KR20240107089A - Steel sheet having a two-layer crystallization structure, and method for manufacturing such steel sheet - Google Patents

Abstract

The present invention relates to a method of manufacturing a steel sheet for packaging, including a two-layer recrystallization structure. Way
● Providing a cold rolled steel sheet made of steel with a carbon content (C) of 10 to 1000 ppm by weight and a specific recrystallization temperature (T _R ), wherein the steel sheet has a first side (a) and a second side (b),
● Applying a barrier layer (3) at least partially impermeable to nitrogen on the first side (a) of the steel sheet, and
● Heating the steel sheet to the heating temperature (T _E )
, where
● The heating process is carried out at least temporarily in a nitrogenizing gas atmosphere until the recrystallization temperature (T _R ) is reached, whereby nitrogen from the gas atmosphere is removed at least from the second side (b ) phase diffuses in the region 2 near the surface and is stored in the region 2, whereby the recrystallization temperature of the steel rises by the value ΔT in the region 2 near the surface,
● The heating temperature (T _E ) is above the recrystallization temperature (T _R ) and below the recrystallization temperature (T _R +ΔT) raised by the value ΔT in the region (2) near the surface.

Description

Steel sheet having a two-layer crystallization structure, and method for manufacturing such steel sheet

The present invention relates to a method for manufacturing a steel sheet with a two-layer crystallization structure, in particular a steel sheet with a two-layer crystallization structure that can be used for the production of packaging by a deep-drawing process, as well as containers made from the steel sheet.

For the production of packaging from steel sheets, such as tin-plated or electrolytic chrome-coated steel (ECCS), increasingly thinner steel sheets with a thickness ranging from 0.1 to 0.25 mm are used for reasons of resource efficiency. To ensure that sufficiently stable packaging can be manufactured from thinner steel sheets, the strength of packaging steel must be increased. In addition, it must be ensured that, despite their lower thickness and higher strength, the steel sheets remain easily formable so that they can be subjected to the severe deformations that occur during the manufacture of packaging in deep drawing and stretch forming processes.

It is generally known from the prior art to increase the strength of steel by introducing dissolved unbound nitrogen into the steel. The introduction of unbound nitrogen into steel is referred to as fixation, nitriding or nitrifying and is a well-known process for solid solution strengthening of steel and steel products.

For steel sheets intended for packaging manufacturing (also known as packaging steel), it is also known to increase the strength by nitriding the steel. For example, from DE 102014116929 B3, a process is known for producing nitrided packaging steel from hot rolled steel products with a carbon content of 400 to 1200 ppm, wherein the hot rolled strip is cold rolled to form flat steel products. The cold rolled flat steel product is then annealed in an annealing furnace, especially a continuous annealing furnace, to recrystallize it, wherein a nitrogen-containing gas is introduced into the annealing furnace and directed onto the flat steel product to remove unbound nitrogen to a concentration of greater than 100 ppm. is introduced into the flat steel product in an amount equivalent to , or the amount of unbound nitrogen in the flat steel product is increased to a concentration of more than 100 ppm. Finally, the annealed and nitrided flat steel product is cooled immediately after recrystallization annealing at a cooling rate of at least 100 K/s. This process can be used to produce cold rolled flat steel products for packaging applications having a tensile strength greater than 650 MPa, especially between 700 MPa and 850 MPa.

From WO 2005/056 841 A1, a process for manufacturing steel sheets with a nitrogen gradient across the thickness of the steel sheet with a high nitrogen concentration at the surface and a lower nitrogen concentration in the core region is known, wherein the nitrogen is released after complete recrystallization of the steel sheet. An ammonia gas atmosphere at a temperature of 550°C to 800°C is introduced during or after annealing of the steel sheet in an annealing furnace.

A process for manufacturing high-strength steel strips for making tinplate and other packaging steels by nitriding the steel strips is known from US 3 219 494, in which the steel strips wound into coils are initially subjected to nitrogen-sustaining outer shells from the steel strips. For nitriding in a bell-type annealing furnace, nitriding in a bell-type annealing furnace is caused by an ammonia gas atmosphere, and over the thickness of the steel strip by diffusion of nitrogen when the steel sheet is heated to a temperature above the recrystallization temperature in an inert gas atmosphere. The nitrogen introduced near the surface is distributed uniformly, whereby nitrogen can diffuse from the nitrogen-sustaining outer shell through the steel strip into its core region and the structure of the steel will be completely recrystallized. Strengths ranging from 439 MPa to up to 527 MPa were achieved for steel plates with a thickness of 0.25 mm.

During the forming of steel sheets in a deep drawing process for the production of packaging, roughening occurs outside the forming zone due to the grain structure of the steel, especially in areas with small bending radii, especially bending radii of less than 14 mm. The degree of roughening depends on the fineness of the steel structure. The smaller the average grain size of the steel structure, the lower the degree of roughening. Packaging steels with low carbon content in the range of 100 to 1000 ppm (0.01 to 0.1% by weight) typically have grain sizes (average grain diameters) in the range of 10 to 30 μm. Due to the (relatively coarse-grained) grain structure of the steel, roughening occurs outside the forming zone during forming because the grains of the steel structure can be forced radially outward during forming and then pushed through the surface of the steel sheet. This is especially noticeable in cold rolled steel sheets that are completely recrystallized after cold rolling. Cold rolled steel sheets for the production of packaging are generally fully recrystallized after cold rolling to restore the original structural state and formability of the steel sheets. However, fully recrystallized steel sheets have a coarse steel structure with a relatively high average grain size and are more ductile than roll-hardened steel sheets. Due to the lower fineness and lower strength of recrystallized steel sheets, the coarser grains of the steel microstructure can be visibly pushed away from the surface during forming, thereby increasing the susceptibility to severe roughening during forming. This roughening often causes problems with stability and corrosion resistance for packaging, e.g. cans, because the roughened areas are located on the outside of the packaging, e.g. on the edges of the base of the can. This is because it is subject to high mechanical stress.

The object of the present invention is to disclose a steel sheet for the production of packaging with high strength and excellent formability, in which roughening does not occur outside the forming zone even in the case of severe forming with small bending radii, especially bending radii of less than 14 mm. It is done. In particular, the steel sheet must have a strength of at least 500 MPa with an elongation at break of at least 5% to meet the mechanical requirements of the forming process even with small thickness steel sheets of less than 0.3 mm. At the same time, the steel sheet must have sufficient formability for its intended use as a packaging steel, for example in deep-drawing or stretch-forming processes, so that packaging such as tin cans or beverage cans is a flat steel product as intended. It can be manufactured from. A further object of the present invention is to provide a method for manufacturing such steel sheets for packaging.

This problem is solved by a method having the characteristics of claim 1 and a steel sheet having the characteristics of claim 12. Preferred embodiments of the process and the steel sheet according to the invention are set out in the dependent claims.

When steel plate is mentioned, it means a flat steel product in sheet or strip form. The values of % or ppm relating to the content or concentration of alloying elements of the steel or cold rolled steel sheet refer in each case to the weight of the steel or sheet. The features of the invention disclosed below in relation to the manufacturing process also relate accordingly to the product of the process (i.e. steel sheet) and vice versa.

In the process according to the invention for the production of steel sheets for packaging, a cold rolled steel sheet with a first side and a second side, produced from steel with a carbon content (C) of 10 to 1000 ppm by weight and subjected to a given crystallization temperature ( The cold rolled steel sheet, having T _R ) (essentially determined by the steel composition), is provided on a first side with a barrier layer at least substantially impermeable to nitrogen and then preferably in a continuous annealing furnace at a temperature of at least recrystallization. is heated to a (maximum) heating temperature as high as It diffuses into the area near the surface on the two sides and deposits in this area, whereby the recrystallization temperature of the steel sheet in the near-surface region on the second side of the steel sheet is raised by the value ΔT, and the heating temperature T _E is on the one hand the recrystallization temperature On the one hand, it is chosen to be below the recrystallization temperature by ΔT (T _R +ΔT) in the near surface region.

The heating temperature T _E is the maximum temperature in the heat treatment of cold rolled steel sheets, i.e. the cold rolled steel sheets before, during or after heat treatment according to the method of the invention are heated to a temperature higher than the (maximum) heating temperature T _E It doesn't work. When a cold rolled steel sheet is heated, nitrogen from the nitriding gas atmosphere is only transferred to the near surface region on the second side of the steel sheet by diffusion of (atomic) nitrogen from the nitriding gas atmosphere onto the near surface region. mixed. On the first side opposite the steel sheet covered by the barrier layer, penetration of nitrogen through the barrier layer is prevented. As a result, the recrystallization temperature of the steel increases by the value ΔT only in the near-surface region on the second side of the steel sheet, while the recrystallization temperature in the region on the first side of the steel sheet remains at least substantially unchanged, i.e. Corresponds to the original recrystallization temperature of the steel determined by the steel composition of the pre-cold rolled steel sheet. Accordingly, the heating temperature (T _E ) is chosen such that T _R ≤ T _E < T _R + ΔT, where T _R is the initial recrystallization temperature of the steel.

As a result, a two-layer crystallization structure is formed in the steel sheet, wherein the steel sheet has a first region on the first side that is at least substantially completely recrystallized and a second region that is not recrystallized or is at most only partially recrystallized on the second side. Includes area. Due to this two-layer structure of the crystallization structure, the process according to the invention takes place because the second region on the second side of the steel sheet (near surface) is either not more nitrided during heating than the second region due to the barrier layer. This is because it has a higher recrystallization temperature due to nitridation during heating, at least compared to the adjacent first region on the first side which is significantly less nitrided. The heating temperature is between the recrystallization temperature of the (near surface) second region (increased by ΔT) and the recrystallization temperature of the adjacent first region (which is not nitrided and therefore has a recrystallization temperature at least substantially equal to the initial recrystallization temperature of the steel). Therefore, only the unnitrided first region will recrystallize during heating, while the nitrided (near surface) second region will not recrystallize or will only recrystallize incompletely, resulting in still roll-hardness. (from cold rolling).

The barrier layer on the first side of the steel sheet prevents atomic nitrogen from diffusing through the surface into the interior of the steel sheet. To ensure this, the material from which the barrier layer is formed is suitably temperature-resistant, in particular to temperatures above 300° C., preferably at least 600° C. or lower, since at these temperatures the steel sheet is nitrided in the annealing furnace; Nitrogen atoms from the gas atmosphere of the annealing furnace are deposited on the hot surface of the steel sheet by a catalytic reaction and diffuse into the steel sheet.

The barrier layer can be formed, for example, by a heat-resistant paint, such as oven paint or silicone resin-based engine paint. To apply the barrier layer, a heat-resistant paint material is applied to one side of the first side of the steel sheet before being placed in an annealing furnace, for example by spraying. The barrier layer can also be formed on one side of the steel sheet by a coating electrolytically deposited, for example on a chromium/chromium oxide layer, especially with a coating weight of chromium in the range from 80 to 140 mg/m ² .

The barrier layer is designed in such a way that it significantly prevents the passage of nitrogen from the nitrogen-containing gas atmosphere of the annealing furnace into the steel sheet, i.e. because, for example, the surface of the first side of the steel sheet is incompletely covered by the barrier layer, or The penetration of nitrogen through the barrier layer on the first side of the steel sheet is suppressed, except for inevitable nitrogen residues that may diffuse into the steel sheet due to defects in the barrier layer on the first side. Preferably, the barrier layer is formed so that at most 10% of the amount of nitrogen diffusing into the second side of the steel sheet can penetrate into the first side of the steel sheet. Therefore, in the process according to the invention, nitrogen diffuses essentially only on the second side of the steel sheet, a gradient of nitrogen concentration is formed over the cross-section of the steel sheet running from the second side to the first side, and the steel sheet is heated at a temperature (T Due to the increase in recrystallization temperature with increasing nitrogen content when heated _with This results in the formation of a two-layer recrystallized structure with a second region.

The barrier layer is preferably a sol-gel layer, especially a layer of SiO ₂ , TiO ₂ and/or ZrO ₂ . This sol-gel layer can be produced by the sol-gel process, wherein the sol used as the coating solution, existing as a colloidal dispersion, is applied to the surface of the first side of the steel sheet, followed by wet-chemically formed layer. It can be efficiently applied to the surface of a steel plate by gelling and drying. For the creation of the SiO ₂ -sol-gel layer, in particular silicone alcoholates can be used as precursors. especially,

- (TMOS),

- (TEOS)

- or (TPOS)

An aqueous solution of can be applied to the surface on the first side of the steel strip. For the creation of ZrO ₂ - or TiO ₂ - sol-gel layers, aqueous solutions of zirconium propylate or titanium (2-propylate) can be applied.

The sol-gel layer is preferably applied wet-chemically to the surface of the steel strip on the first side in a strip coating process. In this way, the sol-gel process for applying a sol-gel layer on the steel sheet surface can be integrated into a continuous strip coating process to achieve efficient process control, where the steel sheet is in strip form and the subsequent nitriding and nitriding of the steel strip. Annealing is performed in a continuous annealing furnace through which the strip passes at a predetermined strip speed. The sol-gel layer is applied in a separate step upstream of the continuous annealing furnace. Gelation and drying of the sol can occur at least partially during heating of the steel strip in a continuous annealing furnace, achieving a very efficient process and reducing the length of the strip line. In particular, it has been shown that the sol-gel process for the creation of SiO ₂ layers can be used to apply a dense and uniform barrier layer on one side of a steel sheet, which is why this process is preferred for applying barrier layers. .

In a particularly preferred embodiment, the silicate-containing barrier layer is electrolytically applied to one of the two sides of the steel sheet from an aqueous basic electrolyte containing silica and sodium salts, wherein the silicate layer acting as barrier layer has 1 to 10 It is applied to one of the two sides of the steel sheet in the range of mg/m ² , particularly preferably in the range of 3 to 6 mg/m ² . During electrolytic application of the barrier layer, the steel sheet surface is simultaneously degreased. Electrolytic application of the barrier layer makes it possible to create a barrier layer that is uniformly distributed over the surface of the steel strip and has a coating weight that is homogeneous and sufficient to prevent nitrogen penetration across the first side of the steel strip during the nitriding process in the annealing furnace. do.

In order to achieve at least substantially complete recrystallization of the first region by the nitriding process in the annealing furnace, the temperature of the steel sheet is preferably adjusted to a heating temperature (T _E ) during a predetermined annealing time (t _G ) after reaching the heating temperature. is maintained in The annealing time is preferably greater than 1 second, especially in the range from 1 to 80 seconds, preferably in the range from 1 to 10 seconds. With longer annealing times, for example, greater than 10 seconds, in addition to complete recrystallization of the first region, the distribution of nitrogen introduced during heating becomes even throughout the thickness of the steel sheet, such that no recrystallization occurs with the first region recrystallizing. or creates a wider transition zone in the core region of the steel sheet between the second region, which is barely recrystallized.

Therefore, it is also an object of the present invention to provide a steel sheet which can be used in particular for the production of packaging and which has a carbon content (C) by weight of 10 to 1000 ppm and is made of steel with a thickness of less than 0.5 mm, It has a two-layer crystallization structure having a first region on one side and a second region on a second side, wherein the first region is at least substantially recrystallized and the second region is not recrystallized, or at least not fully recrystallized. A barrier layer that is at least substantially impermeable to nitrogen is arranged on the surface of the first side. Due to the barrier effect of the barrier layer, in the process according to the invention, nitrogen diffuses essentially only on the second side of the steel sheet, and a gradient of nitrogen concentration is formed over the cross-section of the steel sheet running from the second side to the first side, This causes the formation of a two-layer crystallization structure due to the increase in recrystallization temperature as the nitrogen content increases when the steel sheet is heated to the heating temperature (T _E ). The two-layer crystallization structure of the steel sheet according to the invention is also referred to below as two-layer microstructure.

The steel sheet according to the invention has a very high strength of more than 500 MPa, especially more than 600 MPa, preferably more than 700 MPa, and an elongation (elongation at break) of at least 5%, which is acceptable for deep drawing applications. At the same time, the steel sheet has excellent formability, especially in the deep drawing process. The steel sheet is characterized by the fact that, in particular, even in the case of heavy forming to very small bend radii, no roughening occurs outside the bend radius when the second non-recrystallized region, which is still roll-hard, is located outside the bend radius. Do it as This is achieved by the fact that the coarse grains of the recrystallized first region cannot be pressed through the surface outside the bending radius due to the high strength of the roll-hard second region. Thus, the second region, still roll-hard, forms a barrier against the coarse grains of the recrystallized steel structure of the first region. The second region, which is roll-hard, prevents larger grains from the recrystallized first region from pressing outward against the surface of the steel sheet during forming. This prevents undesirable optical effects and mechanical instability during forming, and also prevents porosity and cracking in the coating applied to the surface of the second side of the steel sheet.

Particularly good formability of the steel sheet is achieved if the second, roll-hardened region is as thin as possible, in particular thinner than the first, recrystallized region. Preferably, the first region has a thickness in the range from 50 μm to 450 μm, particularly preferably in the range from 90 μm to 400 μm, especially in the range from 150 μm to 300 μm, and the thickness of the second region preferably is It is in the range from 1 μm to 50 μm, particularly preferably in the range from 2 μm to 10 μm. Preferably, the thickness of the second region is 1% to 10% of the thickness of the first region. At this low thickness of the non-recrystallized or barely recrystallized second region, the formability of the steel sheet is hardly affected by the high strength of the still roll-hard region. Nevertheless, during forming of the steel sheet, the second region forms a sufficient barrier against coarse grains of the steel structure being pressed through the surface of the steel sheet, preventing roughening of the surface.

The value ΔT at which the recrystallization temperature increases in the second region due to the incorporation of nitrogen when heating the steel sheet can be controlled by the nitrogen content introduced by nitriding in the second region of the steel sheet after the end of heating, where in particular ΔT = A linear relationship can be observed that can be described by a ) is the nitrogen content. By testing on samples with different nitrogen contents and otherwise the same alloy composition, for example a A value of 1.2 K/ppm was calculated. Therefore, ΔT An increase in the recrystallization temperature in the near-surface region in the range from 10 K to 24 K can already be achieved with a low addition of ΔN in the range from 10 ppm to 20 ppm (corresponding to 0.001 to 0.002% by weight). For example, at higher nitrogen loadings of ΔN = 100 ppm (corresponding to 0.01% by weight), the (theoretically achievable) increase in recrystallization temperature in the near-surface region is already approximately ΔT. It is 120 K. Preferably, the value ΔT at which the recrystallization temperature increases in the second region due to the incorporation of nitrogen when heating the steel sheet is greater than K, particularly preferably greater than 100 K, and particularly preferably in the range from 100 K to 250 K. .

In the process according to the invention, the steel sheet can be heated in a single-stage heating step or in a two-stage heat treatment. In the single-stage heating step, the steel sheet is heated from room temperature to the heating temperature ( _TE ) within the heating time (t _E ), and after reaching the heating temperature (T _E ), a predetermined annealing time is performed at the heating temperature (T _E ). (t _G ), the heating time (t _E ) is preferably in the range from 1.0 to 300 seconds and/or the annealing time (t _G ) is preferably in the range from 1.0 to 80 seconds, particularly preferably from 1 second to 1 second. The range is 10 seconds. During heating, the steel sheet is at least temporarily exposed to a nitriding gas atmosphere, especially an ammonia-containing gas atmosphere with a volume fraction of ammonia in the range from 0.1 to 6%.

In a two-stage heating process, nitriding is performed in the first stage at an intermediate temperature, and annealing occurs in the second stage at a heating temperature higher than the intermediate temperature (which is why the heating temperature is also referred to as the annealing temperature below). The intermediate temperature is lower than the original recrystallization temperature T _R . In a two-stage process, the steel sheet is heated from room temperature to an intermediate temperature (T _Z < T _R ) in a first stage within a first heating time and maintained at least approximately at this temperature for a holding time t _H. The intermediate temperature _T This is because dissociation begins at a temperature of about 300°C. In any case, at temperatures below about 550° C., (complete) recrystallization does not yet occur for most alloy compositions according to the invention. Therefore, at the preferred intermediate temperature T _Z of the first stage, diffusion of the dissociated nitrogen from the nitriding gas atmosphere into the second region of the steel sheet occurs, but not yet recrystallizes. Recrystallization of the first region does not occur until a second stage in which the steel sheet is heated to a heating temperature (annealing temperature) T _E above the original recrystallization temperature T _R but below T _R + ΔT. For example, the heating temperature T _E ranges from 650° C. to 800° C., especially around 750° C., depending on the value of the initial recrystallization temperature T _R of the steel used.

As in the single-stage heating of the steel sheet in the first embodiment, in the two-stage process at least substantially only (partial) recrystallization occurs in the first region of the steel sheet, while the second region does not recrystallize. Recrystallization of the second region nitrided in the first stage is prevented by selecting the heating temperature (annealing temperature) T _E so that it is below the recrystallization temperature T _R + ΔT which is increased by ΔT due to nitriding of the second region.

In a two-stage heat treatment, the steel sheet is suitably annealed in a continuous annealing furnace, first in a first stage, in a second region near the surface on the second side of the steel sheet, for a holding time preferably in the range of 10 to 150 seconds (initial heat treatment) of the steel. ) annealed at a lower intermediate temperature T _{Z below the} recrystallization temperature T It is at least partially recrystallized and annealed only in the first region at a heating temperature (annealing temperature T _E ) that is above the (initial) recrystallization temperature T _R .

The first heating time (t _E ¹ ), during which the steel sheet is heated from room temperature to the intermediate temperature in the two-stage process, preferably ranges from 1.0 to 120 seconds, particularly preferably from 10 to 90 seconds, as in the first embodiment. , can be adapted depending on the desired material properties of the steel sheet according to the invention. The holding time (t _H ) during which the steel sheet is maintained at the intermediate temperature here also preferably ranges from 1.0 to 90 seconds, particularly preferably from 10 to 60 seconds, and is likewise selected depending on 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) immediately after cooling or at a second heating time (t _E ² ) without cooling in the second stage, at least approximately by this heating. It can be maintained at temperature T _E for an annealing time (t _G ). Optionally, the annealing furnace may also be provided with a nitriding gas atmosphere during the annealing period, from which the dissociated (atomic) nitrogen is available such that (further) nitriding of the steel sheet in the deeper second region is still possible during the annealing period t _G. It can happen. This increases the size of the second region in the direction of the first side of the steel sheet, thereby increasing the thickness of the second region. In this case the heating temperature T _E (annealing temperature) is also between the original recrystallization temperature T _R and the recrystallization temperature increased to T _R + ΔT by nitriding in the near surface region of the steel sheet. Therefore, in a two-stage process, this also holds for the heating temperature T _E : T ≤ T _RE < T _R + ΔT, where the intermediate temperature (T _Z ) is lower than the initial recrystallization temperature T _R .

In both the single-stage process and the two-stage process, the steel sheet is exposed at least temporarily to a nitriding gas atmosphere in which dissociated (atomic) nitrogen is provided to the annealing furnace during heating and before reaching the recrystallization temperature, which is initially Only a second region on the second side of the steel sheet diffuses into the steel sheet near the surface, raising the recrystallization temperature there, while on the first side of the steel sheet penetration of nitrogen through the barrier layer is prevented.

The steel of the cold rolled steel sheet preferably has the following composition by weight:

● Carbon, C: greater than 0.001% but less than 0.1%, preferably less than 0.06%;

● Manganese, Mn: greater than 0.01% but less than 0.6%;

● Phosphorus, P: less than 0.04%;

● Sulfur, S: less than 0.04%, preferably more than 0.001%;

● Aluminum, Al: less than 0.08%;

● Silicon, Si: less than 0.1%;

● optionally copper, Cu: less than 0.1%;

● optionally chromium, Cr: less than 0.1%;

● Optionally Nickel, Ni: less than 0.1%;

● optionally titanium, Ti: less than 0.1%, preferably more than 0.02%;

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

● optionally molybdenum, Mo: less than 0.08%;

● optionally tin, Sn: less than 0.05%;

● optionally boron, B: less than 0.01%, preferably less than 0.005%, preferably more than 0.0005%;

● optionally nitrogen, N ₀ : less than 0.02%, especially less than 0.016%, preferably more than 0.001%;

● the remaining iron and inevitable impurities,

● Here, after heating the cold-rolled steel sheet in a nitriding gas atmosphere, the average nitrogen content (N) by weight, averaged over the entire thickness of the steel sheet, is at least 0.01%, preferably at least 0.015%.

When reference is made to average nitrogen content (N) or average nitrogen content, the nitrogen concentration averaged over the respective thickness is meant. Therefore, when reference is made to the average nitrogen content (N) of a steel sheet, it means the nitrogen concentration averaged over the thickness of the steel sheet.

The steel of the cold rolled steel sheet may already preferably have an initial nitrogen content N ₀ of greater than 0.001% by weight and less than 0.02% by weight and particularly preferably less than 0.016% by weight. However, steels that do not contain any nitrogen other than the inevitable nitrogen impurities can also be used. Limiting the initial nitrogen content to a value of less than 0.02% by weight ensures that the hot strips produced from steel by hot rolling can be cold rolled without difficulty using conventional cold rolling equipment (rolling mills for cold rolling steel sheets into ultrafine sheets). It can be done. Additionally, the low initial nitrogen content of less than 0.02% by weight _NO in the steel prevents the formation of defects when casting slabs. However, in order to achieve the highest possible (average) nitrogen content in cold rolled steel sheets and thereby high solid solution strengthening, the steel used to produce the hot strip is already preferably 0.001% to 0.02% by weight. , especially preferably with an (initial) nitrogen content in the range from 0.005% to 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 in the second region on the second side of the steel sheet, the nitrogen content is increased to a value greater than the initial nitrogen content (N ₀ ). increases. The nitrogen content by weight in the second region is thereby increased to a value averaged over the thickness of this region which is preferably greater than 50 ppm, particularly preferably greater than 400 to 800 ppm, of the initial nitrogen content ( _NO ) of the steel. In this case, a (sometimes significant) increase in the strength of the steel sheet is observed depending on the thickness of the second region. Strengths of steel sheets exceeding 700 MPa can be achieved at an elongation at break of at least 5%. The (average) nitrogen content of the nitrogen deposited in the second region by the nitriding process can reach a solubility limit of nitrogen in iron of approximately 1000 ppm.

Depending on the setting of the process parameters, in particular the holding time and annealing time during annealing of the steel sheet at the heating temperature (annealing temperature) and the (optional) concentration of nitriding gas (in particular ammonia) under the gas atmosphere of the annealing furnace, the first region of the steel sheet is It is also nitrided to a certain degree. However, preferably, the nitridation of the first region is (quite) small in order to achieve the largest possible difference in the degree of recrystallization of the first and second regions after annealing or at the end of heating. Preferably, the second region has a degree of recrystallization of less than 30%, more preferably less than 20%, and the first region has a degree of recrystallization of greater than 70%, more preferably greater than 80%. Particularly preferably, the first region is completely recrystallized.

In a preferred embodiment, the steel sheet according to the invention is produced in a preliminary stage by hot rolling and then cold rolling and has, by weight, more than 0.001% but less than 0.1% C, more than 0.01% but less than 0.6% Mn, less than 0.04% of P, less than 0.04% S, less than 0.08% Al, less than 0.1% Si, and optionally an initial nitrogen content ( _NO ) of up to 0.020%, preferably not more than 0.016%, with the remainder iron and unavoidable impurities. do. After nitriding, the steel sheet preferably contains an average nitrogen content N > N ₀ of at least 0.020%, particularly preferably at least 0.025%, especially N in the range from 0.040 to 0.080%. At an average nitrogen content of N above 0.040%, the tensile strength of the steel sheet is at least 800 MPa, and the steel sheet simultaneously exhibits an elongation at break in the range of 2% to 10% and thus sufficient formability for deep drawing processes.

Nitrogen incorporated into the second region during heating of the cold rolled steel sheet may be present as nitrides in dissolved and/or bound form (up to the solubility limit). In the presence of strong nitride formers in steel, the intercalated nitrogen exists at least partially as nitrogen bound to nitrides, in particular as TiN and/or NbN and/or AlN. Since the interstitial incorporation of nitrogen introduced into the second region during nitriding is desirable to increase the strength, when high strength steel sheets are to be achieved, the steels are expediently treated with nitride formers such as Ti, Nb, Mo or Al. Contain as little as possible. Accordingly, the following upper limits for the weight percentage of nitride former are preferred:

● Al: less than 0.08% Si:

● Ti: less than 0.1%, preferably greater than 0.02%;

● Nb: less than 0.08%, preferably greater than 0.01%;

● Mo: less than 0.08%;

● B: less than 0.01%, preferably less than 0.005%.

On the other hand, the formation of nitrides during nitriding of the second region promotes a sharp boundary of the first recrystallized region of the steel sheet and the second region that is not recrystallized or at least not fully recrystallized. Therefore, if the highest possible difference in the degree of recrystallization between the first and second regions is desired, a sufficient amount of a strong nitride former such as Ti, Nb, Mo and/or Al is added to the steel. In this context, the steel of the cold rolled sheet preferably contains more than 200 ppm titanium and/or more than 100 ppm niobium and/or more than 0.0005% boron and/or more than 50 ppm aluminum by weight. Particularly preferably, in this case the sum of the weight fractions of the strong nitride formers is greater than 300 ppm, preferably greater than 500 ppm. Strong nitride formers such as titanium, niobium and/or aluminum bind the nitrogen deposited in the first region and prevent the nitrogen initially deposited only near the surface from diffusing further into the interior of the steel sheet towards the first side. Accordingly, a very thin first region with a high nitrogen content compared to the sheet thickness is created on the surface of the steel sheet, which results in a strong increase in the recrystallization temperature in this region. The steel sheet cold rolled according to the process according to the invention is now on the one hand higher than the (initial) recrystallization temperature in the second (non-spiked or only slightly spiked) region of the steel sheet and on the other hand in the nitrided region (increased temperature). ) When heated to a temperature below the recrystallization temperature, only the first region recrystallizes (completely), while the second region remains in a rolled-hard state due to increased recrystallization. This results in a two-layer crystallization of the microstructure in the steel sheet according to the invention with a clear boundary between the first and second regions.

A surface nitride layer, especially an iron nitride layer, can also be formed on the free surface of the second region depending on the concentration of nitrogen in the nitrogen donor. This surface nitride layer is disadvantageous in terms of roughening during forming of the steel sheet because it is brittle and may break during forming. The formation of the surface nitride layer essentially depends on the concentration of nitrogen in the nitriding gas atmosphere of the annealing furnace. When using a gaseous atmosphere with ammonia as nitrogen donor, for example, the formation of a nitride layer on the surface of the first region can be observed on a laboratory scale from an ammonia content of approximately 2 to 3% by volume. The nitride layer is very thin and has a thickness in the range of about 10 μm or less. To avoid a nitride layer on the surface of the second region, the concentration of ammonia in the gas atmosphere is preferably set to 3% by volume or less. The volume fraction of ammonia in a gas atmosphere refers to the conditions of laboratory tests in which steel sheets were heated with an induction heater, and the volume fraction of ammonia in a gas atmosphere in a laboratory furnace was determined at room temperature.

The formation of the two-layer microstructure, which is related to the crystallinity of the first and second regions of the steel sheet, can be influenced in particular by the heating time and the annealing time. Preferably, the cold rolled steel sheet is heated to a heating temperature (T _E ) and maintained at this heating temperature for an annealing time (t _G ) of 1.0 to 90 seconds. The short heating time and short annealing time contribute to a stronger formation of a nitrogen gradient from the second side to the first side of the steel sheet, because with the short heating time and annealing time initially only on the surface of the second region of the steel sheet. This is because the deposited nitrogen cannot diffuse into the core area. The longer the annealing time, the more diffusion of nitrogen from the second region near the surface into the core region can be observed, with the result that the core region of the steel sheet is also at least slightly nitrided.

Accordingly, the thickness of the non-recrystallized second region can be controlled by the heating time, whereby a linear relationship can be observed between the thickness of the second region and the heating time. Accordingly, an adjustable process parameter of the process according to the invention is available through the heating time, through which the thickness of the non-recrystallized and therefore roll-hardened second region can be specifically adjusted. Depending on the selected heating time, a thickness of the second region of 0.1 μm to 150 μm can be set, whereby the thickness of the second region should be as thin as possible, preferably 1 μm to 10 μm, especially 1 μm and It is 2 μm. The barrier layer on the surface of the first region preferably has a small thickness of less than 1 μm, especially less than 0.1 μm, or a weight support of less than 10 mg/m ² and is particularly preferably thinner than the second region. The barrier layer has a sufficiently high thickness to at least substantially completely prevent the diffusion of nitrogen on the first side of the steel sheet, especially at temperatures above 300°C, especially between 300°C and 600°C. At the same time, the barrier layer does not affect, or at least does not significantly affect, the mechanical properties of the steel sheet due to its low density.

When the cold rolled steel sheet is heated in a nitriding gas atmosphere, a gradient of nitrogen content is established in the second region as the nitrogen content steadily decreases from the second side to the first side of the cold rolled steel sheet. The amount of nitrogen incorporated into the second region can be controlled through the nitrogen concentration in the nitriding gas atmosphere and, in particular, through the volume fraction of ammonia in the gas atmosphere of the annealing furnace.

Preferably, the steel sheet is heated in a continuous annealing furnace in a nitrogen-containing gas atmosphere through which the steel sheet in expedient strip form passes. A nitrogen-containing gas atmosphere can be provided in particular by introducing ammonia gas into the annealing furnace, whereby during heating the ammonia molecules thermally dissociate into atomic nitrogen due to a catalytic reaction at the heated surface of the second region of the steel sheet, 2 It spreads into the steel plate through the surface of the area. On the first side of the steel sheet, this diffusion process is at least suppressed or preferably completely inhibited by the barrier layer, which is why no or only a very small amount of nitrogen can diffuse into the steel sheet during heating. The volume concentration of ammonia in the nitrogen-containing gas atmosphere is preferably greater than 0.1%, especially 0.1% to 10%, and more preferably 0.1% to 5%, especially 0.5% to 3%. Particularly preferably, the cold rolled steel sheet is heated in an inert protective gas atmosphere particularly containing HNx, and the volume concentration of HNx in the nitrogen-containing gas atmosphere is preferably between 90% and 99.5%.

Accordingly, the process of the present invention can be used to produce steel sheets from steel with a carbon content (C) by weight of 10 to 1000 ppm and a thickness of less than 0.5 mm, wherein the steel sheets have a first region on the first side of the steel sheet. and a two-layer crystallization structure having a second region on the second side of the steel sheet, wherein the first region is at least substantially recrystallized and the second region is not recrystallized, or at least not completely recrystallized, and the manufacturing process. As a result, a barrier layer that is at least substantially impermeable to nitrogen is present on the surface of the first side. There is a gradient of nitrogen content at least in the second region of the steel sheet with the nitrogen content decreasing from the surface of the second side toward the first side.

Accordingly, the second region of the steel sheet has a significantly higher hardness than the first region. Preferably, the second region has a Vickers hardness of at least 220 HV _0,025 , particularly preferably at least 300 HV _0,025 . Preferably, the Vickers hardness in the first region is at least 100 HV _0,025 and less than 280 HV _0,025 . The ratio of the hardness of the second region to the hardness of the first region is preferably greater than 1.2, particularly preferably greater than 1.4.

Particularly high strength and hardness values can be achieved if the average nitrogen content by weight in the second region of the steel sheet after nitriding is 400 to 800 ppm.

The two regions of the two-layer microstructure differ from each other not only in terms of their hardness or strength, but also in terms of their organization. For example, a first region that is at least substantially recrystallized and a second region that is barely or not recrystallized can also be distinguished from each other by the ratio of {001} and {111} orientations in the ε fibers. The ratio of {001} and {111} orientations in ε fibers can be defined as the “strain index” that characterizes the forming behavior of steel sheets. The {111} orientation allows for excellent formability and has a good Lankford coefficient (r-value), while the {001} orientation is less formable. Here, the ε-fiber is defined by the <110> vector lying parallel to the transverse direction (perpendicular to the rolling direction and the normal direction in the strip plane of the steel strip). In the steel sheet according to the invention, the first recrystallized region in each case has a strain index of less than 0.8 and the second region has a strain index of more than 2.0, in particular between 2.0 and 5.0. Corresponding characterizations of the texture in the first and second regions can also be defined for other fibers of the microstructure, for example α-fibers defined by the <110> vector lying in the rolling direction.

The advantageous sharp boundaries of the first and second regions with respect to each other are such that the steel sheet of the second region has a (re)crystallization of less than 30%, preferably less than 20%, and/or the first region has a degree of (re)crystallization of less than 70%. This is achieved with a degree of (re-)crystallization greater than, preferably greater than 80%. A particularly sharp boundary between the two regions can be achieved if the following applies to the heating temperature (T _E ): T _E = T _R + ΔT/2. Preferably, the heating temperature T _E is in the range from _TR + ΔT/3 to _TR + 2 ΔT/3.

The steel sheet produced by the process according to the invention can be advantageously used for the production of packaging, whereby the steel sheet can be formed in a forming process into the body of a can, for example. In some cases, steel sheets are given to heavy forming with very small bending radii of less than 14 mm. The steel sheet according to the invention is characterized by the fact that virtually no roughening of the surface occurs outside the bending radius during forming, especially if the still roll-hard second region is outside the bending radius. The roughening that occurs during forming can be achieved, for example, by deep drawing a sheet steel specimen to form a cubic vessel with different bend radii at the four corners of the cube and then measuring the roughness on the outside of the bend radius of the vessel using a four-radius cup. Can be detected by test. In the steel sheet according to the invention, the outer surface of the second region after forming the steel sheet with a bending radius in the range of 8 mm to 14 mm preferably has a roughness (Ra) of less than 1.0 μm, particularly preferably less than 0.8 μm. or a roughness coefficient of less than 3, preferably less than 2.5, wherein the roughness coefficient is the ratio of the roughness of the surface of the second area outside the 90° angle and the roughness of the steel sheet of the unstrained section of the container formed from the steel sheet.

Therefore, in addition to the manufacturing process and the steel sheet produced therefrom, the object of the present invention is also to provide at least one convexly shaped section, whereby a second region of the steel sheet according to the invention lies on the convex outside of the shaped section. It is a container manufactured from a steel plate with. In particular, the container may be a can having a can bottom and a body formed thereon, with the transition between the can bottom and the body forming the molded portion.

These and other advantages of the packaging steel and manufacturing process according to the invention arise from the embodiments described in more detail below with reference to the accompanying drawings. The drawing shows:
Figure 1 is a schematic diagram of an embodiment of the heating step of the process according to the invention in the form of a temperature-time diagram.
Figure 2 is a schematic diagram of a cross section of a steel sheet according to the invention with a two-layer microstructure.
Figure 3 is a microscopic cross-sectional image (metallographic section) of a steel sheet according to the invention with a two-layer microstructure.
Figure 4 is a plot of the cross-sectional change in Vickers hardness (HV _0.025 ) versus the thickness of the steel sheet when measured on different samples of the steel sheet according to the invention, heated and nitrided at different heating times (t _E ) in the method according to the invention. .
Figure 5 is a comparison of the stress-strain diagrams [σ(ε)] of steel sheets according to the invention heated and nitrided at different heating times (t _E ).
Figure 6 shows a schematic illustration of the 4-radius cup test with a top view of the bottom of the container manufactured in this test from a steel sheet according to the invention with different bending radii at the corners of the container (Figure 6a) and a section of the side view of the container (Figure 6b) am.
Figure 7 is a schematic of the results of the 4-radius cup test of Figure 6 performed on a specimen of a steel sheet according to the present invention, wherein Figure 7a shows bending radii (R1 to R4) at different positions (P0 to P4) than Figure 6b. The roughness (Ra; unit: μm) for the outside is shown, and in Figure 7b the roughness coefficient at these positions (P0 to P4) (Ra coefficient = Ra/R ₀ , where R ₀ is the (Roughness of the steel plate) is shown.

Hot rolled and subsequently cold rolled steel sheets with a carbon content by weight of 10 to 1000 ppm are used as starting products for the production of the steel sheets according to the invention. The alloy composition of the steel complies with the limits specified by standards for packaging steels (e.g. as specified in the standards ASTM A623-11 “Standard Specification for Tin Mill Products” or the “European Standard EN 10202”). However, there may be deviations from this, especially with regard to the initial nitrogen content, especially if highly nitrided steel sheets with a high nitrogen content of more than 0.02% by weight are to be produced. The composition of the steel from which the steel sheet according to the invention can be manufactured is explained in detail below:

Lecture composition:

● Carbon, C: greater than 0.001% but less than 0.1%, preferably less than 0.06%;

Carbon has the effect of increasing hardness and strength. Therefore, the steel preferably contains more than 0.001% by weight carbon. In order to ensure the rollability of the steel sheet during the first cold rolling without reducing the elongation at break and, if necessary, in the second cold rolling stage (quench rolling), the carbon content should not exceed 0.1% by weight.

● Manganese, Mn: greater than 0.01% but less than 0.6%;

Manganese also has the effect of increasing hardness and strength. Additionally, manganese improves the malleability, weldability, and wear resistance of steel. Moreover, the addition of manganese reduces the red fracture tendency during hot rolling, and manganese causes grain refinement. Therefore, a manganese content of at least 0.01% by weight is preferred. To achieve high strengths, a manganese content of more than 0.1% by weight, especially more than 0.20% by weight, is preferred. However, if the manganese content becomes too high, it harms the corrosion resistance of the steel. Additionally, if the manganese content becomes too high, the strength becomes too high and the steel is no longer cold rollable and formable. Therefore, the preferred upper limit for manganese content is 0.6% by weight.

● Phosphorus, P: less than 0.04%.

Phosphorus is an undesirable by-product of rivers. High phosphorus content particularly causes brittleness of the steel and thus reduces the formability of the steel sheet, which is why the upper limit of the phosphorus content is 0.04% by weight.

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

Sulfur is an undesirable minor element that worsens ductility and corrosion resistance. Therefore, less than 0.04% by weight sulfur should be present in the steel. On the other hand, complex and cost-intensive measures have to be taken to desulfurize the steel, which is why sulfur contents below 0.001% by weight are no longer feasible from an economic point of view. Therefore, the sulfur content preferably ranges from 0.001% to 0.04% by weight, particularly preferably from 0.005% to 0.01% by weight.

● Aluminum, Al: less than 0.08%;

In steel manufacturing, aluminum acts as a deoxidizer in the casting process to calm the steel. Aluminum also increases scale resistance and formability. Additionally, aluminum forms nitrides with nitrogen, which is advantageous in the steel sheet 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 above 0.08% by weight may 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 scale resistance of steel and is a solid solution strengthener. In steel manufacturing, it has the positive effect of making the melt thinner and acts as a deoxidizer. Another positive effect of silicon on steel is that it increases tensile strength, yield strength and scale resistance. Therefore, a silicon content of 0.003% by weight or more is preferred. However, if the silicon content becomes too high, especially exceeding 0.1% by weight, the corrosion resistance of the steel may decrease and surface treatment, especially by electrolytic coating, may become more difficult.

● Optionally nitrogen, N _O : less than 0.02%, especially less than 0.016%, preferably more than 0.001%.

Nitrogen is an optional component in the molten steel from which the steel for the steel sheet according to the invention is produced. It is true that nitrogen acts as a solid solution strengthener, increasing hardness and strength. However, an excessively high nitrogen content in the steel melt of more than 0.02% by weight means that the hot strip produced from the steel melt can no longer be cold rolled. Additionally, the high nitrogen content in molten steel increases the risk of defects in hot rolled strip because hot forming ability is reduced at nitrogen concentrations above 0.016% by weight. According to the present invention, it is contemplated to subsequently increase the nitrogen content of the cold rolled steel sheet by nitriding it in an annealing furnace. Therefore, the introduction of nitrogen into the molten steel can be completely eliminated. However, in order to achieve strong solid solution strengthening, it is desirable for the steel melt to already contain an initial nitrogen content of more than 0.001% by weight, particularly preferably more than 0.010% by weight.

● Optional: Nitride formers, especially niobium, titanium, zirconium, vanadium:

Nitride-forming elements such as aluminum, titanium, niobium, zirconium or vanadium are optionally used to at least partially bind the nitrogen originally contained in the steel and the nitrogen subsequently introduced in the form of nitrides by a subsequent nitriding process in the annealing furnace. It is advantageous in the steel sheet according to the present invention. This improves the forming behavior and makes it possible to produce IF (interstitial-free) steel sheets with almost no aging. Aluminum, titanium and/or niobium are particularly preferred as steel components because, in addition to their properties as strong nitride formers, they also act as microalloying components that increase strength by grain refinement without reducing toughness. Because.

Therefore, in terms of weight, the steel optionally, preferably contains:

● Titanium, Ti: preferably greater than 0.02%, particularly preferably greater than 0.02% but less than 0.1%, and/or

● Niobium, Nb: preferably greater than 0.01% and less than 0.08%, and/or

● Aluminum, Al: preferably greater than 0.005% by weight and less than 0.08% by weight, and/or

● Molybdenum, Mo: less than 0.08%;

Other optional ingredients:

In addition to the remaining iron (Fe) and inevitable impurities, the steel may contain other optional components, such as:

● optionally copper, Cu: less than 0.1%;

● optionally chromium, Cr: less than 0.1%;

● Optionally Nickel, Ni: less than 0.1%;

● optionally tin, Sn: less than 0.05%;

● Optionally boron, B: less than 0.01%, preferably less than 0.005%, preferably more than 0.0005%.

Table 1 shows steel compositions A, from which steel sheets can be produced by hot rolling and cold rolling, which can be processed by the process according to the invention to produce a two-layer crystallized structure in order to form a two-layer microstructure of cold rolled steel sheets. , B and C are shown.

Manufacturing process of steel plate:

With the described composition of the steel, a steel melt is produced whereby, in a preferred embodiment, in order to achieve a high (average) nitrogen content of the steel sheet, the steel is produced by adding nitrogen to the steel melt, for example by blowing in nitrogen gas. and/or by adding solid nitrogen compounds such as lime nitrogen (calcium cyanamide) or manganese _nitride . To prevent the strength of the steel sheet made from the steel melt from becoming too high due to nitrogen solid solution strengthening, and to maintain the hot formability of the steel as well as to avoid defects caused by nitrides in the slabs produced from the steel melt, the initial nitrogen content of the steel ( It is advantageous when N ₀ ) does not exceed 0.02% by weight and is preferably 0.016% by weight or less.

The slab is first cast from molten steel, which is then hot rolled and cooled to room temperature. The hot strips produced in this way have a thickness ranging from 1 to 4 mm and are wound into coils at a predetermined coiling temperature ranging from 500° C. to 750° C., preferably from 650° C. to 750° C. The hot strip is then wound into coils at a predetermined coiling temperature. To produce packaging steel in the form of thin sheets of typical sheet thicknesses of less than 0.5 mm, preferably less than 0.3 mm, hot strips are cold rolled with a thickness reduction ranging from 50% to more than 90%.

The steel sheet is then cold rolled with a barrier layer at least substantially impermeable to (atomic) nitrogen in the form of a silicate layer or a sol-gel layer, in particular a SiO ₂ , TiO ₂ and/or ZrO ₂ layer to a thickness of less than 0.5 mm. It is applied to the surface of a steel plate on one of its two sides. In a coil coating process, for example, a dispersion of silicone alcoholate is applied to the surface of a steel strip on the first side (a). For this purpose, the dispersion is sprayed as a sol on the first side of the steel strip in a wet-chemical coating process using a spray nozzle or applied with a doctor blade and subsequently dried. In the process, molecular chains are initially formed, and after a longer period, fine particles are formed. In the further process, the particles form a network in the sol. In the wet-chemically applied sol layer, a gel state is created due to subsequent hydrolysis and condensation reactions. Gelation can be accelerated by applying heat.

For gelling and drying, the sol-coated steel strip will be placed in an oven. Drying of the sol can advantageously take place at least partially in a continuous annealing furnace, where subsequent heat treatment of the steel sheet for nitriding and partial recrystallization takes place. For this purpose, a cold rolled steel strip provided with a barrier layer on the first side is passed through a continuous annealing furnace, in which the steel strip is heated to a temperature higher than the (initial) recrystallization temperature T _R of the steel.

In a particularly preferred embodiment, the silicate-containing barrier layer is applied electrolytically from an aqueous electrolyte to one of the two sides of the steel sheet. For this purpose, an aqueous basic electrolyte solution containing silica and sodium salts is added to the electrolyte tank, and a steel sheet as cathode is passed through the electrolyte tank at a predetermined strip speed. Table 2 shows the preferred composition of the electrolyte solution, Table 2 shows the preferred parameters of the electrolyte application process, thereby providing a barrier in the range of 1 to 10 mg/m ² , particularly preferably in the range of 3 to 6 mg/m ² The silicate layer, which acts as a layer, is applied from the electrolyte on one side of the steel sheet.

In the process according to the invention, after the barrier layer has been applied, the cold rolled steel sheet is nitrided by heating it under a nitriding gas atmosphere in a continuous annealing furnace before or preferably simultaneously with the recrystallization annealing. Nitriding is preferably carried out simultaneously with recrystallization annealing in an annealing furnace by introducing a nitrogen-containing gas, preferably ammonia (NH ₃ ), into the annealing furnace while the steel sheet is heated to a temperature above the (initial) recrystallization temperature T _R of the steel. do. When ammonia is used as the nitrogen-containing gas, preferably at a temperature of the annealing furnace exceeding 300° C., atomic nitrogen is formed by dissociation of nitrogen from the nitrogen-containing gas due to a catalytic reaction, which occurs on the surface of the steel sheet (interstitial ) can spread into the steel sheet on the second side of the steel sheet. On the first side of the steel sheet, diffusion of nitrogen is prevented by the barrier layer.

To prevent oxidation of the surface of the steel sheet on the second side during heating, it is expedient to use an inert gas atmosphere in the annealing furnace. Preferably, the atmosphere of the annealing furnace consists of a mixture of a nitrogen-containing gas acting as a nitrogen donor and an inert gas such as HNx, the volume fraction of the inert gas is preferably 90% to 99.5%, and the volume of the gas atmosphere is preferably 90% to 99.5%. The remainder of the fraction is formed by nitrogen-containing gases, especially ammonia gas (NH ₃ gas).

Figure 1 schematically shows the temperature-time profile of a heat treatment for nitriding and annealing a steel sheet in an annealing furnace in an example of a preferred embodiment of the process according to the invention. In an example of this first embodiment, the single-stage heat treatment of the steel sheet is performed in a continuous annealing furnace, and the steel sheet is simultaneously recrystallized annealed and nitrided during the single-stage heating. As _shown in Figure 1, in this embodiment, the steel sheet is heated from room temperature to _a heating temperature _T It is maintained at least approximately at this temperature for a period of time (t _G ). The heating temperature T _E corresponds to the annealing temperature at which the steel sheet is annealed and recrystallized (in a given region), the initial recrystallization temperature T _R and the recrystallization temperature increased to T _R + ΔT by nitriding in the near surface region of the steel sheet. It's in between. This holds here for the heating temperature T _E : T ≤ T _RE < T _R + ΔT.

The heating time (t _E ) preferably ranges from 1.0 to 300 seconds, particularly preferably from 10 to 120 seconds, and can be adjusted depending on the desired material properties of the steel sheet according to the invention, as will be explained further below. To adjust the heating time, the heating rate at which the steel sheet is heated in the annealing furnace or the speed at which the steel sheet passes through the continuous annealing furnace can be adjusted according to the desired heating time. To set the preferred heating time (t _E ) in the range of 1.0 to 300 seconds, a heating rate of, for example, 10 K/s to 80 K/s may be selected. During the heating time, the steel sheet in the continuous annealing furnace is exposed to a nitriding gas atmosphere, especially ammonia gas atmosphere. The annealing time (t _G ) preferably ranges from 1.0 to 90 seconds, particularly preferably from 10 to 60 seconds, and is also selected depending on 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 is cooled passively in the environment or cooled to room temperature by active cooling, for example water cooling or gas flow cooling. Suitable cooling rates range from 3 K/s to 20 K/s for gas flow cooling and above 1000 K/s for water cooling.

The (initial) recrystallization temperature T _R of the steel depends on the composition of the steel and typically ranges from 550°C to 720°C.

When a cold rolled steel sheet is heated in an annealing furnace, nitrogen from the nitrogen-containing gas is initially deposited only on the near surface area on the second side of the steel sheet as atomic nitrogen diffuses through the steel sheet surface. Nitrogen that diffuses into the near surface region can become interstitially incorporated into the iron lattice of the steel or become incorporated as nitrides, especially if strong nitride formers such as Al, Nb, Ti, or B are present in the steel. The incorporation of nitrogen increases the recrystallization temperature (T _R ) of the steel in the second region of the root surface by the value ΔT. This increase in recrystallization temperature (T _R ) in the second region near the surface is shown in Figure 1 as ΔT.

According to the invention, the heating temperature (T _E ) or annealing temperature is now selected such that T ≤ T _RE < T _R + ΔT applies. Therefore, the heating temperature (T _E ) or the annealing temperature is the (initial) recrystallization temperature (T _R ) of the steel used for the manufacture of cold rolled steel sheets and the recrystallization temperature increased by the value ΔT due to near-surface nitriding of the steel sheet in the near-surface second region. The temperature is set in the process according to the invention to be between T _R + ΔT. By setting the heating temperature T _E (or annealing temperature) in this way, recrystallization takes place only in a first region on the first side of the steel sheet, adjacent inwardly to a second region near the surface, where, at least initially, In, nitrogen was not (yet) incorporated during the annealing and simultaneous nitriding of the steel sheet. This is because the heating temperature (T _E ) is higher than the recrystallization temperature (T _R ) only in this first region of the steel sheet and only in the second region where the recrystallization temperature is increased by ΔT due to the incorporated nitrogen, and the heating temperature (T _E ) is below the recrystallization temperature increased by T _R + ΔT. Thus, a two-layer microstructure is formed over the cross-section of the steel sheet with a first region (1) and a second region (2) that is at least essentially and preferably substantially completely recrystallized, with the second region (2) being recrystallized. It does not crystallize, or at least does not recrystallize completely.

Accordingly, the microstructure resulting from heating the steel sheet in a nitrogen-containing gas atmosphere consists of at least an essentially completely recrystallized first region 1 and a recrystallized It includes a second area (2) that is not converted. The second region 2 located on the second side b of the steel sheet does not recrystallize or at least does not recrystallize completely and thus remains in the as-rolled state of the cold rolled steel sheet. In contrast, the first region 1 lying on the first side a of the steel sheet is (particularly completely) recrystallized. The barrier layer 3 shown in Figure 2 is located on the first side (a) of the steel sheet.

The degree of recrystallization of the second region 2 and the first region 1 can be adjusted through heating temperature (T _E ) and annealing time (t _G ). A sharp boundary between the core region (2) and the heme region (1) can be achieved, for example, when the annealing time (t _G ) is greater than 10 seconds and the heating temperature (T _E ) is between T _R + ΔT/3 and T _R + 2ΔT/. This can be achieved in case 3. Similarly, the thickness of the hem region 1 can be adjusted by the process parameters of heating temperature (T _E ) and heating time (t _E ).

Accordingly, the process according to the invention can be used to produce a two-layer microstructure with a first region (1) that is at least largely fully recrystallized and a second region (2) that is roll-hard.

After production of the steel sheets according to the invention, they can be coated with a conversion or protective layer on one or both sides in the customary way, in particular by electrolytic tin plating or chrome plating.

Examples:

Examples of steel sheets, their use in the production of containers and embodiments of the method according to the invention are described below.

A steel sheet with a thickness of 0.23 ± 0.01 mm is hot rolled and subsequently cold rolled from steel melt A with the alloy composition listed in Table 1 (ppm figures refer to the weight fraction of alloying elements in the steel from which the cold rolled steel sheet is produced). Manufactured by . The cold rolled steel sheets were heat treated to a heating temperature T _E of 750°C at different heating times t _E under an ammonia-containing inert gas atmosphere with a volume fraction of ammonia of 5% and kept at the heating temperature T _E for an annealing time t _G of 45 s. .

The microstructure of heat-treated steel sheets (Nital followed by cold-embedding with 3% nitric acid, grinding, polishing and etching) was examined microscopically. Figure 3 shows an example of a cross-sectional micrograph of a metallographic section of a steel sheet according to the invention, from which two regions (1, 2) with different degrees of crystallinity and a barrier layer (3) on the first side of the steel sheet (Fig. (not visible in section 3) can be seen. From Figure 3, the steel sheet according to the present invention has a two-layer microstructure with a recrystallized first region (1) and a non-recrystallized second region (2), and the first region (1) is a second region ( It is clear that it is considerably thicker than 2).

In samples of steel sheets according to the invention, the hardness was recorded at various positions across the cross section. Figure 4 shows the Vickers plot of the thickness of a steel sheet specimen at different positions (position 1 to position 7, where position 1 is the surface of the steel sheet on the first side (a) and position 7 is the surface of the steel sheet on the second side (b)) This represents a change in hardness HV _0.025 . From Figure 4, it can be seen that the hardness increases steadily (linearly) with respect to the thickness of the steel sheet specimen from the first side (a) to the second side (b).

This change in hardness with respect to the thickness of the steel sheet occurs during heat treatment in the nitriding and annealing furnace of the steel sheet in the second region 2 as the nitrogen content decreases from the outside towards the core of the steel sheet (complete) in the first region 1. It is due to recrystallization. The second region 2 is still roll-hard and has a high hardness with a maximum hardness at the surface of the steel sheet on the second side b.

Figure 4 also shows the absolute value of Vickers hardness depending on the heating time (t _E ). For example, a sample heat treated with a short heating time of t _E = 60 seconds has a low Vickers hardness of approximately 100 on the first side (a), and a sample treated with a longer heating time, for example, of t _E = 300 seconds. has a Vickers hardness of greater than 150 in the same plane. Therefore, the hardness or strength of the steel sheet according to the present invention can be adjusted through the heating time (t _E ). Furthermore, from Figure 4, it can be seen that for longer heating times of more than 120 seconds, at least partial hardening of the first region 1 also occurs during heating, as nitrogen diffuses into this region and the hardness or strength of the steel sheet increases. You can see that

This can also be confirmed by strength and elongation measurements on the steel sheet according to the invention. Figure 5 shows an example of a stress-strain diagram of two steel sheet samples according to the invention made from the steels of Table 1, heated and nitrided with different heating times of t _E = 60 s and t _E = 300 s, respectively. It represents.

Figure 5 shows that the mechanical parameters of the steel sheet according to the invention, in particular the tensile strength and elongation at break, can be adjusted through heating time, and tensile strengths of more than 600 MPa and elongations at break of more than 7% are achievable.

The process according to the invention can therefore be used to produce (nitrided) steel sheets characterized by very high strengths of more than 600 MPa combined with good elongations at break of more than 5%, preferably more than 7%. These steel sheets can be processed excellently in forming processes for the production of stable packaging such as tin cans and beverage cans, as well as parts thereof such as (tear-off) lids.

The exact composition of the microstructure, in particular the thickness and degree of crystallization of the first and second zones, as well as the nitrogen content in the first and second zones produced by the nitriding process in a continuous annealing furnace and the nitrogen content throughout the thickness of the steel sheet. The gradient can be influenced by changing process parameters. Accordingly, the properties of the steel sheets produced by the process according to the invention can be adapted to different applications.

The behavior of steel sheets according to the invention during forming was investigated in a four-radius cup test by forming steel sheet specimens into cuboidal containers with different bending radii (R1, R2, R3, and R4) at each of the four corners of the container.

Figure 6 shows a schematic diagram of a vessel formed from a steel plate during a four-radius cup test, Figure 6a showing a top view of the base of the vessel and Figure 6b a cross-sectional side view in the region of the vessel wall. Figure 6a shows different bending radii (R1, R2, R3, R4) with the following sizes:

R1 = 8 mm,

R2 = 10 mm,

R3 = 12 mm,

R4 = 14 mm.

To determine the central roughness Ra value, roughness measurements were performed at positions P0, P1, P2, P3, and P4 shown in Figure 6b.

In the four-radius cup test, a steel sheet with a two-layer crystallization structure was used in each case, a first region (1) that was recrystallized and a second region that was not recrystallized (and therefore still mill-hard). Regions (2) were arranged both on the outer and inner sides at the bend radius. In this context, steel sheets with different thicknesses of the second region 2 were also investigated in the 4-radius cup test. Different thicknesses of the second region 2 were created in the manufacturing process by different heating times for the stitching process (sample A: t _E = 1 second, sample B: t _E = 300 seconds).

The results of the 4-radius cup test are shown in Figures 7A and 7B. Figure 7a shows measurements at various bending radii R1, R2, R3, R4 at positions P1 to P4 (see Figure 6b) and in the undeformed bottom region of the vessel (position P0 in Figure 6b) for both deformations of the tested steel sheet specimen. This shows the central roughness (Ra value).

From Figure 7a, it can be seen that at all bending radii R1 to R4 the roughness Ra is higher than the roughness at the undeformed bottom of the container. It can also be seen from Figure 7a that the roughness at the bend radii R1 to R4 is lower in the container in which the non-recrystallized (rolled-hard) second region 2 is located outside the bend radius. In contrast, containers in which the first recrystallized region 1 lies outside the bend radius have a higher roughness Ra. In containers with the second region 2 lying outside the bending radius R1 to R4, the central roughness Ra is less than 1.0 μm, especially less than 0.8 μm.

Figure 7b shows the Ra coefficient defined by the ratio of the Ra value at the bending radius (R1 to R4) and the Ra value in the unstrained bottom area of the container. As can be seen in Figure 7b, the container in which the non-recrystallized second region 2 is located outside the bending radius R1 to R4 has an Ra coefficient that is about 3 times lower. In the case of containers with the second region 2 lying outside the bending radius R1 to R4, the roughness coefficient (Ra coefficient) is less than 3, especially less than 2.5.

From this it follows that when the steel sheet according to the invention is formed, much less roughening occurs outside the bending radius when the second side (b) of the steel sheet with the non-recrystallized second region (2) is on the outside. A conclusion emerges. Therefore, the results of the 4-radius cup test show that the steel sheet according to the invention is very suitable for the production of containers, showing low roughening with low Ra values in the region of the bending radius of the container. Preferably, when the steel sheet is formed into a container, the second side (b) of the steel sheet with the non-recrystallized second region (2) is arranged in such a way that after forming this side lies outside the bending radius of the container. do. In this case, the non-recrystallized second region 2 of the steel sheet represents a barrier to the larger grains of the steel structure, with the grains of the steel structure being pressed through the surface of the second side b in a visually visible manner. This prevents them from forming, where they create undesirable roughening outside the bend radius. Preferably, the thickness of the roll-hardened second region 2 is selected as small as possible, in particular less than 50 μm. This ensures that the mechanical properties of the steel sheet, in particular its formability, are only slightly affected by the roll-hardened second region 2. In particular, this ensures that the formability of the steel sheet is not significantly reduced despite the roll-hard second region 2 having high hardness and strength. In fact, on the inside of the bending radii R1 to R4 there is a softer and more easily formable first side (a) of the steel sheet with a softer and recrystallized first region (1). When the steel sheet is formed to a bend radius, the first region 1 of the steel sheet is compressed inside the bend radius, and the ductile recrystallized first region 1 forms only a low resistance to this forming. Therefore, despite the high hardness and strength on the second side (b), the steel sheet according to the invention can be used in conventional forming processes, especially deep drawing processes, without any detrimental roughening of the steel sheet surface on the outside of the formed area. It can be easily molded into a container. From the comparison of the roughness values of specimen A and specimen B shown in Figure 7a, it can be seen that the thickness of the second region 2 has only a minor effect on the roughness, which prevents roughening during forming the steel sheet. This is the reason why the thickness of the second region may be small from this point of view.

Claims (24)

● Providing a cold rolled steel sheet of steel with a carbon content (C) by weight of 10 to 1000 ppm and a predetermined recrystallization temperature (T _R ), wherein the steel sheet has a first side (a) and a second side ( b) having,
● Applying a barrier layer (3) at least substantially impermeable to nitrogen to the first side (a) of the steel sheet,
● Heating the steel sheet to the heating temperature (T _E )
A method of manufacturing a steel sheet for packaging comprising:
● The heating takes place in a nitriding gas atmosphere until at least temporarily the recrystallization temperature _T diffuses into and is incorporated into region 2, whereby the recrystallization temperature of the steel in near surface region 2 rises by the value ΔT,
● A manufacturing method in which the heating temperature (T _E ) is above the recrystallization temperature (T _R ) and below the recrystallization temperature (T _R + ΔT) increased by ΔT in the near surface region (2). The method of claim 1, wherein the cold rolled steel sheet is
● C: greater than 0.001% but less than 0.1%, preferably less than 0.06%;
● Mn: greater than 0.01% but less than 0.6%;
● P: less than 0.04%;
● S: less than 0.04%, preferably greater than 0.001%;
● Al: less than 0.08%, preferably more than 0.005%;
● Si: less than 0.1%;
● Optionally Cu: less than 0.1%;
● Arbitrarily Cr: less than 0.1%;
● Randomly Ni: less than 0.1%;
● optionally Ti: less than 0.1%, preferably more than 0.02%, especially preferably more than 0.05%;
● Optionally Nb: less than 0.08%, preferably more than 0.01%;
● Arbitrarily Mo: less than 0.08%;
● Arbitrarily Sn: less than 0.05%;
● Optionally B: less than 0.01%, preferably less than 0.005%, preferably more than 0.0005%;
● Optionally N: less than 0.02%, especially less than 0.016%, preferably more than 0.001%;
● Remaining iron and unavoidable impurities
It has a weight-based composition of,
● A manufacturing method wherein the average nitrogen content by weight after heating of the cold-rolled steel sheet in a nitriding gas atmosphere is at least 0.005%, preferably at least 0.015%. 3. The method according to claim 1 or 2, wherein the steel of the cold rolled sheet contains more than 200 ppm by weight, preferably more than 500 ppm of titanium and/or more than 100 ppm of niobium and/or more than 50 ppm of aluminum. Manufacturing method: The method according to any one of claims 1 to 3, wherein the cold rolled steel sheet is heated from room temperature to the heating temperature ( _{TE) within the heating time (t E )} , and after reaching the heating temperature ( _TE ), the cold rolled steel sheet is heated. maintained at a temperature (T _E ) for a predetermined annealing time (t _G ), wherein the heating time (t _E ) is preferably in the range from 1.0 to 300 seconds and/or the annealing time (t _G ) is preferably in the range from 1.0 to 80 seconds. A manufacturing method characterized in that the range of seconds. 5. The method according to any one of claims 1 to 4, wherein the steel of the cold rolled steel sheet has an initial nitrogen content (N ₀ ) and, during heating of the cold rolled steel sheet, the average nitrogen content in the near surface region (2). increases to a value averaged over the thickness of the near surface region 2, which is 50 to 1000 ppm higher than the initial nitrogen content of the steel (N ₀ ), and the gradient of nitrogen content is such that the nitrogen content is ), wherein the barrier layer (3) is established over the cross-section of the cold rolled steel sheet, decreasing from the near surface area (2) on the first side (a) of the provided steel sheet. The method according to claim 1 , wherein during heating and in particular during the holding time (t _H ) nitridation of the near surface region (2) on the second side (b) of the cold rolled steel sheet takes place at least temporarily. , during the annealing time t _G , at least partial recrystallization annealing of the cold rolled steel sheet takes place outside the near surface region 2 , in particular in the first region 1 on the first side a of the steel sheet, while The near surface region (2) on the second side (b) of the steel sheet is not recrystallized. The method according to any one of claims 1 to 6, wherein the value ΔT at which the recrystallization temperature increases in the near surface region (2) due to the incorporation of nitrogen when heating the steel sheet is greater than 30°C, preferably greater than 50°C. and, in particular, a production method in the range of 100°C to 250°C. 8. The method according to any one of claims 1 to 7, wherein the barrier layer (3) is a sol-gel layer, in particular a layer of SiO ₂ , TiO ₂ and/or ZrO ₂ on the surface of the first side (a) of the steel sheet. A manufacturing method, characterized in that formed by applying to. 9. Process according to claim 8, characterized in that the steel sheet is a steel strip and the sol-gel layer is applied to the surface of the steel strip on the first side (a), in particular wet-chemically in a strip coating process. Process according to any one of claims 1 to 7, characterized in that the barrier layer (3) is electrolytically applied to the surface of the first side (a) of the steel sheet from an aqueous electrolyte containing silica. . 11. Process according to any one of claims 1 to 10, wherein the barrier layer (3) has a coating weight in the range of 1 to 10 mg/m ² , preferably in the range of 3 to 6 mg/m ² method. Steel sheet, especially for packaging, made of steel with a carbon content (C) by weight of 10 to 1000 ppm and having a first side (a) and a second side (b) and a thickness of less than 0.5 mm, comprising at least A substantially impermeable barrier layer (3) is present on the surface of the first side (a), and the steel sheet has a first region (1) on the first side (a) and a second region on the second side (b). A steel sheet having a two-layer crystallization structure with (2), wherein the first region (1) is at least substantially recrystallized and the second region (2) is not recrystallized or at least not completely recrystallized. According to clause 12,
● C: greater than 0.001% but less than 0.1%, preferably less than 0.06%;
● Mn: greater than 0.01% but less than 0.6%;
● P: less than 0.04%;
● S: less than 0.04%, preferably greater than 0.001%;
● Al: less than 0.08%, preferably more than 0.005%;
● Si: less than 0.1%;
● Optionally Cu: less than 0.1%;
● Arbitrarily Cr: less than 0.1%;
● Randomly Ni: less than 0.1%;
● optionally Ti: less than 0.1%, preferably more than 0.02%, especially preferably more than 0.02%;
● Optionally Nb: less than 0.08%, preferably more than 0.01%;
● Arbitrarily Mo: less than 0.08%;
● Arbitrarily Sn: less than 0.05%;
● Optionally B: less than 0.01%, preferably less than 0.005%, preferably more than 0.0005%;
● and a nitrogen content averaged over the thickness of the steel sheet of at least 0.005%, preferably greater than 0.015%, particularly preferably greater than 0.02%,
● As well as the remaining iron and inevitable impurities
A steel plate having a weight-based composition of 14. The method according to claim 12 or 13, wherein the first region (1) has a thickness in the range from 50 μm to 450 μm, preferably in the range from 90 μm to 400 μm and in particular in the range from 150 μm to 300 μm, and/ or a steel sheet, characterized in that the second region (2) has a thickness in the range of 1 μm to 50 μm, preferably in the range of 2 μm to 10.0 μm. 15. Steel sheet according to any one of claims 12 to 14, characterized in that it has a tensile strength greater than 500 MPa, preferably greater than 700 MPa and/or an elongation at break greater than 5%, preferably greater than 7%. . 16. Steel sheet according to any one of claims 12 to 15, characterized in that the average nitrogen content of the second region (2) of the steel sheet is 50 to 1000 ppm by weight. 17. The method according to any one of claims 12 to 16, wherein a gradient of nitrogen content exists at least in the second region (2) and/or over the entire thickness of the steel sheet, and the nitrogen content is at least on the second side (b) of the steel sheet. A steel plate characterized in that it decreases from to the first surface (a). 18. The method according to any one of claims 12 to 17, wherein the second region (2) has a higher hardness and/or a higher tensile strength than the first region (1), and the hardness of the first region (1) is The ratio of the hardness of the second region 2 to the hardness is preferably greater than 1.2, particularly preferably greater than 1.4, and the second region 2 of the steel sheet is preferably at least 250 HV _0.025 , particularly preferably at least 300 HV. A steel plate characterized by having a Vickers hardness of _0.025 . 19. The process according to any one of claims 12 to 18, wherein the second region (2) has a degree of recrystallization of less than 30%, preferably less than 20%, and/or the first region (1) has a recrystallization degree of less than 70%. Steel sheet characterized in that it has a degree of recrystallization greater than 80%. 20. Steel sheet according to any one of claims 12 to 19, characterized in that a nitride layer, in particular an iron nitride layer, is present on the surface of the second region (2). 21. Steel sheet according to any one of claims 12 to 20, characterized in that the barrier layer (3) is a sol-gel layer, in particular a layer of SiO ₂ , TiO ₂ and/or ZrO ₂ . 22. The method according to claim 12, wherein the barrier layer (3) on the surface of the first region (1) has a thickness of less than 1 μm, especially less than 0.1 μm, or less than 10 mg/m 2, preferably less than 10 mg/m ² . is a steel sheet characterized in that it has a coating weight of 3 mg/m ² to 6 mg/m ² . 23. The method according to any one of claims 12 to 22, wherein the surface of the second region (2) after forming of the steel sheet has a bend radius in the range of 8 mm to 14 mm, and the second region (2) lies outside the bend radius. ) has a roughness (Ra) of less than 1.0 μm, preferably less than 0.8 μm and/or a roughness coefficient of less than 3, preferably less than 2.5, the surface of the second region 2 outside the bending radius. A steel plate, characterized in that it is defined by the ratio of the roughness of the steel plate and the roughness of the undeformed section. A container, in particular a can, made of a steel sheet according to any one of claims 12 to 23, wherein the container has at least one convexly deformed portion, wherein the second region 2 of the steel sheet has a convex shape of the deformed portion. A container that is located on the outside.