CA2858292C

CA2858292C - Method for improving a metal coating on a steel strip

Info

Publication number: CA2858292C
Application number: CA2858292A
Authority: CA
Inventors: Dirk Matusch; Reiner Sauer; Helmut Oberhoffer; Rainer Thomas; Markus Opper
Original assignee: ThyssenKrupp Rasselstein GmbH
Current assignee: ThyssenKrupp Rasselstein GmbH
Priority date: 2012-01-23
Filing date: 2013-01-22
Publication date: 2016-09-20
Anticipated expiration: 2033-01-22
Also published as: RU2014132568A; AU2013211694B2; WO2013110577A1; BR112014016377A2; PL2807280T3; DE102012100509B4; PT2807280T; CN104066859A; JP2015509144A; CN104066859B; EP2807280B1; CA2858292A1; EP2807280A1; DE102012100509A1; ES2573835T3; AU2013211694A1; RU2590787C2; BR112014016377A8; US20150001089A1

Abstract

The invention relates to a method for improving a metal coating on a steel strip (1) or a steel sheet or plate, wherein the coating is melted to a maximum temperature (PMT) above the melting temperature (TS) of the material of the coating by inductive heating performed by means of at least one induction coil (2) and subsequently cooled to a quenching temperature (TA), lying below the melting temperature, in a cooling device (3). In order to improve the corrosion stability of the coating treated according to the invention, even in the case of thin coating layers, it is provided according to the invention that the coating is kept at a temperature above the melting temperature (TS) during a holding time (th) and that the holding time (th) is adapted to the maximum temperature (PMT) and the thickness of the coating by moving at least one of the induction coils (2) with respect to the cooling device (3), in order to melt the coating completely over its entire thickness to the boundary layer with the steel strip.

Description

METHOD FOR IMPROVING A METAL COATING ON A STEEL STRIP
The invention concerns a method to improve a metal coating on a steel strip or steel sheet according to the preamble of Claim 1 and an apparatus to apply a metal coating on a steel strip, in particular a strip tin-plating unit, according to the preamble of Claim 10.
In the production of galvanically coated steel strips, for example, in the production of tinplate, a method is known to increase the corrosion resistance of the coating by a melting of the coating according to the galvanic coating process. To this end, the coating galvanically deposited on the steel strip is heated to a temperature above the melting point of the coating material and subsequently quenched in a water bath. By the melting of the coating, the surface of the coating receives a shiny appearance and the porosity of the coating is reduced, wherein its corrosion resistance is increased and its permeability for aggressive substances, in particular organic acid, is decreased.
The melting of the coating can, for example, take place by inductive heating of the coated steel strip. From DE 1 186 158-A, an arrangement for the inductive heating of metal strips for the melting, in particular, of electrolytically applied coatings, such as tin layer on steel strips, is known. This arrangement has several rollers, over which the coated strip is conducted, and several induction coils that are arranged one behind the other in groups and comprising a moving strip, with which the coated strip is inductively heated to temperatures above the melting temperature of the coating material so as to melt the coating. In order to make it possible for the melting temperature to be reached uniformly over the entire width of the strip, additional inductors with heating conductors acting in lines are placed on the strip edges of the coated strip.
This measure is to prevent the temperature of the coated strip being raised by the induction coils to temperatures far above the melting temperature of the coating material, so that the coating will be heated uniformly over the entire width of the strip. In this way, in turn, the formation of an alloy intermediate layer is to be avoided, which is composed of iron atoms and atoms of the coating material, for example, tin.
With the known methods for the melting of metal layers on steel strips or sheets, the entire steel strip or sheet, including the applied coating, is, as a rule, heated to temperatures above the melting temperature of the coating material and subsequently again cooled, for example, in a water bath, to normal temperature. For this purpose, there is a considerable energy requirement.
Proceeding from this, the goal of the invention is to indicate a method and an apparatus to improve a metal coating on a steel strip or sheet, which, in comparison to the known methods and apparatuses, make possible a substantially more energy-efficient treatment of the coated

2 steel strip. The method and the apparatus should also attain an increased corrosion stability of the coating treated in accordance with the invention, even with thin coating layers.
These goals are attained with a method with the features of Claim 1 and with an apparatus with the features of Claim 10. Preferred embodiments of the method and the apparatus in accordance with the invention are indicated in the subclaims.
With the method in accordance with the invention, the metal coating is appropriately melted over its entire thickness by heating to a temperature above the melting temperature of the coating material, wherein the heating is carried out by electromagnetic induction by means of an induction furnace with at least one induction coil or one inductor. The maximum temperature of the coating thereby attained is designated below as the maximum temperature.
After the inductive heating, the temperature of the coating is held for a holding time at a temperature above the melting temperature of the coating material before the coated steel strip is quenched in a cooling device at a quenching temperature below the melting temperature. The time period in which the temperature of the coating is above the melting temperature of the coating material is regarded as the holding time. By moving at least one of the induction coils relative to the cooling device, the holding time is thereby adapted to the other process parameters, in particular, the maximum temperature, the strip speed, and the thickness of the coating, so as to completely melt the coating over its entire thickness down to the boundary layer with the steel strip. In this way, the process parameters can be coordinated with one another, so that the coating (in an essentially precise manner) is melted over its entire thickness down to the boundary layer with the steel strip, without the underlying steel strip being substantially heated. The movement of at least one of the induction coils relative to the cooling device provided in accordance with the invention makes possible thereby the adaptation of the holding time to the strip speed (specified by the production process in the galvanic coating method) and the thickness of the coating applied in the coating method. The latter is appropriately recorded by suitable thickness sensors at the end of the coating device. The holding times that are preferably maintained are in the range of 150 ms to 800 ms with the typical strip speeds of strip tin-plating units (which move between 300 m/min and 700 m/min). In order not to worsen the deformability of the strip, it is preferable that the holding time be set as low as possible (however, without thereby setting the maximum temperature to values above 360 C).
The energy input produced by the electromagnetic induction preferably takes place into the melting coating and into the uppermost layers of the underlying steel strip in the method in accordance with the invention. The penetration depth of the induction current can be controlled thereby via the operating frequency of the induction coil or the inductor. The range of the frequencies that can be used with the required induction performances is thereby in the high

3 frequency range (50 kHz to 1 MHz), wherein frequencies are preferably around 150 kHz to attain penetration depths in the range 10 to 100 gm.
It has been shown that the coated steel strips have particularly good values for their corrosion resistance if the metal coating is inductively heated to a maximum temperature of more than 310 C so as to melt the coating over the holding time. The range from 310 C to 360 C has proved to be particularly advantageous, and the range from 320 to 350 is particularly preferred for the maximum temperature. With a heating to temperatures above 360 C, the deformability of the strips or sheets treated in accordance with the invention worsens as a result of a reduction of the yield strength.
By comparative experiments, it was surprisingly possible to show that in maintaining a maximum temperature of more than 310 C, essentially independent of the selected holding time, an alloy layer that is thin (in comparison to the thickness of the coating) and that consists of iron atoms and atoms of the coating material is formed on the boundary layer between the coating and the steel strip or the steel sheet, if the coating is completely melted over its entire thickness down to the boundary layer with the steel strip. With tin-plated steel strips (tinplate), therefore, a very thin iron-tin alloy layer (FeSn2) is formed, for example, on the boundary layer of the tin coating with the steel.
By measuring the ATC value ("Alloy Tin Couple" value), which, as an electrochemical test, is a measure for the porosity of the alloy layer, it was determined that the alloy layer formed by the inductive melting has a lower porosity and a substantially higher density in comparison to the alloy layers that result during the traditional process operation (that is, the melting of the coating in an annealing furnace, for example, by electrical resistance heating at temperatures just above the tin melting temperatures of 232 C). Therefore, it is suspected that this thin and low-pore alloy layer influences the corrosion stability in a particularly positive manner. The method in accordance with Claim 2 is therefore regarded as a stand-alone invention, independent of the features of the characterizing part of Claim 1.
The method parameters for the inductive melting of the coating, in particular, the maximum temperature and the holding time, are appropriately selected and adapted to the strip speed and the thickness of the coating in such a manner that only one part of the coating is alloyed with the iron atoms of the steel strip or the steel sheet and, therefore, after the melting, still unalloyed coating and, underneath, a thin alloy layer are present.
Depending on the selected process parameters, the thickness of the alloy layer thereby corresponds to approximately a weight per unit area or a coating of only 1.3 gim2 or less. With regard to the corrosion stability and the formability, alloy layers that are thinner than 1.0 g/m2 have proved to be particularly suitable, and alloy layers with a thickness in the range of 0.05 to 0.6 g/m2 have proved to be particularly preferred. With thicker alloy layers, corresponding to a coating of more than

4 1.3 g/m2, the formability of the coated steel sheet worsens, for example, for the production of cans for beverages or food.
With the method in accordance with the invention, it is possible to ensure that, for example, in the tin-plating of steel sheets, even those with thin total tin coatings of 1.0 g/m2 or less, a thin and, at the same time, essentially pore-free and thus very dense alloy layer with an optically attractive (that is, shiny) coating surface is attained. The alloy layer, which, in comparison to the thickness of the coating, is very thin and at the same time dense, leads to an increased corrosion resistance of the coated steel and to an improved adhesion of the coating on the steel strip or sheet. In accordance with the invention, this is made possible in that the process parameters can be adapted to one another during the melting of the coating, so as to undertake a purposeful adjustment of the thickness of the alloy layer forming during the melting of the coating. In particular, with the method in accordance with the invention, according to Claim 1, the thickness of the forming alloy layer is decoupled from the distance between the melting device and the cooling device, which has been firmly established in the method up to now. In the method in accordance with the invention, on the other hand, the distance from the induction coil to the cooling device can be appropriately adjusted continuously so as to adjust the holding time to the desired value. Via an adaptation of the holding time to the other process parameters, such as the maximum temperature and the thickness of the coating deposited on the steel strip, it is finally possible to purposefully control the thickness of the alloy layer and thus, ultimately, the material characteristics of the coated steel strip, such as its corrosion resistance and formability.
The best results can thereby be attained if the maximum temperature was established at values between 310 C and 360 C and the holding time, between 0.1 sand 1.0 s, and preferably between 0.2 s and 0.3 s.
The goal of the invention is, furthermore, attained with an apparatus to apply a metal coating on a steel strip. In the apparatus, an endless steel strip is moved at a strip speed in the movement direction of the strip and is electrolytically provided with a metal coating in a coating device. The apparatus can be, in particular, a strip tin-plating unit with an electrolytic coating device in which the steel strip is moved through a tin-containing electrolyte at the strip speed, so as to deposit a tin layer on the steel strip. In the movement direction of the strip, a melting device in which the coating is melted by inductive heating at a maximum temperature above the melting temperature of the material of the coating comes subsequent to the coating device. A cooling device in which the coating steel strip is cooled to a quenching temperature below the melting temperature follows the melting device in the movement direction of the strip.
In accordance with the invention, the melting device can move, relative to the cooling device, so as to be able to adjust the distance between the melting device and the cooling time to a desired value in the movement direction of the strip.

For this purpose, the melting device comprises at least one induction coil arranged so it can move in the movement direction of the strip. In addition to this movable induction coil, the melting device can also contain additional induction coils, which are arranged one behind the other in the movement direction of the strip. These additional induction coils can be thereby fixed in situ relative to the cooling device or can also be movable.
Appropriately, however, in an arrangement of several induction coils connected one behind the other, at least the last induction coil, which is next to the cooling device, or the entire coil device are designed so they can move.
With the induction coil(s), the coated steel strip can be heated inductively to the maximum temperature at adjustable heating rates. For the purpose, heating rates between 600 K/s and 1300 K/s, and preferably between 900 K/s and 1100 K/s, have proved to be appropriate.
The cooling device can be a quenching tank, filled with a cooling liquid, for example, water. However, another cooling device, for example, blower cooling or gas cooling, in particular, an air cooling, can also be used.
The invention is explained in more detail below with the aid of an embodiment example, with reference to the accompanying figures. The figures show the following:
Figure 1: schematic representation of an apparatus for the application of a metal coating on a steel strip;
Figure 2: schematic representation of the melting device and the cooling device of the apparatus of Figure 1;
Figure 3: perspective representation of the movable melting device of the apparatus of Figure 1.
The apparatus shown schematically in Figure 1 is, for example, a strip tin-plating unit with a coating device, in which a tin coating is deposited on a fine or very fine sheet, in which the steel strip is conducted through a tin-containing electrolyte at a strip speed vB. The application area of the invention, however, is not limited to this embodiment example. The invention can also be used appropriately, for example, in methods for the electrolytic coating of steel strips with other metals, such as zinc, so as to produce a so-called special, very fine, zinc-plated sheet. The use of the method in accordance with the invention is also not limited to the coating of steel strips in strip zinc-plating units, but rather can also be appropriately used, for example, in the immersion coating of strip sheets in the form of tablets, in which the metal coating is not applied electrolytically on the steel strip.
The strip tin-plating unit for the electrolytic tin-plating, shown schematically in Figure 1, comprises a decoiler group 10, in which a steel strip, cold-rolled to form a fine or very fine sheet, is drawn off from a roll (coil) and is welded together, in a welding device 11, to form an endless steel strip. The endless strip is conducted in a loop tower 12 in order to form a supply of strips.

The supply of strips held by the loop tower 12 also makes possible a continuous passage of the strip through the strip tin-plating unit at a prespecified strip speed during the necessary idle times in the welding together or, later, during the separation of the coated steel strip and the rolling onto wound coils. The loop tower 12 is followed by a pretreatment device 13 and a coating device 4. In the pretreatment device 13, there is a cleaning and degreasing of the steel strip surface, which is described in more detail below, and in the coating device, 4, the strip that is moving through the strip tin-plating unit at the strip speed (vB) is conducted through a tin-containing electrolyte so as to deposit a tin layer on the steel strip. The coating device 4 is followed in the movement direction of the strip by a melting device 5, in which the coating deposited on the steel strip is heated to temperatures above the melting temperature of the coating material (with tin, this is 232 C), so as to melt the deposited coating. The melting device is followed by a cooling device 3 and a post-treatment device 14 and a second loop tower 15.
Finally, the coated steel strip is wound, in a winding group 16, on rollers (coils).
The still uncoated steel strip coming from the first loop tower 12 is first subjected to a pretreatment in the pretreatment device 13 before it is provided with a tin layer in the coating device 4. In the pretreatment device 13, the uncoated steel strip is first degreased and then pickled. In addition, the still uncoated steel strip is conducted through an alkaline degreasing bath, for example, a sodium carbonate or sodium hydroxide solution at the strip speed (vB). The degreasing bath was freed at regular intervals of soiling that was produced by the introduction of grease and iron wear. It was shown that for the subsequent carrying out of the improving method in accordance with the invention, a sufficient cleanliness of the degreasing bath is present; if the bath murkiness (bath extinction) of the degreasing bath has an extinction value of < 1 (according to the Lambert-Beer Law, corresponding to a light weakening of less than factor 10), with an optical measurement with light with a wavelength of 535 nm.
After the degreasing, a first rinsing takes place with a rinsing liquid and subsequently, the steel strip is pickled in an acidic solution, for example, in a sulfuric acid solution, and rinsed once again. For the subsequent carrying out of the improving method in accordance with the invention, it is appropriate to rinse the steel strip after the degreasing and pickling with a rinsing liquid, which preferably has a conductivity of < 20 p.S/cm.
In the coating device 4, which follows the pretreatment device 13, the degreased and pickled steel strip is conducted through a tin-containing electrolyte bath and is connected there as a cathode and conducted through between two rows of tin anodes. In this way, the tin of the anodes is dissolved and deposited on the steel strip as a tin coating. The tin can be thereby applied in any thickness and, if required, on both sides of the steel strip.
The thickness of the applied tin layer is regularly between 1.0 g/m2 and 5.6 g/m2. However, a coating of the steel strip with thinner or with thicker tin layers is also possible.

To increase the corrosion resistance of the coated steel strip, it is subjected to an improving method in accordance with the invention after the coating process in the coating device 4. The improving method is carried out in the melting device 5 and the cooling device 3, which follows it in the movement direction of the strip. The details of the improving method in accordance with the invention and the devices used for the purpose are described in detail below with reference to Figures 2 and 3.
Figure 2 schematically shows the melting device 5 and the cooling device 3, which follows in the movement direction of the strip. The moved steel strip is moved at the strip speed over deflection rollers 19 and conducted into the melting device 5 and, from there, into the cooling device 3. The moved steel strip essentially moves between the melting device 5 and the cooling device 3 in a vertical direction from top to bottom, as shown in Figure 2. The melting device 5 is an induction furnace with at least one induction coil 2. The induction furnace can also comprise several induction coils or inductors, arranged one behind the other in the movement direction of the strip. The assumption below is that the induction furnace contains only one induction coil 2. The induction coil 2 is impinged on by an electric alternating current, preferably in the high frequency range (50 kHz to 30 MHz), and the coated steel strip 1 is moved through the induction coil 2 at the strip speed (vB). In this way, alternating currents are induced in the coated steel strip that heat the coated steel strip. In order to melt the coating applied on the steel strip, the coated steel strip is heated in the induction furnace to temperatures above the melting temperature of the coating material T9; this is 232 C with tin). The maximum temperature thereby attained is designated as the maximum temperature (peak metal temperature, PMT). It has been shown that for the execution of the improving method in accordance with the invention, maximum temperatures that are higher than 310 C are to be preferred, and are preferably in the range between 320 C and 350 C. The maximum temperature can be controlled by the output of the induction coil 2. The penetration depth of the induction current produced by the electromagnetic induction into the surface of the coated steel strip can be controlled by the frequency of the electromagnetic alternating current with which the induction coil 2 is impinged.
The outputs of the induction coil 2 required for the carrying out of the improving method in accordance with the invention are in the range of 1500 to 2500 kW.
With the induction furnace, the coated steel strip can be heated to temperatures above the melting temperature T, of the coating material at heating rates between 600 K/s and 1300 K/s.
The heating rates of the induction furnace are appropriately set between 900 K/s and 1100 K/s.
The melting device 5 (induction furnace) or the induction coil 2 extends in the movement direction of the strip, between the coil inlet 2a and the coil outlet 2b, over a length L, which is appropriately in the range from 2 to 3 m. This length L represents the effective heating zone in which the coated steel strip is heated in the melting device 5.

A cooling device 3 follows the melting device 5 in the movement direction of the strip and at a distance to the melting device 5. In the embodiment example shown here graphically, the cooling device 3 comprises a quenching tank 6 filled with a cooling liquid. Another deflection roller 19 is located in the quenching tank 6; the quenched steel strip is conducted out of the cooling device 3 by means of this deflection roller. The liquid level of the cooling liquid is designated, in Figure 2, with the reference symbol 7. On the stretch between the rinsing outlet 2b and the liquid level 7, the melted coating is slightly cooled by heat conduction and convection between the melting device 5 and the cooling device 3. Since the coating, however, was heated to temperatures far above the melting temperature T, in the melting device 5, the melted coating still remains in a melted state on its way between the melting device 5 and the cooling device.
The time over which a prespecified point on the strip traverses between the rinsing outlet 2b and the liquid level 7 of the cooling liquid is determined by the distance D
between the rinsing outlet 2b and the liquid level 7 and the strip speed (vB), and is calculated as tH =
D/vB. This time period tH is designated below as the holding time.
If the strip is immersed in the cooling liquid, there is a rapid quenching of the strip heated in the melting device 5 to the temperature of the cooling liquid, which, as a rule, is in the area of the room temperature. By the melting and rapid quenching of the coating, a shiny surface of the coated strip is produced. Furthermore, the adhesive capacity of the applied coating on the steel strip is increased by the melting and the rapid quenching.
In accordance with the invention, provision is then made so that the entire melting device or at least one induction coil 2, located therein, can be moved relative to the cooling device 5 so as to be able to set the distance D between the rinsing outlet 2b and the inlet of the cooling device 3, in particular the liquid level 7, at a desired value suitable for carrying out the method in accordance with the invention. To this end, the entire melting device 5, or at least its induction coil 2, is arranged so it can move in a frame 8, as shown in Figure 3.
Appropriately, the entire melting device 5 is arranged on the frame 8 so that it can be moved continuously in the movement direction of the strip. When using a melting device 5 with an induction coil series (consisting of a plurality of induction coils that are appropriately arranged, one behind the other, in the movement direction of the strip), at least the induction coil that is last seen in the movement direction of the strip (that is, the induction coil that is adjacent to the cooling device 3) is to be designed so that it can be moved in the movement direction of the strip, so as to be able to set its distance to the adjacent cooling device 3 at a suitable value.
The suitable distance between the melting device 5 or the (last) induction coil of an induction rinsing series is thereby determined so that the coating is melted just so over its entire thickness down to the boundary layer with the steel strip without thereby introducing (by the electromagnetic induction) excess energy into the coating.

Figure 3 shows the frame 8 with the melting device 5 (induction furnace) arranged thereon. The melting device 5 thereby comprises a housing 9, in which the induction coil 2 is located. The housing 9 is located on the frame 8 over sliding tracks so that it can move between an upper end position 2c and a lower end position 2d. The movement of the frame 9 appropriately takes place via a motor drive.
With this arrangement, it is now possible to adapt the holding time after the melting of the coating to the quenching of the melted coating in the cooling device 3 to the other process parameters, such as the maximum temperature, the strip speed, and the thickness of the coating applied in the coating device 4. In this way, it is possible to set the aforementioned process parameters and the holding time so that the coating is melted under defined conditions. It is possible, in particular, for the coating to be melted (right) over its entire thickness down to the boundary layer with the steel strip. It has been shown that a melting of the coating down to the boundary layer with the steel strip is very advantageous, because, simultaneously, a very dense and thin, in comparison to the thickness of the coating, alloy layer is formed thereby on the boundary layer between the coating and the steel strip. This alloy layer consists of iron atoms of the steel strip and atoms of the coating material (that is, for example, with a tin coating consisting of tin and iron atoms, in the FeSn2 stoichiometry). The formation of this alloy intermediate layer has a considerable effect on the characteristics of the coated steel strip. In particular, the formation of the alloy layer increases the corrosion resistance of the coated steel strip and improves the adhesion of the coating to the steel strip.
By comparative experiments, it was possible to determine that with the improving method in accordance with the invention, especially if the maximum temperature is higher than 310 C, a particularly stable and dense alloy layer is formed. By measuring the ATC value, it was possible to determine that this alloy layer is particularly low-pore and thus dense in comparison to the intermediate layers formed with a traditional method operation. This dense alloy layer with a lower porosity leads to an improved corrosion stability of the coated steel strip.
For comparison purposes, tinplates produced according to traditional methods were compared to tinplates which were improved with the method in accordance with the invention.
To this end, tinplates coated with a tin coating of 2.0 to 8.6 g/m2 were treated in accordance with the invention, wherein in one embodiment example, a heating rate of 963 C/s and a maximum temperature (PMT) of 330 C were established in the inductive melting of the coating. The distance of the movable melting device to the cooling device was set at D =
3.9 m and the strip was moved at a strip speed of 700 m/min through the strip tin-plating unit. An alloy layer with a layer thickness was thereby produced; it corresponds to a coating of 0.8 g/m2.
The tinplate thus produced was tested with the standardized ATC method with regard to its corrosion resistance and compared to the traditionally produced tinplate. A traditionally produced tinplate has typical values of 0.12 ttA/cm2 or more for the ATC value ("Alloy Tin Couple" value).
The tinplates treated in accordance with the invention, on the other hand, have substantially lower ATC values of less than 0.08 [tA/cm2. With the improving method in accordance with the invention, it was even possible to produce tinplates that now have ATC values of merely 0.04 A/em2. By comparative experiments, it was possible to determine that such low ATC values can be attained especially if the maximum temperature (PMT) is above 310 C.

Claims

1. Method for improving a metal coating on a steel strip (1) or steel sheet, wherein the coating is melted by inductive heating, with at least one induction coil (2), to a maximum temperature (PMT) above the melting temperature (T s) of the material of the coating, and is subsequently cooled, in a cooling device (3), to a quenching temperature (T A) below the melting temperature, characterized in that the coating is held during a holding time (t h) at a temperature above the melting temperature (T s) and that the holding time (t h) is adapted to the maximum temperature (PMT) and the thickness of the coating by moving at least one of the induction coils (2) relative to the cooling device (3) so as to completely melt the coating over its entire thickness down to the boundary layer with the steel strip.

2. Method according to Claim 1, characterized in that the maximum temperature is higher than 310°C and that the coating is completely melted over its entire thickness down to the boundary layer with the steel strip.

3. Method according to Claim 1 or 2, characterized in that the maximum temperature (PMT) is between 310°C and 360°C.

4. Method according to Claim 1 or 2, characterized in that the maximum temperature (PMT) is between 320°C and 350°C.

5. Method according to any one of claims 1 to 4, characterized in that the heating rate of the inductive heating is between 600 K/s and 1300 K/s.

6. Method according to any one of claims 1 to 4, characterized in that the heating rate of the inductive heating is between 900 K/s and 1100 K/s.

7. Method according to any one of claims 1 to 6, characterized in that the coated steel strip (1) is moved at a strip speed (v B) relative to the induction coil (2).

8. Method according to any one of claims 1 to 7, characterized in that the distance of the induction coil (2) to the cooling device (3) can be adjusted continuously, so as to set the holding time (t h) at a desired value.

9. Method according to claims 1 or 8, characterized in that the holding time (t h) is between 0.1 s and 1.0 s.

10. Method according to claims l or 8, characterized in that the holding time (t h) is between 0.2 s and 0.3 s.

11. Method according to any one of claims 1 to 10, characterized in that a thin alloy layer, which essentially consists of iron atoms and atoms of the coating material, is formed at the boundary layer between the coating and the steel strip.

12. Method according to Claim 11, characterized in that the alloy layer has a coating weight lower than 1.3 g/m2.

13. Method according to Claim 11, characterized in that the alloy layer has a coating weight lower than 1.0 g/m2.

14. Apparatus for the application of a metal coating on a steel strip, in which a continuous steel strip (1) is moved at a strip speed (v B) in a movement direction and is electrolytically coated by a coating device (4) with a metal coating, wherein a melting device (5) follows the coating device (4) downstream in the movement direction of the strip, and in the melting device, the coating is melted by inductive heating at a maximum temperature (PMT) above the melting temperature (T s) of the material of the coatings, and wherein a cooling device (3) follows downstream the melting device (2) and in the cooling device, the coated steel strip (1) is quenched to a quenching temperature (T A) that is below the melting temperature, characterized in that the melting device (5) can be moved relative to the cooling device (3) so as to set the distance between the melting device (5) and the cooling device (3) in the movement direction of the strip.

15. Apparatus according to Claim 14, characterized in that the melting device (5) contains at least one induction coil (2), which is moveable in the movement direction of the strip.

16. Apparatus according to Claim 14, characterized in that the melting device (5) contains a plurality of induction coils arranged one behind the other in the movement direction of the strip, wherein at least the last induction coil, which is closest to the cooling device (3), can move relative to the cooling device (3).

17. Apparatus according to Claim 14, characterized in that the cooling device (3) comprises a quenching tank (6) filled with a cooling liquid.