METHOD FOR PRODUCING A CORROSION-RESISTANT, WORKABLE SHEET METAL WITH FULL-SURFACE COATING OF THE JOINED, THERMALLY TREATED STEEL SHEETS

Abstract

The invention relates to a method of producing corrosion-resistant and workable sheet metal consisting of uncoated steel sheets (1, 1′), comprising the following steps: —placing the steel sheets (1, 1′) in abutting relationship, —welding the or each joint groove (14) by butt joint welding by means of a welding beam (13) for forming a weld (2) along the respective joint groove (4), —thermal treatment of the or each weld (2) directly after or even during the forming of the weld (2) by means of an annealing beam (15), —full-surface coating of the joined steel sheets (1, 1′), including the or each weld (2), after cooling the welds (2), with a metallic coating.

Description

FIELD OF THE INVENTION

The present invention relates to a method of producing corrosion-resistant and workable sheet metals and also relates to a device for applying a metallic coating to butt-joint welds in steel sheets.

BACKGROUND OF THE INVENTION

Sheet metals produced with this method can be used for the production of containers for packing foodstuffs, for example, cans of tinned food or beverage cans or even in other fields in which corrosion-resistant sheet metals with good workability are required, for example for production of automotive body panels. The sheet metals must be corrosion-resistant for use of sheet metals in the production of food packages or automotive body panels. In the case of food packages, the sheet metal must be corrosion-resistant such that the sheet metal surface is not attacked by the aggressive filling. For this reason, steel sheets that are used for the production of food packages are provided with a corrosion-resistant coating. In this case, it can be a metallic coating as is the case with tin plate (tinned steel sheet).

For producing food packages, the coated sheet metal must then be reshaped, for example, in a deep-drawing process. During the production of coated sheet metals, defective or damaged spots in the sheet metal are cut out and the defect-free sheet metal parts are subsequently welded together to form a sheet metal strip of standard length that is wound on to a reel. Such sheet metal strips that have been welded together from many pieces of sheet metal have unsatisfactory forming behavior because the sheet metal in the region of welds cannot be shaped in a satisfactory manner. For example, corrosion-resistant steel sheets are used for producing tin cans which comprise one welded body and two folded lids. In the region of welds, the sheet metal which is composed of individual steel sheets has a distinctly higher hardness than the individual steel sheets, resulting in inferior workability of sheet metal in the region of welds.

Therefore, in the prior art, methods have been recommended for producing workable steel sheets with which reducing the hardness of welds of at least two steel sheets that are joined together is achieved in order to improve their workability. EP 540 382 A1 describes a method for producing a deep-drawn steel sheet formed from at least two elements in which the edges of the elements are welded by beam welding wherein the welded edges of the elements are swept with a laser beam before welding to reduce the hardness of the weld, and oxygen is fed simultaneously. Thus, decarburization and oxidation of edges is achieved.

The preparatory laser beam affects the geometrical formation of the sheet edge in the case of thin sheet thicknesses in a disadvantageous manner, since the material is stretched above its plastic limit due to thermal energy inputs. This leads to increased internal stresses and leads to component warping at the sheet edge. An accurate implementation of subsequent laser welding without height offsets, and alternating weld widths, and with uniform seam geometry seems improbable to achieve, but it must be available for optimum shaping results. Furthermore, this type of heat treatment in combination with material decarburization leads to the formation of distinct heat-affected zones with very soft microstructures at which the material always fails during the shaping operations. The introduction of oxygen in the welding region also disguises the danger of material embrittlement and of surface oxidation phenomena in the regions close to the weld where the temperature remains below the decomposition temperature of FeO₂ into Fe and O₂.

SUMMARY OF THE INVENTION

Based on this, the task of the invention is to produce as much as possible a corrosion-resistant and easily workable sheet metal which is put together from individual steel sheets that are welded to each other.

This problem is solved with the method as set forth in the claims. Preferred embodiments of this method are to be taken from the subclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is better explained in the following with the help of embodiments with reference to the attached drawings. The drawings show:

FIG. 1: Schematic representation of a partial step of the inventive method by means of a longitudinal section through a weld of two steel sheets joined together;

FIG. 2: Top view of the steel sheets joined together during a partial step of the inventive method;

FIG. 3: Representation of temperature-time cooling diagram for a weld during a procedural step of the inventive method;

FIG. 4 a: Cross section through a device for electrodeposition on a weld in a steel sheet with a metallic coating, shown in the process step during the electrodeposition on the weld;

FIG. 4 b: sectional representation of device of FIG. 4a during the implementation of cleaning process for cleaning the electrocoated the weld;

FIG. 4 c: Top view of the upper anode strip of the device of FIG. 4a;

FIG. 5: Schematic representation of a device for automated regeneration of an application device of the device of FIG. 1 for applying electrolyte onto the weld;

FIG. 6: schematic representation of the cleaning unit of the device of FIG. 4;

FIG. 7: sectional representation of the cleaning unit of FIG. 6;

FIG. 8: sectional representation of a possible embodiment of the anode of the device of FIG. 4;

FIG. 9: schematic representation of an application device of the device of FIG. 4, for applying an electrolyte onto the weld;

FIG. 10: sectional representation of the device of FIG. 4 during implementation of a drying step for drying the electrocoated and cleaned weld.

DETAILED DESCRIPTION OF THE INVENTION

According to a method of the invention, a corrosion-resistant and easily workable sheet metal is produced from two or more uncoated steel sheets with sheet thicknesses between 0.10 mm and 0.70 mm in which the bare steel sheets are first placed next to each other in a butting relationship, and subsequently each joint groove is welded by butt-joint welding by means of a welding beam forming a weld. The welding beam can be a laser welding beam or an electron welding beam. A thermal treatment of each weld is effected by means of an annealing beam during or directly after welding; this annealing beam is preferably formed from a laser beam. After cooling of the welds, the sheet metal which is formed from the steel sheets welded together is coated with a metallic coating at least on one side or even on both sides, wherein the or each weld is also coated on one side or both sides.

According to another method of the invention, a corrosion-resistant and easily workable sheet metal is produced from two or more steel sheets that are coated with a metallic coating, wherein the coated steel sheets are first placed next to each other in an abutting relationship, and subsequently the joint grooves or each joint groove is welded by butt-joint welding by means of a welding beam forming a weld along the respective joint grooves. During or directly after welding, each weld is treated thermally by means of an annealing beam. After cooling of the welds, each weld is covered with a strip-shaped, metallic coating on one side or both sides. The coating of welds is in this case preferably effected by electrodeposition on the respective weld.

Because of the thermal treatment of welds during or directly after welding, the temperature changes of the weld during cooling can be passed through over a designated period of time in a controlled manner. It has been observed that there is an increase in hardness of welds due to martensite and bainite formation, and also due to residual welding stresses during cooling of the weld. The cooling range between 800° C. and 500° C. is especially critical for martensite formation. If this temperature range is controlled during cooling of the weld and passed through more slowly than was specified at the natural cooling rate, martensite and bainite formation can be suppressed and the internal welding stresses can be reduced, and thus an increase in weld hardness can be avoided. Due to lower weld hardness, the sheet metal which has been welded together from steel sheets then has better workability.

Finally, the welds are protected against corrosion by metallic coating of the welds in the subsequent process step, such that an easily workable and simultaneously corrosion-resistant sheet metal can be produced with the inventive method.

It has been shown that the martensite and bainite formation in the welds can be suppressed very efficiently if the temperature of the weld during cooling is held in the temperature range between 800° C. and 500° C. that is critical for structural composition for a period of 1-3 sec, preferably 1.5-2 sec, by the annealing beam.

The laser beam can be preferably a welding beam or even an annealing beam, wherein the welding beam is directed in the form of a highly focused point in the region of the joint groove on the surface of the steel sheets to be joined, while the annealing beam is directed onto the surface of the weld as a line focus or even as a round, rectangular or elliptical focus with the application area being considerably greater compared to the focus of the welding beam. Preferably, the annealing beam follows the welding beam in the welding direction. Then, a thermal post-treatment of the weld is undertaken in short time intervals after the end of the welding period. The diameter of the annealing beam focus is preferably selected such that at least the entire width of the weld is covered by the annealing beam focus. The diameter of the annealing beam focus can be adjusted to be even bigger in order to encompass, for example, the heat-affected zone in the lateral direction around the weld or even regions outside its heat-affected zone. The application time of the annealing beam can be adjusted either via the speed of welding or via the length of the annealing beam focus at a constant welding speed. The annealing beam is preferably formed from a laser beam because great variation of the thermal energy coupled into the weld is thus possible. Inductive or conductive heat sources can also be used for producing the annealing beam, wherein these require distinctly more energy for achieving the same results due to their lower energy densities. This has proven to be disadvantageous due to the higher internal stresses and warping or creasing generated in the weld region of.

A double laser system is preferably used for the thermal treatment of the weld during welding. This system provides the welding beam in the form of a welding laser beam and the annealing beam in the form of an annealing laser beam. Thus the thermal treatment can take place directly at and synchronously with the front of the weld pool advancing in the direction of welding during welding.

FIG. 1 is a schematic representation of the longitudinal section of a joint groove 14 between two steel sheets 1, 1′ placed next to each other in an abutting relationship. It is butt-welded by irradiation with a welding beam 13; this beam is directed as a highly focused point in the joint groove 14. The welding beam 13 thus advances in the welding direction S with a pre-set welding speed. A weld pool 16 is formed in the region of the advancing welding beam 13 in the form of liquefied steel. The annealing beam 15 is directed onto the surface of the steel sheets 1, 1′ along the developing weld 2 in the welding direction S directly behind the welding beam 13, wherein the diameter of annealing beam focus is considerably greater when compared to the diameter of welding beam focus.

In FIG. 2, the arrangement of the steel sheets 1, 1′ joined next to each other of FIG. 1 is shown in top view. The shape of the annealing beam focus—as can be seen from FIG. 2—can take the shape of an oval and/or an elliptical shape. Preferably, the focus of the annealing beam 15 has the shape of a line focus, wherein the long main axis of the line focus runs lengthwise to the formed weld 2. An annealing region 17 is formed around the annealing beam focus. In this annealing region 17, the weld 2 formed by welding of the joint grooves 14 experiences thermal post-treatment directly after the end of the welding period, i.e. during the cooling of weld 2. The incident angle α, at which the annealing beam 15 strikes the surface of the steel sheet, is preferably adjustable. The application time, the intensity and the incident angle of the annealing beam 15 are preferably adjusted such that the weld 2 remains in a temperature range of 500° C. to 800° C. during cooling for a period of 1-3 sec. The welding beam 13 is preferably incident—as shown in FIG. 1—perpendicular to the surface of steel, it can also be incident slantwise.

In FIG. 3, the time curve of the temperature of weld 2 at a specified spot in the joint groove 14 is shown during and after welding. During the welding period, the temperature first increases tremendously when the welding beam 13 travels across this spot. The temperature reaches values above the melting temperature T_S of the steel sheets such that these are fused. After passing through the welding beam 13, i.e., after the end of the welding period, the temperature drops again, wherein the shape of temperature-time curve which is generated by the effect of welding beam in the weld 2 has the shape of a Gaussian curve in an idealized manner, as is seen from FIG. 3. The heat input due to the annealing beam 15 takes place in the weld 2 during and even after the effect of the welding beam according to the inventive method in order to maintain the temperature of the weld within the temperature range between 500° C. and 800° C. for a longer period of time. This temperature range between 500° C. and 800° C. represents the transformation range in which martensite formation takes place during weld cooling for quick passage through this temperature range according to the natural cooling rate. The effect of annealing beam 15 holds the temperature of weld 2 in the temperature range of this transformation range for a longer period of time of, for example, 1-3 sec. This follows from the resultant temperature-time curve shown in FIG. 3. Depending on the duration and intensity of the incidence of the annealing beam, the temperature of the weld is found in the transformation range for a shorter or longer time period. It has been observed that martensite formation can be largely suppressed if the temperature of the weld can be held in the transformation range for at least a period of 1.5-2 sec.

The butt-joint welding method described above with simultaneous or subsequent thermal treatment of the weld by the annealing beam is applied in the same way for both of the inventive methods. In one inventive method, two uncoated steel sheets 1, 1′ are first welded to each other in this way and are subsequently coated over the full surface with a metallic coating after welding, wherein the or each weld 2 is also coated with the metallic coating. The full surface coating of the joined steel sheets 1, 1′ is in this case effected in the known way, for example, by galvanic tinning or chrome plating in a known strip tinning and/or strip chrome plating plant.

In the second inventive method, steel sheets 1, 1′ that have already been provided with a metallic coating are welded by the butt-joint welding method described above and treated thermally during or after the welding by means of annealing beam 15. After cooling of the welds, these are provided with a strip-shaped metallic coating by electrodeposition. In FIG. 4, a device is shown for the two-sided application of metallic coating on the weld 2, wherein FIG. 4a shows the device in the process step during the electrodeposition on the weld and FIG. 4b shows the same device during the implementation of a cleaning step for cleaning the electrocoated weld.

The device shown in FIG. 4 comprises two anode strips 4, 5 that are directed vertically and are movable perpendicular to the surface of the steel sheets 1, 1′. Both of the anode strips 4,5 are arranged aligned with each other on opposite sides of the steel sheets 1, 1′ and are preferably floating with respect to each other. The device further comprises a feed device for feeding fluid electrolyte which is used for electrodeposition on weld 2. The feed device is coupled to an electrolyte reservoir. Furthermore, the device comprises an application device for application of electrolyte onto the weld 2, and also comprises an anode 8 at which an electric potential difference can be applied relative to the steel sheets 1, 1′. A clamping device 11 is provided for clamping and fixing the steel sheets 1, 1′, by means of which the steel sheets 1, 1′ are positioned and fixed such that the two anode strips 4, 5 are arranged on the top and bottom side's of the steel sheets 1, 1′ running parallel to these sheets and lengthwise to the joint grooves 14.

The length of the two anode strips 4, 5 is at least the maximum width of steel sheet 1 to be treated (corresponding to the maximum length of the weld 2 to be treated). In FIG. 4c, the top anode strip 4 of the two anode strips shown in FIG. 4a is shown in top view with the steel sheet 1 arranged below and two vertical guides 30 in which the anode strip 4 is guided. The anode strips 4, 5 are produced from a corrosion-stable material, for example, from metal, especially acid-resistant stainless steel, or from nonmetallic materials, for example ceramic. For practical purposes, the anode strips 4, 5 are produced from an electrically insulating material since a protective device against electrical charging of anode strips 4, 5 can be omitted. The anode strips 4, 5 preferably have a hollow profile which can be seen from the sectional representations of FIG. 4a.

The application device for applying electrolyte onto the weld 2 is arranged inside the hollow anode strips 4, 5, wherein this application device of the embodiment shown in FIG. 4a comprises a pad 7 which is made from electrically nonconductive, open-cell material. The pad 7 can be open-cell foam or a felt or fleece material. The pad 7 projects out of an opening which runs lengthwise to each anode strip 4, 5 and protrudes slightly opposite the lower edges of anode strips 4,5 which face the steel sheets 1, 1′. The pad 7 is connected to a tube 6 inside the hollow profile of each anode strip 4, 5. Outlet openings are incorporated in the tube 6 through which the electrolyte fluid flowing in the tube 6 flows into the pad 7 and can be absorbed from it. Furthermore, an anode 8 is arranged within each anode strip 4, 5. In the embodiment shown in FIG. 4a, the anode 8 is formed from an electrically conductive tube, preferably a metallic tube, which runs within each anode strip 4, 5 in the longitudinal direction.

For electrodeposition on a weld 2, the steel sheets 1, 1′ which are joined to each other by the earlier described butt-joint welding method, are introduced into the treatment device and positioned such that the weld 2 runs parallel to the two anode strips 4, 5. The treatment device comprises a clamping device 11 for positioning and fixing the steel sheets 1, 1′ in this position. This clamping device 11 comprises two pairs of clamping jaws 11a and 11b as shown in the embodiment of FIG. 4a. These are arranged on the top and/or bottom side of the steel sheets 1, 1′. The clamping jaws are moveable perpendicular to the surface of the steel sheets 1, 1′ just like the anode strips 4, 5, and each have a gasket 12 at their lower edge facing the steel sheets 1, 1′. These are clamped between the pairs of clamping jaws 11a, 11b for positioning and fixing of the steel sheets 1, 1′, wherein the gasket 12 prevents mechanical damage to the surfaces of the steel sheets when setting them, and moreover seals the processing space in the region around the weld 2 in a liquid-tight manner. During clamping of the steel sheets 1, 1′ in the clamping device 11, the surface of the steel sheet, which has a waviness perpendicular to the direction of welding and of rolling due to internal stresses, is smoothed out.

After (or even before) positioning and clamping the steel sheet in the clamping device 11, the two anode strips 4, 5 are placed on either side of the steel sheets 1, 1′ in the region of weld 2, with the anode strips 4, 5 being moved in their respective guide 30 in the direction perpendicular to the surface of the steel sheets 1, 1′. In that case, the pad 7 is pressed onto the weld 2 with a prescribed pressure. Subsequently, an electrical voltage is applied between anode 8 and steel sheets 1, 1′, and simultaneously an electrolyte fluid is led through the feed device to the application device, by means of which the pad 7 is soaked with electrolyte fluid. An electrolyte film is formed on the surface of the steel sheets 1, 1′. This is shown in FIG. 4a with the reference number 3. Due to the voltage that has been applied between steel sheets 1, 1′ (and/or weld 2) and anode 8, the cations in the electrolyte migrate to the cathodically connected steel sheet and especially to the weld 2. The electrolyte cations on the sheet surface and/or weld surface are deposited by uptake of electrons. The sheet thickness of the metal coat that has been deposited on the weld 2 is determined exclusively by the processing time, given constant electrical characteristic values and stable cation concentrations in the electrolyte that has a prescribed electrical conductivity. Said processing time can be preset via a timer.

To ensure constant process parameters for each electrodeposition process of a weld 2, the pad 7 must be renewed at regular time intervals and/or regenerated, since the open-cell structure of pad 7 can be adversely affected, especially due to salting out, and thus, a regular flow of electrolyte onto the surface of the steel sheet could be locally prevented. The replacement and/or regeneration of pad 7 can be effected by the replacement of anode strips 4, 5. Alternatively, an automated pad changing unit can be used, as is shown schematically in FIG. 5. This pad changing unit comprises a supply roll 18 on which the pad material is wound. This supply roll 18 is arranged next to each anode strip 4, 5. When the pad 7 is used up, fresh pad material can be drawn from the supply roll 18 by manual activation or automatically via pre-set time control and drawn into the pad-receiver of each anode strip 4, 5, while the used-up region of pad 7 is simultaneously wound onto a take-up roll 19. A take-up roll 19 is arranged next to each anode strip 4, 5 on the side opposite the supply roll 18.

As soon as a sufficiently thick metallic layer is electrodeposited on the weld, the electrical voltage between the steel sheets 1, 1′ and anode 8 is switched off and the supply of electrolyte is shut off. The electrolyte fluid which remains on the steel sheet surface is kept in circulation to avoid salting out, and the anode strips 4, 5 are driven to their initial position away from the sheet surface after turning off the electrical voltage and electrolyte supply, as is shown in FIG. 4b.

Finally, cleaning of the surface of the steel sheet is performed by removal of electrolyte residues that remain on the surface of the steel sheet. For this purpose, the device has a cleaning device 13 with cleaning slides 13a, 13b that run in the horizontal direction parallel to the surface of the steel sheet, wherein a cleaning slide 13a and 13b, respectively is assigned to the top side and bottom side of the steel sheet. In FIG. 6, the travel of cleaning slides 13a, 13b is shown schematically. The cleaning slides 13a, 13b are guided in channels in the clamping jaws 11. The channels are designed in the shape of grooves 27 in which the guide webs 28 of the cleaning slide 13 engage, as shown in the sectional representation of FIG. 4b.

In FIG. 7, a cleaning slide 13a is shown in cross section. Each cleaning slide 13a, 13b bears a high-pressure spray nozzle 15 through which water can be sprayed onto the surface of steel sheet 1. Each cleaning slide 13a, 13b further has a suction device 16 operated by vacuum. Gaskets 14 are arranged on the bottom of each cleaning slide 13 which contacts the steel sheet surface, thus preventing damage to the surface and sealing the process space around the weld 2 in a leak-proof manner. Because of the leak-proof seal, the cleansing water cannot escape and external air cannot be sucked in from outside.

To implement the cleansing step, each cleaning slide 13a, 13b is driven over the steel sheet surface from its original position shown in FIG. 6 horizontally and lengthwise to the weld 2. For cleaning the steel sheet surface, water is sprayed through the nozzle 15 onto the steel sheet surface and the water-electrolyte mixture which is thereby formed is simultaneously sucked out by the suction device 16. If necessary, the cleaning process can be repeated several times. The required number of cleaning cycles can be preset by a control device. After the last cleaning process, each cleaning slide 13a, 13b is brought back to its initial position.

The electrolyte fluid is supplied through the tube 6 in the embodiment of the coating device shown in FIG. 4a. In an alternative embodiment, the electrolyte fluid can also be supplied through a tube-shaped anode 8, wherein the tube-shaped anode 8 likewise has outlet openings via which the electrolyte fluid can penetrate into the pad 7 that surrounds the anode 8. The anode tube 8 is preferably produced from an acid-resistant, electrically conductive material, for example, stainless steel in this embodiment.

In another embodiment, which is shown in FIGS. 8 and 9, the anode 8 is formed as slotted rectangular tube 20 (FIG. 8) on whose bottom side are arranged sealing strips 21. The rectangular tube 20 is used as an anode 8 and for this purpose, has an electrical voltage applied to it referenced to the steel sheet 1 and is used simultaneously as a supply device for supplying electrolyte fluid and as an application device for applying the electrolyte onto the weld. Such a rectangular tube 20 is arranged on the top side of the steel sheet running along the weld 2, and a structurally identical rectangular tube 26 is arranged on the bottom side, as shown in FIG. 9.

Both of the rectangular tubes 20, 26 are connected to a reservoir 22 of an electrolyte fluid (FIG. 9). The electrolyte fluid is pumped into the rectangular tube 20 via a pump and sprayed onto the steel sheet surface in the region of weld 2 via spray nozzles. Thus, the process space, which is sealed around the weld 2 by the gaskets 21, is flooded with electrolyte fluid. Excess electrolyte fluid can be sucked from the surface of steel sheet 1 during the process by a suction device and is delivered to reservoir 22 for recycling. As in the earlier described embodiment, cleaning of the steel sheet surface is performed by water cleansing after the completion of electrodeposition on weld 2 and suctioning off of the electrolyte-water solution. For this purpose, each rectangular tube 20, 26 is connected to water circulation unit 25 in this embodiment (FIG. 9). After the cleaning cycle, drying of the steel sheet surface preferably takes place, especially in the region of the electrocoated weld 2. The drying process can be implemented with compressed air or hot air. For this purpose, air nozzles 17 are provided in the clamping jaws 11a, 11b, as is shown in FIG. 10. Dry air can be blown onto steel sheet 1 at adjustable angles with these air nozzles 17. The degree of drying is controlled via the drying time, which is adjustable by means of a control device.

The welds 2 can be provided with a metallic strip-shaped coating with the described device in order to protect the welds 2 from corrosion. Due to thermal treatment of the welds during or after welding, the joined steel sheets 1, 1′ can be easily workable even in the region of welds 2, and are simultaneously protected against corrosion.

Claims

1. Method for producing corrosion-resistant and workable sheet metal consisting of uncoated steel sheets comprising the following steps: placing the steel sheets in abutting relationshipwelding the or each joint groove by butt-joint welding by means of a welding beam for forming a weld along the respective joint groove,thermal treatment of the or each weld directly after or even during the forming of weld by means of an annealing beam, wherein, during cooling after welding, the temperature of weld is held in a temperature range between 800° C. and 500° C. by the annealing beam for a time period of 1 to 3 seconds in order to prevent martensite formation in the region of weld.full-surface coating of the joined steel sheets, including the or each weld, after cooling the welds, with a metallic coating.

2. Method for producing corrosion-resistant and workable sheet metal consisting of the steel sheets coated with a metallic coating comprising the following steps: placing the coated steel sheets in abutting relationshipwelding the or each joint groove by butt-joint welding by means of a welding beam for forming a weld along the respective joint groove,thermal treatment of the or each weld directly after or even during the forming of weld by means of an annealing beam, wherein, during cooling after welding, the temperature of weld is held in a temperature range between 800° C. and 500° C. by the annealing beam for a time period of 1 to 3 seconds in order to prevent martensite formation in the region of weld.applying a strip-shaped metallic coating onto each weld after cooling the weld.

3. Method according to claim 1, wherein the coating of the steel sheets takes place by electrodeposition, especially by galvanic tinning, zinc coating or galvanic chrome-plating.

4. Method according to claim 1 wherein the annealing beam follows the welding beam in the welding direction.

5. Method according to claim 1, wherein the annealing beam is incident on the weld as a line focus.

6. Method according to claim 1, wherein the temperature of weld is held in a temperature range between 800° C. and 500° C. during cooling after welding for a time period of 1.5 to 2 seconds by means of an annealing beam.

7. Method according to claim 2, wherein the coating of weld is effected by electrodeposition on weld.

8. Method according to claim 7, wherein an electrical voltage is applied on the steel sheets referenced to an anode which runs along the weld at a distance from it for electrodeposition on weld, and an electrolyte is simultaneously applied onto the weld.

9. Method according to claim 7, wherein the coated steel sheets can be tin-plated or special chrome-plated steel sheets, and in that the electrolyte contains dissolved tin cations or chromium cations.

10. Method according to claim 8 wherein the electrolyte has an electrical conductivity of 50-500 mS/cm.

11. Method according to claim 8, wherein the electrolyte is applied onto the weld via a pad; this pad contacts the weld and is made from electrically nonconductive open-cell material.

12. Method according to claim 11, wherein the pad contacts the surface of weld at a prescribed pressure and covers it completely.

13. Method according to claim 8, wherein the electrolyte is sprayed onto the weld via a tube provided with at least one spray opening or spray nozzle.

14. Method according to claim 7, wherein the steel sheets are held mostly flat in one plane during the electrodeposition on weld by means of clamping elements in the region around the weld.

15. Method according to claim 8, wherein the steel sheets are cleaned of electrolyte residues that remain on the surface after electrodeposition on weld at least in the region around the weld.

16. Method according to claim 1, wherein the cooling time of weld is prolonged by the annealing beam such that the cooling rate with which the weld cools is held below the cooling rate at which martensite formation could have taken place in the region of the weld without the effect of annealing beam.

17. Method according to claim 2 wherein the annealing beam follows the welding beam in the welding direction.

18. Method according to claim 2, wherein the annealing beam is incident on the weld as a line focus.

19. Method according to claim 1, wherein the temperature of weld is held in a temperature range between 800° C. and 500° C. during cooling after welding for a time period of 1.5 to 2 seconds by means of an annealing beam.