Method and device for welding steel strips

Abstract

The invention relates to a device and a method for welding steel strips (1, 2). The method comprises at least the following steps: - joining (S100), in a butt joint, a first steel strip edge (7) of a first steel strip (1) with a scale layer (3, 5) and a second steel strip edge (8) of a second steel strip (2); - processing (S200) a boundary zone (10) around the butt joint on at least one surface (A1, A2) of the first steel strip (1) and of the second steel strip (2) by means of at least one plasma torch (111-116), in order to reduce the thickness of the scale layer (3, 5); and - welding (S300) the first steel strip edge (7) and the second steel strip edge (8) along the butt joint from the at least one surface (A1, A2) by means of a welding head (121); wherein the processing (S200) of the boundary zone (10) is carried out at least partially simultaneously with the welding (S300).

Description

[0001] title

[0002] Method and apparatus for welding steel strips

[0003] Description

[0004] Technical field

[0005] The present invention relates to a method for welding steel strips, in particular to form an endless strip, and a corresponding device.

[0006] Technical background

[0007] In steel rolling mills, individual steel strips are typically welded together at their ends to form a so-called "endless strip," which can then be further processed, for example, by cold rolling. The endless strip is so named because a seemingly endless strip is processed within the cold rolling mill, to which new steel strips are welded before each cold rolling process. After cold rolling, the endless strip is cut again at the weld seams, and the resulting new steel strips are used as "coils."

[0008] During cold rolling, the welds must withstand considerable stress, as an unintentional break in the continuous strip (e.g., due to a weld crack) within the cold rolling mill requires lengthy and therefore very costly troubleshooting until an intact continuous strip can be produced and fed back into the mill. Accordingly, high demands are placed on the quality of the welding process.

[0009] Depending on the equipment, the steel strips can be pickled before welding, which simplifies the process, or they can be unpickled and thus covered with a layer of scale after hot rolling. Welding scaled steel strips presents additional challenges, as the scale layer interacts unfavorably and sometimes unpredictably with the welding process.

[0010] A typical measure would be to mechanically remove the scale layer before welding, for example by grinding it off. However, this process produces metal shavings that can damage the strip. The targeted, complete, and trouble-free removal of such metal shavings is very complex. On the one hand, mechanical descaling must be carried out as close as possible to the welding process; on the other hand, the welding equipment, which has to move across the strip, is particularly sensitive to such metal shavings.

[0011] Summary of the invention

[0012] Based on the above, there is therefore a need for an improved method and an improved device for welding steel strips, especially into an endless strip.

[0013] A solution is provided by the features of the independent patent claims.

[0014] According to a first aspect, the invention provides a method for welding steel strips, comprising at least the steps of:

[0015] - Joining together, in a butt joint, a first steel strip edge of a first steel strip with a scale layer and a second steel strip edge of a second steel strip;

[0016] - Machining a boundary zone at or around the butt joint on at least one surface of the first steel strip and the second steel strip using at least one plasma torch to condition the scale layers, in particular to reduce the thickness of the scale layers on the at least one surface; and

[0017] - Welding of the first steel strip edge and the second steel strip edge along the butt joint from at least one surface by means of a welding head; wherein the processing of the boundary zone is carried out at least partially simultaneously with the welding.

[0018] The joining of the first and second steel strip edges can be carried out in particular indirectly or immediately after hot rolling of the first and / or the second steel strip, whereby hot rolling can also be a process step of the process.

[0019] One of several advantageous effects of the method according to the invention is the reduction of the porosity of a weld seam (in particular a laser weld seam) that results from the welding (in particular laser welding) of the steel strip edges after the processing according to the invention. That is, a weld seam produced by the method according to the invention advantageously has a lower porosity than a conventionally produced weld seam.

[0020] More than one welding head can be provided, for example, one for welding the butt joint from a first surface (e.g., from above), and one for welding the butt joint from a second, opposite surface (e.g., from below). The welding head (or heads) can be, in particular, a laser welding head of a laser welding torch. However, the application of other welding techniques is also conceivable, for example, resistance pressure welding processes, especially resistance seam welding, such as roll seam welding. Accordingly, the welding head (or several or all welding heads) can also be designed as a resistance seam welding head, roll seam welding head, etc. The plasma torch used for processing and the welding head used for welding are advantageously designed as separate assemblies.In this way, their parameters can be set more flexibly and precisely.

[0021] In this context, "surfaces" always refers to the two largest, planar sides of the steel strips, which are stretched by length and width, and not to the comparatively much smaller edges, which are stretched using the depth (i.e. the smallest dimension) of the steel strips.

[0022] The distance between the nozzle of the respective plasma torch and the respective surface on which the boundary zone is processed can be, for example, between 1 mm and 11 millimeters, in particular between 3 mm and 8 mm, for example 5 mm.

[0023] A scale layer that needs to be reduced can be present on one or both sides of the first steel strip, and optionally also on one or both sides of the second steel strip.

[0024] If only the first steel strip has a scale layer to be reduced, the interface to be treated can be located on one side of the butt joint. If both steel strips have a scale layer on the same surface, the interface can be located on both sides of this surface around the butt joint, for example, symmetrically around it.

[0025] If both the first and second steel strips have a scale layer to be reduced on the same surface (i.e., directly adjacent to each other), any processing of the scale layers is preferably carried out in the same way for the scale layer of the first steel strip as for the scale layer of the second steel strip. For example, the reduction of both scale layers is carried out for the same time and / or to the same degree of reduction.

[0026] According to some preferred embodiments, variants, or refinements of embodiments, the machining of the interface zone is carried out at least at the same speed along the butt joint as the welding. The machining of the interface zone advantageously begins no later than the start of the welding, and advantageously earlier, e.g., 120 seconds or less, preferably 50 seconds or less, particularly 20 seconds or less, and most preferably 5 seconds or less, for example, 1 second or less before the start of the welding. In this way, it is ensured that the welding always takes place on an already machined (e.g., reduced) scale layer.

[0027] According to some preferred embodiments, variants or refinements of embodiments, the method comprises moving the at least one plasma torch to process the boundary zone and the welding head (e.g. of a laser welding torch) to weld the first steel strip edge and the second steel strip edge uniformly at a constant distance along the butt joint.

[0028] According to some preferred embodiments, variants, or refinements of embodiments, the movement of the at least one plasma torch and the welding head is accomplished by moving a common carriage (or cart) on which the at least one plasma torch and the welding head are mounted. If welding from both sides (i.e., from both surfaces, e.g., top and bottom) is intended, a single carriage may be provided on which one plasma torch for the top and one for the bottom and / or one welding head for the top and one for the bottom are arranged, or two separate carriages may be provided. The two carriages may move at the same speed or at different speeds and may be arranged in the same or offset positions. If the welding head is also arranged on the carriage, it may also be referred to as a welding cart.

[0029] According to some preferred embodiments, variants, or refinements of embodiments, welding is carried out at a speed of at least 2 meters per minute, preferably at least 7 meters per minute, particularly preferably at least 10 meters per minute, for example 12 meters per minute or more. A faster welding speed is made possible by the treatment of the scale layer described herein and increases the throughput of the process.

[0030] According to some preferred embodiments, variants or refinements of embodiments, the processing is carried out such that the scale layer of the first steel strip (and optionally, if present, also a scale layer of the second steel strip, each) is reduced by between 10% and 90%, preferably between 20% and 70%, particularly preferably between 35% and 65%.

[0031] It has surprisingly turned out that the scale layer does not need to be completely removed to significantly improve weld quality. This saves time and effort by reducing, rather than completely removing, the scale layers, which in turn increases the throughput of the process. Furthermore, this method also reduces the amount of material or particles that may be removed from the weld zone.

[0032] According to some preferred embodiments, variants or refinements of embodiments, the processing is carried out such that the scale layer of the first steel strip (and optionally, if present, also a scale layer of the second steel strip, each) is reduced by a thickness between 0.5 micrometers and 15 micrometers, preferably by a thickness between 1 micrometer and 5 micrometers, for example by a thickness between 1 micrometer and 2 micrometers.

[0033] Reducing scale layers by x% or y micrometers can each mean reducing them by a corresponding average value across the treated boundary zone.

[0034] Preferably, the scale layers of both steel bands are reduced in the same way, in particular to the same degree, although in some variants a different reduction of the scale layers on the two steel bands can also be carried out.

[0035] According to some preferred embodiments, variants or refinements of embodiments, there is a period of 120 seconds or less, preferably 50 seconds or less, in particular 20 seconds or less, particularly preferably 5 seconds or less, for example 1 second or less, between machining a point of the boundary zone and welding it.

[0036] According to some preferred embodiments, variants or refinements of embodiments, the boundary zone on both surfaces of the first steel strip and the second steel strip is treated with at least one plasma torch each in order to treat the scale layers on both sides, in particular to reduce them on both sides.

[0037] According to some preferred embodiments, variants, or refinements of embodiments, the machinable (i.e., the area to be machined or subsequently machined) boundary zone has a greater width (in the longitudinal dimension of the steel strips) than the diameter of a zone that can be machined (i.e., machined or subsequently machined) by the welding head, i.e., welded. The machinable zone can, for example, be the diameter of a laser beam from a laser welding torch on the boundary zone. This ensures that welding only takes place on previously machined areas of the steel strips.

[0038] According to some preferred forms of execution, variants or refinements of forms of execution, the procedure also includes the following steps:

[0039] - Cutting off a steel strip end (in the process, in particular a trailing end facing the second steel strip) of the first steel strip to produce the first steel strip edge; and

[0040] - Separating a steel strip end (in the process, in particular, preceding the first steel strip) of the second steel strip to produce the second steel strip edge.

[0041] The cutting can be done, for example, by means of mechanical scissors (or several mechanical scissors) or by means of a welding machine (or several welding machines), for example a laser welding machine suitable for laser cutting.

[0042] According to some preferred forms of execution, variants or refinements of forms of execution, the procedure further comprises the following steps:

[0043] - Recording the properties of the first steel strip and the second steel strip;

[0044] - Determine, based at least on the recorded properties, a porosity measure which indicates the probability of pore formation during welding with the welding head (e.g., using a laser), wherein the porosity measure is higher the more carbon is contained in the composition of the first steel strip and the second steel strip, and lower the more at least one predetermined element is contained in the composition, wherein the at least one predetermined element comprises silicon and / or aluminum; and

[0045] - Check whether the specified porosity measure exceeds a predetermined threshold.

[0046] Preferably, processing of the boundary zone is only carried out if the check shows that the specified porosity measure exceeds the predetermined threshold, and is prevented if the specified porosity measure does not exceed the predetermined threshold.

[0047] Recording the properties of the first and second steel strips can include, for example, recording their respective compositions, process parameters from preceding process steps, or process parameters from the production of the steel strips, and / or similar data. This recording can involve reading values ​​from a database and / or taking measurements. Process parameters can include, for example, temperatures, durations, power outputs, or similar data, such as the duration the steel strip spent in an annealing process.

[0048] The at least one predetermined element may, in addition to silicon and / or aluminium, also contain at least one of the following elements: manganese, niobium, titanium, and / or chromium.

[0049] In particular, a porosity measure P can be defined, for example, as follows: where the element symbols each represent the mass fraction of the respective element in the steel strip. For a porosity measure defined in this way, the predefined threshold value can be, for example, 0.1 or greater, such as 0.13 or greater, in particular 0.15 or greater, for example 0.15.

[0050] In general, conditioning the scaled surface is particularly beneficial for low-alloy steel grades. Steel grades with low levels of deoxidizing elements, such as aluminum and silicon, tend to form pores during laser welding. Dissolved carbon from the steel reacts with oxygen introduced by the scale in the weld pool, for example, during a laser welding process, to form CO or CO2 if no more reactive alloying elements are present, according to the reaction equation FeO + C → Fe + CO. These CO or CO2 gas bubbles are trapped in the weld seam when the weld pool solidifies and negatively affect the joint properties. Furthermore, moisture on the surface of the steel strips can lead to the formation of H2 pores in the weld seam.The processing of the boundary zone according to the invention thus also results in an advantageous drying of the surface of the steel strips in the boundary zone, as well as a reduction of pore formation in the weld seam, i.e., porosity of the finished weld seam.

[0051] Based on the properties recorded for the first and second steel strips, a parameter for processing the interface zone can advantageously be set automatically. This could include, for example, the processing duration, the distance between the plasma torch nozzle and the surface, the plasma torch current, or the degree of scale reduction (either relative in percent or absolute in micrometers). For instance, the scale layer thickness depends on the silicon content in the steel strip's composition. A higher silicon content, all other things being equal (e.g., under the same cooling conditions for the steel strip), results in a thinner scale layer.

[0052] According to some preferred embodiments, variants, or refinements of embodiments, another steel strip is provided, and after welding the first and second steel strips together in a first iteration of the process, one result of the welding is used as the first steel strip for a further iteration of the process, and the additional steel strip is used as the second steel strip for the subsequent iteration. It is understood that this procedure can be continued indefinitely to produce an endless steel strip.

[0053] According to some preferred embodiments, variants, or refinements of embodiments, the first steel strip is processed in the boundary zone using at least one first plasma torch, and the second steel strip is processed in the boundary zone using at least one second plasma torch. In other words, the part of the boundary zone belonging to the first steel strip is processed using a first plasma torch, and the part of the boundary zone belonging to the second steel strip is processed using a second plasma torch, preferably without an overlap.

[0054] According to some preferred embodiments, variants or refinements of embodiments, the entire boundary zone on one side (i.e., on one surface) of the first steel strip and the second steel strip is processed by means of a single plasma torch.

[0055] The invention also provides a device for welding steel strips, comprising: a conveying device configured to bring together a first steel strip edge of a first steel strip with a scale layer and a second steel strip edge of a second steel strip with a scale layer in a butt joint; at least one plasma torch configured to process a boundary zone at or around the butt joint on at least one surface of the first steel strip and the second steel strip, in particular to reduce the scale layers on the at least one surface; and a welding device configured to weld the first steel strip edge and the second steel strip edge along the butt joint from the at least one surface.wherein the at least one plasma torch and the welding device are arranged such that the processing of the boundary zone by the at least one plasma torch takes place at least partially simultaneously with the welding by the welding device.

[0056] This device can be adapted to carry out the method described above, as well as any of its preferred embodiments, variants or refinements of embodiments, and vice versa.

[0057] The welding device can, for example, include at least one laser welding device and / or one resistance seam welding device, which accordingly includes at least one laser welding head or at least one resistance seam welding head (e.g., a roller seam welding head).

[0058] If multiple welding heads are provided, the device can comprise either a welding unit with multiple welding heads, multiple welding units each with one welding head, or a combination thereof. Further advantageous embodiments, variants, and refinements of embodiments will become apparent from the following detailed description with reference to the figures.

[0059] Brief description of the characters

[0060] The invention is explained in more detail below with reference to exemplary embodiments shown in the figures of the drawings. These show:

[0061] Fig. 1 is a schematic flowchart to explain a method according to one embodiment of the present invention;

[0062] Figs. 2 to 6 show schematic situations illustrating possible variants of the method from Fig. 1;

[0063] Fig. 7 is a schematic block diagram to explain a device according to a further embodiment of the present invention;

[0064] Figs. 8A to 10B are schematic and experimentally determined representations of cross-sections through steel strips to illustrate the effect of the processing of the steel strips according to the invention;

[0065] Fig. 11A shows a laser weld seam on a workpiece produced using a conventional method and therefore with comparatively high porosity; and Fig. 11B shows a laser weld seam on the same workpiece as in Fig. 11A, but produced using the method according to the invention and therefore with comparatively low porosity.

[0066] In all figures, identical or functionally equivalent elements and devices are designated with the same reference numerals unless otherwise indicated. The designation and numbering of the process steps do not necessarily imply a sequence, but serve for better differentiation, although in some variants the sequence may correspond to the numbering sequence.

[0067] Detailed description of the figures

[0068] Fig. 1 shows a schematic flowchart to explain a method according to one embodiment of the present invention, i.e., a method for welding steel strips.

[0069] Fig. 2 and Fig. 6 show schematic situations to illustrate possible variants of the method from Fig. 1, whose reference numerals will therefore be used in the following.

[0070] In step S100, a first steel strip edge 7 of a first steel strip 1 with a scale layer 3, 5 and a second steel strip edge 8 of a second steel strip 2 with a scale layer 4, 6 are joined in a butt joint. Typically, scaled steel strips 1, 2 are scaled on all their surfaces Al, A2 and thus exhibit, in particular, a first scale layer 3, 4 on a (not defined) first surface Al (in Fig. 2, the top side) and a second scale layer 4, 6 on a second surface A2 opposite the first surface Al (in Fig. 2, the bottom side).

[0071] As explained above, a scaled first steel strip 1 can also be welded to a non-scaled (e.g., pickled) steel strip 2 (or vice versa, i.e., the designation as first or second steel strip can be interchangeable). The following description primarily focuses on the case where both steel strips 1 and 2 are scaled, particularly on both surfaces (Al, A2). However, it is understood that all described methods and devices can also be adapted for welding a scaled steel strip to a non-scaled steel strip.

[0072] In the joined state, the steel bands 1, 2 can be fixed, for example, by clamps 11, 12 of a device for welding steel bands 1, 2 (in particular the device according to the invention described herein).

[0073] The merging S100 can, in particular, include conveying S110 of the steel bands 1, 2 towards a cutting device, for example, a mechanical shear or a welding (cutting) device, for example, a laser welding (cutting) device set up for laser cutting. Alternatively or additionally, the corresponding cutting device, or several cutting devices, can also be moved.

[0074] Subsequently, in step S120, a steel strip end of the first steel strip 1 can be cut off to create the first steel strip edge 7, and in step S130, a steel strip end of the second steel strip 2 can be cut off to create the second steel strip edge 8, wherein the first and second steel strip ends are advantageously facing each other, so that the same applies to the resulting steel strip edges 7, 8. In this way, a precise and clean butt joint can be formed. Steps S120 and S130 can be carried out simultaneously (e.g., by two separate cutting devices) or sequentially, wherein either the cutting device, or at least one steel strip 1, 2, or both are moved between step S120 and step S130.

[0075] In step S200, a boundary zone 10 around the butt joint is processed (or conditioned) on at least one surface Al of the first steel strip 1 and the second steel strip 2 using at least one plasma torch 111, in particular to reduce the thickness of the scale layers 3, 4. This is shown in Fig. 2 for the first surface Al; however, it is understood that this can also be done for the second surface A2 or both.

[0076] The plasma torch 111 can, for example, be designed and set up as described in the published patent application EP 3 751 967 Al .

[0077] If a scaled steel strip is welded to a non-scaled steel strip, the boundary zone 10j to be processed can be limited to a section of the scaled steel strip and thus, for example, abut on one side of the butt joint. If both steel strips are scaled, the boundary zone 10 is advantageously arranged on both sides, particularly symmetrically, around the butt joint. The following section explains in more detail the case of welding two scaled steel strips together, without limiting this to the specific case.

[0078] Preferably, the processing of S200 is carried out such that the scale layer 3 of the first steel strip 1 and the scale layer 4 of the second steel strip 2j are each reduced by at least 30%, preferably between 30% and 90%, more preferably between 50% and 80%, and particularly preferably between 60% and 75%. Equivalently, or alternatively, the processing of S200 can be carried out such that the scale layer 3 of the first steel strip 1 and the scale layer 4 of the second steel strip 2j are each reduced by a thickness between 0.5 micrometers and 15 micrometers, more preferably by a thickness between 1 micrometer and 5 micrometers.

[0079] Surprisingly, it was found that complete removal of the scale layers 3, 4 is not necessary to significantly improve the welding properties of the steel strips 1, 2 in the area of ​​the boundary zone 10. It is assumed, without this being meant as a limitation, that this is due to near-surface porosity of the scale layers 3, 4, the removal of which thus improves the welding properties.

[0080] As indicated in Fig. 2, the boundary zone 10 comprises areas of both steel strips 1, 2 adjoining the respective steel strip edges 7, 8, and its dimensions are defined by the extent of the machining by the (at least one) plasma torch 111. Preferably, the (at least one) plasma torch 111 is directed towards the gap between the steel strip edges 7, 8, and the boundary zone 10 is thus centered on the gap.

[0081] In step S300, the first steel strip edge 7 and the second steel strip edge 8 are welded along the butt joint from the at least one surface Al by means of a welding head, for example a laser or seam welding head s. Advantageously, the processing S200 of the boundary zone 10 is carried out at least partially simultaneously, preferably predominantly simultaneously, with the welding, although not necessarily at the same location in the boundary zone. Fig. 3, for example, illustrates that the (at least one) plasma torch 111 and (at least) one welding head 121 (shown here as a laser welding head) are arranged on (at least) a common slide 131 and thus move at a fixed distance (approximately between 10 cm and 20 cm, for example 16 cm) from each other along the gap between the steel strips 1, 2 over the boundary zone 10.

[0082] For this purpose, the carriage 131 is moved to the gap between the steel bands 1, 2, or, preferably, the steel bands 1, 2 are advanced until the gap is in line with the linear path of movement of the carriage 131. The plasma torch 111 then begins processing S200 of the boundary zone 10 at a starting point S of the gap, and the carriage 131 moves linearly along the gap until the welding head 121 also reaches the starting point S and begins welding S300. The processing S200 of the boundary zone 10 ends at an endpoint E of the gap, whereupon the carriage 131 moves further until finally the welding head 121 also reaches the endpoint E and completes the welding S300 there.

[0083] Thus, the welding S300 of the steel strips 1 , 2 can in particular comprise a uniform movement S310 of the at least one plasma torch 111 and the at least one welding head 121 by moving the common slide 131 .

[0084] The steel strips 1, 2 can then be moved further in their welded state, in particular perpendicular to the gap and parallel to the surfaces Al, A2, while the carriage 131 can be returned to its initial position. The welded steel strips 1, 2 can now be considered a new first steel strip 1, and a subsequent further steel strip a new second steel strip 2, and the process can begin again with the joining S 100 of the first and the second steel strips 1, 2. In this way, an endless strip can be produced. The method according to the invention can therefore also be described as a method for producing an endless steel strip.

[0085] Welding S300 is performed, for example, at a speed of at least 2 meters per minute, preferably at least 7 meters per minute, particularly preferably at least 10 meters per minute, for example at least 12 meters per minute. If the at least one plasma torch 111 and the at least one welding head 121 are moved uniformly, for example because they are mounted on the same carriage 131, the processing S200 of the boundary zone 10 is also performed at the same speed. Alternatively, the plasma torch 111 and the welding head 121 can also be arranged on different carriages; in this case, the carriage with the plasma torch 111 moves forward, and the processing S200 is performed at the same or a higher speed than the welding S300, so that the welding S300 never has to be delayed due to incomplete processing of the boundary zone 10.

[0086] To ensure that the welding process S300 advantageously encompasses only machined areas of the steel strips 1, 2, and thus in particular excludes unmachined areas of scale layers 3, 4, 5, 6, the boundary zone 10 is designed such that it has a greater width than the diameter of a zone that can be machined by the welding head 121. Thus, for example, the diameter of the plasma flame of the plasma torch 111 on the first surface Al can be larger than the diameter of a laser spot of a laser welding head on the first surface Al, with both following the same path across the boundary zone 10 (e.g., centered on the gap). It is also possible that the plasma flame and the laser spot on the first surface Al are of the same size.Advantageously, a period of 120 seconds or less, preferably 50 seconds or less, in particular 20 seconds or less, particularly preferably 5 seconds or less, for example 1 second or less, ...

[0087] The main parameters for the S200 plasma processing step are the distance between the nozzle of the plasma torch 111 and the surface Al of the steel strips Al, S2, and the current of the plasma torch 111. The smaller the distance between the nozzle of the plasma torch 111 and the surface Al, and the higher the current, the more the scale layer thickness is processed, and in particular, reduced. In this specific application, the processing speed is typically determined by the welding speed and can therefore only be varied within a small range. The goal is a high throughput of steel strips 1, 2 through the process, and therefore it is undesirable if this throughput were further reduced by the S200 processing step, in addition to the necessary welding. Optimization of the S200 processing step is also possible via the nozzle geometry.

[0088] There are many alternatives to the situations or implementation methods shown in Fig. 2 and Fig. 3.

[0089] Fig. 4 illustrates a variant in which a first plasma torch 112 is provided and used for processing the boundary zone 10 on the first surface Al of the first steel strip 1, and a second plasma torch 113 is provided and used for processing the boundary zone 10 on the first surface Al of the second steel strip 2. The first and second plasma torches 112, 113 can advantageously be moved uniformly and, in particular, mounted on the same carriage. In this variant, for example, a wider boundary zone 10 can be formed and processed more easily than if a very wide plasma flame had to be generated with a single plasma torch 111.

[0090] The two plasma torches 112, 113 can be directed perpendicularly to the surface Al, as shown schematically in Fig. 4, or they can be inclined (i.e. at an angle less than 90°) to the surface Al, in particular in an orientation inclined towards each other.

[0091] This variant is also suitable, for example, if the scale layers 3, 4 of different steel strips 1, 2 are to be processed in different ways, i.e., if their layer thickness is to be reduced to different degrees. The two plasma torches 112, 113 can then be used with different power and / or for different durations. Alternatively, especially in the case of two identical scaled steel strips 1, 2, they can also be used with the same power, the same duration, etc.

[0092] If only one of the two steel bands 1, 2 is scaled, the corresponding one of the two plasma torches 112, 113 can remain unused or be omitted entirely, so that the boundary zone 10 would then only be in contact with one side of the butt joint.

[0093] Fig. 5 illustrates a variant in which a first plasma torch 111 is provided and used for processing the boundary zone 10 of both the first steel strip 1 and the second steel strip 2 on the first surface Al of the first steel strip 1, and a second plasma torch 114 is provided and used for processing the boundary zone 10 of both the first steel strip 1 and the second steel strip 2 on the second surface A2. The first and second plasma torches 111, 114 can advantageously be moved uniformly and, in particular, mounted on the same carriage. This variant is advantageous, for example, if welding is also to be carried out from both surfaces Al, A2. However, even when welding through the butt joint from one surface Al, A2, it is advantageous if the scale layer present on the other surface A2, A2 is processed according to the invention, in particular reduced.

[0094] The reduction of the scale layers 3, 4, 5, 6 on both sides can be carried out to the extent described above in both relative and absolute values.

[0095] Fig. 6 shows a variant that combines the variants from Fig. 4 and Fig. 5, namely an arrangement and use of one plasma torch 112, 113, 115, 116 for each steel strip 1, 2 and for each Al, A2 surface, for a total of four. This variant is suitable, for example, if the scale layers 3, 4, 5, 6 of different steel strips 1, 2 and / or on different Al, A2 surfaces are to be processed differently, e.g., their layer thickness is to be reduced to different degrees. Furthermore, the variants described with reference to Fig. 4 can also be applied here, according to which, if one of the steel strips (on one or both sides) is free of scale, a corresponding plasma torch 112, 113, 115, 116 can remain unused or be omitted.

[0096] Returning to Fig. 1, the method can optionally include further steps, which are particularly aimed at ensuring that the method is only carried out if properties of the steel strips 1, 2 so require or suggest. For this purpose, properties of the first steel strip 1 and / or the second steel strip 2 can be determined in a step S10. In the following, it is assumed that an endless strip is produced in the method according to the invention, i.e., that the respective first steel strip 1 and the respective second steel strip have essentially the same composition. In cases where the steel strips 1, 2 have different compositions, a separate procedure can be performed for each steel strip 1, 2.

[0097] In step S20, a porosity measure P is determined based on at least the recorded properties. This measure (explicitly or implicitly) indicates the probability of pore formation during welding S300 with the welding head 121. The value of the porosity measure P is higher the more carbon is contained in the composition of the first steel strip 1 or the second steel strip 2, and lower the more of at least one predetermined element is contained in the composition. The at least one predetermined element is from the following list: silicon, aluminum, manganese, niobium, titanium, and / or chromium.

[0098] In particular, a porosity measure P can be defined, for example, as follows: where the element symbols each represent the mass fraction of the respective element in the steel strip.

[0099] In step S30, it is checked whether the specified porosity measure P exceeds a predetermined threshold. For the porosity measure P defined above, the predefined threshold can be, for example, 0.1 or greater, such as 0.13 or greater, and in particular 0.15 or greater. In this variant, machining S200 of the boundary zone 10 before welding S300 is only carried out if the check S30 shows that the specified porosity measure P exceeds the predetermined threshold (indicated by the "+" symbol in Fig. 1). If the predetermined threshold is not exceeded (indicated by the "-" symbol in Fig. 1), machining S200 is not carried out, and instead, for example, welding S300 is performed directly after step S100.

[0100] For example, a low-alloy steel strip with the composition 0.04% C, 0.008% Si, 0.2% Mn, 0.05% Al (and the remainder Fe) has a porosity value of 0.16 calculated according to the formula above and would therefore require machining S200 to avoid porosity (“+” symbol in Fig. 1).

[0101] It is understood that an analogous definition with an inversely defined threshold and its being undercut can also be used; likewise, the case of the value of the porosity measure P being equal to the threshold can also trigger the execution of processing S200.

[0102] Fig. 7 shows a schematic representation of a device according to a further embodiment of the present invention, namely a device 100 for welding steel strips 1, 2. The device 100 can be configured to carry out the method according to the invention, in particular according to Fig. 1, but can also be used independently thereof. Accordingly, the device 100 can be adapted according to all embodiments, variants, options, and further developments of embodiments described with respect to the method according to the invention, and vice versa.

[0103] The device 100 comprises a conveying device 140, which is configured to bring together a first steel strip edge 7 of a first steel strip 1 with (at least) one scale layer 3, 5 and a second steel strip edge 8 of a second steel strip 2 with (at least) one scale layer 4, 6 in a butt joint. For this purpose, the device may include conveyor belts, rollers, cylinders, clamps 11, 12 and the like to convey, grip, fix, guide, and perform the same actions on steel strips 1, 2 (and optionally an endless steel strip produced therefrom).

[0104] The device 100 can also include a cutting device 150, which is configured to cut off a steel strip end of the first steel strip 1 to produce the first steel strip edge 7, and to cut off a steel strip end of the second steel strip 2 to produce the second steel strip edge 8, for example, as described above with reference to steps S 120 and S 130. The cutting device 150 can comprise a mechanical shear and / or a welding cutter, such as a laser welding cutter. It can be provided that the same cutting device 150 performs both cuts and that the steel strips are moved between them, for example by the conveyor 140, or that a separate cutting device 150 is provided for each of the two cutting steps S 120 and S 130.

[0105] The device 100 further comprises at least one plasma torch 111-116, which is configured to process a boundary zone 10 around the butt joint on at least one surface Al of the first steel strip 1 and the second steel strip 2, in particular to reduce the scale layers 3, 4, 5, 6, at least on one surface Al, A2, for example as described above with reference to step S200. The device 100 can in particular have one or more plasma torches 111-116, which are arranged and configured as described above and in particular according to any of the variants shown in and described with reference to Figures 2 to 6.

[0106] The device 100 also includes a welding unit 120, which is configured to weld the first steel strip edge 7 and the second steel strip edge 8 along the butt joint from the at least one surface Al. The welding unit 120 includes at least one welding head 121, but can also have several welding heads, as already explained above. For example, the welding S300 of the butt joint can be carried out on a first surface Al with a first welding head 121 (e.g., a laser welding head) and on the second, opposite surface A2 with a second welding head. The welding heads can be arranged on a common slide 131 or each on separate slides. Each of the slides can be connected to the plasma torch(es) 111-116 provided for the respective surface Al, A2.

[0107] The at least one plasma torch 111-116 and the welding device are configured such that the processing S200 of the boundary zone 10 by the at least one plasma torch 111-116 is carried out at least partially simultaneously, preferably predominantly simultaneously, with the welding S300 by the welding device 130. As already described above, this can be achieved in particular by the uniform movement S310 of the plasma torch(s) 111-116 and the welding head(s) 131. Predominantly simultaneous means that the predominant part (more than 50%, in particular at least 75%, particularly preferably at least 85%) of the boundary zone 10 is processed by one plasma torch 111-116, while at the same time another area of ​​the boundary zone is processed, i.e., welded, by the welding head(s) 131.The device 100 can also have a control unit 160 which is configured to carry out the process steps S 10 , S20 and S30, so that the device 100 can be configured to carry out the welding S300 only if the properties of the first and / or the second steel strip 1 , 2 , meet certain conditions, for example with regard to their composition or the parameters of their manufacturing process, as explained above.

[0108] Such a control device 160 can be implemented as any device capable of performing calculations, and in particular of executing software, an application, or an algorithm. The control device 160 can, for example, have at least one processor unit, such as a central processing unit (CPU) and / or a graphics processing unit (GPU) and / or a field-programmable logic gate (FPGA) and / or an application-specific integrated circuit (ASIC) and / or a combination thereof. The control device 160 can also have a working memory that is operationally coupled with the at least one processor unit, as well as a non-volatile memory that is operationally coupled with the at least one processor unit and the working memory.The control unit 160 can be implemented wholly or entirely in a local device and / or wholly or entirely in a remote system such as a remotely located server and / or a cloud computing platform.

[0109] Figures 8A to 10B show schematic (A) and experimentally determined (B) representations of cross-sections through steel strips to illustrate the effect of the machining process S200 of the steel strips 1, 2 according to the invention. Figures 8A and 8B each show a cross-section through a section of the upper half of a scaled steel strip 1, where Figure 8A is a schematic representation and Figure 8B is a corresponding experimentally determined representation. Theoretically, the scale layer 3 is of uniform thickness as in Figure 8A; however, in reality, both the surface of the steel strip 1 itself and that of the scale layer 3 are uneven, resulting in different diameters, as shown by way of example (though not necessarily to scale) in Figure 8B.

[0110] Fig. 9A and Fig. 9B each show the condition after processing S200 of the steel strip 1, in which in particular the scale layer 3 was significantly reduced, and essentially uniformly, for example everywhere by 50%-60%, on average by 55%.

[0111] Figures 10A and 10B show the condition after processing S200 according to one variant or under different conditions (e.g., a different composition of the steel strip 1): here, the thickness of the scale layer 3 is not uniformly reduced, but section by section. The average reduction is approximately 35%.

[0112] Fig. 11A shows an X-ray examination of a laser weld 13 on a workpiece made of scaled, untreated steel, on which no machining S200 of the boundary zone 10 according to the inventive method took place after hot rolling. The relatively high porosity of this laser weld 13 is recognizable by a multitude of pores 14 (darker areas in Fig. 11A, not all of which are explicitly marked with reference numerals).

[0113] Fig. 11B, in contrast, shows a laser weld 13 on the same workpiece as in Fig. 11A after the

[0114] The boundary zone 10 of S200 was processed according to the inventive method after hot rolling, whereby the scale layer 3, 5 in the boundary zone 10 was reduced by between 50% and 60%, as shown in the X-ray examination. The significantly smaller total area in Fig. 11B compared to Fig. 11A, which is covered with darker areas, shows the significantly lower number of pores 14 and thus the significantly reduced porosity of the laser weld 13 after application of the inventive method. Accordingly, the laser weld 13 in Fig. 11B is again of a significantly higher quality than that in Fig. 11A.

[0115] For these examples, steel strips of steel grade DX51D according to standard EN 10346 were used, which, in mass %, contain max. 0.18% carbon, max. 0.5% silicon, max.

[0116] It contains 1.2% manganese, max. 0.12% phosphorus, max. 0.045% sulfur, and max. 0.3% titanium.

[0117] Reference character list

[0118] 1 first steel band

[0119] 2 second steel band

[0120] 3 First layer of scale on the first steel strip

[0121] 4 first scale layer of the second steel strip

[0122] 5 second scale layer of the first steel strip

[0123] 6 second scale layer of the second steel strip

[0124] 7 first steel band edge of the first steel band

[0125] 8 second steel band edge of the second steel band

[0126] 10 Border zone

[0127] 11 terminal

[0128] 12 terminals

[0129] 13 Laser weld seam

[0130] 14 pores

[0131] 100 Device

[0132] 111 Plasma torches

[0133] 112 plasma torches

[0134] 113 Plasma torches

[0135] 114 Plasma torches

[0136] 115 plasma torches

[0137] 116 plasma torches

[0138] 120 welding equipment

[0139] 121 Welding head

[0140] 131 sleds

[0141] 140 funding facility

[0142] 150 separating device

[0143] 160 Control unit

[0144] Al first surface

[0145] A2 second surface

[0146] E Endpoint

[0147] S Starting point

[0148] S 10 . . S300

[0149] Procedural steps

Claims

Patent claims 1. Method for welding steel strips (1, 2) , comprising at least the steps: - Joining (S100) , in a butt joint, a first steel strip edge (7) of a first steel strip (1) with a scale layer (3, 5) and a second steel strip edge (8) of a second steel strip (2) ; - Machining (S200) of a boundary zone (10) at or around the butt joint on at least one surface (Al, A2 ) of the first steel strip (1) and the second steel strip (2) using at least one plasma torch (111-116) to reduce the thickness of the scale layer (3, 5, ) on the at least one surface (Al, A2 ); and - Welding (S300) of the first steel strip edge (7) and the second steel strip edge (8) along the butt joint from the at least one surface (Al, A2) by means of a welding head (121) ; wherein the machining (S200) of the boundary zone (10) is carried out at least partially simultaneously with the welding (S300).

2. Method according to claim 1, wherein the machining (S200) of the boundary zone (10) is carried out at least at the same speed along the butt joint as the welding (S300) and begins at the latest with the commencement of the welding (S300).

3. The method of claim 2, further comprising: a movement (S310) of the at least one plasma torch (111— 116) for processing (S200) the boundary zone (10) and of the welding head (121) for welding (S300) the first steel strip edge (7) and the second steel strip edge (8) uniformly at a constant distance along the butt joint .

4. Method according to claim 3, wherein the movement (S310) of the at least one plasma torch (111-116) and the welding head (121) is carried out by moving a common slide (131) on which the at least one plasma torch (111-116) and the welding head (121) are mounted.

5. Method according to any one of claims 1 to 4, wherein the welding (S300) is carried out at a speed of at least 2 meters per minute, preferably at least 7 meters per minute, particularly preferably at least 10 meters per minute.

6. Method according to any one of claims 1 to 5, wherein the processing (S200) of the boundary zone (10) is carried out such that the scale layer (3, 5) is reduced by between 10% and 90%, preferably between 20% and 70%, particularly preferably between 35% and 65%.

7. Method according to any one of claims 1 to 6, wherein the processing (S200) of the boundary zone (10) is carried out such that the scale layer (3, 5) is reduced by a thickness between 0.5 The thickness is reduced from micrometers to 15 micrometers, preferably by a thickness between 1 micrometer and 5 micrometers.

8. Method according to any one of claims 1 to 7, wherein a period of 120 seconds or less, preferably 50 seconds or less, in particular 20 seconds or less, particularly preferably 5 seconds or less, for example 1 second or less, lies between the machining (S200) of a place of the boundary zone (10) and its welding (S300).

9. Method according to any one of claims 1 to 8, wherein the boundary zone (10) on both surfaces (Al, A2 ) of the first steel strip (1) and the second steel strip (2) is treated with at least one plasma torch (111-116) each to reduce the scale layer (3, 4, 5, 6) on both sides.

10. Method according to any one of claims 1 to 9, wherein the second steel strip (2) also has a scale layer (4, 6), and the processing (S200) of the boundary zone (10) to reduce the thicknesses of the scale layers (3, 4, 5, 6) is carried out on the at least one surface (Al, A2 ) of both the first steel strip (1) and the second steel strip (2).

11. Method according to any one of claims 1 to 10, wherein the machinable boundary zone (10) has a greater width than a diameter of a zone machinable by the welding head (121).

12. A method according to any one of claims 1 to 11, wherein the method further comprises the steps: - Cutting (S120) one steel strip end of the first steel strip (1) to produce the first steel strip edge (7) ; and - Cutting (S130) one steel strip end of the second steel strip (2) to produce the second steel band edge (8) .

13. A method according to any one of claims 1 to 12 further comprising: Recording (S10) properties of the first steel strip and the second steel strip; Determine (S20) , based at least on the recorded properties, a porosity measure which gives a probability of pore formation during welding (S300) with the welding head (121), wherein a value of the porosity measure is higher the more carbon is contained in the composition of the first steel strip (1) or the second steel strip (2) and lower the more at least one predetermined element is contained in the composition, wherein the at least one predetermined element comprises silicon and / or aluminum; and Check (S30) whether the determined porosity measure exceeds a predetermined threshold; wherein the processing (S200) of the boundary zone (10) is only carried out if the check (S30) shows that the determined porosity measure exceeds the predetermined threshold.

14. Method according to any one of claims 1 to 13 wherein a further steel strip is provided; and wherein, after welding the first and the second steel strip (1, 2) together in a first iteration of the method, a result of the welding is used as the first steel strip (1) for a further iteration of the method and the further steel strip is used as the second steel strip (2) for the further iteration of the method.

15. Method according to any one of claims 1 to 14, wherein the first steel strip (1) is processed in the boundary zone (10) by means of at least one first plasma torch (111, 112, 115), and the second steel strip (2) in the boundary zone (10) is processed by means of at least one second plasma torch (112, 113, 116).

16. Method according to one of claims 1 to 14, wherein the entire boundary zone (10) of the first steel strip (1) and the second steel strip (2) is processed by means of a single plasma torch (111, 114).

17. Device (100) for welding steel strips (1, 2) , comprising: a conveying device (150) which is configured to bring together a first steel strip edge (7) of a first steel strip (1) with a scale layer (3, 5) and a second steel strip edge (8) of a second steel strip (2) in a butt joint (S100) ; at least one plasma torch (111-116) which is configured to process a boundary zone (10) at or around the butt joint on at least one surface (Al, A2) of the first steel strip (1) and the second steel strip (2) (S200) in order to remove the scale layer (3, 5) on the at least one to reduce the surface area (Al, A2); and a welding device (120) which is configured to weld the first steel strip edge (7) and the second steel strip edge (8) along the butt joint from the at least one surface (Al, A2) (S300); wherein the at least one plasma torch (111-116) and the welding device (120) are configured such that the processing (S200) of the boundary zone (10) by the at least one plasma torch (111-116) is carried out at least partially simultaneously with the welding (S300) by the welding device (120).

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