METHOD FOR PRODUCTION OF A NITRIDED PACKAGING STEEL

Abstract

A method for producing a nitrided packaging steel from a hot-rolled steel product with a carbon content of 400 to 1200 ppm, utilizing a cold-rolling of the steel product to a flat steel product, subsequent recrystallization annealing of the cold-rolled flat steel product in an annealing furnace, in particular a continuous annealing furnace. A nitrogen-containing gas is supplied into the annealing furnace and is directed at the flat steel product to introduce unbonded nitrogen into the flat steel product in an amount corresponding to a concentration of more than 100 ppm, or to increase the amount of unbonded nitrogen in the flat steel product to a concentration of more than 100 ppm, and subsequent cooling of the recrystallized annealed flat steel product at a cooling rate of at least 100 K/s directly after the recrystallization annealing. Using this method, cold-rolled flat steel products may be produced for packaging purposes with a tensile strength of more than 650 MPa and in particular between 700 and 850 MPa.

Claims

1-19. (canceled)

20. A method for production of a nitrided packaging steel from hot-rolled steel product with a carbon content from 400 to 1200 ppm, the method comprising: cold rolling of the steel product to a flat steel product; recrystallization annealing of the cold-rolled flat steel product in an annealing furnace, especially a continuous annealing furnace, in which a nitrogen-containing gas is introduced into the annealing furnace and directed at the flat steel product in order to introduce unbonded nitrogen into the flat steel product in an amount corresponding to a concentration of more than 100 ppm, or to increase the amount of unhanded nitrogen in the flat steel product to a concentration of more than 150 ppm; cooling of the recrystallized annealed flat steel product at a cooling rate of at least 100 K/s immediately after recrystallization annealing.

21. The method according to claim 20, wherein the steel product is a hot-rolled steel nitrided to a nitrogen content of maximum 160 ppm by nitriding of a steel melt.

22. The method according to claim 21, wherein the nitriding of the steel melt occurs by introducing a nitrogen-containing gas and/or a nitrogen-containing solid into the steel melt, especially by introducing nitrogen gas (N.sub.2) and/or calcium cyanamide (CaCN.sub.2) and/or manganese nitride (MnN) into the steel melt.

23. The method according to claim 1, wherein during recrystallization annealing of the cold-rolled flat steel product, ammonia gas (NH.sub.3) is introduced into the annealing furnace and preferably directed onto the flat steel product by at least one spray nozzle.

24. The method according to claim 23, wherein an ammonia equilibrium with a concentration of less than 15 wt % and preferably in the range of 0.05 to 1.5 wt % is set in the annealing furnace by introducing ammonia gas (NH.sub.3).

25. The method according to claim 19, wherein the ammonia equilibrium concentration set in the annealing furnace by introducing ammonia gas (NH.sub.3) is detected with an ammonia sensor and the detected measured value of ammonia equilibrium concentration is used to control the amount of ammonia gas introduced into the annealing furnace per unit time.

26. The method according to claim 20, wherein the weight amount of unbonded nitrogen after nitriding of the cold-rolled flat steel product in the annealing furnace is at least 210 ppm, and especially between 210 and 350 ppm.

27. The method according to claim 20, wherein the concentration distribution of unbonded nitrogen in the nitrided flat steel product over the thickness of the flat steel product deviates by less than ±10 ppm about the value of mean concentration (weight amount) of introduced nitrogen, in which the mean concentration (weight amount) of unbonded nitrogen is more than 150 ppm, and especially between 210 and 350 ppm.

28. The method according to claim 20, wherein the recrystallization annealing of the cold-rolled flat steel product occurs by passing the flat steel product through a continuous annealing furnace in which the flat steel product is heated at least briefly to temperatures above the Ac1 temperature.

29. The method according to claim 20, wherein the cold-rolled flat steel product in the annealing furnace is initially heated in a first heating step to a temperature (T.sub.h) below the Ac1 temperature and especially in the range of 600 to 650° C., and in a subsequent holding step is held at this holding temperature (T.sub.h) in order to expose the cold-rolled flat steel product to nitriding with the nitrogen-containing gas at the holding temperature (T.sub.h).

30. The method according to claim 20, wherein the cold-rolled flat steel product in the annealing furnace is initially heated in a heating step to a holding temperature (T.sub.h) above the Ac1 temperature and especially in the range of 740 to 760° C., and held in a subsequent holding step, at this holding temperature (T.sub.h), in which the cold-rolled flat steel product is exposed to nitriding with the above-mentioned nitrogen-containing gas during the heating step and/or during the holding step.

31. The method according to claim 29, wherein the cold-rolled flat steel product immediately after nitriding in the annealing furnace is heated in the second heating step to an annealing temperature (T.sub.g) above the Ac1 temperature and especially between 740° C.≦T.sub.g≦760° C., and then cooled at a cooling rate of more than 100 K/s.

32. The method according to claim 29, wherein the cold-rolled flat steel product in the (first) heating step is heated from ambient temperature to the holding temperature (T.sub.h) at a heating rate of 15 to 25 K/s and especially 20 K/s.

33. The method according to claim 31, wherein the cold-rolled flat steel product in the second heating step is heated from the holding temperature (T.sub.h) to the annealing temperature (T.sub.g) at a heating rate of more than 100 K/s, wherein the heating in the second heating step preferably occurs at a heating rate of more than 150 K/s, and especially inductively at a heating rate between 500 K/s and 1500 K/s.

34. The method according to claim 20, wherein the steel of the hot-rolled steel product contains less than 0.4 wt % manganese, less than 0.04 wt % silicon, less than 0.1 wt % aluminum, and less than 0.1 wt % chromium.

35. A Cold-rolled flat steel product for use as packaging steel and especially produced with the method according to claim 20, with a carbon content from 0.04 to 0.12 wt %, a weight fraction of unbonded nitrogen of more than 0.01 wt %, and preferably more than 0.021 wt %, as well as the following upper limits for the weight fraction of alloy components; Mn: max. 0.4%, Si: max. 0.04%, preferably less than 0.02%; Al: max. 0.1%, preferably less than 0.08%; Cr: max. 0.1%, preferably less than 0.08%; P: max. 0.03%, preferably less than 0.02%; Cu: max. 0.1%, preferably less than 0.08%; Ni: max. 0.15%, preferably less than 0.08%; Sn: max. 0.04%, preferably less than 0.02%; As: max. 0.02%, S: max. 0.03%, preferably less than 0.02%; Mo: max. 0.05%, preferably less than 0.02%; V: max. 0.04%; Ti: max. 0.05%, preferably less than 0.02%; Nb: max. 0.05%, preferably less than 0.02%; B: max. 0.005%; other alloy components, including contaminant: max. 0.05%, which has a multiphase structure that includes ferrite and at least one of the structure components martensite, bainite and/or troostite as well as optionally residual austenite.

36. The flat steel product according to claim 35, wherein the tensile strength is more than 650 MPa and especially between 700 and 850 MPa, and the elongation at break is more than 5% and especially between 7 and 15%.

37. A device for recrystallization annealing and for nitriding of a flat steel product, especially a steel strip, the device comprising an annealing furnace, through which the flat steel product is passed in a strip running direction, wherein the annealing furnace comprises: a heating zone for heating of the flat steel product, a holding zone, in which the heated flat steel product is held at a holding temperature (T.sub.h), a gassing zone with at least one nozzle to expose the flat steel product to a nitrogen-containing gas, and a cooling device for cooling of the annealed and nitrided flat steel product is arranged downstream the holding zone.

38. The device according to claim 18, wherein the flat steel product, is heated at least in an upstream area of the heating zone by thermal radiation with a heating rate of up to 25 K/s, and wherein a downstream area of the heating zone or in the holding zone, an induction or conduction heating device is arranged, with which the flat steel product can be heated at a heating rate of more than 150 K/s.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] These and additional advantages of the packaging steel produced according to the invention are apparent from the following embodiment example described with reference to the accompanying drawings. The drawings show:

[0027] FIG. 1: schematic depiction of a first embodiment example of an annealing furnace, in which nitriding and recrystallization annealing of a flat steel product is conducted according to the method of the invention;

[0028] FIG. 2: schematic depiction of a second embodiment example of an annealing furnace, in which nitriding and recrystallization annealing of a flat steel product is conducted according to the method of the invention;

[0029] FIGS. 3a to 3c: graphic depiction of the temporal temperature profile of the annealing cycles conducted in the annealing furnace of FIG. 1 during performant of the method of the invention.

DETAILED DESCRIPTION

[0030] In a first embodiment example of the method according to the invention, a steel product produced and hot rolled in continuous casting with a thickness in the range of 2 to 15 mm thickness and a carbon content of 400 to 1200 ppm is used as starting product. The alloy composition of the steel then expediently fulfills the limit values stipulated by standards for packaging steel (as defined, for example, in ASTM standard A623-11 “Standard Specification for Tin Mill Products” or in European standard EN 10202). The steel product used as starting product preferably has the following upper limits for the weight fraction of alloy components (in order to make the end product consistent with the cited standards for packaging steel): [0031] C: max. 0.12%, [0032] Mn: max. 0.4%, [0033] Si: max. 0.04%, preferably less than 0.02%; [0034] Al: max. 0.1%, preferably less than 0.08%; [0035] Cr: max. 0.1%, preferably less than 0.08%; [0036] P: max. 0.03%, preferably less than 0.02%; [0037] Cu: max. 0.1%, preferably less than 0.08%; [0038] Ni: max. 0.15%, preferably less than 0.08%; [0039] Sn: max. 0.04%, preferably less than 0.02%; [0040] As: max. 0.02%, [0041] S: max. 0.03%, preferably less than 0.02%; [0042] Mo: max. 0.05%, preferably less than 0.02%; [0043] V: max. 0.04%; [0044] Ti: max. 0.05%, preferably less than 0.02%; [0045] Nb: max. 0.05%, preferably less than 0.02%; [0046] B: max. 0.005%; [0047] other alloy components, including impurities: max. 0.05%; [0048] remainder iron.

[0049] A steel product with such an alloy composition is cold rolled in the method according to the invention initially with a thickness reduction from 50% to 96% to a final thickness in the fine or ultrafine sheet range (about 0.1 to 0.5 mm) to a flat steel product (steel sheet or steel strip). The steel product is expediently rolled to a steel strip and wound as a coil. According to the invention, the flat steel product is then introduced into an annealing furnace for recrystallization annealing, on the one hand, and nitriding, on the other, to nitrogen concentrations of more than 100 ppm, and preferably more than 210 ppm. Immediately after recrystallization annealing, the flat steel product is cooled according to the invention at a cooling rate of at least 100 K/s expediently to room temperature.

[0050] To restore the crystal structure of the steel destroyed during cold rolling of the steel product, the cold-rolled steel strip must be recrystallization annealed. This occurs according to the invention by passing the cold-rolled steel strip through an annealing furnace, which is expediently designed as a continuous annealing furnace through which the flat steel product expediently present as a steel strip is passed with a speed of more than 200 m/min. The flat steel product subsequently referred to as steel strip is heated in the annealing furnace to temperatures above the recrystallization point of steel and at least briefly above the Ac1 temperature. Nitriding of the steel strip occurs in the method according to the invention simultaneously with recrystallization annealing. This is also conducted in the annealing furnace by introducing a nitrogen-containing gas, preferably ammonia (NH.sub.3), to the annealing furnace and passing it to the surface of the steel strip. In order to create optimal (temperature) conditions in the annealing furnace for both recrystallization annealing and nitriding, the steel strip is subjected in the annealing furnace to an annealing cycle (annealing process with a temperature trend of the steel strip alternating with time). Preferred annealing cycles are explained below with reference to FIG. 3:

[0051] A first embodiment example for an appropriate annealing cycle in the form of the time profile of the temperature (T) of the steel strip passed through the annealing furnace is shown in FIG. 3(a). The steel strip is heated in this annealing cycle in the annealing furnace initially in a first heating step I from room temperature to a holding temperature (T.sub.h). Heating of the steel strip in the first heating step then occurs by thermal radiation in the heated annealing furnace with a (comparatively limited) heating rate of 15 to 25 K/s, and especially about 20 K/s. The holding temperature (T.sub.h) then expediently lies just above the recrystallization temperature of steel and preferably in the range of 600 to 650° C. The temperature of the steel strip is held in a holding step II following the first heating step I after the holding temperature (T.sub.h) for a first holding time (t.sub.h1) of about 60 to 200 seconds, and preferably about 80 to 150 seconds, especially about 100 to 110 seconds. During the holding step II, the steel strip is exposed in a nitriding phase A, preferably over the entire holding time (t.sub.h1), to a nitrogen-containing gas in order to nitride the steel strip (i.e., to increase the concentration of unbonded nitrogen in the steel to values above 100 ppm and preferably more than 210 ppm). After completion of the first holding time (t.sub.h1), the steel strip is heated in a second (short) heating step III very quickly (preferably within a short heating time between 0.1 s and 10) at a heating rate of more than 100 K/s and preferably more than 500 K/s to a temperature above the Ac1 temperature of steel, especially to temperatures in the range of 725 to 800° C. The second heating step III is then conducted in the annealing furnace for inductive or conductive heating of the steel strip. For this purpose, induction or conduction heating is arranged within the annealing furnace through which the steel strip is passed in the second heating step III. A short second holding step IV follows the second heating step III, in which the steel strip is passed through induction or conduction heating and its temperature is held for a (short) second holding time (t.sub.h2), which lies in the range of a few seconds (between 0.1 and 10 seconds and especially about 2 seconds), above the Ac1 temperature.

[0052] After annealing, the steel strip is removed from the annealing furnace and cooled in a cooling step V outside the annealing furnace at a cooling rate of at least 100 K/s to room temperature. The cooling step V is then immediately followed by the (short) second holding time (t.sub.h2). Cooling can then occur by means of a cold gas stream, which is directed at the surface of the steel strip, or by introducing the steel strip into a cooling liquid, for example, into a water bath. When a cooling liquid is used, higher cooling rates in the range much greater than 1000 K/s can be achieved. Quenching of the steel strip with a cooling liquid, however, is more demanding in terms of equipment.

[0053] The at least brief heating of the flat steel product at temperatures above the Ac1 temperature (about 723° C.) ensures the entry of the steel during the annealing cycle of FIG. 3(a) into the two-phase region (alpha- and gamma-iron), which again permits formation of a multiphase structure during subsequent (rapid) cooling. At the same time, nitriding can be conducted in this annealing cycle during the first holding step II at much lower temperatures (in the range of 600 to 650° C.), which makes possible higher efficiency of nitriding and lower consumption of nitrogen-containing gas.

[0054] Two additional embodiment examples for appropriate annealing cycle are shown in FIGS. 3(b) and 3(c). In these annealing cycles, the steep strip is heated in the annealing furnace initially in a (single) heating step I from room temperature to a holding temperature (T.sub.h), in which the holding temperature (T.sub.h)—unlike in the first embodiment example of FIG. 3(a)—then lies above the Ac1 temperature of the steel and preferably is in the range 740 to 800° C. Heating of the steel strip in the (single) step I then occurs again by thermal radiation in the heated annealing furnace with a (comparatively limited) heating rate from 15 to 25 K/s and especially about 20 K/s. The temperature of the steel strip is held in a holding step II following the (single) heating step I at the holding temperature (T.sub.h>Ac1) for a holding time (t.sub.h1) from about 60 to 200 seconds and preferably 80 to 150 seconds. The steel strip—as in the embodiment example of FIG. 3(a)—is then cooled in the annealing furnace in a cooling step V at a cooling rate of at least 100 K/s to room temperature. The cooling step V then immediately follows the holding step II.

[0055] The steel strip is already exposed in the annealing cycle according to FIG. 3(b) to a nitrogen-containing gas during the (single) heating step I. The nitriding phase A in this annealing cycle therefore coincides with heating of the steel strip in the (single) heating step I. The steel strip is preferably exposed at least essentially over the entire heating time of the heating step I to a nitrogen-containing gas in order to nitride the steel strip.

[0056] In contrast to this, the steel strip in the annealing cycle according to FIG. 3(c) is only exposed to a nitrogen-containing gas during the holding step II. The nitriding phase A in this annealing cycle therefore coincides with the holding step II.

[0057] It has been shown that it is advantageous to hold the steel strip following gassing with a nitrogen-containing gas (for example, NH.sub.3 treatment) over a holding time of preferably more than 5 seconds at temperatures above 600° C. before it is cooled. Homogenization of nitrogen distribution over the cross section of the steel strip thereby occurs and improved deformation properties of the steel strip. In particular, a reduction in elongation during bake hardening can also be avoided. The annealing cycles according to FIGS. 3(a) and 3(b) are therefore preferred, and in the annealing cycle according to FIG. 3(c) the nitriding phase is expediently ended before the end of the holding time.

[0058] Two embodiment examples of a continuous annealing furnace for performing recrystallization annealing and nitriding are schematically shown in FIGS. 1 and 2, which differ from each other (only) by the design of the cooling device. Different zones are formed in the continuous annealing furnace between an inlet E and an outlet A, which are arranged one behind another in the passage direction (strip passage direction v in FIG. 1 from right to left) of the steel strip S passed through the continuous annealing furnace. The continuous steel strip S is brought to different temperatures in the individual zones in order to pass through the annealing cycles described above.

[0059] In the embodiment examples of FIGS. 1 and 2, a heating zone 1 follows the inlet E of the continuous annealing furnace, in which the steel strip S is heated at least in the front region (only) by thermal radiation at a heating rate of up to 25 K/s and, depending on the selected annealing cycle, to temperatures just below or just above the Ac1 temperature and especially in the range of 600 to 800° C. Heating zone 1 is followed by a holding zone 2, in which the temperature of the steel strip S is held at the holding temperature (T.sub.h), which lies below or above the Ac1 temperature, depending on the selected annealing cycle.

[0060] A gassing zone 4 is formed in holding zone 2, in which a continuous steel strip is exposed to a nitrogen-containing gas. The gassing zone 4 has several cascades 3 of spray nozzles, which are arranged one behind another in the moving direction of the strip. In the embodiment example of FIG. 1, the spray nozzle cascades 3 are arranged (only) in the area of the holding zone 2. However, they can also be arranged in the area of heating zone 1, so that gassing zone 4 extends either only in heating zone 1 or over heating zone 1 and holding zone 2. For performing the annealing cycle according to FIG. 3(b) the spray nozzle cascades 3 are expediently arranged in the heating zone 1. For performance of the annealing cycle according to FIGS. 3(a) and 3(c), the spray nozzle cascades 3 are expediently arranged in holding zone 2. For performance of the annealing cycle according to FIG. 3(a), induction or conduction heating 5 is additionally arranged in the downstream area of holding zone 2.

[0061] Each spray nozzle cascade 3 then includes a number of nozzles that are arranged at a spacing relative to each other across the running direction of the strip. The nozzles are connected to a gas supply line, via which they are exposed to a nitrogen-containing gas. Ammonia gas has proven to be particularly suitable for the nitriding of steel strips. This is applied via the nozzles of cascades 3 onto the surface of the continuous steel strip S, where it penetrates the near-surface region of the steel strip and diffuses uniformly into the steel strip at the high temperatures of the annealing furnace. A uniformly homogeneous nitrogen distribution is therefore formed over the thickness of the steel strip, whose concentration distribution over the sheet thickness in steel sheets with a thickness of less than 0.4 mm deviates by at most ±10 ppm and regularly by only ±5 ppm about the mean value.

[0062] The design of the preferred nozzles of the spray nozzle cascades 3 is described in German patent application DE 102014106135 of Apr. 30, 2014, whose disclosure content is hereby incorporated by reference. A nozzle device for treatment of a flat steel product is described in this patent application, in which the nozzle device includes an outer tube and inner tube arranged therein with a primary opening to supply a gas flowing through the nozzle device into the outer tube, and the outer tube is provided with a secondary opening through which the gas can emerge. The primary opening of the inner tube and the secondary opening of the outer tube are then arranged offset relative to each other. A very homogeneous gas flow onto the surface of the flat steel product is thereby made possible. When this type of nozzle device is used in the method according to the invention, homogeneous gassing of the surface of the steel strip in a continuous annealing furnace can be achieved with the nitrogen-containing gas (for example, ammonia), so that homogeneous diffusion of nitrogen into the steel sheet can be achieved over the surface of the steel strip, especially over its width, and the nitrogen is added there interstitially.

[0063] The method of direct exposure of the steel strip (gassing) to a nitrogen-containing gas by means of nozzles then has two essential advantages: in the first place, only a limited nitrogen concentration (NH.sub.3 concentration) is required, which leads to limited consumption of nitrogen-containing gas (for example, NH.sub.3 consumption). In the second place, formation of a nitride layer does not occur because of a very short exposure time.

[0064] In order to also ensure the most homogeneous formation possible of a nitrogen-enriched surface layer of the length of the steel strip S, during passage of the steel strip S through the gassing zone 4 of the continuous annealing furnace, a nitrogen-containing atmosphere must be maintained with the most uniform nitrogen equilibrium concentration possible. In order to ensure this, the nitrogen concentration formed in the area of the spray nozzle cascades 3 is recorded. When ammonia is used as nitrogen-containing gas, the ammonia concentration generated by the introduction of ammonia is measured for this purpose in the gassing zone 4. For this purpose, a concentration sensor arranged outside the continuous annealing furnace is provided, which can be a laser spectroscopy sensor. A gas sample taken from the gassing zone 4 is fed to it in order to determine the ammonia concentration and the nitrogen concentration of the gas atmosphere in the gassing zone. The concentration of nitrogen in the gas atmosphere of the gassing zone 4 recorded by the concentration sensor is fed to a control device and used by it in order to keep the amount of nitrogen-containing gas (ammonia) sprayed into the gassing zone 4 via the nozzles of cascades 3 constant at a stipulated target value.

[0065] Target values for the equilibrium concentration of ammonia in the range of 0.05 to 1.5% and preferably below 1%, especially below 0.2%, have proven particularly expedient when ammonia is used as nitrogen-containing gas. The equilibrium concentration of ammonia preferably lies in the range of 0.1 to 1.0% and especially between 0.1 and 0.2%. These low ammonia or nitrogen concentrations in the annealing furnace are sufficient in order to introduce the desired amounts of nitrogen into the flat steel product during nitriding at temperatures in the range of 600 to 650° C. At higher temperatures, especially above 700° C., as occur during nitriding in the annealing cycle of FIG. 3(c), higher ammonia concentrations must then be generated in the annealing furnace of up to 15 wt % in order to achieve the desired amount of nitriding.

[0066] To avoid oxidation processes on the surface of the steel strip S, an inert gas is introduced into the annealing furnace in the gassing zone 4, in addition to the nitrogen-containing gas (ammonia). This can be nitrogen gas or/or [sic; and/or] hydrogen gas. A mixture of about 95% nitrogen and about 5% hydrogen gas is preferably used.

[0067] A first cooling zone is provided in the strip running direction v after holding zone 2 (and optionally after the induction or conduction heating 5 arranged at the end of holding zone 2), in which the continuous steel strip S is rapidly cooled at a cooling rate of at least 100 K/s. The first cooling zone 6 contains a cooling device 7, which is designed in the embodiment example of FIG. 1 as a gas cooling device 7a, in which the steel strip is exposed to a cold gas stream, especially an inert gas. In the embodiment example of FIG. 2, the cooling device 7 is designed as a liquid cooling device, in which the steel strip is quenched by introduction into a cooling liquid, especially a water bath 7b. Cooling rates in the range of 100 to 1000 K/s can be achieved with the gas cooling device. Much higher cooling rates (far above 1000 K/s) are attainable with the liquid cooling device. In the cooling device 7, which is arranged in the inert atmosphere of the annealing furnace, the steel strip S is initially rapidly cooled to a temperature that is above room temperature, for example, to temperature of around 100° C. A second cooling zone 8 expediently follows the first cooling zone 6 downstream, in which the continuous steel strip S is finally slowly cooled to room temperature (23° C.) at a cooling rate in the range of 10 to 20 K/s. The steel strip S is then removed from the annealing furnace at outlet A. The steel strip S between inlet E and outlet A is continuously found in the inert atmosphere of the annealing furnace, so that oxidation processes cannot occur on the surface of steel strip S during recrystallization annealing, nitriding and cooling.

[0068] After cooling, the steel strip S, if necessary, can be temper-rolled dry (dressed) in order to impart to the strip the deformation properties required for production of packaging. The degree of temper rolling then varies between 0.4 and 2%, depending on the application of the packaging steel. If necessary, the steel strip can also be temper-rolled wet, in order to generate a further thickness reduction of up to 43% (double-reduced steel strip). An additional increase in strength occurs during temper rolling. The steel strip S is then optionally fed to a coating unit, in which the surface of the steel strip is provided, for example, electrolytically with a tin or chromium/chromium dioxide coating (ECCS) or varnishing to increase corrosion resistance. During coating of the surface of the packaging steel, baking of the coating ordinarily occurs by heating the coated packaging steel, in which case an additional strength increase known as “bake hardening” is observed by this baking process. It has been found that packing steels produced with the method according to the invention not only have higher strengths, but also better properties in terms of corrosion resistance than the known flat steel products.

[0069] Heat treatment in the annealing furnace according to the invention leads to the formation of a multiphase structure in the steel of the cold-rolled flat steel product. The structural composition can then be controlled via the process parameters. It was found that during annealing of the flat steel product (steel strip S) above the Ac1 temperature and subsequent rapid cooling (quenching) at a cooling rate in the range of 100 K/s to about 1000 K/s, a multiphase structure consisting of ferrite and troostite (finely striated perlite) is formed. If the flat steel product is annealed above the Ac1 temperature and then quenched at a very high cooling rate of (much) more than 1000 K/s (for example, by introduction into a cooling liquid, especially a water bath 7b, as shown in FIG. 2), on the other hand, a structure with ferrite and martensite is formed as essential structure components, which largely corresponds to the dual-phase structure known from auto body design. Both the multiphase structure from ferrite and troostite and the multiphase structure from ferrite and martensite are characterized by increased strength relative to the monophase steel structure. The high strength of the packaging steel produced according to the invention is therefore achieved, on the one hand, by the strength-increasing content of unbonded nitrogen incorporated interstitially by nitriding and, on the other hand, by the formation of a multiphase structure during heat treatment in an annealing furnace.

[0070] Nitrided flat steel products characterized by very high strength of more than 650 MPa with simultaneously good elongation at break of more than 5% and especially between 7 and 15% as well as good deformation properties can be produced with the method according to the invention. The strength values of the packaging steel produced according to the invention are optionally further increased in a firing process of an applied coating layer (yield point increase by bake hardening), in which strengths of up to 850 MPa are obtainable.

[0071] The strength and elongation at break increased by nitriding are then very homogeneous over the cross section of the steel strip both in and across the rolling direction of the cold-rolled steel strip. This results from very homogeneous introduction of unbonded nitrogen into the steel during nitriding in the annealing phase. Melt analyses on flat steel products produced according to the invention have shown that the nitrogen concentration introduced by nitriding over the thickness of the flat steel product, in each case in ultrafine sheets, only deviates over a narrow band of at most ±10 ppm and regularly only around ±5 ppm about the mean concentration.

[0072] The hot-rolled steel product, which is used as starting product of the method according to the invention, can already contain a fraction of nitrogen. The following method is conducted to produce a corresponding starting product (as expanded embodiment example of the invention):

[0073] A nitrided steel melt is initially generated in a converter and/or in a subsequent ladle treatment, which has a content of free, unbonded (i.e., dissolved in steel) nitrogen of up to 160 ppm. The alloy composition of the produced steel then expediently satisfies the limit valued stipulated by standards for packaging steel (like ASTM standard A623-11 “Standard Specification for Tin Mill Products” or as defined in European standard EN 10202), except for the upper limit for nitrogen content (which lies at N.sub.max=80 ppm in standard EN 10202 and at N.sub.max=200 ppm in the AST ASTM standard 623), which can be surpassed owing to nitriding in the method according to the invention. The carbon fraction of the produced steel then preferably lies in the range of 400 to 1200 ppm and especially between 600 and 900 ppm.

[0074] To produce the steel melt, the converter is filled with scrap and crude iron and the melt blown with oxygen gas and nitrogen gas, in which case the oxygen gas (O.sub.2) is blown from above and the nitrogen gas (N.sub.2) by means of bottom nozzles from below into the converter. A nitrogen content in the steel melt from 70 to 120 ppm is thus established, in which case saturation occurs. During production of the steel melt the composition and especially the nitrogen content of the melt are recorded. If the stipulated analysis is not correct (if the percentage of phosphorus is too high) oxygen gas is blown in through an oxygen lance and argon gas (Ar) through the bottom nozzles. Since virtually no more carbon (C) is present in the steel, no overpressure is formed and the nitrogen of the air is drawn in so that additional nitriding can occur.

[0075] If the desired amount of (dissolved) nitrogen in the steel melt (which regularly lies at about 120 ppm) is still not achieved by blowing in of nitrogen gas, during emptying of the converter (tapping) calcium cyanamide (CaCN.sub.2) can additionally be introduced into the steel stream emerging from the converter. The calcium cyanamide is then added in the form of a granulate (5-20 mm).

[0076] The ladle then goes to first argon scavenging, where it is scavenged with argon for about 3 minutes with an introduced refractory lance. After control analysis, if necessary, a second argon scavenging occurs for about 3 minutes. The ladle then goes to a third argon scavenging. This represents the last stage before casting. If the nitrogen content does not lie in the stipulated target range, manganese nitride (MnN, for example, in the form of wires of MnN powder in a steel shell) can be added during the third argon scavenging. The amount of missing nitrogen is then converted into the required amount of MnN (for example, through the required length of MnN wire), which is added to the melt. The MnN is added until the stipulated nitrogen target content or an MnN upper limit of the steel is reached.

[0077] Finally, the melt is introduced into a distribution trough in order to cast a slab from the steel melt. The nitrogen content can then rise by about 10 ppm because of leaks and diffusion of atmospheric nitrogen. An upper limit of the amount of dissolved nitrogen in the cast steel slab of about 160 ppm should not be surpassed, because defects in the slab, like cracks or pores, can form at higher nitrogen contents, which lead to undesired oxidation.

[0078] The slab cast from the steel melt is then hot rolled and cooled to room temperature. The produced hot strip then has thicknesses in the range of 1 to 4 mm and is optionally wound into a coil. To product packaging steel in the form of a flat steel product in the usual fine and ultrafine thicknesses, the hot strip must be cold rolled, in which a thickness reduction in the range of 50 to more than 90% occurs. Fine sheet is then understood to mean sheet with a thickness of less than 3 mm, and ultrafine sheet has a thickness of less than 0.5 mm. For performance of cold rolling, the hot strip, optionally wound as a coil, is unwound from the coil, pickled and introduced into a cold-rolling device, for example, a cold rolling train. The cold-rolled flat steel product already nitrided to nitrogen concentrations of up to 160 ppm is then used as starting product for subsequent treatment according to the method of the invention, in which the cold-rolled flat steel product is recrystallization annealed in the annealing furnace and at the same time further nitrided in order to increase the nitrogen concentrations to values above 100 ppm and preferably to more than 150 ppm.