otherwise the same alloy composition (according to example “A” from Table 1). The sample labeled “untreated” was not embroidered in the annealing furnace (ie no nitrogen donor was present during the annealing in the annealing furnace, the steel of this sample had a nitrogen content of 25 ± 5 ppm by weight), whereas the one with “71.9 ppm N2 ”and“ 107 ppm N2 ”were embroidered in the annealing furnace at approx. 550 ° C to a total nitrogen content of 71.9 ppm and 107 ppm, respectively. The sample “71.9 ppm N2” was heated within 1 s to 550 ° C under a 1% ammonia atmosphere in the annealing furnace (bell-type furnace process) and then the nitrogen was evenly distributed over the cross-section (under an argon atmosphere for 5.5 hours of heating to 550 ° C, 5 h hold and 36 h cooling). The sample “107 ppm N2” was generated similarly to sample sample “71.9 ppm N2”, but with an embroidery time of 10 s. Then recrystallization curves were created on the embroidered samples. It can be seen from this that the nitrogen introduced during embroidery has increased the recrystallization temperature, with an increase in the recrystallization temperature of approx.

FIG. 14 shows the relationship between the hardness (Vickers HV hardness _0.025 , the hardness being recorded in each case in the hard-rolled state of the samples) and the Recrystallization temperature of the nitrogen content of the different embroidered samples from Figure 13. A linear relationship can be seen for both the microhardness and the recrystallization temperature, ie as the nitrogen content of the embroidered samples increases, the hardness and the recrystallization temperature rise linearly.

Cold-rolled steel sheets with the composition A according to Table 1 were thermally treated in tests with the parameters given in Table 4 in a continuous annealing furnace according to the invention. Results of these tests are shown in FIGS. 16 to 18. Figure 16 shows a microscopic cross-sectional image of the three-layer microstructure, Figure 17 shows the microhardness curve over the cross-section and Figure 18 shows strength-elongation diagrams of samples of steel sheets that were skin-tempered with different degrees of skin-pass (DG) after the thermal treatment in the annealing furnace are.

Furthermore, texture measurements were carried out on the steel sheets produced according to the invention. FIG. 19 shows the orientation density distribution function f (g) along the e-fiber of a steel sheet according to the invention with a three-layer structure (sandwich structure). The e -fiber is defined by an orientation of the <110> vector parallel to the transverse direction (QR). From this diagram, a deformation index VI can be determined for the individual areas of the three-layer structure from the ratio of the orientation density distribution f (g) as follows:

Deformation index VI = {001} -orientation / {111} -orientation

The deformation index VI represents a measure of the deformation behavior of the steel sheet, because a {III} -orientation enables good formability and has a good Lankford coefficient (r-value), whereas the {001} -orientation is less malleable . As can be seen from the diagram in FIG. 19, the individual areas (layers) of the three-layer structure (i.e. the recrystallized core area and the two outer, non-recrystallized border areas) differ from one another with regard to the orientation density profile in the sheet metal plane and thus in their texture. In the example shown in Fig. 19, the deformation index is in the recrystallized one Core area VI = 0.46 and in the two outer seam areas the deformation index is VI = 3.09 and VI = 4.31.

In the steel sheets according to the invention, the recrystallized first layer (which is on the inside in a three-layer sandwich structure and represents the core area) generally has a deformation index of less than 0.8 and the second layer (which is on the outside in a three-layer sandwich structure and the Represents the hem area) has a deformation index of more than 2.0 and in particular in the range from 2.0 to 5.0.





Claims

Expectations

1. A method of manufacturing a steel sheet for packaging with the following steps:

• cold rolling a hot-rolled steel sheet made from a steel with a carbon content (C) by weight of 10 to 1000 ppm, the steel of the steel sheet having a recrystallization temperature (T _R ),

• Heating of the cold-rolled steel sheet to a specified heating temperature (T _E ), for which the following applies: T _R ≤ T _E ,

• where the heating takes place at least temporarily until the recrystallization temperature (T _R ) is reached in the presence of a nitrogen donor, whereby when the cold-rolled steel sheet is heated, nitrogen from the nitrogen donor diffuses at least into an area (1) of the cold-rolled steel sheet close to the surface and in the area close to the surface ( 1), whereby the recrystallization temperature (T _R ) of the steel in the near-surface area (1) is increased by a value ΔT, characterized in that the following also applies to the heating temperature (T _E ): T _E <T _R + ΔT.

2. The method of claim 1, wherein the steel of the cold-rolled steel sheet has the following composition based on weight:

• C: more than 0.001% and less than 0.1%, preferably less than 0.06%;

• Mn: more than 0.01% and less than 0.6%;

• P: less than 0.04%; • S: less than 0.04% and preferably more than 0.001%;

• Al: less than 0.08%;

• Si: less than 0.1%;

• optional Cu: less than 0.1%;

• optional Cr: less than 0.1%;

• optional Ni: less than 0.1%;

• optional Ti: less than 0.1% and preferably more than 0.02%;

• optional Nb: less than 0.08% and preferably more than 0.01%;

• optional Mo: less than 0.08%;

• optional Sn: less than 0.05%;

• optional B: less than 0.01%, preferably less than 0.005% and preferably more than 0.0005%;

• optional N: less than 0.02%, in particular less than 0.016%, and preferably more than 0.001%;

• remainder of iron and unavoidable impurities,

• wherein the weight average nitrogen content after heating the cold-rolled steel sheet in the presence of the nitrogen donor is at least 0.005% and preferably at least 0.015%.

3. The method according to claim 1 or 2, wherein the steel of the cold-rolled steel sheet, based on the weight, contains more than 200 ppm and preferably more than 500 ppm titanium and / or more than 100 ppm niobium and / or more than 50 ppm aluminum.

4. The method according to any one of the preceding claims, characterized in that the heating of the cold-rolled steel sheet takes place in stages and comprises at least one holding phase in which the temperature of the steel sheet during a holding time (t _H ) at an at least substantially constant intermediate _{temperature (T Z} ) is held and after the hold time (t _H ) is further heated until the heating temperature (T _E ) is reached.

5. The method according to claim 4, characterized in that the cold-rolled steel sheet is exposed at least temporarily to the nitrogen donor _{at least before the start and / or during the holding time (t H).}

6. The method according to any one of the preceding claims, characterized in that the steel sheet after reaching the heating temperature (T _E) for a predetermined annealing time (t _G) is maintained at the heating temperature (T _E). 7. The method according to any one of claims 4 to 6, characterized in that the

Intermediate temperature (T _Z ) is less than the recrystallization temperature (T _R ).

8. The method according to any one of the preceding claims, characterized in that the cold-rolled steel sheet is heated from room temperature to the heating temperature (T _E _{) within a heating time (t E) of 1.0 to 300 seconds.}

9. The method according to any one of claims 4 to 8, characterized in that the holding time (t _H ) is between 1.0 and 300 seconds. 10. The method according to any one of the preceding claims, characterized in that the steel of the cold-rolled steel sheet has an initial nitrogen content (N ₀ ) and when the cold-rolled steel sheet is heated, the mean nitrogen content (N _S ) in the area near the surface (1) increases to over the thickness of the near-surface area (1) averaged value, which is between 50 and 1000 ppm above the initial nitrogen content (N ₀ ) of the steel, is increased.

11. The method according to any one of the preceding claims, characterized in that when the cold-rolled steel sheet is heated in the near-surface area (1), a gradient of the nitrogen content (N _S ) is established, the nitrogen content from the surface to a core area (2) of the cold-rolled

Steel sheet decreases.

12. The method according to claim 11, characterized in that the nitrogen content (N _K ) in the core region (2) is greater than the initial nitrogen content (N ₀ ) of the steel, the difference in the nitrogen content based on weight (N _K ) in the core region (2) and the initial nitrogen content (N ₀ ) of the steel is preferably more than 30 ppm.

13. The method according to any one of the preceding claims, wherein the nitrogen stored in the near-surface area (1) when the cold-rolled steel sheet is heated is present in dissolved form and in particular interstitially in the lattice of the

Steel is embedded, or bound as nitride, in particular as A1N and / or TiN and / or NbN.

14. The method according to any one of the preceding claims, characterized in that the nitrogen donor is formed by a nitrogen-containing gas atmosphere which in particular contains ammonia, the volume concentration of the ammonia in the nitrogen-containing gas atmosphere preferably more than 0.1% and in particular between 0.1 % and 3%. 15. The method according to any one of the preceding claims, characterized in that the heating of the cold-rolled steel sheet takes place in an inert gas atmosphere, which in particular contains HNx, the volume concentration of the HNx in the nitrogen-containing gas atmosphere is preferably between 97% and 99.9%.

16. The method according to any one of the preceding claims, wherein during the heating and in particular during the holding time (ΪH) an embroidery of the near-surface area (1) of the cold-rolled steel sheet takes place at least temporarily.

17. The method according to any one of claims 11 to 16, wherein a recrystallization annealing of the core region (2) of the cold-rolled steel sheet takes place at least partially _{during the annealing period (t u).} 18. The method according to any one of the preceding claims, characterized in that the nitrogen donor comprises a nitrogen-containing liquid and / or a nitrogen-containing solid, which is applied to one or both sides of the surface of the cold-rolled steel sheet before or during the heating. 19. The method according to any one of the preceding claims, wherein the value ΔT by which the recrystallization temperature in the near-surface area (1) is increased by the inclusion of nitrogen when heating the steel sheet, of which after the end of the heating in the near-surface area (1) of the Steel sheet depends on the nitrogen content introduced by embroidery, in particular with the following relationship: ΔT = a DN (%), where a is a constant of proportionality and DN (%) relates to the nitrogen content in% introduced when the steel sheet is heated by embroidering in the area near the surface (1) on the weight of the steel that is. 20. The method according to any one of the preceding claims, wherein the value ΔT by which the recrystallization temperature in the near-surface area (1) increases due to the inclusion of nitrogen when heating the steel sheet, is greater than 30 ° C and preferably greater than 50 ° C and is in particular in the range from 100 ° C to 250 ° C.

2E steel sheet, in particular for packaging, which is made of a steel with a carbon content (C) based on weight of 10 to 1000 ppm and a thickness of less than 0.5 mm, characterized in that the steel sheet has a multilayer structure with at least a first layer and a second layer, the first layer at least in Substantially recrystallized and the second layer is not or at least not completely recrystallized.

22. Steel sheet according to the preceding claim, wherein the multi-layer microstructure has an inner, at least substantially recrystallized core area (2) and a near-surface area (1) surrounding the core area (2) on both sides, the near-surface area (1) not or at least not is not fully recrystallized.

23. Steel sheet according to claims 21 and 22, wherein the steel sheet has the following composition based on weight:

• C: more than 0.001% and less than 0.1%, preferably less than 0.06%;

• Mn: more than 0.01% and less than 0.6%;

• P: less than 0.04%;

• S: less than 0.04% and preferably more than 0.001%;

• Al: less than 0.08%;

• Si: less than 0.1%;

• optional Cu: less than 0.1%;

• optional Cr: less than 0.1%;

• optional Ni: less than 0.1%;

• optional Ti: less than 0.1% and preferably more than 0.02%;

• optional Nb: less than 0.08% and preferably more than 0.01%;

• optional Mo: less than 0.08%;

• optional Sn: less than 0.05%;

• and a nitrogen content averaged over the thickness of the steel sheet of at least 0.005%, preferably more than 0.015%, particularly preferably more than 0.02%, and the remainder contains iron and unavoidable impurities.

24. Steel sheet according to one of claims 21 to 23, characterized in that the steel sheet is produced by cold rolling from a hot-rolled steel with a predetermined recrystallization temperature (T _R ) and the cold-rolled steel sheet in the presence of a nitrogen donor to a predetermined heating temperature (T _E ) above has been heated to the recrystallization temperature (T _R), wherein the first layer through the heating at the predetermined heating temperature (T _e), has been, at least substantially fully recrystallized particular the core region (2) and the second layer, in particular the near-surface region ( 1), at least not completely recrystallized.

25. Steel sheet according to one of claims 21 to 24, characterized in that the steel sheet has an initial nitrogen content (N ₀ ) of the steel which is preferably in the range from 0 to 0.0160% by weight, and that when the cold-rolled steel sheet is heated Nitrogen from the nitrogen donor has diffused into the near-surface area (1) of the cold-rolled steel sheet and has been stored in the near-surface area, as a result of which the nitrogen content (N _S ) in the near-surface area (1) is above the initial nitrogen content (N ₀ ) of the Stahls lying value has increased and the recrystallization temperature (T _R ) of the steel in the near-surface area has been increased by a value ΔT, wherein ΔT is preferably greater than 50 ° C and particularly preferably in the range of 100 ° C to 250 ° C.

26. Steel sheet according to one of claims 21 to 25, wherein the steel of the steel sheet, based on the weight, contains more than 500 ppm titanium and preferably between 550 ppm and 900 ppm titanium and / or more than 100 ppm niobium.

27. Steel sheet according to one of claims 21 to 26, characterized in that the thickness of the cold-rolled steel sheet is in the range from 0.1 mm to 0.3 mm, preferably in the range from 0.15 mm to 0.25 mm. 28. Steel sheet according to one of claims 22 to 27, characterized in that the area (1) close to the surface has a thickness in the range from 5 pm to 150 pm, preferably in the range from 10 pm to 100 pm and in particular between 30 pm and 80 pm . 29. Steel sheet according to one of claims 21 to 28, characterized in that the

Steel sheet has a tensile strength of more than 600 MPa and preferably of more than 800 MPa.

30. Steel sheet according to one of claims 21 to 29, characterized in that the steel sheet has an elongation at break of more than 4% and preferably of more than 5%.

31. Steel sheet according to one of claims 21 to 30, characterized in that the average nitrogen content (N _S ) in the near-surface area (1) of the steel sheet is based on the weight between 50 and 1000 ppm.

32. Steel sheet according to one of claims 22 to 31, characterized in that there is a gradient of the nitrogen content in the area near the surface (1) and / or across the thickness of the steel sheet, the nitrogen content from the surface to the core area (2) of the steel sheet decreases.

33. Steel sheet according to one of claims 21 to 32, characterized in that the first layer, in particular the area (1) of the steel sheet close to the surface, has a higher hardness and / or higher tensile strength than the second layer, in particular the core area (2) , where the ratio of the hardness of the first layer to The hardness of the second layer is preferably greater than 1.2 and particularly preferably greater than 1.4.

34. Steel sheet according to one of claims 21 to 33, characterized in that the first layer, in particular the area (1) of the steel sheet _{close to the surface, has a Vickers hardness of at least 250 HV 0.025} and preferably of at least 300 HVO, O25.

35. Steel sheet according to one of claims 22 to 34, characterized in that both the first layer, in particular the area close to the surface (1), and the second layer, in particular the core area (2), have a nitrogen content ( N ₀ ) of the steel has a higher nitrogen content, the nitrogen content (N _S ) generated by embroidering the steel in the first layer, in particular in the near-surface area (1), higher than the nitrogen content (N _K ) in the second layer, in particular in the Core area (2), wherein the ratio of the nitrogen content (N _S ) of the first layer to the nitrogen content (N _K ) of the core area (2) is preferably greater than 1.5 and particularly preferably greater than 2.0.

36. Steel sheet according to one of claims 21 to 35, characterized in that the first layer, in particular the near-surface area (1) of a three-layer structure, has a degree of recrystallization of less than 30%, preferably less than 20%, and / or that the second layer, in particular the core area (2) of a three-layer structure, has a degree of recrystallization of more than 70%, preferably of more than 80%.

37. Steel sheet according to one of claims 21 to 36, characterized in that the first layer, in particular the area (1) close to the surface, contains a nitride layer, in particular an iron nitride layer, on the free surface.

38. Steel sheet according to one of claims 21 to 37, characterized in that the first layer, in particular the area near the surface (1), has a deformation index of less than 0.8 and the second layer, in particular the core area (2), has a deformation index of has more than 2.0, the deformation index being defined by the ratio of the {001 {orientation and the

{111 {-orientation in the e-fiber.