STEEL SHEET HAVING A TWO-LAYER CRYSTALLIZATION STRUCTURE AND PROCESS FOR PRODUCING SUCH A STEEL SHEET

Abstract

A method for producing a steel sheet having a two-layer recrystallization structure includes: providing a cold-rolled steel sheet made of a steel with a carbon content of 10 to 1000 ppm, based on the weight, and a specified recrystallization temperature; applying a barrier layer which is at least partly impermeable to nitrogen onto a first face; and heating the steel sheet to a heating temperature. The heating is carried out at least temporarily in a nitrogenizing gas atmosphere at least until the recrystallization temperature is reached, whereby nitrogen from the gas atmosphere is diffused at least in a region near the surface on a second face of the steel sheet upon heating and is stored in the region, whereby the recrystallization temperature of the steel is raised in the region. The heating temperature is higher than or equal to the specified recrystallization temperature and lower than the raised recrystallization temperature.

Claims

1-24. (canceled)

25. A method of manufacturing a steel sheet for packaging, the method comprising: providing a cold rolled steel sheet of a steel having a carbon content by weight of 10 to 1000 ppm and a predetermined recrystallization temperature, the steel sheet having a first side and a second side; applying a barrier layer, which is substantially impermeable to nitrogen, to the first side of the steel sheet; and heating the steel sheet to a heating temperature, wherein the heating takes place in a nitriding gas atmosphere until the recrystallization temperature is at least temporarily reached, whereby during heating of the steel sheet, nitrogen diffuses from the gas atmosphere at least into a near surface region on the second side of the steel sheet and is incorporated in the near surface region, as a result of which the recrystallization temperature of the steel in the near surface region is raised by a value of T; wherein the heating temperature is greater than or equal to the recrystallization temperature and less than the recrystallization temperature increased by the value of T in the near surface region.

26. The method of claim 25, wherein the steel of the cold-rolled steel sheet has the following composition by weight: C: more than 0.001% and less than 0.1%; Mn: more than 0.01% and less than 0.6%; P: less than 0.04%; S: less than 0.04%; Al: more than 50 ppm and less than 0.08%; Si: less than 0.1%; an average nitrogen content by weight after heating of the cold-rolled steel sheet in the nitriding gas atmosphere of at least 0.005%; residual iron and unavoidable impurities.

27. The method of claim 25, wherein the steel of the cold-rolled steel sheet has the following composition by weight: C: more than 0.001% and less than 0.1%; Mn: more than 0.01% and less than 0.6%; P: less than 0.04%; S: less than 0.04%; Al: more than 50 ppm and less than 0.08%; Si: less than 0.1%; Cu: less than 0.1%; Cr: less than 0.1%; Ni: less than 0.1%; Ti: less than 0.1% and more than 0.02%; Nb: less than 0.08% and more than 0.01%; Mo: less than 0.08%; Sn: less than 0.05%; B: less than 0.01% and more than 0.0005%; N: more than 0.001% and less than 0.016% before the heating and at least 0.005% after the heating of the cold-rolled steel sheet in the nitriding gas atmosphere; residual iron and unavoidable impurities

28. The method of claim 25, wherein the cold-rolled steel sheet is heated from room temperature to the heating temperature within a predetermined heating time and, after reaching the heating temperature, the cold-rolled steel sheet is kept at the heating temperature for a predetermined annealing time, wherein the heating time is in the range from 1.0 to 300 seconds and the annealing time is in the range from 1.0 seconds to 80 seconds.

29. The method of claim 25, wherein the steel of the cold-rolled steel sheet has an initial nitrogen content and, during heating of the cold-rolled steel sheet, the average nitrogen content in the near surface region increases to a value averaged over the near surface region being between 50 and 1000 ppm above the initial nitrogen content of the steel, wherein a gradient of the nitrogen content is established with a decreasing nitrogen content from the near surface region on the second side of the steel sheet to the first side of the steel sheet.

30. The method of claim 29, wherein during the heating at least temporarily a nitriding of the near surface region on the second side of the cold-rolled steel sheet takes place and during the annealing time at least partially a recrystallization annealing of the cold-rolled steel sheet takes place in a first region on the first side of the steel sheet outside the near surface region, whereas the near surface region on the second side of the steel sheet is not recrystallized.

31. The method of claim 25, wherein the value by which the recrystallization temperature in the near surface region is increased by incorporation of nitrogen during heating of the steel sheet, is greater than 30 C.

32. The method of claim 25, wherein the barrier layer is formed by applying a sol-gel layer on the surface of the first side of the steel sheet.

33. A steel sheet for packaging having a first side, a second side and a thickness of less than 0.5 mm, and a carbon content by weight of 10 to 1000 ppm, wherein a barrier layer which is substantially impermeable to nitrogen is present on a surface of the first side, wherein the steel sheet has a two-layer crystallization structure with a first region on the first side and a second region on the second side, wherein the first region is at least substantially recrystallized and the second region is not or at least not completely recrystallized.

34. The steel sheet of claim 33, wherein the steel sheet has the following composition by weight: C: more than 0.001% and less than 0.1%, preferably less than 0.06%; Mn: more than 0.01% and less than 0.6%; P: less than 0.04%; S: less than 0.04% and preferably more than 0.001%; Al: less than 0.08% and preferably more than 0.005%; Si: less than 0.1%; a nitrogen content averaged over the thickness of the steel sheet of at least 0.005%; rest iron and unavoidable impurities.

35. The steel sheet of claim 33, wherein the first region has a thickness in the range from 50 m to 450 m and the second region has a thickness in the range from 1 m to 50 m.

36. The steel sheet of claim 33, wherein the steel sheet has a tensile strength of more than 500 MPa and an elongation at break of more than 5%.

37. The steel sheet of claim 33, wherein a gradient of the nitrogen content is present at least in the second region with the nitrogen content decreasing from the second side to the first side of the steel sheet.

38. The steel sheet of claim 33, wherein the second region has a higher hardness and a higher tensile strength than the first region.

39. The steel sheet of claim 33, wherein the second region has a degree of recrystallization of less than 30% and the first region has a degree of recrystallization of more than 70%.

40. The steel sheet of claim 33, wherein the barrier layer comprises a sol-gel layer containing at least one of SiO.sub.2, TiO.sub.2 and ZrO.sub.2.

41. The steel sheet of claim 33, wherein the barrier layer has a thickness of less than 1 m or a coating weight of less than 10 mg/m.sup.2.

42. The steel sheet of claim 33, wherein, after a forming of the steel sheet, the surface of the second region has a bending radius in the range of 8 mm to 14 mm, wherein the second region is lying on the outside of the bending radius and has a roughness of less than 1.0 m.

43. A container made of a steel sheet according to claim 34, wherein the container has at least one convexly deformed portion and the second region of the steel sheet is located on a convex outer side of the deformed portion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0076] 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:

[0077] FIG. 1: schematic representation of an embodiment of the heating step of the process according to the invention in the form of a temperature-time diagram;

[0078] FIGS. 2A and 2B: Schematic representation of the cross-section of a steel sheet according to the invention with a two-layer microstructure;

[0079] FIG. 3: Microscopic cross-sectional image (metallographic section) of a steel sheet according to the invention with a two-layer microstructure;

[0080] FIG. 4: Plot of the cross-sectional variation of Vickers hardness (HV.sub.0.025) versus thickness of the steel sheet, measured on different samples of steel sheets according to the invention, which have been heated and nitrided with different heating times (t.sub.E) in the method according to the invention;

[0081] FIG. 5: Comparison of stress-strain diagrams [()] of steel sheets according to the invention which have been heated and nitrided at different heating times (t.sub.E);

[0082] FIGS. 6A and 6B: Schematic representation of the 4-radius cup test with a top view of the bottom of a container produced in this test from the steel sheets according to the invention (FIG. 6A) with different bending radii at the corners of the container and a section of a side view of the container (FIG. 6B);

[0083] FIGS. 7A and 7B: Representation of the results of the 4-radius cup test from FIGS. 6A and 6B, performed on specimens of steel sheets according to the invention, where in FIG. 7A the roughness (Ra in m) on the outside of the bending radii (R1 to R4) at the different positions (P0 to P4) from FIG. 6B is shown and in FIG. 7B the roughness factor (Ra factor=Ra/R.sub.0 with R.sub.0 as roughness of the steel sheet in an undeformed section of the container) at these positions (P0 to P4) is shown; and

[0084] FIG. 8: Table 1

DETAILED DESCRIPTION

[0085] 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: [0086] Carbon, C: more than 0.001% and less than 0.1%, preferably less than 0.06%;

[0087] 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. %. [0088] Manganese, Mn: more than 0.01% and less than 0.6%;

[0089] 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. [0090] Phosphorus, P: less than 0.04%.

[0091] 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. [0092] Sulfur, S: less than 0.04% and preferably more than 0.001%.

[0093] 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. %. [0094] Aluminum, Al: less than 0.08%

[0095] 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. [0096] Silicon, Si: less than 0.1%;

[0097] Silizium erhht im Stahl die Zunderbestndigkeit und ist ein Mischkristallhrter. 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. [0098] optionally nitrogen, N.sub.0: less than 0.02%, in particular less than 0.016%, and preferably more than 0.001%.

[0099] 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. [0100] optional: nitride formers, especially niobium, titanium, zirconium, vanadium: Nitride-forming elements such as aluminum, titanium, niobium, zirconium or vanadium are optionally advantageous in the steel of the steel sheets according to the invention in order to bind off, at least in part, the nitrogen originally contained in the steel and the nitrogen subsequently introduced in the form of nitrides by the subsequent nitriding process in the annealing furnace. This improves the forming behavior and makes it possible to produce IF (Interstitial Free) steel sheets which are virtually free of 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 constituents to increase strength by grain refinement without reducing toughness

[0101] Therefore, in terms of weight, the steel optionally and preferably contains [0102] titanium, Ti: preferably more than 0.02%, particularly preferably more than 0.02% but less than 0.1%, and/or [0103] Niobium, Nb: preferably more than 0.01% but less than 0.08%, and/or [0104] aluminum, Al: preferably more than 0.005 wt. % but less than 0.08 wt. %, and/or [0105] Molybdenum, Mo: less than 0.08%;

Other Optional Components:

[0106] In addition to the residual iron (Fe) and unavoidable impurities, the steel may contain other optional constituents, such as. [0107] optionally copper, Cu: less than 0.1%; [0108] optionally chromium, Cr: less than 0.1%; [0109] optionally nickel, Ni: less than 0.1%; [0110] optional Tin, Sn: less than 0.05%; [0111] optionally boron, B: less than 0.01%, preferably less than 0.005% and preferably more than 0.0005%; [0112] in order to give the steel further advantageous properties, if any, which can be achieved by these additional constituents.

[0113] Table 1 (FIG. 8) shows embodiments of steel compositions A, B and C from which steel sheets can be produced by hot rolling and cold rolling, which can be treated with the process according to the invention to produce a two-layer crystallization structure in order to form a two-layer microstructure in the cold-rolled steel sheet.

Manufacturing Process of the Steel Sheet:

[0114] 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.

[0115] 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%.

[0116] 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 SiO.sub.2, TiO.sub.2 and/or ZrO.sub.2, 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.

[0117] 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 T.sub.R of the steel.

[0118] 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/m.sup.2 and particularly preferably in the range from 3 to 6 mg/m.sup.2 is applied from the electrolyte on one side of the steel sheet.

[0119] 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 (NH.sub.3), into the annealing furnace while the steel sheet is heated to a temperature above the (initial) recrystallization temperature T.sub.R 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.

[0120] 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 HN.sub.X, 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 (NH.sub.3 gas).

[0121] FIG. 1 schematically shows a temperature-time profile of the thermal treatment for nitriding and annealing the steel sheet in the annealing furnace in a preferred embodiment example for the process according to the invention. In this first embodiment example, a single-stage heat treatment of the steel sheet is carried out in a continuous annealing furnace, and the steel sheet is simultaneously recrystallization annealed and nitrided during the single-stage heating. As shown in FIG. 1, in this embodiment the steel sheet is heated from room temperature within a heating time (t.sub.E) at a preferred (average) heating rate of 10 to 15 C./s to a heating temperature T.sub.ET.sub.R and held at least approximately at this temperature during an annealing time (t.sub.G). The heating temperature T.sub.E corresponds to the annealing temperature at which the steel sheet is annealed to recrystallize (in certain regions) and lies between the initial recrystallization temperature T.sub.R and the recrystallization temperature increased to T.sub.R+T by the nitriding in the near-surface area of the steel sheet. It holds here for the heating temperature T.sub.E: TT.sub.RE<T.sub.R+T.

[0122] The heating time (t.sub.E) 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 (t.sub.E) 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 (t.sub.G) is preferably in the range from 1.0 to 90 seconds, particularly preferably between 10 and 60 seconds, and is also selected according to the desired material properties of the steel sheet according to the invention. After the annealing time (t.sub.G) has elapsed, the steel sheet leaves the annealing furnace and either cools passively in the environment or is cooled to room temperature by active cooling, 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.

[0123] The (initial) recrystallization temperature T.sub.R of the steel depends on the composition of the steel and is typically in the range of 550 to 720 C.

[0124] 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 (T.sub.R) of the steel in the near-surface second region by a value T. This increase in the recrystallization temperature (T.sub.R) in the second region near the surface is shown in FIG. 1 with T.

[0125] According to the invention, the heating temperature (T.sub.E) or the annealing temperature is now selected so that TT.sub.RE<T.sub.R+T applies. The heating temperature (T.sub.E) or the annealing temperature is thus set in the process according to the invention so that it lies between the (initial) recrystallization temperature (T.sub.R) of the steel used for the production of the cold-rolled steel sheet and the recrystallization temperature (T.sub.R+T) increased by the value T due to the near-surface nitriding of the steel sheet in the near-surface second region. By setting the heating temperature (T.sub.E) (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 (T.sub.E) is above the recrystallization temperature (T.sub.R) only in this first region of the steel sheet, and in the second region, in which the recrystallization temperature has been increased by T due to the incorporated nitrogen, the heating temperature (T.sub.E) is below the recrystallization temperature increased to T.sub.R+T. 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.

[0126] 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 FIGS. 2A and 2B. The second region 2 located on the second side b of the steel sheet is not recrystallized or at least not completely recrystallized and therefore remains in the as-rolled condition of the cold-rolled steel sheet. In contrast, the first region 1 lying on the first side a of the steel sheet is (in particular completely) recrystallized. The barrier layer 3 shown in FIG. 2A is located on the first side a of the steel sheet.

[0127] The degree of recrystallization of the second region 2 and the first region 1 can be adjusted via the heating temperature (T.sub.E) and the annealing time (t.sub.G). A sharp demarcation of the core region 2 and the hem region 1 can be achieved, for example, if the annealing time (t.sub.G) is greater than 10 seconds and the heating temperature (T.sub.E) is between T.sub.R+T/3 and T.sub.R+2T/3. Similarly, the thickness of the hem region 1 can be adjusted by the process parameters of the heating temperature (T.sub.E) and the heating time (t.sub.E).

[0128] 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

[0129] 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

[0130] Examples of embodiments of the steel sheet, its use in the manufacture of containers and the method according to the invention are explained below.

[0131] Steel sheets with a thickness of 0.230.01 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 T.sub.E of 750 C. at different heating times the and held at the heating temperature T.sub.E for an annealing time to of 45 seconds.

[0132] The microstructure of the heat-treated steel sheets was examined microscopically (cold-embedded, ground, polished and etched with 3% nitric acid after Nital). FIG. 3 shows an example of a cross-sectional microscopic micrograph of a metallographic section of a steel sheet according to the invention, from which the two regions 1, 2 with different degrees of crystallization and the barrier layer 3 on the first side of the steel sheet, which is not visible in the section of FIG. 3, can be seen. It is clear from FIG. 3 that the steel sheet according to the invention has a two-layer microstructure with a recrystallized first region 1 and a non-recrystallized second region 2, the first region 1 being significantly thicker than the second region 2.

[0133] On samples of steel sheets according to the invention, the hardness was recorded at various positions across the cross-section. FIG. 4 shows the variation of the Vickers hardness HV.sub.0.025 over the thickness of the steel sheet specimens at different positions (position 1 to position 7, where position 1 is the surface of the steel sheet at the first side a and position 7 is the surface of the steel sheet at the second side b). From FIG. 4 it can be seen that the hardness increases steadily (linearly) over the thickness of the steel sheet specimens from the first side a to the second side b.

[0134] 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.

[0135] FIG. 4 also shows that the absolute values of the Vickers hardness depending on the heating time t.sub.E. While the samples heat-treated with a short heating time of, for example, t.sub.E=60 seconds have a low Vickers hardness on the first side a of approx. 100, the samples treated with a longer heating time of, for example, t.sub.E=300 seconds have a Vickers hardness on the same side of more than 150. Therefore, the hardness or strength of the steel sheets according to the invention can be adjusted via the heating time t.sub.E. It can also be seen from FIG. 4 that with the longer heating times of more than 120 seconds, at least partial stiffening of the first region 1 also takes place during heating, with nitrogen diffusing into this area and increasing the hardness or strength of the steel there.

[0136] This can also be confirmed by strength and elongation measurements on the steel sheets according to the invention. FIG. 5 shows examples of stress-strain diagrams of two samples of steel sheets according to the invention made from the steel of Table 1, which were heated with different heating times of t.sub.E=60 seconds and t.sub.E=300 seconds, respectively, and nitrided during these heating times.

[0137] FIG. 5 shows that the mechanical parameters of the steel sheets according to the invention, in particular the tensile strength and elongation at break, can be adjusted via the heating time, with tensile strengths of >600 MPa and elongations at break of more than 7% being attainable.

[0138] 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.

[0139] 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.

[0140] 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.

[0141] FIGS. 6A and 6B show a schematic diagram of the container formed from the steel sheets during the 4-radius cup test, with FIG. 6A showing a top view of the container base and FIG. 6B a sectional view in a side view in the region of the container wall. FIG. 6A shows the different bending radii (R1, R2, R3, R4), which have the following size: [0142] R1=8 mm, [0143] R2=10 mm, [0144] R3=12 mm, [0145] R4=14 mm.

[0146] Roughness measurements were performed at positions P0, P1, P2, P3 and P4 shown in FIG. 6B to determine the center roughness Ra value.

[0147] 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: t.sub.E=1 second, sample B: t.sub.E=300 seconds) for the stitching process.

[0148] The results of the 4-radius cup test are shown in FIGS. 7A and 7B. FIG. 7A shows the center roughness (Ra values) measured at the various bending radii R1, R2, R3, R4 at positions P1 to P4 (see FIG. 6B) and in the area of the undeformed bottom of the container (position P0 in FIG. 6B) for both variants of the steel sheet specimens tested.

[0149] From FIG. 7A it can be seen that the roughness Ra at all bending radii R1 to R4 is higher than the roughness in the undeformed bottom of the container. Furthermore, it can be seen from FIG. 7A that the roughness at the bending radii R1 to R4 is lower in the containers in which the non-recrystallized (roll-hard) second region 2 is located on the outside of the bending radius. In contrast, the containers in which the recrystallized first region 1 lies on the outside of the bending radii have a higher roughness Ra. In the containers with the second region 2 lying on the outside of the bending radii R1 to R4, the center roughness (Ra) is less than 1.0 m and in particular less than 0.8 m.

[0150] FIG. 7B shows a Ra factor defined by the ratio of the Ra values at the bending radii R1 to R4 and the Ra value in the undeformed bottom region of the container. As can be seen from FIG. 7B, the containers in which the non-recrystallized second region 2 is located on the outside of the bending radii R1 to R4 have Ra factors lower by a factor of about 3. In the case of the containers with the second region 2 lying on the outside of the bending radii R1 to R4, the roughness factor (Ra factor) is less than 3, in particular less than 2.5.

[0151] 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 m. 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 FIG. 7A, it can be seen that the thickness of the second region 2 has only a minor influence on the roughness, which is why, from the viewpoint of avoiding roughening during forming of the steel sheet, the thickness of the second region can be small.

TABLE-US-00001 TABLE 2 Electrolyte A Electrolyte B Ingredients Silica, sodium salt Silica, sodium salt SiO.sub.2/Na.sub.2O SiO.sub.2/Na.sub.2O (concentration 40-60%) (concentration 35-<40%) Density [g/cm.sup.3] 1.47-1.52 1.32-1.36 pH value >12 10.2-10.8

TABLE-US-00002 TABLE 3 Min. Max. Silica coating [mg/m.sup.2] 1 10 Electrolyt temperature [ C.] 70 90 Total Current [A] 8000 10000 Current Density [A/dm.sup.2] 10 15 Strip Speed [m/min] 50 1050 Electrolyt Conductivity [mS] 40 60