Abstract
A method for producing a steel sheet for packaging includes: cold-rolling a hot-rolled steel sheet made from a steel having a carbon content of 10 to 1000 ppm by weight, the steel of the hot-rolled steel sheet having a predetermined recrystallization temperature (T.sub.R); heating the cold-rolled steel sheet to a predetermined heating temperature (T.sub.E), where T.sub.R≤T.sub.E, the heating performed at least partially in the presence of a nitrogen donor at least until T.sub.R is reached such that when the cold-rolled steel sheet is heated, nitrogen from the nitrogen donor is diffused at least into a near-surface region of the cold-rolled sheet steel and incorporated in the near-surface region, as a result of which the T.sub.R in the near-surface region is increased by a value ΔT, where T.sub.E<T.sub.R+ΔT. Using this method, high-strength steel sheets having a multilayer microstructure can be produced.
Claims
1-38. (canceled)
39. A method of manufacturing a steel sheet for packaging, the method comprising: cold rolling of a hot rolled steel sheet made of a steel having a carbon content (C) by weight of 10 to 1000 ppm, wherein the steel of the steel sheet has a recrystallization temperature (T.sub.R), heating the cold-rolled steel sheet to a predetermined heating temperature (T.sub.E) for which applies: T.sub.R≤T.sub.E, wherein the heating takes place at least until the recrystallization temperature (T.sub.R) is reached at least temporarily in the presence of a nitrogen donor, as a result of which, when the cold-rolled steel sheet is heated, nitrogen diffuses from the nitrogen donor at least into a region of the cold-rolled steel sheet close to the surface and is incorporated in the region close to the surface, as a result of which the recrystallization temperature (T.sub.R) of the steel in the region close to the surface is raised by a value ΔT, wherein the following condition applies to the heating temperature (T.sub.E): T.sub.E<T.sub.R+ΔT.
40. The method according to claim 39, 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: less than 0.08%; Si: less than 0.1%; optional Cu: less than 0.1%; optional Cr: less than 0.1%; optional Ni: less than 0.1%; optionally Ti: less than 0.1%; optional Nb: less than 0.08%; optional Mo: less than 0.08%; optional Sn: less than 0.05%; optional B: less than 0.01%; optional N: less than 0.02%; Residual iron and unavoidable impurities, wherein the average nitrogen content by weight after heating of the cold-rolled steel sheet in the presence of the nitrogen donor is at least 0.005%.
41. The method according to claim 39, wherein the steel of the cold rolled steel sheet contains at least one of titanium, niobium and aluminum with more than 200 ppm by weight titanium, more than 100 ppm by weight niobium and more than 50 ppm by weight aluminum.
42. The method according to claim 39, wherein the heating of the cold-rolled steel sheet comprises at least one holding phase in which the temperature of the steel sheet is held during a holding time (t.sub.H) at an at least substantially constant intermediate temperature (T.sub.Z), which is lower than the recrystallization temperature (T.sub.R), and the cold-rolled steel sheet is at least temporarily exposed to the nitrogen donor before the start of and/or during the holding time (t.sub.H) and, after the holding time (t.sub.H) has elapsed, is heated further until the heating temperature (T.sub.E) is reached, wherein the holding time (t.sub.H) is between 1.0 and 300 seconds.
43. The method according to claim 42, wherein the cold-rolled steel sheet is heated from room temperature to the heating temperature (T.sub.E) within a heating time (t.sub.E) of 1.0 to 300 seconds, wherein, after reaching the heating temperature (T.sub.E), the steel sheet is kept at the heating temperature (T.sub.E) for a predetermined annealing time (t.sub.G).
44. The method according to claim 39, wherein a gradient of the nitrogen content is established in the region close to the surface during heating of the cold-rolled steel sheet, the nitrogen content decreasing from the surface to a core region of the cold-rolled steel sheet.
45. The method according to claim 44, wherein the steel of the cold-rolled steel sheet has an initial nitrogen content and the nitrogen content in the core region is greater than the initial nitrogen content of the steel.
46. The method according to claim 39, wherein the nitrogen incorporated in the region close to the surface during heating of the cold-rolled steel sheet at least partly is present in dissolved form and is interstitially incorporated in the lattice of the steel.
47. The method according to claim 39, wherein the nitrogen donor is constituted by a nitrogen-containing gas atmosphere containing ammonia, the volume concentration of the ammonia in the nitrogen-containing gas atmosphere being greater than 0.1%, and the rest of the gas atmosphere is an inert gas.
48. The method according to claim 39, wherein the value ΔT depends on the nitrogen content introduced into the region close to the surface of the steel sheet by nitriding after completion of the heating, with the following relationship: ΔT=aΔN(%), where a is a proportionality constant and ΔN(%) is the nitrogen content in % based on the weight of the steel introduced into the region (1) close to the surface during heating of the steel sheet by nitriding.
49. The method according to claim 39, wherein the value ΔT by which the recrystallization temperature in the region close to the surface is increased by the incorporation of nitrogen is greater than 30° C.
50. A steel sheet for packaging, which is made from a steel having a carbon content by weight of 10 to 1000 ppm and a thickness of less than 0.5 mm, wherein the steel sheet contains a multilayer microstructure having at least a first layer and a second layer, the second layer being at least substantially recrystallized and the first layer being not recrystallized or at least not completely recrystallized.
51. The steel sheet according to claim 50, wherein the multilayer microstructure has an inner core region, which is at least substantially recrystallized, and a region close to the surface surrounding the core region on both sides, wherein the region close to the surface is not or at least not completely recrystallized.
52. The steel sheet according to claim 50, wherein the steel sheet has the following composition by weight: C: more than 0.001% and less than 0.1%; Mn: more than 0.01% and less than 0.6%; P: less than 0.04%; S: less than 0.04%; Al: less than 0.08%; Si: less than 0.1%; optional Cu: less than 0.1%; optional Cr: less than 0.1%; optional Ni: less than 0.1%; optionally Ti: less than 0.1%; optional Nb: less than 0.08%; optional Mo: less than 0.08%; optional Sn: less than 0.05%; optional B: less than 0.01%; and a nitrogen content averaged over the thickness of the steel sheet of at least 0.005%, residual iron and unavoidable impurities.
53. The steel sheet according to claim 50, wherein the steel sheet has been produced by cold rolling from a hot-rolled steel having a predetermined recrystallization temperature (T.sub.R) and the cold-rolled steel sheet has been heated in the presence of a nitrogen donor to a predetermined heating temperature (T.sub.E) above the recrystallization temperature (T.sub.R), wherein by heating to the predetermined heating temperature (T.sub.E), the second layer has been at least substantially completely recrystallized and the first layer has been at least not completely recrystallized.
54. The steel sheet according to claim 50, wherein the steel of the steel sheet contains at least one of titanium and niobium with more than 500 ppm by weight titanium and more than 100 ppm niobium.
55. The steel sheet according to claim 51, wherein the region close to the surface has a thickness in the range from 5 μm to 150 μm.
56. The steel sheet according to claim 50, wherein the steel sheet has a tensile strength of more than 600 MPa and an elongation at break of more than 4%.
57. The steel sheet according to claim 50, wherein the first layer has a higher hardness than the second layer, wherein the ratio of the hardness of the first layer to the hardness of the second layer is greater than 1.2.
58. The steel sheet according to claim 50, wherein the first layer has a degree of recrystallization of less than 30%, and the second layer has a degree of recrystallization of more than 70%.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0068] These and other advantages of the packaging steel according to the invention and of the manufacturing method become apparent from the embodiments described in more detail below with reference to the accompanying drawings. The drawings show:
[0069] FIGS. 1A-1D: Schematic representation of temperature-time diagrams of the method according to the invention in three different embodiments, the embodiment example of FIG. 1A showing a single-stage heating and the embodiment examples of FIGS. 1B and 1C and FIG. 1D each showing a two-stage heating of the steel sheet in an annealing furnace;
[0070] FIGS. 2A and 2B: Schematic representation of the microstructure of the steel sheet according to the invention in a first embodiment with a three-layer microstructure (FIG. 2A) and in a second embodiment with a two-layer microstructure (FIG. 2B);
[0071] FIGS. 3A and 3B: Comparison of microscopic cross-sectional images of the microstructure of a conventional steel sheet (FIG. 3a) and a steel sheet according to the invention with a three-layer microstructure (FIG. 3B);
[0072] FIGS. 4A and 4B: Comparison of microscopic images of the microstructure of a conventional steel sheet (FIG. 4A) and a steel sheet according to the invention with a three-layer microstructure (FIG. 4B), in each case showing the cross-sectional variation of Vickers hardness (HV 0.05) over the thickness of the steel sheet;
[0073] FIG. 5: Comparison of the stress-strain diagram measured on a comparative example (conventional steel sheet) and on an embodiment of the steel sheet according to the invention;
[0074] FIGS. 6A and 6B: Microscopic cross-sectional images of the microstructure of embodiments of steel sheets according to the invention with a three-layer microstructure, which were nitrided at different heating temperatures (T.sub.E, FIG. 6A) or with different annealing times (t.sub.G, FIG. 6B);
[0075] FIGS. 7A and 7B: Microscopic cross-sectional images of the microstructure of examples of steel sheets according to the invention with a three-layer microstructure, which were nitrided in an annealing furnace in the presence of a nitrogen donor (NH.sub.3-gas atmosphere) with different nitrogen concentrations (volume fraction of NH.sub.3 in the gas atmosphere);
[0076] FIG. 8: Representation of the course of the thickness of the fringe region formed in the steel sheet when nitriding on steel sheets using the method according to the invention, as a function of the nitrogen concentration of the nitrogen donor used for nitriding (ammonia content of the gas atmosphere);
[0077] FIG. 9: Representation of the course of the microhardness measured on 0.25 mm thick steel sheets according to the invention over the cross-section of the steel sheet;
[0078] FIG. 10: Representation of the course of the thickness of the fringe region formed in the steel sheet when nitriding steel sheets using the method according to the invention, as a function of the annealing time (t.sub.G) for which the steel sheet has been kept at the heating temperature (t.sub.E=750° C.);
[0079] FIG. 11: Representation of the course of strength versus elongation, of steel sheets according to the invention, which have been nitrided with the method according to the invention at a heating temperature t.sub.E=750° C. and different heating times;
[0080] FIG. 12: Gradient curve of the nitrogen content over the cross-sectional profile of a steel sheet according to the invention;
[0081] FIG. 13: Illustration of the influence of the nitrogen content of steel sheets nitrided at different levels on the hardness and recrystallization temperature using hardness diagrams as a function of the heating temperature during recrystallization annealing;
[0082] FIG. 14: Depiction of the dependence of microhardness (Vickers hardness HV.sub.0.025) and recrystallization temperature on the nitrogen content of steel sheets nitrided to different levels;
[0083] FIG. 15: Representation of the course of the microhardness over the cross-section of embodiments of steel sheets according to the invention which have been nitrided in the annealing furnace with different high ammonia contents (NH.sub.3 in vol. %);
[0084] FIG. 16: Illustration of a microscopic cross-sectional image of the microstructure of an embodiment of a steel sheet according to the invention with a three-layer microstructure,
[0085] FIG. 17: Representation of the course of the microhardness of a steel sheet according to the invention over the cross-section;
[0086] FIG. 18: Plot of the strength-elongation diagram of steel sheets according to the invention with different degrees of skin pass (DG);
[0087] FIG. 19: Representation of the orientation density distribution function f(g) along the ε-fiber of a steel sheet according to the invention with a three-layer microstructure (sandwich structure);
[0088] FIG. 20: Table 1;
[0089] FIG. 21: Tables 2A and 2B;
[0090] FIG. 22: Tables 3A and 3B; and
[0091] FIG. 23: Tables 4A and 4B.
DETAILED DESCRIPTION
[0092] 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 original nitrogen content, if in particular highly nitrided steel sheets with a high nitrogen content of more than 0.02% by weight are to be produced. The components of the steel from which steel sheets according to the invention can be produced are explained in detail below:
[0093] Composition of the Steel: [0094] Carbon, C: more than 0.001% and less than 0.1%, preferably less than 0.06%; Carbon has a hardness- or strength-increasing effect. 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) and not to reduce the elongation at break, the carbon content should not exceed 0.1 wt. %. [0095] Manganese, Mn: more than 0.01% and less than 0.6%;
[0096] 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. [0097] Phosphorus, P: less than 0.04%.
[0098] 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. [0099] Sulfur, S: less than 0.04% and preferably more than 0.001%.
[0100] 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. %. [0101] Aluminum, Al: less than 0.08%
[0102] In steel production, aluminum acts as a deoxidizing agent in the casting method to calm the steel. Aluminum also increases scale resistance and formability. In addition, aluminum forms nitrides with nitrogen, which are beneficial 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. [0103] Silicon, Si: less than 0.1%;
[0104] Silicon increases the scale resistance in steel and is a solid solution hardener. In steel production, it has the positive effect of making the melt thinner and serves as a 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. [0105] optionally nitrogen, N.sub.0: less than 0.02%, in particular less than 0.016%, and preferably more than 0.001%.
[0106] 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 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 with entirely. However, to achieve 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. [0107] optional: nitride formers, especially niobium, titanium, zirconium, vanadium:
[0108] 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 method 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. Aluminium, 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.
[0109] Therefore, in terms of weight, the steel optionally and preferably contains [0110] titanium, Ti: preferably more than 0.02%, particularly preferably more than 0.02% but less than 0.1%, and/or [0111] Niobium, Nb: preferably more than 0.01% but less than 0.08%, and/or [0112] aluminum, Al: preferably more than 0.005 wt. % but less than 0.08 wt. %, and/or [0113] Molybdenum, Mo: less than 0.08%;
[0114] Other Optional Components:
[0115] In addition to the residual iron (Fe) and unavoidable impurities, the steel may contain other optional constituents, such as [0116] optionally copper, Cu: less than 0.1%; [0117] optionally chromium, Cr: less than 0.1%; [0118] optionally nickel, Ni: less than 0.1%; [0119] optional tin, Sn: less than 0.05%; [0120] optionally boron, B: less than 0.01%, preferably less than 0.005% and preferably more than 0.0005%;
[0121] in order to give the steel further advantageous properties, if any, which can be achieved by these additional constituents.
[0122] Manufacturing Method of the Steel Sheet:
[0123] 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 (N.sub.0) of the steel does not exceed 0.02 wt. % and is preferably 0.016 wt. % or less.
[0124] 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 tentatively wound into a coil at a predetermined coiling temperature of 500 to 750° C., preferably in the range from 650° C. to 750° C. 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%. To restore the crystal structure of the steel destroyed during cold rolling, the cold-rolled steel strip is annealed in an annealing furnace to recrystallize it. This is done, for example, by passing the steel strip in the form of a cold-rolled steel strip through a continuous annealing furnace in which the steel strip is heated to temperatures above the (original) recrystallization temperature T.sub.R of the steel. In the method according to the invention, before or preferably simultaneously with the recrystallization annealing, the cold-rolled steel strip is nitrided by heating the steel strip in the presence of a nitrogen donor. Nitriding is preferably carried out simultaneously with recrystallization annealing in the annealing furnace by introducing a nitrogen donor, in particular in the form of a nitrogen-containing gas, preferably ammonia (NH.sub.3), into the annealing furnace and heating the steel sheet to a temperature above the (original) recrystallization temperature T.sub.R of the steel. The nitrogen donor is selected so that, at the temperatures in the annealing furnace, atomic nitrogen is formed by dissociation of the nitrogen donor and can diffuse (superficially) into the steel sheet. To prevent oxidation of the steel sheet surface during annealing, an inert gas atmosphere is expediently used 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 (NH.sub.3 gas).
[0125] FIG. 1A schematically shows a temperature-time profile of the thermal treatment for nitriding and recrystallization annealing of the steel sheet in the annealing furnace in a first embodiment example for the method 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, whereby the steel sheet is simultaneously recrystallization annealed and nitrided during the (single-stage) heating. As shown in FIG. 1A, 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.E≥T.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 areas) and lies between the original recrystallization temperature T.sub.R and the recrystallization temperature increased to T.sub.R+ΔT by the nitriding in the fringe region of the steel sheet. It is therefore valid here for the heating temperature T.sub.E: T.sub.R≤T.sub.E<T.sub.R+ΔT.
[0126] The heating time (t.sub.E) is preferably in the range from 1.0 to 120 seconds, more preferably between 10 and 90 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 speed at which the steel sheet passes through a continuous annealing furnace can be adjusted in accordance with the desired heating time. For example, to set the preferred heating times (t.sub.E) in the range of 1.0 to 120 seconds, a heating rate of 10 K/s to 80 K/s can be selected. The annealing time (t.sub.G) is preferably in the range from 1.0 to 90 seconds, more 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.
[0127] In the embodiment examples shown in FIGS. 1B and 1C and FIG. 1D on the basis of schematic temperature-time profiles of the thermal treatment, a two-stage thermal treatment of the steel sheet is carried out in the annealing furnace, whereby in a first stage a nitriding and in a second stage a recrystallization annealing of the core region takes place. In the embodiment example of FIGS. 1B and 1C, the steel sheet is heated in the first stage from room temperature within a first heating time (t.sub.E.sup.1) to an intermediate temperature T.sub.Z<T.sub.R and held at least approximately at this intermediate temperature during a holding time (t.sub.H). The intermediate temperature T.sub.Z is preferably in the range from 300° C. to 600° C., particularly preferably between 500° C. and 600° C. The first heating time (t.sub.E.sup.1) is again preferably in the range from 1.0 to 120 seconds, more preferably between 10 and 90 seconds, and can be adapted according to the desired material properties of the steel sheet according to the invention, as in the first embodiment. The holding time (t.sub.H) here is also preferably in the range from 1.0 to 90 seconds, more preferably between 10 and 60 seconds, and is also selected according to the desired material properties of the steel sheet according to the invention. After the holding time has elapsed, the steel sheet leaves the annealing furnace and is either cooled passively in the environment or is actively cooled to room temperature, the cooling rates being expediently in the range from 3 K/s to 20 K/s. The steel sheet is then brought back into an annealing furnace (FIG. 1C), in particular a continuous annealing furnace, and heated in a second stage to the heating temperature (annealing temperature T.sub.E) in a second heating time (t.sub.E.sup.2) and kept at least approximately at the heating temperature T.sub.E for an annealing time (t.sub.G). During the annealing time (t.sub.G), a nitrogen donor can optionally be present in the annealing furnace. However, the heating of the steel sheet can also take place in the first stage in a first chamber of the (continuous annealing) furnace, in which a nitrogen-containing gas atmosphere with dissociated (“atomic”) nitrogen prevails, while the second stage takes place in a second chamber of the furnace, in which, e.g. a pure inert gas (such as HNx, consisting for example of 95% N.sub.2 and 5% H.sub.2) forms the furnace atmosphere without dissociated (“atomic”) nitrogen being present. According to the invention, the heating temperature (annealing temperature T.sub.E) is between the original recrystallization temperature T.sub.R and the recrystallization temperature increased to T.sub.R+ΔT by the nitriding in the fringe region of the steel sheet. Thus, the following also applies here for the heating temperature T.sub.E: T.sub.R≤T.sub.E<T.sub.R+ΔT, where the intermediate temperature (T.sub.Z) is lower than the (original) recrystallization temperature T.sub.R.
[0128] FIG. 1D shows a modified example of a two-stage thermal treatment of the steel sheet on the basis of a schematic temperature-time diagram. As in the example shown in FIGS. 1B and 1C, the steel sheet is heated in the first stage from room temperature within a first heating time (t.sub.E.sup.1) to an intermediate temperature T.sub.Z<T.sub.R and held at least approximately at this intermediate temperature during a holding time (t.sub.H) by passing the steel sheet through a continuous annealing furnace. Unlike in the embodiment example of FIGS. 1B and 1C, however, in the embodiment example of FIG. 1D there is no cooling of the steel sheet after the holding time has elapsed, but immediately in the second stage during a second heating time (t.sub.E.sup.2) there is further heating from the intermediate temperature to the heating temperature T.sub.E, which is higher than the intermediate temperature T.sub.Z and lies between the original recrystallization temperature T.sub.R and the recrystallization temperature increased to T.sub.R+ΔT by the nitriding in the fringe region of the steel sheet.
[0129] Heating up to the heating temperature T.sub.E can expediently take place within a very short second heating time (t.sub.E.sup.2) of less than 5 seconds by means of an induction heater arranged in the downstream region of the continuous annealing furnace. As in the second embodiment example of FIGS. 1B and 1C, in the third embodiment example of FIG. 1D, the heating temperature T.sub.E is: T.sub.R≤T.sub.E<T.sub.R+ΔT, where the intermediate temperature (T.sub.Z) is lower than the (original) recrystallization temperature T.sub.R. This third embodiment is characterized by improved efficiency compared to the second embodiment because cooling of the steel sheet at the end of the first stage and reheating at the beginning of the second stage from room temperature is omitted. The efficiency can be further improved if induction heating is used for heating in the second stage, with which very fast heating with high heating rates can be realized. The third embodiment can usefully be carried out in a continuous annealing furnace with two chambers arranged one behind the other, with the heating of the steel sheet in the first stage taking place in a first chamber of the continuous annealing furnace in which there is a nitrogen-containing gas atmosphere with dissociated (“atomic”) nitrogen, and the steel sheet is heated in the second stage by means of the induction heater arranged in the second chamber of the continuous annealing furnace, wherein in the second chamber either a pure inert gas (such as HNx) forms the furnace atmosphere without dissociated (“atomic”) nitrogen being present therein, or here also, further nitriding takes place in a gas atmosphere with atomic nitrogen. The induction heater is conveniently arranged in the downstream region of the second chamber.
[0130] 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. Accordingly, the heating temperature T.sub.E is preferably in the range 630° C. to the Curie temperature of about 768° C. The preferred upper limit value of the Curie temperature of 768° C. for the heating temperature T.sub.E results from apparatus-related reasons, since induction heating can only be carried out up to this limit temperature. If heating in the annealing furnace is carried out conductively or by thermal radiation, heating can also be carried out to heating temperatures T.sub.E above the Curie temperature.
[0131] When the cold-rolled steel sheet is heated in the annealing furnace, nitrogen from the nitrogen donor is initially deposited only in a fringe region of the steel sheet close to the surface, as atomic nitrogen from the nitrogen donor diffuses into the fringe region. The nitrogen diffused into the fringe region can either be interstitially deposited in the iron lattice of the steel or is bound as nitride, especially if strong nitride formers such as Al, Nb, Ti, or B are present in the steel. The incorporation of the nitrogen raises the recrystallization temperature (T.sub.R) of the steel in the fringe region by a value ΔT. This increase in the recrystallization temperature (T.sub.R) in the fringe region is shown in FIGS. 1A to 1D with ΔT.
[0132] According to the invention, the heating temperature (T.sub.E) or the annealing temperature is now selected so that T.sub.R≤T.sub.E<T.sub.R+ΔT applies. The heating temperature (T.sub.E) or the annealing temperature is thus set in the method according to the invention so that it lies between the (original) 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+ΔT) increased by the value ΔT due to the near-surface nitriding of the steel sheet in the fringe region. By setting the heating temperature (T.sub.E) (or the annealing temperature) in this way, recrystallization only takes place in a core region of the steel sheet adjoining the outer fringe region towards the inside, in which at least initially no nitrogen has (yet) been incorporated during annealing and simultaneous nitriding of the steel sheet. The heating temperature (T.sub.E) is only above the recrystallization temperature (T.sub.R) in the core region, and in the fringe region, where the recrystallization temperature has been increased by ΔT due to the incorporated nitrogen, the heating temperature (T.sub.E) is below the recrystallization temperature, which has been increased to T.sub.R+ΔT. Therefore, a three-layer microstructure in the form of a “sandwich” is formed over the cross-section of the steel sheet with an at least essentially, preferably largely completely recrystallized core region and a near-surface fringe region surrounding the core region, the fringe region not being recrystallized or at least being not completely recrystallized (which is why this three-layer microstructure is also referred to as a “sandwich microstructure”).
[0133] The microstructure resulting from the heating of the steel sheet in the presence of the nitrogen donor therefore comprises an at least essentially completely recrystallized core region 2 and a fringe region 1 surrounding the core region 2 on both sides, as shown in the schematic sectional view of a steel sheet according to the invention in FIG. 2A. The fringe region 2 is not recrystallized or at least not completely recrystallized and therefore remains in the as-rolled condition of the cold-rolled steel sheet. The respective degree of recrystallization of the core region 2 and the fringe region 2 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 fringe 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+ΔT/3 and T.sub.R+2ΔT/3. Similarly, the thickness of the fringe region 1 can be adjusted by the method parameters of the heating temperature (T.sub.E) and the heating time (t.sub.E), which will be explained in more detail below.
[0134] The method according to the invention can also be used to produce two-layer microstructures with an at least largely completely recrystallized core region 2 and an overlying, roll-hard fringe region 1 if, instead of a gaseous nitrogen donor, a liquid or solid nitrogen donor is used which is applied to only one side of the steel sheet. In this embodiment of the method according to the invention, the liquid or solid nitrogen donor is applied to one side of the steel sheet before annealing and the steel sheet thus coated on one side with the nitrogen donor is then annealed in the annealing furnace in the manner described above. The microstructure shown schematically in FIG. 2B is formed with a lower region (core region) 2 and an upper region (fringe region) 1, the lower region (core region) being recrystallized and the upper region (fringe region) remaining uncrystallized and thus roll-hard. Nitrogen-containing liquids which can be used as nitrogen donors include nitrogen compounds dissolved in water, such as guanidine or guanidine hydrochloride, urea or melamine. Aqueous solutions of these nitrogen-containing compounds can be applied, for example by means of a CO.sub.2 cartridge, as a fine spray mist on one or both sides of the surface of the steel sheet, which can then be dried before the steel sheet thus coated with a dry coating of the nitrogen-containing compound is brought into the annealing furnace for annealing. The annealing furnace can be a batch-type annealing furnace or a continuous annealing furnace, preferably with an inert gas atmosphere, e.g. 100% HNx. The nitrogen concentration of the nitrogen donor thus applied to the surface of the steel sheet can be adjusted via the concentration of the nitrogen-containing compound in the aqueous solution or via the thickness of the dried layer (dry coating).
[0135] The nitrogen donor can also be applied as a nitrogen-containing powder or granules to one or both sides of the steel sheet. Nitrides or nitrates, for example, can be used as nitrogen-containing powder or granules. Melamine resins can also be used as nitrogen donors, which can be applied to the surface of the steel sheet as a viscous mass.
[0136] 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
[0137] Examples of embodiments of the steel sheet and the method according to the invention are explained below.
[0138] Steel sheets with a thickness of 0.22±0.01 mm were produced by hot rolling and subsequent cold rolling from steel melts with the alloy compositions A, B and C listed in Table 1 shown in FIG. 20 (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 thermal treatment in a laboratory furnace with an induction heater in an inert gas atmosphere containing ammonia at different method parameters in terms of the heating temperature T.sub.E, the heating time t.sub.E, the annealing time t.sub.G, and the volume concentration of ammonia in the furnace. The atmosphere in the continuous annealing furnace was composed of the ammonia gas and, as the remainder, HNx protective gas, with the volume fraction of ammonia in the gas atmosphere of the furnace determined at room temperature and maintained constant by influx of ammonia during the thermal treatment of the steel sheet. When the experiments are carried out on a commercial scale in a continuous annealing furnace, the ammonia concentrations required for nitriding will probably shift to higher values, since at the high temperatures in a continuously heated continuous annealing furnace only part of the total ammonia atmosphere is effectively available for nitriding the steel sheet due to dissociation and recombination effects of the ammonia to atomic and molecular nitrogen.
[0139] The microstructure of the heat-treated steel sheets was examined microscopically (cold-embedded, ground, polished and etched with 3% nitric acid after Nital). After cooling, the furnace heat-treated steel sheets were subjected to a second cold rolling step (skin pass) with a reduction level of 1.5%. Table 2A shown in FIG. 21 lists, for various examples of steel sheets with alloy composition A from Table 1, the parameters of the heat treatment, such as the heating time t.sub.E, the annealing time t.sub.G, the heating temperature T.sub.E and the volume concentration of ammonia NH.sub.3 (%) in the gas atmosphere of the furnace.
[0140] FIGS. 3A and 3B show a comparison of the microstructures of heat-treated steel sheets of the examples in Table 2A, with FIG. 3A showing the example “fully recrystallized” and FIG. 3B the example “sandwich structure” according to the invention. It is clear that in the “sandwich structure” example treated in accordance with the invention, a three-layer microstructure (“sandwich structure”) has been formed with a recrystallized core region 2 and fringe regions 1 surrounding it on both sides, the fringe region 1 in each case having a thickness of approx. 65 μm. In the comparative example 2, on the other hand, the steel sheet is largely completely recrystallized over the entire thickness of the steel sheet.
[0141] The tensile strength Rm, the 0.2% proof stress (Rp0.2) and the 0.5% proof stress (Rp0.5) and the elongation at break A, the uniform elongation Ag and the hardness (according to Vickers and Rockwell) were measured on the specimens from Table 2A. The course of the hardness was also measured over the profile of the steel sheets (in the thickness direction). The properties determined on the measured specimens of the examples from Table 2A are listed in Table 2B (also shown in FIG. 21) and exemplary hardness profiles over the thickness of the steel sheets are compared in FIGS. 4A and 4B. It can be seen that the sample according to the invention (“sandwich structure”) of FIG. 4B has a clearly pronounced, inverted Gaussian profile with a hardness maximum of approx. 350 HV 0.005 at the surfaces of the steel sheet and a minimum Vickers hardness of approx. 260 HV 0.05 in the center of the steel sheet profile. In contrast, the sample of the comparative example not according to the invention in FIG. 4A (completely recrystallized) has a largely constant hardness value of approx. 100 HV 0.05 over the entire thickness of the steel sheet. It thus follows that the heat-treated steel sheet of the example of FIG. 4B according to the invention has a substantially higher hardness both in the fringe region 1 and in the core region 2 than the sample of the comparative example of FIG. 4A, as well as a clear hardness gradient with a hardness decreasing from the outside towards the core region 2. This gradient of hardness across the thickness of the steel sheet is due to the steel sheet of the example according to the invention being nitrided both in the fringe region 1 and in the core region 2 with a nitrogen content decreasing from the outside towards the center of the steel sheet, as well as to the (complete) recrystallization of the core region 2 during the thermal treatment in the annealing furnace. The hardness profile of the specimen of the example according to the invention shown in FIG. 4B and the comparison of the absolute hardness values of this specimen in the fringe and core regions with the measured hardness of the specimen of the comparative example shown in FIG. 4A show that in the specimen according to the invention there has been a nucleation (increase in nitrogen content) both in the core region 2 and in the fringe region 1 and a resulting increase in hardness and strength due to solid solution strengthening. The increase in the nitrogen content in the fringe region 1 was so high that the recrystallization temperature in the fringe region was raised by a value ΔT to a final value of T.sub.R+ΔT, which was above the heating temperature T.sub.E, with the result that no recrystallization or only incomplete recrystallization took place in the fringe region 1. The fringe region 1 of the specimen of example 1 is therefore still roll-hard and exhibits a high hardness with a hardness maximum at the surface of the steel sheet.
[0142] Strength and strain measurements were made on the specimens of the examples in Table 2A. FIG. 5 shows examples of the stress-strain diagrams of the specimens. The specimens compared in FIG. 5 are cold-rolled steel sheets with a composition according to Example A of Table 1 and were subjected to a thermal treatment in an annealing furnace after cold rolling as follows: [0143] “Sandwich structure” (sample according to the invention): one-step heating of the sample by the method according to the invention in an annealing furnace to a heating temperature T.sub.E of 750° C., with a heating time t.sub.E of 62 seconds and an annealing time t.sub.G of 45 seconds (according to the temperature-time diagram of FIGS. 1A-1D), wherein (at least) during the heating time a nitrogen donor was present in the annealing furnace; [0144] “roll-hard” (comparative example): no annealing; [0145] “completely recrystallized” (comparative example): Annealing in a standard annealing method at an annealing temperature of 750° C. without a nitrogen-containing gas atmosphere (i.e. without nitrogen donor, esp. without ammonia addition in the annealing furnace and therefore without nitrogen buildup in the annealing furnace); [0146] “completely recrystallized and nitrided” (comparative example): initially recrystallizing annealing at an annealing temperature of 750° C. without a nitrogen-containing gas atmosphere (i.e. without a nitrogen donor) in the annealing furnace, whereby the sample was first completely recrystallized and nitrided in the annealing furnace with an ammonia atmosphere, whereby the nitriding in the annealing furnace was analogous to the “sandwich structure” sample, but in the completely recrystallized state.
[0147] FIG. 5 shows that the specimen according to the invention (“sandwich structure”) has a much higher tensile strength of >1000 MPa (compared to <800 MPa of the comparative examples) at about the same elongation at break A of approx. 7.8% compared to the specimens of the comparative examples (“roll-hard”, “fully recrystallized”, “fully recrystallized and nitrided on”).
[0148] The method according to the invention can thus be used to produce (nitrided) steel sheets characterized by a very high strength of more than 800 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 cans and beverage cans as well as parts thereof such as (tear-off) lids.
[0149] The exact composition of the microstructure, in particular the thickness of the fringe region and the nitrogen content in the fringe region and in the core region generated by the nitriding method in the continuous annealing furnace, as well as the gradient of the nitrogen content across the thickness of the steel sheet can be influenced by varying the method parameters. Therefore, the properties of the steel sheets produced by the method according to the invention can be tailored to different applications.
[0150] To illustrate this, FIGS. 6A and 6B and FIGS. 7A and 7B show the microstructures of samples of steel sheets according to the invention which have each been heat treated by varying a method parameter in the single-stage method described above. FIG. 6A shows a sequence of microstructures of samples (identical in terms of composition) which have been heated to a different heating temperature T.sub.E (at a constant heating time of t.sub.E=62 seconds and a constant annealing time of t.sub.G=45 seconds) and FIG. 6B shows a sequence of microstructures of samples which have been kept at the heating temperature for different lengths of time at a constant heating temperature of T.sub.E=750° C. and a constant heating time of t.sub.E=62 seconds (i.e. have had different annealing times t.sub.G for the same heating time).
[0151] From the images of the microstructure in FIG. 6A it can be seen that a multilayer microstructure with a roll-hard fringe region 1 and a recrystallized core region 2 is formed as soon as the heating temperature of T.sub.E is higher than the recrystallization temperature T.sub.R of the steel. In the first image from the left of FIG. 6A, the heating temperature T.sub.E is still below the recrystallization temperature T.sub.R and therefore no recrystallization takes place over the entire thickness of the steel sheet. In the second picture from the left, T.sub.E>T.sub.R and recrystallization only occurs in the core region. In the fringe region 1, the (original) recrystallization temperature T.sub.R was raised by ΔT, which is why the heating temperature T.sub.E there is below the increased recrystallization temperature (T.sub.R+ΔT) and no recrystallization has occurred. The thickness of the fringe region in the second image from the left of FIG. 6A is about 67±5 μm. If the heating temperature T.sub.E is further increased up to the preferred upper limit of 768° C. (Curie temperature, up to which temperature induction heating can be used), the microstructure does not change significantly as long as the heating temperature T.sub.E is lower than the increased recrystallization temperature (T.sub.R+ΔT) in the fringe region, i.e. no recrystallization takes place, i.e. a three-layer microstructure is formed with a fringe thickness of the roll-hard, non-recrystallized fringe region of approx. 67±5 μm (images 3 to 6 of FIG. 6A).
[0152] FIG. 6B shows the influence of the annealing time on the formation of the microstructure. The composition of the three-layer microstructure is largely independent of the annealing time t.sub.G, i.e. in particular the fringe thickness of the roll-hard fringe region remains approximately the same. In the examples shown in FIG. 6B with an annealing time between 30 seconds and 90 seconds, the fring thickness is approx. 67±5 μm. However, at higher annealing times, it can be observed that nitrogen diffuses from the fringe region into the core region. As a result, on the one hand the transition between the fringe region and the core region becomes less sharp and on the other hand, due to solid solution strengthening caused by the incorporation of nitrogen in the core region, there is an increase in strength, so that at higher annealing times with t.sub.H>40 seconds a noticeable increase in strength can also be observed in the core region, as is also clear from the comparison of the hardness measurements of FIGS. 4A and 4B.
[0153] The nitrogen concentration of the nitrogen donor (ammonia content in the gas atmosphere of the annealing furnace) has a much greater influence on the microstructure. FIGS. 7A and 7B show images of the microstructure of samples of steel sheets according to the invention that have been heat treated in a single-stage method using a different volume concentration of the ammonia gas in the gas atmosphere of a laboratory furnace with induction heating (with otherwise identical parameters, namely T.sub.E=750° C., t.sub.E=62 seconds and t.sub.H=45 seconds). The volume fraction of ammonia in the gas atmosphere of the laboratory furnace has been determined at room temperature. As shown in FIG. 7A, already at an ammonia concentration of 0.5% by volume, a three-layer microstructure is obtained with a recrystallized core region and a fringe region surrounding it on both sides, the fringe thickness here being approx. 34±3 μm (FIG. 7A, top left). With increasing ammonia content, the fringe thickness initially increases linearly and then saturates at an ammonia content of approx. 2-3 vol. % at a fringe thickness of approx. 65 μm (see FIG. 8). A further increase of the ammonia content no longer leads to an expansion of the fringe region. At an ammonia content of 10% by volume, the fringe thickness of approx. 61 μm (FIG. 7A, far right) is even somewhat below the maximum value of approx. 66 μm observed at an ammonia content of 2.5% by volume.
[0154] A closer look at the micrographs with higher resolution reveals that at ammonia concentrations >2 vol. % a very thin nitride layer, in particular an iron nitride layer, has formed on both sides of the surface of the steel strip. The nitride layer can be seen in the enlarged images of the microstructures in FIG. 7B. In particular, in the images at the high ammonia contents ≥5 vol. %, it can be seen that the surface nitride layer is composed of several superimposed individual layers. The individual layers are probably different phases (γ-, γ′-, ε-phase) of the iron nitrides. The formation of the (iron) nitride layer on the surface of the steel sheet influences the fringe thickness of the fringe region. Up to the formation of a surface nitride layer, the fringe thickness increases with increasing nitrogen content of the nitrogen donor (ammonia gas) and as soon as a nitride layer is formed (from approx. 2% by volume ammonia content), the fringe region no longer extends further toward the center of the steel sheet, as can be seen in FIG. 8. Thus, in the method according to the invention, the structure of the multilayer structure and, in particular, the fringe thickness of the fringe region can be adjusted via the nitrogen content of the nitrogen donor. As soon as a surface nitride layer forms, it continues to grow in thickness as the ammonia content increases. However, the fringe region no longer increases in size, i.e. the fringe region is limited to a maximum thickness of approx. 65 μm, which is already reached at an ammonia content of about 2% by volume.
[0155] The undesirable surface layer of (iron) nitride is conveniently removed by chemical analysis before the composition of the steel sheets is determined (in accordance with the normative specification in Clause 4.4.1. of DIN EN ISO 14284). The alloy composition of the tested specimens listed in Table 1 was determined in a corresponding manner after removal of the nitride surface layer in order to avoid falsification of the measured nitrogen content of the specimen.
[0156] FIG. 9 shows hardness profiles of specimens with the alloy composition according to Example A of Table 1, where the specimens have been thermally treated after cold rolling like the corresponding examples of FIG. 5 (see page 32, last paragraph).
[0157] FIG. 9 shows the course of the microhardness over the sheet thickness. It has been shown that there is a correlation between the microhardness and the nitrogen content for all nitrided specimens. In the non-nitrided condition, the sample with the alloy composition according to Example A of Table 1 has a hardness around 250 HV in the as-rolled condition and around 100 HV in the fully recrystallized condition. The hardness increases in both textures due to the nitriding, as can be seen from the hardness curves of the samples “sandwich structure” (sample according to the invention) and “completely recrystallized and nitrided”.
[0158] From a comparison of the sample according to the invention (“sandwich structure”) with the sample “completely recrystallized and nitrided”, it can be seen from FIG. 9 that the course of the microhardness over the sample cross-section (thickness of the sample) differs. In particular, it can be seen that the sample according to the invention (“sandwich structure”) has a higher microhardness over the entire sample cross section. In particular, in the core region of the specimen, the microhardness is significantly higher compared to the specimen “completely recrystallized and nitrided”, with the difference in microhardness of these two specimens being even greater in the core region than in the outer edge areas (fringe region). From this it can be seen that in the specimen according to the invention (“sandwich structure”) not only the fringe region but also the core region has been nitrided, because an increase in hardness can be observed in the center of the specimen due to the nitrogen apparently also introduced in the core region. In contrast, in the “completely recrystallized and nitrided” sample (which has undergone the same nitriding cycle as the “sandwich structure” sample, but in the already completely recrystallized state), only a slight increase in microhardness is observed in the core region compared to the “completely recrystallized” sample, which suggests that in the “completely recrystallized and nitrided” sample, hardly any nitrogen has been incorporated during nitriding in the core region of this sample. It can be seen from this that the diffusion of nitrogen during nitriding in the annealing furnace is strongly dependent on the microstructure of the steel, and that a non-recrystallized microstructure leads to faster diffusion of nitrogen into the core region of a steel sheet. In the case of a (completely or partially) recrystallized microstructure, on the other hand, the diffusion method is slowed down, which means that in the case of an already recrystallized steel sheet, nitrogen is essentially only incorporated in the outer edge areas when it is nitrided in the annealing furnace.
[0159] In the method according to the invention, therefore, the diffusion of nitrogen into the steel sheet during nitriding in the annealing furnace is much more efficient than in the case of fully recrystallized steel sheets due to nitriding in the roll-hard region. In particular, the method according to the invention can be used to produce steel sheets with a three-layer microstructure (“sandwich structure”) which have an increased nitrogen concentration both in the core region and in the outer edge areas (fringe region) and thus a significantly increased overall microhardness and tensile strength.
[0160] In the method according to the invention, the nitrogen content of the nitrogen donor can therefore also be used to control the average total hardness or tensile strength of the steel sheet as well as the hardness profile over the cross-section of the steel sheet. By comparing hardness profiles of samples of steel sheets according to the invention which have been nitrided in the annealing furnace with different ammonia contents, as shown in FIG. 15, it can be seen that the average hardness averaged over the thickness of the steel sheet increases with increasing nitrogen content (ammonia content) of the nitrogen donor. In the case of the sample nitrided according to the invention (“sandwich structure”), a hardness profile is formed in each case—irrespective of the ammonia content—over the thickness of the steel sheet, with a maximum hardness at the surface and a minimum hardness value in the center, as can be seen from FIG. 9 for the “sandwich structure” sample. At a low ammonia content of, for example, 0.5% by volume, the hardness in the center of the steel sheet, i.e. in the middle of the core region, is approximately the same as that of a completely recrystallized, non-nitrided sample (comparative example “completely recrystallized” from FIG. 9). This shows that at a low ammonia content of 0.5% by volume, noticeable nitriding occurs only in the surface-near fringe region, but not in the core region. At a higher ammonia content, in particular 1% by volume or more, nitrogen is incorporated both in the fringe region and in the core region, with the result that the core region is also hardened by solid solution strengthening. The ratio of the average hardness values in the fringe region and in the core region is thereby about 3:1 or more for the low ammonia contents (of <1 vol. %) and about 1.3 or less for the higher ammonia contents (of ≥5 vol. %). A sharp demarcation of the core region from the fringe region can be achieved if the ratio of the average hardness values in the fringe region and in the core region is 1.2 or more. Preferably, the ratio of the average hardness values in the fringe region and in the core region is 1.4 or more.
[0161] The hardness profiles of FIG. 9 also show that the average nitrogen content of the specimen according to the invention generated by the nitriding method in the annealing furnace is significantly higher in the fringe region (N.sub.S) than in the core region. It can be assumed that the increase in hardness is due solely to the solidification of the mixed crystal and thus to the increased nitrogen content caused by the nitriding. Based on this, it can be concluded from the hardness profiles of FIG. 9 that the ratio of the average nitrogen content of the fringe region (N.sub.S) to the average nitrogen content of the core region (N.sub.K) is also about 1.3 for the higher ammonia contents (of ≥5 vol. %) and about 3:1 or more for the low ammonia contents (of <1 vol. %). This could be confirmed by measurements of the profile of the nitrogen content of samples of steel sheets according to the invention under successive pickling of the layer near the surface. A clearly pronounced gradient of the nitrogen content over the thickness of the steel sheet was found, with nitrogen contents based on weight of approx. 900 ppm at the surface of the steel sheet (in which case the surface iron nitride layer was removed before measuring the nitrogen content) and nitrogen contents of approx. 550 ppm in the core region (FIG. 12). The nitrogen concentration in the nitrogen donor can thus be used to adjust the degree of nitriding in the fringe region and in the core region.
[0162] The thickness of the fringe region can be controlled by the heating time t.sub.E. This is clearly shown in the diagram in FIG. 10, which shows the dependence of the fringe thickness on the heating time t.sub.E at two different ammonia contents (1 vol. % and 5 vol. %). In both cases, an approximately linear increase of the fringe thickness with increasing heating time can be observed. The straight lines of the linear curve have approximately the same slope at different ammonia contents and are only shifted by approx. 10 μm in the absolute value of the fringe thickness, as can be seen in FIG. 10. As a result, the composition of the microstructure and in particular the thickness of the outer, roll-hard fringe region can be adjusted in a defined manner via the method parameter of the heating time. The heating time method parameter can be very easily controlled by the heating rate or the speed at which the steel sheet is passed through the continuous annealing furnace, so that a specific and precise setting of the desired fringe thickness can be achieved. The fringe thickness, in turn, can be used to set the average hardness of the steel sheet (averaged over the entire thickness) or its tensile strength as well as its formability (via the thickness of the core region and its average nitrogen content).
[0163] FIG. 11 shows strength-elongation curves of specimens nitrided according to the invention with composition A of Table 1. The specimens were heated to a heating temperature of t.sub.E=750° C. with different heating times t.sub.E and kept at the heating temperature with the same annealing time of t.sub.G=45 seconds in a 5% ammonia atmosphere. The material properties obtained in this way are shown in Table 3A (FIG. 22) and graphically in FIG. 11. It can be seen that by increasing the heating time, the strength Rm can be increased, but that the elongation at break A is (slightly) reduced. By varying the heating time t.sub.E, it is therefore possible to set a value for tensile strength and elongation at break that is optimized for the particular application, or the working capacity for cold forming that results from this as the product of tensile strength and elongation at break. With the steel sheets according to the invention, values for the working capacity W=Rm (in MPa) A (in %) of more than 5000 MPa % can be achieved.
[0164] FIG. 12 shows the gradient of the nitrogen concentration over the thickness of a steel sheet (with a composition according to example A from Table 1) is shown. In order to record the nitrogen profile over the cross-section of the steel sheet resulting from nitriding at 750° C. in a 5% ammonia atmosphere with a heating time of 62 s and an annealing time of 45 s, the sample was pickled in hydrochloric acid in stages and the nitrogen content was measured over the entire remaining sheet thickness by means of hot carrier gas extraction.
[0165] The diagram in FIG. 13 shows the influence of the nitrogen content of steel sheets with different nitrogen contents on the hardness and recrystallization temperature by means of hardness diagrams as a function of the heating temperature during recrystallization annealing. The diagram in FIG. 13 shows the course of hardness as a function of heating temperature (annealing temperature) for three different specimens with different nitrogen contents but otherwise the same alloy composition (according to Example “A” from Table 1). In this case, the sample labeled “untreated” was not nitrided in the annealing furnace (i.e., no nitrogen donor was present during annealing in the annealing furnace, and the steel of this sample had a nitrogen content of 25±5 ppm by weight), whereas the samples labeled “71.9 ppm N2” and “107 ppm N2” were nitrided in the annealing furnace at about 550° C. to a total nitrogen content of 71.9 ppm and 107 ppm, respectively. For the “71.9 ppm N2” sample, the nitrogen was heated to 550° C. within 1 s under 1% ammonia atmosphere in the annealing furnace (batch-type furnace process) and then the nitrogen was uniformly distributed over the cross section (under argon atmosphere for 5.5 h heating to 550° C., 5 h holding and 36 h cooling). Sample “107 ppm N2” was generated similarly to sample sample “71.9 ppm N2”, but with 10 s of nitriding time. Recrystallization curves were then plotted on the nitrided samples. This shows that the nitrogen introduced during the nitriding method increased the recrystallization temperature, whereby, for example, an increase in the recrystallization temperature of approx. 20° C. was observed with a nitriding of 17 ppm nitrogen (based on the weight).
[0166] FIG. 14 shows the dependence of the hardness (Vickers HV hardness.sub.0.025, where the hardness was measured in the as-rolled condition of the specimens) and the recrystallization temperature on the nitrogen content of the specimens from FIG. 13, which were nitrided at different levels. A linear relationship can be seen for both the microhardness and the recrystallization temperature, i.e. the hardness and the recrystallization temperature increase linearly with increasing nitrogen content of the nitrided specimens.
[0167] Cold-rolled steel sheets with composition A according to Table 1 were thermally treated in tests with the parameters given in Table 4 shown in FIG. 23 in a continuous annealing furnace according to the invention. FIGS. 16 to 18 show the results of these tests. FIG. 16 shows a microscopic cross-sectional image of the three-layer microstructure, FIG. 17 shows the microhardness profile over the cross-section, and FIG. 18 shows strength-elongation diagrams of samples of the steel sheets which were skin-passed with different degrees of skin-passing (DG) after thermal treatment in the annealing furnace.
[0168] Furthermore, texture measurements were carried out on the steel sheets produced according to the invention. FIG. 19 shows the orientation density distribution function f(g) along the ε-fiber of a steel sheet according to the invention with a three-layer microstructure (sandwich structure). The ε-fiber is defined by an orientation of the <110> vector parallel to the transverse direction (QR). From this diagram, a deformation index VI can be determined for the individual regions of the three-layer microstructure from the ratio of the orientation density distribution f(g) as follows:
Deformation index VI={001}-orientation/{111}-orientation
[0169] The deformation index VI is a measure of the forming behavior of the steel sheet, because a {111} orientation allows good formability and exhibits a good Lankford coefficient (r value), whereas the {001} orientation is less formable. As can be seen from the diagram in FIG. 19, the individual regions (layers) of the three-layer microstructure (i.e. the recrystallized core region and the two outer, non-recrystallized fringe regions) differ from one another in terms of the orientation density gradient in the sheet plane and thus in their texture. In the example shown in FIG. 19, the deformation index in the recrystallized core region is VI=0.46 and in the two outer fringe regions the deformation index is VI=3.09 and VI=4.31, respectively.
[0170] In the steel sheets according to the invention, in each case the recrystallized first layer (which, in the case of a three-layer sandwich structure, lies on the inside and represents the core region) generally has a deformation index of less than 0.8 and the second layer (which, in the case of a three-layer sandwich structure, lies on the outside in each case and represents the fringe region) has a deformation index of more than 2.0 and, in particular, in the range from 2.0 to 5.0.
