Patent Application
20250011893
The invention relates to a process for the production of a steel sheet with a two-layer crystallization structure and to a steel sheet with a two-layer crystallization structure which can be used in particular for the production of packaging by deep-drawing processes, as well as to a container made from the steel sheet.
For the production of packaging from steel sheets, such as tinplate or electrolytic chromium coated steel (ECCS), more and more thinner steel sheets with thicknesses in the range of 0.1 to 0.25 mm are used for reasons of resource efficiency. To ensure that sufficiently stable packaging can be produced from thinner steel sheets, the strength of packaging steels must be increased. Furthermore, it must be ensured that the steel sheets remain readily formable despite their lower thickness and higher strength, so that the steel sheet can be subjected to the severe deformations occurring during the production of packaging in deep drawing and stretch forming processes.
It is generally known from the prior art to increase the strength of steels by introducing unbound nitrogen dissolved in the steel. The introduction of unbound nitrogen into the steel is referred to as sticking, nitriding or nitrifying and is a well-known process for solid solution strengthening of steel and steel products.
In the case of steel sheets intended for the production of packaging (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 for producing a nitrided packaging steel from a hot-rolled steel product with a carbon content of 400 to 1200 ppm is known, in which a hot-rolled strip is cold-rolled to form a steel flat product and the cold-rolled steel flat product is then annealed in an annealing furnace, in particular in a continuous annealing furnace, to recrystallize it, wherein a nitrogen-containing gas is introduced into the annealing furnace and directed onto the steel flat product to introduce unbound nitrogen into the steel flat product in an amount corresponding to a concentration of more than 100 ppm or to increase the amount of unbound nitrogen in the steel flat product to a concentration of more than 100 ppm. Finally, the annealed and nitride steel flat product is cooled at a cooling rate of at least 100 K/s immediately after recrystallizing annealing. This process can be used to produce cold-rolled steel flat products for packaging purposes with a tensile strength of more than 650 MPa and, in particular, between 700 MPa and 850 MPa.
From WO 2005/056 841 A1 a process for the production of a steel sheet 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 introduced during or after an annealing of the steel sheet in an annealing furnace with an ammonia gas atmosphere at temperatures between 550° C. and 800° C. after a complete recrystallization of the steel sheet.
A process for the production of high-strength steel strips for the manufacture of tinplate and other packaging steels by nitriding the steel strip is known from U.S. Pat. No. 3,219,494, in which the steel strip wound into a coil is nitrided in a bell-type annealing furnace in order to initially achieve a nitrogen-rich outer shell in the steel strip, the nitriding in the bell-type annealing furnace being effected by an ammonia gas atmosphere, with uniform distribution of the nitrogen introduced near the surface over the thickness of the steel strip by diffusion of the nitrogen upon heating of the steel sheet in an inert gas atmosphere to temperatures above the recrystallization temperature, whereby the nitrogen can diffuse from the nitrogen-rich outer shell through the steel strip into its core region and the structure of the steel will be completely recrystallized. Strengths in the range from 439 MPa up to 527 MPa were achieved for steel sheets with a thickness of 0.25 mm.
During the forming of steel sheets in deep drawing processes for the production of packaging, roughening occurs on the outside of the formed area, particularly in the areas with a small bending radius, especially with bending radii smaller than 14 mm, due to the granular structure of the steel. 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 contents in the range of 100 to 1000 ppm (0.01-0.1 wt. %) typically have a grain size (mean grain diameter) in the range of 10 to 30 μm. Due to the (relatively coarse-grained) grain structure of the steel, roughening occurs on the outside of the formed area during forming because the grains of the steel structure are forced radially outward during forming and can then push through on the surface of the steel sheet. This is particularly noticeable in cold-rolled steel sheets that have been completely recrystallized after cold rolling. Cold-rolled steel sheets for the production of packaging are usually completely recrystallized after cold rolling in order to restore the original structural condition and formability of the steel sheet. However, fully recrystallized steel sheets have a coarse steel structure with a comparatively high average grain size and are softer than roll-hardened steel sheets. Due to the lower fineness and lower strength of recrystallized steel sheets, the susceptibility to severe roughening during forming increases because the coarser grains of the steel microstructure can visibly push through at the surface during forming. Such roughening often leads to problems in terms of stability and corrosion resistance on the packaging, e.g. a can, because the roughened areas are located on the outside of the packaging, which is subject to high mechanical stress, e.g. on the edges of can bases.
One aspect of the invention relates to a steel sheet for the manufacture of packaging with high strength and good formability, in which roughening does not occur on the outside of the formed area even in the case of severe forming with small bending radii, in particular with bending radii of less than 14 mm. In particular, the steel sheet should have strengths of at least 500 MPa with an elongation at break of at least 5% in order to meet the mechanical requirements in the forming processes even with small thicknesses of the steel sheet of less than 0.3 mm. At the same time, the steel sheet should have sufficient formability for its intended use as packaging steel, for example in deep-drawing or stretch-forming processes, so that packaging, such as tin cans or beverage cans, can be produced from the steel flat product as intended. Another aspect of the invention relates to a method for producing such a steel sheet for packaging.
Accordingly, preferred embodiments of the process according to the invention and of the steel sheet according to the invention are disclosed herein.
When referring to a steel sheet, a flat steel product in the form of a sheet or strip is meant. Values in % or ppm relating to a content or concentration of an alloying constituent of the steel or of the cold-rolled steel sheet refer in each case to the weight of the steel or of the steel sheet. Features of the invention which are disclosed below in relation to the manufacturing process also relate accordingly to the product of the process (i.e. the steel sheet), and vice versa.
In the process according to the invention for producing a steel sheet for packaging, a cold-rolled steel sheet having a first side and a second side, which is produced from a steel having a carbon content (C) of 10 to 1000 ppm by weight and which has a given recrystallization temperature (TR) (essentially predetermined by the steel composition), is provided on the first side with a barrier layer which is at least substantially impermeable to nitrogen and is then preferably heated in a continuous annealing furnace to a (maximum) heating temperature which is at least as high as the recrystallization temperature, wherein the heating takes place at least until the recrystallization temperature is reached at least temporarily in a nitriding gas atmosphere, whereby during heating of the steel sheet nitrogen diffuses from the nitriding gas atmosphere at least into a region near the surface on the second side of the steel sheet and is deposited in this region, whereby the recrystallization temperature of the steel in the near-surface region on the second side of the steel sheet is raised by a value ΔT, the heating temperature TE being selected such that it is on the one hand greater than or equal to the recrystallization temperature and on the other hand smaller than the recrystallization temperature increased by ΔT (TR+ΔT) in the near-surface region.
The heating temperature TE is the maximum temperature in the thermal treatment of the cold-rolled steel sheet, i.e. neither before nor during or after the thermal treatment according to the process of the invention is the cold-rolled steel sheet heated to temperatures higher than the (maximum) heating temperature TE. When the cold-rolled steel sheet is heated, nitrogen from the nitriding gas atmosphere is incorporated only into the near-surface region on the second side of the steel sheet by diffusion of (atomic) nitrogen from the nitriding gas atmosphere on the second side of the steel sheet into the near-surface region. On the opposite first side of the steel sheet, which is covered by the barrier layer, the penetration of nitrogen through the barrier layer is prevented. As a result, the recrystallization temperature of the steel is raised by a value ΔT only in the near-surface region on the second side of the steel sheet, while the recrystallization temperature in a 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 cold-rolled steel sheet before heating. The heating temperature (TE) is thus selected such that TR≤TE<TR+ΔT, where TR is the initial recrystallization temperature of the steel.
As a result, a two-layer crystallization structure is formed in the steel sheet, the steel sheet comprising on the first side a first region which is at least substantially completely recrystallized and on the second side a second region which is not recrystallized or at most partially recrystallized. This two-layer structure of the crystallization structure results in the process according to the invention because the (near-surface) second region on the second side of the steel sheet has a higher recrystallization temperature due to the nitriding during heating, compared to the adjacent first region on the first side, which is not nitrided or at least significantly less nitrided during heating than the second region due to the barrier layer. Since the heating temperature is between the recrystallization temperature (increased by ΔT) of the (near-surface) second region and the recrystallization temperature of the adjacent first region (which is not nitrided and therefore has a recrystallization temperature which is at least substantially equal to the initial recrystallization temperature of the steel), only the unnitrided first region will be recrystallized during heating, while the nitided (near-surface) second region will not be recrystallized or will be only incompletely recrystallized and, as a result, will still be roll-hard (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 for temperatures above 300° C. and preferably up to at least 600° C., because at these temperatures the steel sheet is nitrided in the annealing furnace, with nitrogen atoms from the gas atmosphere of the annealing furnace being deposited on the hot surface of the steel sheet by a catalytic reaction and diffusing into the steel sheet.
The barrier layer can, for example, be formed by a heat-resistant paint, such as an oven paint or a silicone resin-based engine paint. To apply the barrier layer, the heat-resistant paint material is applied to one side of the first side of the steel sheet before it is placed in the annealing furnace, for example by spraying. The barrier layer can also be formed by a coating deposited electrolytically on one side of the steel sheet, e.g. a chromium/chromium oxide layer, in particular with a coating weight of chromium in the range from 80 to 140 mg/m2.
The barrier layer is designed in such a way that it largely prevents the passage of nitrogen from the nitrogen-containing gas atmosphere of the annealing furnace into the steel sheet, i.e. apart from unavoidable nitrogen residues which may diffuse into the steel sheet, for example due to incomplete coverage of the surface of the first side of the steel sheet with the barrier layer or due to defects in the barrier layer on the first side, the penetration of nitrogen through the barrier layer on the first side of the steel sheet is suppressed. Preferably, the barrier layer is formed such that at most 10% of the amount of nitrogen that diffuses into the second side of the steel sheet can penetrate into the first side of the steel sheet. Accordingly, in the process according to the invention, nitrogen diffuses essentially only at the second side of the steel sheet and a gradient of the nitrogen concentration is formed over the cross section of the steel sheet, running from the second side to the first side, which, due to an increase in the recrystallization temperature with increasing nitrogen content when the steel sheet is heated to the heating temperature (TE), leads to the formation of a two-layer crystallization structure with an at least substantially recrystallized first region on the first side and a second region which is not or at least not completely recrystallized on the second side.
The barrier layer is preferably a sol-gel layer, in particular a layer of SiO2, TiO2 and/or ZrO2. Such sol-gel layers can be produced by sol-gel processes and applied efficiently to a steel sheet surface by applying the sols used as coating solutions, which are present as colloidal dispersions, to the surface of the first side of the steel sheet and a subsequently gelling and drying the wet-chemically formed layer. For the preparation of an SiO2—sol-gel layer, in particular silicon alcoholates can be used as precursors. In particular, an aqueous solution of
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 the sol-gel layer to the steel sheet surface can be integrated into a continuous strip coating process to achieve efficient process control, in which the steel sheet is in strip form and the subsequent nitriding and annealing of the steel strip takes place in a continuous annealing furnace through which the strip is passed at a predetermined strip speed. The sol-gel layer is applied in a separate step upstream of the continuous annealing furnace. The gelation and drying of the sol can take place at least partly during the heating of the steel strip in the continuous annealing furnace, thus achieving a very efficient process and reducing the length of the strip line. It has been shown that sol-gel processes, in particular for the production of SiO2 layers, can be used to apply dense and uniform barrier layers on one side of a steel sheet, which is why this process is preferred for applying the barrier layer.
In a particularly preferred embodiment, a silicate-containing barrier layer is applied electrolytically to one of the two sides of the steel sheet from an aqueous, basic electrolyte containing silica and a sodium salt, wherein a silicate layer acting as a barrier layer is applied to one of the two sides of the steel sheet in the range from 1 to 10 mg/m2 and particularly preferably in the range from 3 to 6 mg/m2. During electrolytic application of the barrier layer, the steel sheet surface is simultaneously degreased. Electrolytic application of the barrier layer makes it possible to produce a barrier layer which is uniformly distributed over the surface of the steel sheet and has a homogeneous and sufficient coating weight to prevent nitrogen penetration over the first side of the steel strip during the nitriding process in the annealing furnace.
In order to achieve at least substantially a complete recrystallization of the first region by the nitriding process in the annealing furnace, the temperature of the steel sheet is preferably maintained at the heating temperature (TE) for a predetermined annealing time (tG) after the heating temperature has been reached. The annealing time is preferably more than 1 second, in particular in the range from 1 to 80 seconds, preferably in the range from 1 to 10 seconds. With longer annealing times of, for example, more than 10 seconds, in addition to complete recrystallization of the first region, the distribution of the nitrogen introduced during heating is levelled out over the thickness of the steel sheet, resulting in a wider transition zone in the core region of the steel sheet between the recrystallized first region and the non-recrystallized or barely recrystallized second region.
Another aspect of the invention therefore relates to a steel sheet which can be used in particular for the manufacture of packaging and is made from a steel having a carbon content (C) by weight of from 10 to 1000 ppm and having a thickness of less than 0.5 mm, the steel sheet having a two-layer crystallization structure with a first region on the first side and a second region on the second side, the first region being at least substantially recrystallized and the second region being not recrystallized or at least not completely recrystallized, and a barrier layer which is at least substantially impermeable to nitrogen being 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 in essentially only at the second side of the steel sheet and a gradient of the nitrogen concentration is formed over the cross section of the steel sheet running from the second side to the first side, which leads to the formation of a two-layer crystallization structure due to an increase in the recrystallization temperature with increasing nitrogen content when the steel sheet is heated to the heating temperature (TE). The two-layer crystallization structure of the steel sheets according to the invention is also referred to below as the two-layer microstructure.
The steel sheet according to the invention has a very high strength of more than 500 MPa, in particular more than 600 MPa and preferably more than 700 MPa, with an elongation (elongation at break) of at least 5%, which is acceptable for deep-drawing applications. At the same time, the steel sheet has good formability, especially in deep drawing processes. The steel sheet is particularly characterized by the fact that, even in the case of severe forming with very small bending radii, no roughening occurs on the outside of the bending radius when the still roll-hard, non-recrystallized second area is located on the outside of the bending radius. This is achieved by the fact that the coarse grains of the recrystallized first area cannot press through to the surface of the outside of the bending radius due to the high strength of the roll-hard second area. The still roll-hard second area thus forms a barrier for the coarse grains of the recrystallized steel structure of the first area. The roll-hard second area prevents larger grains from the recrystallized first area from being pressed outward to the surface of the steel sheet during forming. This prevents undesirable optical effects and mechanical instabilities during forming and also prevents porosity and cracks in a coating applied to the surface of the second side of the steel sheet.
Particularly good formability of the steel sheet is achieved if the roll-hard second region is as thin as possible and in particular thinner than the recrystallized first 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 and especially between 150 μm and 300 μm, and the thickness of the second region is preferably in the range from 1 μm to 50 μm, and particularly preferably in the range from 2 μm to 10 μm. Preferably, the thickness of the second region is between 1% and 10% of the thickness of the first region. At these low thicknesses 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 second region. Nevertheless, during forming of the steel sheet, the second area forms a sufficient barrier for coarse grains of the steel structure to be pressed through the surface of the steel sheet, thus preventing roughening of the surface.
The value ΔT, by which the recrystallization temperature in the second region increases due to the incorporation of nitrogen upon heating the steel sheet, can be controlled by the nitrogen content introduced by nitriding in the second region of the steel sheet after the heating is ended, wherein particularly a linear relationship can be observed which can be described by ΔT=a·ΔN(ppm), where a is a proportionality constant and ΔN(ppm) is the nitrogen content in ppm (based on the weight of the steel) introduced into the second region during heating of the steel sheet by nitriding. Tests on samples with different nitrogen contents and otherwise the same alloy composition yielded a value of a≅1.2 K/ppm, as an example. Accordingly, an increase in the recrystallization temperature in the near-surface region in the range of ΔT≅10 K to 24 K can already be achieved with a low addition of ΔN in the range of 10 ppm to 20 ppm (corresponding to 0.001 to 0.002 wt. %). At a higher nitrogen loading of, for example, ΔN=100 ppm (corresponding to 0.01 wt. %), the (theoretically achievable) increase in the recrystallization temperature in the near-surface region is already approx. ΔT≅120 K. Preferably, the value ΔT by which the recrystallization temperature in the second region is increased by the incorporation of nitrogen upon heating of the steel sheet is greater than K and particularly preferably greater than 100 K and is in particular in the range from 100 K to 250 K.
In the process according to the invention, the steel sheet can be heated either in a single-stage heating step or in a two-stage thermal treatment. In a single-stage heating step, the steel sheet is heated from room temperature to the heating temperature (TE) within a heating time (tE) and, after reaching the heating temperature (TE), is held at the heating temperature (TE) for a predetermined annealing time (tG), the heating time (tE) preferably being in the range from 1.0 to 300 seconds and/or the annealing time (tG) preferably being in the range from 1.0 seconds to 80 seconds, particularly preferably between 1 and 10 seconds. During heating, the steel sheet is exposed at least temporarily to a nitriding gas atmosphere, in particular 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 carried out in a first stage at an intermediate temperature, and annealing takes place in a second stage at a heating temperature which is higher than the intermediate temperature (which is why the heating temperature in the following is also referred to as the annealing temperature). The intermediate temperature is lower than the original recrystallization temperature TR. In the two-stage process, the steel sheet is heated in the first stage from room temperature within a first heating time to the intermediate temperature TZ<TR and held at least approximately at this temperature during a holding time tH. The intermediate temperature TZ is preferably in the range from 300° C. to 600° C., particularly preferably between 400° C. and 550° C., because dissociation to atomic nitrogen on metallic surfaces begins at a temperature of about 300° C. when ammonia gas is used as a nitriding component of the gas atmosphere of the annealing furnace. In any case, at temperatures up to about 550° C., (complete) recrystallization does not yet occur for most of the alloy compositions according to the invention. At the preferred intermediate temperatures TZ in the first stage, therefore, diffusion of the dissociated nitrogen from the nitriding gas atmosphere into the second region of the steel sheet does occur, but not yet recrystallization. Recrystallization of the first region does not occur until the second stage, in which the steel sheet is heated to the heating temperature (annealing temperature) TE, which is equal to or greater than the original recrystallization temperature TR but below TR+ΔT. For example, the heating temperature TE is in the range of 650° C. to 800° C., depending on the value of the initial recrystallization temperature TR of the steel used, and in particular about 750° C.
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 of the first region of the steel sheet takes place, whereas the second region is not recrystallized. Recrystallization of the second region nitrided in the first stage is prevented by selecting the heating temperature (annealing temperature) TE such that it is below the recrystallization temperature TR+ΔT increased by ΔT due to the nitriding of the second region.
In the two-stage thermal treatment, the steel sheet is suitably annealed in a continuous annealing furnace first in the first stage at a lower intermediate temperature TZ, which is below the (initial) recrystallization temperature TR of the steel, during a holding time which is preferably in the range from 10 to 150 seconds, in the second region near the surface on the second side of the steel sheet, and in a second stage is at least partially recrystallizing annealed only in the first region at a heating temperature (annealing temperature TE) which is higher than the intermediate temperature and above the (initial) recrystallization temperature (TR) of the steel, during an annealing time tG, which is preferably in the range from 1 to 300 seconds, particularly preferably from 1 to 10 seconds.
The first heating time (tE1), in which the steel sheet is heated from room temperature to the intermediate temperature in the two-stage process, is preferably in the range from 1.0 to 120 seconds, particularly preferably between 10 and 90 seconds, and can be adapted in accordance with the desired material properties of the steel sheet according to the invention, as in the first embodiment. The holding time (tH), during which the steel sheet is held at the intermediate temperature, here is also preferably in the range from 1.0 to 90 seconds, particularly preferably between 10 and 60 seconds, and is likewise selected according to the desired material properties of the steel sheet according to the invention. After the holding time has elapsed, the steel sheet can be heated to the heating temperature TE (annealing temperature), either after cooling or immediately without cooling in the second stage in a second heating time (tE2), and held at least approximately at this heating temperature TE during an annealing time (tG). Optionally, the annealing furnace can also be provided with a nitriding gas atmosphere during the annealing period, from which dissociated (atomic) nitrogen is made available so that (further) nitriding of the steel sheet in the deeper second area can still take place during the annealing period tG. This leads to an increase in the size of the second region in the direction of the first side of the steel sheet and thus increases the thickness of the second region. The heating temperature TE (annealing temperature) in this case is also between the original recrystallization temperature TR and the recrystallization temperature increased to TR+ΔT by the nitriding in the near-surface region of the steel sheet. Thus, in the two-stage process, it also holds for the heating temperature TE: T≤TRE<TR+ΔT, wherein the intermediate temperature (TZ) is lower than the initial recrystallization temperature TR.
In both the single-stage and the two-stage process, the steel sheet is exposed at least temporarily during heating and before the recrystallization temperature is reached to the nitriding gas atmosphere from which dissociated (atomic) nitrogen is provided in the annealing furnace, which initially diffuses into the steel sheet near the surface only in the second region on the second side of the steel sheet and raises the recrystallization temperature there, while on the first side of the steel sheet the penetration of nitrogen through the barrier layer is prevented.
The steel of the cold-rolled steel sheet preferably has the following composition by weight:
When talking about an average nitrogen content (N) or an average nitrogen content, the nitrogen concentration averaged over the respective thickness is meant. When talking about the average nitrogen content (N) of the steel sheet, therefore, the nitrogen concentration averaged over the thickness of the steel sheet is meant.
The steel of the cold-rolled steel sheet may already have an initial nitrogen content No of preferably more than 0.001 wt. % and less than 0.02 wt. % and particularly preferably less than 0.016 wt. %. However, a steel may also be used which, apart from unavoidable nitrogen impurities, does not contain any nitrogen. Limiting the initial nitrogen content to values of less than 0.02 wt. % enables the hot strip produced from the steel by hot rolling to be cold rolled without difficulty using the usual cold rolling equipment (rolling mills for the cold rolling of steel sheets into ultra-fine sheets). Furthermore, low initial nitrogen contents of N0<0.02 wt. % in the steel prevent the formation of defects when casting a slab. However, in order to achieve the highest possible (average) nitrogen content in the cold-rolled steel sheet, and thereby to achieve a high solid solution strengthening, it is advantageous if the steel used to produce the hot strip already has an (initial) nitrogen content which is preferably in the range from 0.001 wt. % to 0.02 wt. % and particularly preferably between 0.005 wt. % and 0.016 wt. %.
If the steel of the cold-rolled steel sheet has an initial nitrogen content (No), the nitrogen content increases to a value above the initial nitrogen content (No) when the cold-rolled steel sheet is heated in the second region on the second side of the steel sheet. If the nitrogen content by weight in the second region thereby increases to a value averaged over the thickness of this region, which is preferably more than 50 ppm and particularly preferably between 400 and 800 ppm above the initial nitrogen content (No) of the steel, a (sometimes considerable) increase in the strength of the steel sheet is observed-depending on the thickness of the second region. Strengths of the steel sheet of more than 700 MPa at elongations at break of at least 5% can be achieved. The (average) nitrogen content of the nitrogen deposited in the second area by the nitriding process can reach up to the solubility limit of nitrogen in iron of approx. 1000 ppm.
Depending on the setting of the process parameters, in particular the holding time and the annealing time, and the (optional) concentration of the nitriding gas (in particular ammonia) in the gas atmosphere of the annealing furnace during annealing of the steel sheet at the heating temperature (annealing temperature), the first region of the steel sheet is also nitrided to a certain extent. Preferably, however, the nitriding of the first region is (considerably) less in order to achieve the greatest possible difference in the degree of recrystallization of the first region and the second region after annealing or at the end of heating. Preferably, the second region has a degree of recrystallization of less than 30% and more preferably of less than 20% and the first region has a degree of recrystallization of more than 70% and more preferably of more than 80%. Particularly preferably, the first region is fully recrystallized.
In preferred embodiments, the steel from which the steel sheet according to the invention is produced in preliminary stages by hot rolling followed by cold rolling contains, by weight, more than 0.001% and less than 0.1% C, more than 0.01% and less than 0.6% Mn, less than 0.04% P, less than 0.04% S, less than 0.08% Al, less than 0.1% Si, and optionally an initial nitrogen content (N0) of up to 0.020% and preferably 0.016% or less, with the balance iron and unavoidable impurities. After nitriding, the steel sheet preferably contains an average nitrogen content N>N0 of at least 0.020%, particularly preferably of 0.025% or more and especially in the range from 0.040 to 0.080% N. The tensile strength of the steel sheet at an average nitrogen content of N>0.040% is at least 800 MPa, the steel sheet simultaneously exhibiting an elongation at break in the range from 2% to 10% and thus sufficient formability for deep drawing processes.
The nitrogen incorporated in the second region during heating of the cold-rolled steel sheet can be present (up to the solubility limit) in dissolved form and/or in bound form as nitrides. In the presence of strong nitride formers in the steel, the intercalated nitrogen is at least partly present as nitrogen bound in nitrides, in particular as TiN and/or NbN and/or AlN. Since interstitial incorporation of the nitrogen introduced into the second region during nitriding is preferred to increase strength, the steel expediently contains as few nitride formers as possible, such as Ti, Nb, Mo or Al, if high strength of the steel sheet is to be achieved. Therefore, the following upper limits for the weight percentages of the nitride formers are preferred:
On the other hand, the formation of nitrides during the nitriding of the second region promotes a sharp demarcation of the recrystallized first region and the not or at least not completely recrystallized second region of the steel sheet. Therefore, a sufficient amount of strong nitride formers such as Ti, Nb, Mo and/or Al is added to the steel if the highest possible difference in the degree of recrystallization of the first region and the second region is desired. In this context, the steel of the cold-rolled steel sheet preferably contains, by weight, 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. Particularly preferably, the sum of the weight fractions of the strong nitride formers in this case is more than 300 ppm and preferably more than 500 ppm. The strong nitride formers, such as titanium, niobium and/or aluminum, bind the nitrogen deposited in the first region and thus prevent the nitrogen, which is initially deposited only near the surface, from diffusing further into the interior of the steel sheet towards the first side. Thus, a very thin first region with a high nitrogen content is created at the surface of the steel sheet compared with the sheet thickness, which leads to a strong increase in the recrystallization temperature in this region. If, in accordance with the process according to the invention, the cold-rolled steel sheet is now heated to temperatures which, on the one hand, are higher than the (initial) recrystallization temperature of the steel in the (not or only slightly spiked) second region of the steel sheet and, on the other hand, are lower than the (increased) recrystallization temperature of the nitrided region, only the first region is (completely) recrystallized and the second region remains roll-hard due to the increased recrystallization temperature. This results in a two-layer crystallization of the microstructure in the steel sheet according to the invention with sharp boundaries between the first region and the second region.
A surface nitride layer, in particular 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 superficial nitride layer is disadvantageous in terms of roughening during forming of the steel sheet, since it is brittle and can break off during forming. The formation of a surface nitride layer depends essentially on the concentration of nitrogen in the nitriding gas atmosphere of the annealing furnace. When using a gas atmosphere with ammonia as nitrogen donor, for example, the formation of a nitride layer on the surface of the first area can be observed on a laboratory scale from an ammonia content of approx. 2 to 3 Vol. %. 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 Vol. % or less. The volume fraction of ammonia in the gas atmosphere refers to the conditions in a laboratory test in which steel sheets have been heated with an induction heater, the volume fraction of ammonia in the gas atmosphere of the laboratory furnace having been determined at room temperature.
The formation of the two-layer microstructure in relation to the degree of crystallization 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 from room temperature within a heating time of 1.0 to 120 seconds (if necessary in steps, with holding phases inserted in between, in which the temperature is kept constant during a holding time) to the heating temperature (TE) and held at the heating temperature for an annealing time (tG) of between 1.0 and 90) seconds. A short heating time and a short annealing time contribute to a stronger formation of a nitrogen gradient from the second side to the first side of the steel sheet, since with short heating times and annealing times the nitrogen initially deposited only on the surface in the second region of the steel sheet cannot diffuse into the core region. With longer annealing times, diffusion of the 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.
The thickness of the non-recrystallized second region can therefore be controlled by the heating time, whereby a linear relationship can be observed between the thickness of the second region and the heating time. Thus, an adjustable process parameter of the process according to the invention is available via the heating time, by means of which the thickness of the non-recrystallized and therefore roll-hard second region can be specifically adjusted. Depending on the selected heating time, thicknesses of the second region of between 0.1 μm and 150 μm can be set, whereby the thickness of the second region should be as thin as possible and is preferably between 1 μm and 10 μm and in particular between 1 μm and 2 μm. The barrier layer on the surface of the first region has a small thickness of preferably less than 1 μm, in particular less than 0.1 μm, or a weight support of less than 10 mg/m2 and is particularly preferably thinner than the second region. The barrier layer has a thickness which is sufficiently high to at least substantially completely prevent the diffusion of nitrogen on the first side of the steel sheet, in particular at temperatures above 300° C. and in particular between 300° C. and 600° C. At the same time, the barrier layer does not or at least not significantly affect the mechanical properties of the steel sheet due to its low density.
When the cold-rolled steel sheet is heated in the nitriding gas atmosphere, a gradient of the nitrogen content is established in the second region, with the nitrogen content decreasing steadily from the second side to the first side of the cold-rolled steel sheet. The amount of nitrogen incorporated in the second region can be controlled via the nitrogen concentration of the nitriding gas atmosphere, in particular via the volume fraction of ammonia in the gas atmosphere of the annealing furnace.
Preferably, the steel sheet is heated in a continuous annealing furnace with a nitrogen-containing gas atmosphere, through which the steel sheet, which is expediently in the form of a strip, is passed. The 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 to atomic nitrogen due to a catalytic reaction at the heated surface of the second region of the steel sheet, and diffuse through the surface of the second region into the steel sheet. On the first side of the steel sheet, this diffusion process is at least inhibited or preferably completely suppressed by the barrier layer, which is why no or only very little nitrogen can diffuse into the steel sheet there during heating. The volume concentration of ammonia in the nitrogen-containing gas atmosphere is preferably more than 0.1%, in particular between 0.1% and 10%, and more preferably between 0.1% and 5%, in particular between 0.5% and 3%. Particularly preferably, the cold-rolled steel sheet is heated in an inert protective gas atmosphere which contains in particular HNX, the volume concentration of HNX in the nitrogen-containing gas atmosphere preferably being between 90% and 99.5%.
Accordingly, the process according to the invention can be used to produce steel sheets from a steel having a carbon content (C) by weight of 10 to 1000 ppm and a thickness of less than 0.5 mm, which steel sheets have a two-layer crystallization structure with a first region on a first side of the steel sheet and a second region on a second side of the steel sheet, the first region being at least substantially recrystallized and the second region not being recrystallized or at least not completely recrystallized, and—as a result of the production process—a barrier layer which is at least substantially impermeable to nitrogen being present on the surface of the first side. A gradient of the nitrogen content is present at least in the second region of the steel sheet, the nitrogen content decreasing from the surface of the second side towards the first side.
The second region of the steel sheet therefore has a significantly higher hardness than the first region. Preferably, the second region has a Vickers hardness of at least 220 HV0.025 and particularly preferably of at least 300 HV0.025. Preferably, the Vickers hardness in the first region is at least 100 HV0.025 and less than 280 HV0.025. The ratio of the hardness of the second region to the hardness of the first region is preferably greater than 1.2 and particularly preferably greater than 1.4.
If the average nitrogen content by weight in the second region of the steel sheet after nitriding is between 400 and 800 ppm, particularly high strength and hardness values can be achieved.
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 texture. For example, the at least substantially recrystallized first region and the barely or not recrystallized second region can also be distinguished from each other by the ratio of the {001} orientation and the {111} orientation in the ε fiber. The ratio of the {001} orientation and the {111} orientation in the ε fiber can be defined as a “deformation index” that characterizes the forming behavior of the steel sheet. A {111} orientation allows good formability and has a good Lankford coefficient (r-value), whereas the {001} orientation is less formable. Here, the E-fiber is defined by the <110> vector lying parallel to the transverse direction (perpendicular to the rolling direction and to the normal direction in the strip plane of the steel strip). In the steel sheets according to the invention, in each case the recrystallized first region has a deformation index of less than 0.8 and the second region has a deformation index of more than 2.0 and, in particular, between 2.0 and 5.0. Corresponding characterizations of the textures in the first and second region can also be defined for other fibers of the microstructure, for example for the a-fiber defined by the <110> vector lying in the rolling direction.
An advantageous sharp demarcation of the first and second region from one another is achieved if the steel sheet in the second region has a degree of (re) crystallization of less than 30%, preferably less than 20%, and/or if the first region has a degree of (re) crystallization of more than 70%, preferably more than 80%. A particularly sharp demarcation between the two regions can be achieved if the following applies to the heating temperature (TE): TE=TR+ΔT/2. Preferably, the heating temperature TE is in the range from TR+ΔT/3 to TR+2 ΔT/3.
The steel sheets produced by the process according to the invention can be used advantageously for the production of packaging, whereby the steel sheet can be formed in forming processes, for example into a body of a can. In some cases, the steel sheet is subjected to severe forming with very small bending radii of less than 14 mm. The steel sheets according to the invention are characterized in particular by the fact that hardly any roughening of the surface occurs on the outside of a bending radius during forming if the still roll-hard second area is on the outside of the bending radius. The roughening that occurs during forming can be detected, for example, by means of a 4-radius cup test in which a steel sheet specimen is deep-drawn to form a cuboid container with different bending radii at the four corners of the cuboid and the roughness on the outside of the bending radii of the container is then measured. In the steel sheets according to the invention, the outer surface of the second area after forming the steel sheet by a bending radius in the range from 8 mm to 14 mm preferably has a roughness (Ra) of less than 1.0 μm and particularly preferably of less than 0.8 μm and/or a roughness factor of less than 3, preferably less than 2.5, the roughness factor being defined by the ratio of the roughness of the surface of the second region on the outside of the 90° angle and the roughness of the steel sheet in an undeformed section of the container formed from the steel sheet.
Another aspect of the invention is therefore, in addition to the manufacturing process and the steel sheet produced therefrom, also a container produced from the steel sheet which has at least one convexly formed section, the second region of the steel sheet according to the invention thereby coming to lie on the convex outer side of the formed section. In particular, the container may be a can having a can bottom and a body formed thereon, the transition between the can bottom and the body forming the formed portion.
These and other advantages of the packaging steel according to the invention and of the manufacturing process result from the embodiments described in more detail below with reference to the accompanying drawings. The drawings show:
Hot-rolled and subsequently cold-rolled steel sheets with a carbon content by weight of 10 to 1000 ppm are used as the starting product for the production of steel sheets according to the invention. The alloy composition of the steel expediently complies with the limits specified by standards for packaging steel (as defined, for example, in ASTM A623-11 “Standard Specification for Tin Mill Products” or in “European Standard EN 10202”), but may deviate from these, particularly with regard to the initial nitrogen content, if in particular highly nitrided steel sheets with a high nitrogen content of more than 0.02% by weight shall be produced. The components of the steel from which steel sheets according to the invention can be produced are explained in detail below:
Composition of the steel:
Carbon has the effect of increasing hardness and strength. Therefore, the steel preferably contains more than 0.001 wt. % carbon. In order to ensure the rollability of the steel sheet during primary cold rolling and, if necessary, in a second cold rolling step (skin pass rolling) without reduction of the elongation at break, the carbon content should not exceed 0.1 wt. %.
Manganese also has the effect of increasing hardness and strength. In addition, manganese improves the forgeability, weldability and wear resistance of steel. Furthermore, the addition of manganese reduces the tendency to red fracture during hot rolling, and manganese leads to grain refinement. Therefore, a manganese content of at least 0.01 wt. % is preferable. To achieve high strengths, a manganese content of more than 0.1 wt. %, in particular 0.20 wt. % or more, is preferable. However, if the manganese content becomes too high, this is to the detriment of the corrosion resistance of the steel. In addition, if the manganese content becomes too high, the strength becomes too high, resulting in the steel no longer being cold-rollable and formable. Therefore, the preferred upper limit for the manganese content is 0.6% by weight.
Phosphorus is an undesirable by-product in steels. A high phosphorus content leads in particular to embrittlement of the steel and therefore deteriorates the formability of steel sheets, which is why the upper limit for the phosphorus content is 0.04% by weight.
Sulfur is an undesirable concomitant element that deteriorates ductility and corrosion resistance. Therefore, no more than 0.04 wt % sulfur should be present in the steel. On the other hand, complex and cost-intensive measures have to be taken to desulfurize steel, which is why a sulfur content of less than 0.001 wt. % is no longer justifiable from an economic point of view. The sulfur content is therefore preferably in the range from 0.001 wt. % to 0.04 wt. %, particularly preferably between 0.005 wt. % and 0.01 wt. %.
In steel production, aluminum acts as a deoxidizing agent in the casting process to calm the steel. Aluminum also increases scale resistance and formability. In addition, aluminum forms nitrides with nitrogen, which are advantageous in the steel sheets according to the invention. Therefore, aluminum is preferably used in a concentration of 0.005 wt % or more. On the other hand, aluminum concentrations of more than 0.08 wt. % can lead to surface defects in the form of aluminum clusters, which is why this upper limit for the aluminum content should preferably not be exceeded.
Silizium erhöht im Stahl die Zunderbeständigkeit und ist ein Mischkristallhärter. In steel production, it has the positive effect of making the melt thinner and serves as a deoxidizing agent. 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 wt % or more is preferable. However, if the silicon content becomes too high, and in particular exceeds 0.1 wt. %, the corrosion resistance of the steel may deteriorate and surface treatments, especially by electrolytic coatings, may become more difficult.
Nitrogen is an optional component in the molten steel from which the steel for the steel sheets according to the invention is produced. It is true that nitrogen acts as a solid solution strengthener to increase 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. Furthermore, a high nitrogen content in the molten steel increases the risk of defects in the hot-rolled strip, since at nitrogen concentrations of 0.016 wt. % or more the hot forming capability is reduced. In accordance with the invention, it is envisaged to subsequently increase the nitrogen content of the steel sheet by nitriding the cold-rolled steel sheet in an annealing furnace. Therefore, the introduction of nitrogen into the molten steel can be dispensed entirely. However, to achieve a strong solid solution strengthening, it is preferable if the steel melt already contains an initial nitrogen content of more than 0.001% by weight, particularly preferably 0.010% by weight or more.
Therefore, in terms of weight, the steel optionally and preferably contains
In addition to the residual iron (Fe) and unavoidable impurities, the steel may contain other optional constituents, such as.
Table 1 (
With the described composition of the steel, a steel melt is produced, whereby in preferred embodiment examples, in order to achieve a high (average) nitrogen content of the steel sheet, the steel can already have an initial nitrogen content No by adding nitrogen to the steel melt, for example by blowing in nitrogen gas and/or by adding a solid nitrogen compound such as lime nitrogen (calcium cyanamide) or manganese nitride. In order to prevent the strength of the steel sheet produced from the steel melt from becoming too high due to nitrogen solid solution strengthening, and in order to maintain the hot formability of the steel as well as to avoid defects caused by nitrides in the slab produced from the steel melt, it is advantageous if the initial nitrogen content (No) of the steel does not exceed 0.02 wt. % and is preferably 0.016 wt. % or less.
A slab is first cast from the molten steel, which is then hot rolled and cooled to room temperature. The hot strip produced in this way has thicknesses in the range from 1 to 4 mm and is wound into a coil at a predetermined coiling temperature of 500 to 750° C., preferably in the range from 650° C. to 750° C. The hot strip is then coiled into a coil at a predetermined coiling temperature. To produce a packaging steel in the form of a thin steel sheet in the usual sheet thicknesses of less than 0.5 mm, preferably less than 0.3 mm, the hot strip is cold-rolled, with a thickness reduction in the range from 50% to over 90%.
A barrier layer which is at least substantially impermeable to (atomic) nitrogen in the form of a silicate layer or a sol-gel layer, in particular a layer of SiO2, TiO2 and/or ZrO2, is then applied to the surface of the steel sheet on one of the two sides of the steel sheet which has been cold-rolled to a thickness of less than 0.5 mm. In a coil coating process, for example, a dispersion of a silicon alcoholate is applied to the surface of the steel strip on a first side (a). For this purpose, the dispersion is sprayed as a sol onto the first side of the steel strip in a wet-chemical coating process using spray nozzles or applied with doctor blades and subsequently dried. In the process, molecular chains are initially formed and, after a longer period of time, minute particles are formed. In the further course, the particles form a network in the sol. In the wet-chemically applied sol layer, a gel state is then generated due to hydrolysis and condensation reactions. The gelation can be accelerated by adding heat.
For gelling and drying, the steel strip coated with the sol will be placed in an oven. The drying of the sol can advantageously take place at least partially in a continuous annealing furnace, in which the subsequent thermal treatment of the steel sheet for nitriding and partial recrystallization of the steel sheet takes place. For this purpose, the cold-rolled steel strip provided with the barrier layer on the first side is passed through a continuous annealing furnace in which the steel strip is heated to temperatures above the (initial) recrystallization temperature TR of the steel.
In a particularly preferred embodiment, a 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 a sodium salt is added to an electrolyte tank and the steel sheet is passed through the electrolyte tank as a cathode at a predetermined strip speed. Table 2 shows preferred compositions of the electrolyte solutions and Table 2 shows preferred parameters of the electrolytic application process, by which a silicate layer acting as a barrier layer in the range from 1 to 10 mg/m2 and particularly preferably in the range from 3 to 6 mg/m2 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 in an nitriding gas atmosphere in the continuous annealing furnace before or preferably simultaneously with the recrystallization annealing. The nitriding is preferably carried out simultaneously with the recrystallization annealing in the annealing furnace by introducing a nitrogen-containing gas, preferably ammonia (NH3), into the annealing furnace while the steel sheet is heated to a temperature above the (initial) recrystallization temperature TR of the steel. At the temperatures in the annealing furnace, which are preferably higher than 300° C. when ammonia is used as the nitrogen-containing gas, atomic nitrogen is formed by dissociation of the nitrogen from the nitrogen-containing gas due to a catalytic reaction, which can diffuse into the steel sheet on the second side of the steel sheet (interstitially) at the surface of the steel sheet. On the first side of the steel sheet, the diffusion of nitrogen is prevented by the barrier layer.
In order to prevent oxidation of the steel sheet surface on the second side during heating, it is expedient to use an inert gas atmosphere in the annealing furnace. Preferably, the atmosphere in the annealing furnace consists of a mixture of the nitrogen-containing gas acting as a nitrogen donor and an inert gas such as HNX, the volume fraction of the inert gas preferably being between 90% and 99.5% and the remainder of the volume fraction of the gas atmosphere being formed by the nitrogen-containing gas, in particular ammonia gas (NH3 gas).
The heating time (tE) is preferably in the range from 1.0 to 300 seconds, particularly preferably between 10 and 120 seconds, and can be adjusted according to 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 rate at which the steel sheet passes through a continuous annealing furnace can be adjusted according to the desired heating time. To set the preferred heating times (tE) in the range of 1.0 to 300 seconds, for example, a heating rate of 10 K/s to 80 K/s can be selected. During the heating time, the steel sheet in the continuous annealing furnace is exposed to the nitriding gas atmosphere, in particular an ammonia gas atmosphere. The annealing time (tG) is preferably in the range from 1.0 to 90 seconds, particularly preferably between 10 and 60 seconds, and is also selected according to the desired material properties of the steel sheet according to the invention. After the annealing time (tG) has elapsed, the steel sheet leaves the annealing furnace and either cools passively in the environment or is cooled to room temperature by active cooling, e.g. water cooling or gas flow cooling. Suitable cooling rates are in the range from 3 K/s to 20 K/s for gas flow cooling and more than 1000 K/s for water cooling.
The (initial) recrystallization temperature TR of the steel depends on the composition of the steel and is typically in the range of 550 to 720° C.
When the cold-rolled steel sheet is heated in the annealing furnace, nitrogen from the nitrogen-containing gas is initially deposited only in a region near the surface on the second side of the steel sheet, as atomic nitrogen diffuses through the steel sheet surface. The nitrogen diffused into the near-surface region can either be interstitially incorporated into the iron lattice of the steel or is bound as a nitride, especially if strong nitride formers such as Al, Nb, Ti, or B are present in the steel. The incorporation of the nitrogen raises the recrystallization temperature (TR) of the steel in the near-surface second region by a value ΔT. This increase in the recrystallization temperature (TR) in the second region near the surface is shown in
According to the invention, the heating temperature (TE) or the annealing temperature is now selected so that T≤TRE<TR+ΔT applies. The heating temperature (TE) or the annealing temperature is thus set in the process according to the invention so that it lies between the (initial) recrystallization temperature (TR) of the steel used for the production of the cold-rolled steel sheet and the recrystallization temperature (TR+ΔT) increased by the value ΔT due to the near-surface nitriding of the steel sheet in the near-surface second region. By setting the heating temperature (TE) (or the annealing temperature) in this way, recrystallization only takes place in a first region on the first side of the steel sheet, which is inwardly adjacent to the second region near the surface and in which, at least initially, no nitrogen has (yet) been incorporated during annealing and simultaneous nitriding of the steel sheet. This is because the heating temperature (TE) is above the recrystallization temperature (TR) only in this first region of the steel sheet, and in the second region, in which the recrystallization temperature has been increased by ΔT due to the incorporated nitrogen, the heating temperature (TE) is below the recrystallization temperature increased to TR+ΔT. Therefore, a two-layer microstructure with an at least essentially, preferably largely completely recrystallized first region 1 and a second region 2 is formed over the cross-section of the steel sheet, the second region 2 not being recrystallized or at least not completely recrystallized.
The microstructure resulting from the heating of the steel sheet in the nitrogen-containing gas atmosphere therefore comprises an at least essentially completely recrystallized first region 1 and a non-recrystallized second region 2, as shown in the schematic sectional view of a steel sheet according to the invention in
The degree of recrystallization of the second region 2 and the first region 1 can be adjusted via the heating temperature (TE) and the annealing time (tG). A sharp demarcation of the core region 2 and the hem region 1 can be achieved, for example, if the annealing time (tG) is greater than 10 seconds and the heating temperature (TE) is between TR+ΔT/3 and TR+2ΔT/3. Similarly, the thickness of the hem region 1 can be adjusted by the process parameters of the heating temperature (TE) and the heating time (tE).
The process according to the invention can therefore be used to produce two-layer microstructures with a first region 1 that is at least largely completely 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 conversion or protective layers on one or both sides in the usual way, in particular by electrolytic tin plating or chromium plating.
Examples of embodiments of the steel sheet, its use in the manufacture of containers and the method according to the invention are explained below.
Steel sheets with a thickness of 0.23±0.01 mm were produced by hot rolling and subsequent cold rolling from the steel melt A with the alloy composition listed in Table 1 (the ppm figures refer to the weight fraction of the alloy constituents in the steel from which the cold-rolled steel sheet was produced). The cold-rolled steel sheets were subjected to a thermal treatment in an ammonia-containing inert gas atmosphere with a volume fraction of ammonia of 5% to a heating temperature TE of 750° C. at different heating times the and held at the heating temperature TE for an annealing time to of 45 seconds.
The microstructure of the heat-treated steel sheets was examined microscopically (cold-embedded, ground, polished and etched with 3% nitric acid after Nital).
On samples of steel sheets according to the invention, the hardness was recorded at various positions across the cross-section.
This variation in hardness over the thickness of the steel sheet is due to nitriding of the steel sheet in the second region 2 with a nitrogen content decreasing from the outside towards the core of the steel sheet and the (complete) recrystallization of the first region 1 during thermal treatment in the annealing furnace. The second region 2 is still roll-hard and has a high hardness with a hardness maximum at the surface of the steel sheet on the second side b.
This can also be confirmed by strength and elongation measurements on the steel sheets according to the invention.
The process according to the invention can thus be used to produce (nitrided) steel sheets characterized by a very high strength of more than 600 MPa combined with good elongation at break of more than 5%, preferably more than 7%. Such steel sheets can be excellently processed 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 regions, as well as the nitrogen content in the first and second regions generated by the nitriding process in the continuous annealing furnace and the gradient of the nitrogen content across the thickness of the steel sheet can be influenced by varying the process parameters. Therefore, the properties of the steel sheets produced by the process according to the invention can be tailored to different applications.
The behavior of steel sheets according to the invention during forming was investigated in a 4-radius cup test by forming steel sheet specimens into a cuboid container with different bending radii (R1, R2, R3, and R4) at each of the four corners of the container.
Roughness measurements were performed at positions P0, P1, P2, P3 and P4 shown in
In the 4-radius cup test, a steel sheet with a two-layer crystallization structure was used in each case, with the recrystallized first region 1 and the non-recrystallized (and therefore still mill-hard) second region 2 arranged both outside and inside at the bending radii. In this context, steel sheets having a different thickness of the second region 2 were also examined in the 4-radius cup test. The different thicknesses of the second region 2 were generated in the manufacturing process by different heating times (sample A: tE=1 second, sample B: tE=300 seconds) for the stitching process.
The results of the 4-radius cup test are shown in
From
It follows from this that when steel sheets according to the invention are formed, much less roughening occurs on the outside of bending radii when the second side b of the steel sheet with the non-recrystallized second region 2 is on the outside. The results of the 4-radius cup test therefore show that the steel sheets according to the invention are excellently suited for the production of containers which exhibit low roughening with low Ra values in the region of the bending radii of the containers. 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 on the outside at the bending radii of the container. In this case, the non-recrystallized second region 2 of the steel sheet represents a barrier to larger grains of the steel structure and prevents the grains of the steel structure from pressing through to the surface of the second side b in a visually visible manner, where they produce an undesirable roughening on the outside of the bending radius. Preferably, the thickness of the roll-hard second region 2 is selected to be as small as possible and in particular less than 50 μm. This ensures that the mechanical properties of the steel sheet, in particular its formability, are only insignificantly influenced 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, which has a high hardness and strength. Indeed, on the inner side of the bending radii R1 to R4 there is the soft and more easily formable first side a of the steel sheet with the softer and recrystallized first region 1. When the steel sheet is formed into a bending radius, the first region 1 of the steel sheet is compressed on the inner side of the bending radius, with the soft, recrystallized first region 1 forming only a low resistance to this forming. Despite their high hardness and strength on the second side b, the steel sheets according to the invention can therefore be easily formed into containers in conventional forming processes, in particular in deep drawing processes, without any detrimental roughening of the surface of the steel sheet on the outside of formed areas. From a comparison of the roughness values of specimen A and specimen B shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2021 129 191.7 | Nov 2021 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/080635 | 11/3/2022 | WO |
