ES2725896T3 - Nozzle device, use thereof and procedure for the treatment of a steel product - Google Patents

Abstract

Nozzle device (10) for the treatment of a flat steel product (3), wherein the nozzle device (10) comprises an outer tube (2) and an inner tube (1) disposed within the outer tube (2) , wherein the inner tube (1) has a primary opening (11) to feed a gas through the nozzle device (10) into an outer tube (2) and the outer tube (2) has a secondary opening ( 12) for the gas outlet of the nozzle device (10) in the direction of the flat steel product (3), wherein the primary opening (11) and the secondary opening (12) are arranged displaced from each other along a perimeter direction, where the secondary opening (12) has a gas conduction system (5) to orient a flow direction (5) of the gas, where the gas conduction system (5) is part of the secondary opening (12) and presents an extension (23).

Description

DESCRIPTION

Nozzle device, use thereof and procedure for the treatment of a steel product

State of the art

The present invention relates to a nozzle device, its use and a method for the treatment of a flat steel product.

A nozzle device of this type is intended to transport gas or a gas mixture as close as possible to a flat steel product, so that the gas or gas mixture transported to the flat steel product conditions the product in a controlled manner. flat steel, particularly its surface, for example nitrure or whip or carbure. Normally, the flat steel product is presented as a continuous belt, which is transported through a continuous furnace for hot treatment. By treating, for example, nitriding or flogging or carbureting, the strength of the flat steel product that finally leaves the continuous furnace can be advantageously increased, without negatively influencing its expansion capacity.

The quality improvement caused by the surface conditioning in the flat steel product depends, in this respect, decisively on the gas supply and the composition of the gas through the nozzle device. In this regard, the ability of the nozzle device to "radiate" the flat steel product in the most homogeneous manner possible with the gas or gas mixture and how large is the part of the gas or gas mixture responsible for the surface conditioning, for example nitriding, which reaches a surface of the flat steel product. Usually, the nozzle devices are made up of tubes with an outlet opening at which end the gas or gas mixture leaves. By feeding on one side into the tube, ultimately, an undesirable fall in the flow profile along the outlet opening occurs.

The prior art is known as a satisfactory measure to homogenize the flow profile of the gas or gas mixture, which affects the flat steel product, from DE 102011 056823 A1, an arrangement of openings with different opening sizes dimensions. In this regard, the individual opening sizes are configured in such a way that they offset a potential pressure drop along the flow direction in the outlet zone. From JP 08176793 A a slot-shaped gas outlet opening is known.

Disclosure of the invention

The objective of the present invention is to further improve existing nozzle devices for the surface conditioning of a flat steel product. In this regard, it would be especially desirable to implement a gas flow or mixture of gases that had a spatially homogeneous impact on the flat steel product, thus increasing the quality of the flat steel product finally produced.

The objective of the present invention is achieved by means of a nozzle device for surface conditioning or treatment of a flat steel product, wherein the nozzle device comprises an outer tube and an inner tube disposed within the outer tube, wherein The inner tube has a primary opening to feed a gas through the nozzle device into an outer tube and the outer tube has a secondary opening for the gas outlet from the nozzle device in the direction of the flat steel product, where the primary opening and the secondary opening are arranged displaced from each other along a perimeter direction, wherein the secondary opening has a gas conduction system to orient a direction of gas flow, where the gas conduction system forms part of the secondary opening and presents an extension.

With respect to the state of the art, by means of the displaced arrangement of the primary opening with respect to the secondary opening, a gas flow can be implemented that spatially homogeneously leaves the secondary opening, which "radiates" the flat steel product, preferably in all its width, in the most homogeneous way possible. In particular, it is used in this respect that, due to the chosen geometry, the residence time in the nozzle device is distributed evenly between the gas or the gas mixture. Without the construction with inner and outer tube, otherwise, an unwanted gradual flow rate profile would be obtained along an outlet opening, due to a feed on one side of the gas to the nozzle device. In addition, the flow output behavior is influenced in the nozzle devices according to the state of the art by the different temperatures of the gas or gas mixture such that different flow rates are obtained along the opening. output With the nozzle device according to the invention, instead, a homogeneous flow profile can be implemented along the secondary opening, which assumes the function of the outlet opening.

In this respect, the perimeter direction is essentially given by the perimeter of the inner tube or the outer tube. In particular, the perimeter of the outer tube or the inner tube is given by a cutting plane that runs along a direction perpendicular to a longitudinal direction, the longitudinal direction being essentially predefined by the general development of the inner and outer tube. In particular, a flow direction of the gas or mixture of bases that passes through the nozzle device essentially runs to the primary opening in parallel to the longitudinal direction and then deviates, for example 90 °. Preferably, the primary opening and the secondary opening have outlet surfaces, through which the gas or gas mixture passes, when it is fed from the inner tube into the outer tube or out of the outer tube. Preferably, the outlet surface of the inner tube is smaller than that of the outer tube. It is further provided that the outlet surface of the inner tube is circularly shaped and that the outlet opening of the outer tube is configured in the form of a groove. In particular, the outer tube has a groove with a groove width of less than 8 mm, preferably less than 6 mm, and especially preferably less than 4 mm.

Advantageous configurations and improvements of the invention can be deduced from the dependent claims, as well as from the description referring to the drawings.

In another embodiment, it is provided that the primary opening and the secondary nozzle opening are displaced from each other essentially from 90 ° to 180 ° along the perimeter direction. Through this geometric configuration, an especially homogeneous profile of the flow velocity can be produced for the gas leaving the secondary opening.

In another embodiment, it is provided that the primary opening and / or the secondary opening have a gas conduction system to orient a flow direction of the gas flow. Preferably, the gas conduction system is part of the secondary opening and has extensions that are configured as a peak forming an outlet groove or converging in the direction of the gas flow. In this way the gas flow can be directed or focused in a controlled manner over an area of the continuous band.

In another embodiment it is provided that the nozzle device has a plurality of primary openings and / or a plurality of secondary openings. Preferably, the nozzle device comprises several primary openings, which are arranged on opposite sides in the inner tube and in each case in the perimeter direction around for example 90 ° with respect to the secondary opening. Preferably, the primary openings are disposed within an area that extends parallel to the secondary opening, on a regular basis, preferably equidistant, from one another. In particular, the primary openings are oriented in their position towards the secondary opening. For example, the primary openings are symmetrically distributed around a first central point in the inner tube, which is located in the same plane, which runs perpendicular to the longitudinal extension of the tubes, than a second central point of the secondary opening. . Preferably, there are more primary openings than secondary openings. In particular, there are an even number of primary openings, preferably four primary openings, which are always arranged in pairs along the longitudinal extension of the inner tube and are facing each other, (being arranged offset from each other in each case in the perimeter direction around for example 90 ° with respect to the secondary opening). By means of the constructive composition of various primary openings in the area covered by the secondary opening, the flow velocity profile along the secondary opening can be further advantageously improved.

In another embodiment it is provided that

- the primary opening is arranged in the area of the secondary opening and / or

- the inner tube and the outer tube are arranged coaxially. In this way it is possible, in an advantageous manner, to ensure that, as far as possible, all parts of the gas or gas mixture travel the same route in the nozzle device. This ultimately impacts positively on the flow velocity profile along the secondary opening.

In another embodiment, it is provided that the secondary opening is oriented towards a continuous band comprising the flat steel product, the secondary opening being less than 25 cm, preferably less than 15 cm and especially preferably less than 10 cm, of the continuous band. In particular, the continuous belt moves along a rolling direction passing next to the nozzle device, and the secondary opening, preferably with its gas conduction system, is oriented such that the gas or gas mixture that exits presents a flow direction that runs essentially perpendicular to the continuous band. By reducing the distance, the proportion of gas that reaches a surface of the flat steel product can be advantageously extended, to deal there with the desired surface conditioning, for example nitriding or carbureting. The higher this percentage for nitriding or carbureting by way of example, the greater the resistance, without significantly affecting the expansion capacity of the flat steel product produced. In particular, it is then possible to reduce the thickness of flat steel products, without fear of resistance losses. In this way, finally, material can be saved in the manufacture of a product made from the flat steel product.

In another embodiment, it is provided that the secondary opening essentially extends along a width of the continuous band, the width of the continuous band running essentially along a direction perpendicular to a rolling direction of the continuous band. Through the spatially homogeneous flow profile, it can be ensured that the flat steel product produced is conditioned on its surface as uniformly as possible along its width. In this way, it is advantageously avoided that resistance oscillations occur along the width of the flat product of steel produced.

In another preferred embodiment, it is provided that the gas passing through the nozzle device comprises ammonia and, if necessary, nitrogen and / or an inert gas. Through the use of nitrogen and / or inert gas, the impulse of the gas is advantageously increased in such a way that a boundary layer carried by the continuous band can be interrupted and a direct reaction of the ammonia with the continuous band is possible, is say with the flat steel product.

In another embodiment, it is provided that the nozzle device has a static mixer for mixing ammonia with, if necessary, nitrogen and / or an inert gas, a gas pipe system redirecting the gas into the inner tube. In this way, a homogeneous gas mixture can be implemented in a simple and uncomplicated manner, in order to provide a constant gas mixture throughout the width of the secondary opening.

In another embodiment it is provided that the tube system supplies another inner tube with gas, wherein the other inner tube comprises another primary opening for the introduction of the gas into another outer tube that envelops the other inner tube at least partially, in where the outer tube has another secondary opening for the gas outlet of the nozzle device in the direction of the flat steel product. In this respect, it is conceivable that the continuous band is advanced, for example, by way of deflection pulleys, such that the other secondary opening is arranged at another point other than a continuous band section of the continuous band.

In another embodiment it is provided that the gas leaving the secondary opening conditions the surface of a first face of the flat steel product, the gas leaving the other secondary opening conditioning the surface of a second face of the flat steel product . In this way the surface of the flat steel product can be conditioned, advantageously, on both sides.

In another embodiment, it is provided that the nozzle device has a supply tube system for supplying ammonia, nitrogen and / or inert gas to the static mixer, the supply tube system presenting in each case means for measuring and controlling the flow volumetric

- of individual gases and / or

- of a mixture of gases formed by ammonia, nitrogen and / or inert gas. The quantity and / or composition of the gas or mixture of gases can be advantageously controlled with the aid of the measuring and control devices and adapted to the optimum requirements for satisfactory nitriding.

Furthermore, the use of a nozzle device in a continuous furnace for the hot treatment of the steel product according to claim 9 is provided.

Another object of the present invention is a method according to independent claim 10 for conditioning the surface of a continuous steel band in a continuous furnace for heat treating the steel product with a nozzle device according to the invention. . With a nozzle device of this type, a flat steel product whose surface conditioning is more efficient compared to that known in the state of the art can be implemented.

In this regard it is provided that, in a first process steps, the gas or gas mixture is supplied to the inner tube, in a second process steps, the gas or gas mixture is introduced through the primary opening into the interior of the outer tube and, in a third process steps, the gas or gas mixture exits the secondary opening of the outer tube outside the nozzle device and conditions the surface of the flat steel product. In this way, a process can be made available in an advantageous manner in which, due to, for example, effective nitriding or carbureting and the greater resistance derived therefrom, thinner flat steel products can be produced. The material savings obtained with this, preferably in a series production of a product from the flat steel product, lead to a significant cost savings in production.

Other details, features and advantages of the invention are apparent from the drawings, as well as from the following description of preferred embodiments with the aid of the drawings. The drawings illustrate in this respect only exemplary embodiments of the invention, which do not limit the idea of the invention.

Brief description of the figures

Figure 1 shows a nozzle device according to a first exemplary embodiment of the present invention.

Figure 2 shows, in a perspective view, a nozzle device according to a second exemplary embodiment of the present invention.

Figure 3 shows, in two different plan views, the nozzle device according to the second exemplary embodiment of the present invention.

Figure 4 shows a sectional view of the nozzle device according to the second exemplary embodiment of the present invention.

Embodiments of the invention

In the different figures, the same parts always bear the same references and therefore they are only mentioned or commented as a rule once in each case.

In Figure 1 there is shown a nozzle device 10 for the treatment or conditioning of surface of a flat steel product 3 according to a first embodiment exemplary of the present invention. A nozzle device 10 of this type is an integral part, preferably, of a continuous furnace, through which the flat steel product is passed as a continuous band 33, ie as a steel band. In the continuous furnace, the flat steel product undergoes, for the improvement of the material, a hot treatment, in which a gas or gas mixture is directed towards the flat steel product, for example for nitriding. For example, in the continuous oven at least the recrystallization temperature of the flat steel product reigns. By means of, for example, nitriding or flogging or carburation, the strength of the flat steel product can be increased, without essentially influencing its expansion capacity. For equal strength requirements in a produced product, thinner steel flat products 3 can then be implemented. The material savings that result from it can then lead to significant cost savings.

It is decisive for quality improvement through surface conditioning, for example nitriding or carburation, the type and mode of gas supply to the flat steel product 33, which is essentially given by the nozzle device 10. For nozzle device 10 is provided, in this case, that the gas conducted through the nozzle device 10 for surface conditioning, for example nitriding, is first supplied to an inner tube 1 of the nozzle device 10. A Through a primary opening 11 in the inner tube 1, the gas leaves the inner tube 1 and is fed to an outer tube 2, which envelops the inner tube 1 at least partially. From there the gas then leaves the nozzle device 10 through a secondary opening 12. In this case, the primary opening 11 and the secondary opening 12 are arranged displaced relative to each other along a perimeter direction that is essentially given by the perimeter (which runs along the cross section) of the inner tube 1 or the outer tube 2. In particular, the primary opening 11 is displaced for example 90 ° with respect to the secondary opening 12. It is preferably provided, to in this regard, that the primary opening 11 and the secondary opening 12 are disposed in a common area of the nozzle device 10. In particular, the secondary opening 12 is arranged in the part of the outer tube 2 that surrounds the primary opening 11 at least partially . In this regard, it is further provided that the secondary opening 12 is directed towards the continuous band 33, that is to say the flat steel product. By means of the relative arrangement of the primary opening 11 with respect to the secondary opening 12, it is advantageously avoided that - as one would expect without an inner tube 1 - the gas leaves the secondary opening 12 which serves as an asymmetrically outlet opening, that the gas present along the secondary opening 12 and by a width the band continues 33 different temperatures and that, finally, different output speeds of the secondary opening 12 are obtained. Instead, by means of a nozzle device 10 with the primary opening 11 in the inner tube 1 and the secondary opening 12 in the outer tube 2 a homogeneous output behavior can be implemented through the secondary opening 12 or by the width of the continuous band 33, that is to say uniform and regular, of gas In this way the gas supply from the nozzle device 10 to the flat steel product 3 is improved. Furthermore, it is provided that the secondary opening 12 be at least as wide as the width of the continuous band 33. In this respect, the secondary opening 12 is oriented in relation to the continuous band such that its longer side essentially runs perpendicular to the direction of advance of the continuous band 33.

In this regard, it is further provided that the secondary opening 12 has a gas conduction system 5. The gas conduction system 5 serves, in this case, to orient the gas or gas mixture leaving the secondary opening 12 and with it to place in a controlled manner the gas that leaves the nozzle device 10 on the flat steel product. The gas conduction system 5 preferably has two areas of the outer tube 2 that approximate each other, and that form a peak-like opening. A groove between the two areas of the outer tube which are close to each other in this respect, the secondary opening 12. Furthermore, it is provided that next to the inner tube 1 and the outer tube 2 there is also still another inner tube 1 ', essentially constructed the same, with another primary opening 11' as well as another outer tube 2 'with another secondary opening 12'. With the aid of this second arrangement of another secondary opening 12 'and another primary opening 11' it is possible, advantageously, for example to nitrate the flat steel product on a first face with a gas leaving the secondary opening 12 and also nitrure it on a second face with a gas leaving the other secondary opening 12 ', whereby the flat steel product for example is advantageously nitrided on both sides. For a nitriding on both sides of this type, the flat steel product is preferably conducted through bypass pulleys 31, the other secondary opening 12 'and the secondary opening 12 being spatially separated along a direction of product advancement steel plane.

Furthermore, in this regard it is provided that the gas is supplied to the inner tube 1 and / or the other inner tube 1 'through a tube system 14 from a static mixer 18. Preferably, the ammonia 18 is preferably mixed in the static mixer 18 for nitriding of the flat steel product with a mixture of gases formed by nitrogen and / or a gas inert. The addition of nitrogen and inert gas improves nitriding, by being in charge of a greater impulse of the gas, with which a boundary layer dragged along a surface of the continuous web can be interrupted. In this way an immediate reaction of the flat steel product with the ammonia can be implemented, which ultimately leads to more efficient nitriding and, thereby, the quality of the flat steel nitrated product is further improved.

In this regard, it is also provided that static mixer 18 be supplied, through a supply tube system 13, ammonia, nitrogen and / or an inert gas. Preferably, the inert gas comprises a mixture of hydrogen-nitrogen. For a specially controlled supply of gases, in this regard, it is provided that the flow rate is monitored by means to measure the volumetric flow rate 17 or flow meters. In particular, the nozzle device 10 also comprises means for controlling the volumetric flow and the amount of the respective gas that is supplied to the static mixer. For example, such a means is a valve 16, which is integrated in the supply tube system 13. In this way, a composition of substances in the secondary opening 12 or in the other secondary opening 12 'can be adapted from advantageously, simply and directly to the dosage or to the desired composition for the respective nitride. In this regard, it is provided that the nozzle device 10 receives the gases from a first tank 21, for example for ammonia, and from a second tank 22, for example for nitrogen. To ensure that an identical volumetric flow flows through both nozzle devices shown, the laying of pipes after the static mixer is identical / symmetrical. Therefore, the pressure losses are identical in both branches, in order to achieve equivalent impulses on both sides of the continuous band. Alternatively, regulation devices not shown can also be installed to obtain equal pulses.

In figure 2 , a nozzle device 10 according to a second exemplary embodiment of the present invention is shown in a perspective view. While the outer tube 2 is essentially seen in the illustration on the left, the outer tube 2 is shown transparently on the right side such that the inner tube 1 can be seen in the illustration inside the outer tube 2. It is preferably provided that the inner tube 1 and the outer tube 2 are arranged coaxially. However, it can also be conceived in an alternative embodiment that the inner tube 1 is arranged displaced with respect to a central axis B inside the outer tube 2. For example, the inner tube 1 could be arranged in an area opposite to the secondary opening 12 inside the outer tube 2. Furthermore, it is provided that in the inner tube 1 with the primary opening 11 and in the outer tube 2 with the secondary opening 12, a tube system 14 is provided at the end, which is supplied by a side the gas or gas mixture. In this regard, the gas leaves the inner tube 1 for example through four primary openings or other primary openings, not shown, preferably arranged symmetrically, which are arranged in pairs in opposite areas of the inner tube 1. The primary openings 11 arranged in pairs are arranged, in this respect, in the perimeter direction in each case at for example 90 ° with respect to the secondary opening 12. Furthermore, it is provided that a total or integral exit surface of the primary openings 11 is smaller than one outlet surface of the secondary opening 12. The outlet surface is to be understood as the surface through which the gas is fed from the inner tube 1 into the outer tube 2, or the surface through which it leaves the gas the nozzle device 10. It is also provided that the flow direction of the gas or gas mixture is directed, with the help of the conduction system of gas 5, in a controlled manner, towards the flat steel product. For this, the gas conduction system 5 preferably has two extensions 23 of the outer tube 2, whose distance from one another narrows along a direction that runs parallel to the flow direction A of the gas leaving the secondary opening 2. In this respect, the extensions 23 may be shaped straight or curved.

In figure 3 two different plan views of the nozzle device 10 according to the second exemplary embodiment of the present invention are shown. The upper surface shows the outlet surface of the primary openings 11 and the lower illustration shows the secondary opening 12 in plan view. Preferably, the primary openings 11 or the primary opening 11 are oriented centered with respect to the secondary opening (in relation to a longitudinal direction established by the outer tube and the inner tube). In this regard, the projection (in a direction that runs perpendicular to the longitudinal direction) of a central point of the secondary opening 12 essentially lies in an area of the inner tube 12 arranged centered between two primary openings 11. Furthermore, it is provided that the distance between two primary openings 11 along the longitudinal direction is less than the extension of the secondary opening 12 along the longitudinal direction. In particular, the gas conduction system 5 extends along the entire length of the secondary opening 12. Furthermore, it is provided that, by means of the arrangement of the primary opening 11 and the secondary opening 12, the flow direction of the gas or gas mixture deviates essentially for example 90 °.

In Figure 4 the cross section of the nozzle device 10 according to the second exemplary embodiment of the present invention is shown. In this regard, it is provided that the secondary opening 12 is arranged between two imaginary planes 24, which in each case comprise at least one primary opening 11. Preferably, the secondary opening 12 is arranged centered between these two imaginary planes. In addition, it is provided that a groove width 25 of the secondary opening 12 is given by the extensions 23 that approach each other of the gas conduction system 5. Preferably, the groove width 25 is smaller than the extension of the tube interior 1 along a direction that runs in perpendicular to the longitudinal direction. In particular, the groove width 25 is less than 8 mm, preferably less than 6 mm and especially preferably less than 4 mm. It is further provided that the extensions 23 of the gas conduction system 5 are preferably made at least partially from the same material as the outer tube 2. For example, it is provided that the inner tube 1 occupies, in cross-section (along of a direction that runs perpendicular to the longitudinal direction) less than 45%, preferably less than 40% and especially preferably less than 35% of the construction space given by the outer tube 2. For example, it is provided that an inner diameter of the outer tube 2 is less than 300 mm, preferably less than 280 mm and especially preferably less than 260 mm and that an inner diameter of the inner tube 1 is less than 100 mm, preferably less than 90 mm.

Reference List

1 inner tube

1 'other inner tube

2 outer tube

2 'other outer tube

3 flat steel product

5 gas conduction system

10 nozzle device

11 primary opening

11 'other primary opening

12 secondary opening

12 'other secondary opening

13 supply pipe system

14 tube system

16 valve

17 means to measure volumetric flow

18 static mixer

21 first deposit

22 second deposit

23 extension

24 imaginary planes

25 slot width

31 deflection pulley

33 continuous band / flat steel product

To flow direction

B central axis

Claims (14)

1. Nozzle device (10) for the treatment of a flat steel product (3), wherein the nozzle device (10) comprises an outer tube (2) and an inner tube (1) disposed within the outer tube ( 2), wherein the inner tube (1) has a primary opening (11) to feed a gas through the nozzle device (10) into an outer tube (2) and the outer tube (2) has an opening secondary (12) for the gas outlet of the nozzle device (10) in the direction of the flat steel product (3), wherein the primary opening (11) and the secondary opening (12) are arranged displaced from each other along of a perimeter direction, where the secondary opening (12) has a gas conduction system (5) to orient a flow direction (5) of the gas, where the gas conduction system (5) is part of the secondary opening (12) and has an extension (23). 2. Nozzle device (10) according to claim 1, wherein the primary opening (11) and the secondary opening (12) are arranged displaced from each other from 90 ° to 180 ° along the perimeter direction. 3. Nozzle device (10) according to one of the preceding claims, wherein the primary opening (11) has a gas conduction system (5) to orient a flow direction (5) of the gas. 4. Nozzle device (10) according to one of the preceding claims, wherein the nozzle device (10) has a plurality of primary openings (11) and / or a plurality of secondary openings (12). 5. Nozzle device (10) according to one of the preceding claims, wherein - the primary opening (11) and the secondary opening (12) are arranged in a common area of the nozzle device (10) and / or - the inner tube (1) and the outer tube (2) are arranged coaxially. 6. Nozzle device (10) according to one of the preceding claims, wherein a tube system (14) redirects the gas to the inner tube (1). 7. Nozzle device (10) according to one of the preceding claims, wherein the tube system (14) supplies another inner tube (1 ') with gas, wherein the other inner tube (1') comprises another opening primary (11 ') for the introduction of the gas into another outer tube (2') that envelops the other inner tube (1 ') at least partially, where the outer tube (2') has another secondary opening (12 ') for the gas outlet of the nozzle device (10) in the direction of the flat steel product (3). 8. Nozzle device (10) according to one of the preceding claims, wherein the nozzle device (10) has a supply tube system (13) for supplying ammonia, nitrogen and / or inert gas to the static mixer ( 18), presenting the supply tube system (13) in each case means for measuring and / or controlling the volumetric flow (16.17) - of individual gases and / or - of a mixture of gases formed by ammonia, nitrogen and / or inert gas. 9. Use of a nozzle device (10) according to one of the preceding claims in a continuous oven for hot treatment of the steel product (3). 10. Procedure for surface conditioning, in particular for nitriding a flat steel product (3) in a continuous furnace for hot treatment of flat steel product (3) with a nozzle device (10) according with one of the preceding claims, wherein, in a first process stage, gas is supplied to the inner tube (1), in a second process stage gas is introduced through the primary opening (11) into the tube outside (2) and where, in a third process stage, gas flows through the secondary opening (12) of the outer tube (2) outside the nozzle device (10) and nitrides the flat steel product (3) . 11. A method according to claim 10, wherein the gas passing through the nozzle device (10) comprises ammonia and, where appropriate, nitrogen and / or an inert gas. 12. Method according to one of claims 10 or 11, wherein a static mixer (18) is used to mix ammonia, nitrogen and / or an inert gas. 13. Method according to one of claims 10 to 12, wherein the secondary opening (12) is oriented towards a continuous band (33) comprising the flat steel product (3), the secondary opening (12) being less than 25 cm, preferably less than 15 cm and especially preferably less than 10 cm, of the continuous band (33). 14. Method according to one of claims 10 to 13, wherein the secondary opening (12) extends over a width of the continuous band (33), the width of the continuous band (33) running along a perpendicular direction to a rolling direction of the continuous web (33).