WO2022234029A1

WO2022234029A1 - A system and a method for plasma surface treatment

Info

Publication number: WO2022234029A1
Application number: PCT/EP2022/062180
Authority: WO
Inventors: Ruud Johannes WESTERWAAL; Christiaan BOELSMA; Erwin BOUWENS; Adrianus Jacobus Wittebrood; Edzo ZOESTBERGEN; Jennifer ROLLS
Original assignee: Tata Steel Nederland Technology B.V.
Priority date: 2021-05-06
Filing date: 2022-05-05
Publication date: 2022-11-10
Also published as: EP4334488A1; KR20240005825A

Abstract

A method for treating the surface of a steel strip is provided where the method comprises the steps of providing a steel strip (5), plasma treatment of the steel strip by a magnetron plasma sputter unit (1); transferring of the plasma treated steel strip in atmospheric conditions to a post-processing unit (35) within 30 mins; and post processing the steel strip after the plasma treatment wherein the post-processing unit (35) is a continuous hot dip galvanizing unit.

Description

A SYSTEM AND A METHOD FOR PLASMA SURFACE TREATMENT

Field of the invention

The present invention relates to a method for treating the surface of a steel strip by plasma surface treatment before performing a post process. In a further aspect, the present invention relates to a system for treating thereof.

Background of the invention

Alloying oxides and other contaminants on the surface of a material such as on a steel surface cause adhesion issues with respect to a subsequent post processing step. Removal of these oxides is advantageous for improving the coatability of the surface as they inhibit the ability of a coating to adhere to the surface or to form a coherent inhibition layer. Specifically this is a problem in high alloyed steels. To remove these oxides and/or contaminants from a steel surface various cleaning methods that include a chemical and/or a mechanical cleaning step can be used. For most of the applications where a post processing step is required such as an annealing step, a thin film deposition step or a coating step, it is often insufficient to use only simple mechanical and/or chemical cleaning methods. After performing one or more initial cleaning steps, a plasma cleaning step may be performed as a subsequent or final cleaning step prior to performing a post processing step. A physical method such as using a plasma cleaning method is a very effective surface preparation method compared to using a chemical cleaning method such as using an acid cleaning step. This could be attributed to the fact that in plasma cleaning as long as the energy activation barrier is reached, the oxides would be removed whereas in a chemical process, the reaction time, temperature, and concentrations are the controlling factors. Another reason is that plasma cleaning is conducted in-situ whereas in chemical process there is always a time delay between acid cleaning or etching step prior to the subsequent post processing step. This time delay provides an opportunity for more oxides and/or contaminants to be formed on the steel surface. By performing plasma cleaning of the substrate, the remaining contaminants or oxides present on the substrate can be removed and the substrate surface is activated. After such a plasma cleaning step, the adhesion of a subsequently applied thin film or coating or a paint layer to the substrate is increased considerably. Moreover, a plasma cleaning step can be used to activate the steel surface prior to a post processing step to avoid formation of any diffuse regions that are prone to a potential spallation.

When a post processing unit is distanced from the plasma cleaning unit, it is often required to transfer the activated and/or cleaned surface that is treated with plasma in a protective environment in order to reduce or delay the formation of oxides or contaminants on the activated surface. However, the addition of a protective environment limits the flexibility of performing the post process within a limited period of time as well as within a limited vicinity of the plasma cleaning unit. Since employing an additional protective environment also requires additional equipment, it increases the overall cost and the footprint of the combined unit.

US patent publication US9321077 B2 describes a method and apparatus for plating high strength steel after irradiating it with at least one of laser light and plasma. This is done to remove Si/Mn/AI oxides from the steel surface to make it suitable for a post process such as a zinc plating process. To avoid carbonization of the surface layer, the steel sheet is irradiated using a laser at room temperature in an air atmosphere where the spot size of laser is 1mm or less or to it is performed within a nitrogen atmosphere. Thus, US9321077 B2 describes a localized irradiation on the surface of a steel strip where a laser beam is required for performing the process at room temperature in an air atmosphere.

European patent publication EP0506304B1 describes a method and apparatus for continuously hot-dipping steel strip. The surface of the steel is activated by sputtering-etching in a vacuum chamber and then passed directly into a coating metal bath where the outlet of the vacuum chamber is immersed in the hot dipping bath. EP0506304B1 always requires a vacuum maintained environment to transfer the steel to the hot dipping bath.

The state of the art solutions (US 9,321,077 B2, EP0506304 B1) describes a plasma cleaning step that is always positioned directly in front of the post processing step such as an hot dip galvanising (HDG) step. Further, the treated steel strip is always kept under a protective atmosphere or requires additional instrumentation. This strongly restricts the applicability of the plasma cleaning step. Furthermore, it complicates the plasma cleaning setup as special measures need to be undertaken in order to integrate the plasma cleaning unit into a continuous coating line. For example, this might require addition of entry-exit locks to the plasma cleaning unit. Previously reported plasma cleaning options also often are based on a localized plasma “jet”, which is sometimes used in combination with laser light. Hence, the application of such prior art processes are either limited to small surface areas or require additional instrumentation to carry out on larger surfaces. This might results in an extensive, complicated and expensive plasma cleaning process. Objectives of the invention

It is an object of the present invention to improve the flexibility of placing a plasma cleaning unit further away from a post processing unit without having the need for employing an additional protective environment.

It is also an object of the invention to provide a method to transfer a cleaned and/or activated steel surface in atmospheric conditions from a plasma cleaning unit to a post processing unit.

It is another object of the present invention to provide a method to improve the wettability of a steel surface to a subsequent post processing step in a cost effective manner. It is another object of the present invention to provide a method to improve the coating quality in a cost effective manner.

It is another object of the present invention to provide a method to improve the coating quality of a steel surface on large surface areas.

One or more of these objectives are reached with a method according to claims 1 - 9, and with a coating unit according to claims 10 - 15.

Description of the invention

In a first aspect of the invention there is provided a method for treating the surface of a steel strip, wherein the method comprises the steps of providing a steel strip, plasma treatment of the steel strip by a magnetron plasma sputter unit; transferring of the plasma treated steel strip in atmospheric conditions to a post-processing unit within 30 mins; and post processing the steel strip after the plasma treatment wherein the post-processing unit is a continuous hot dip galvanizing (HDG) unit. The method of the invention is not limited to a steel strip but the method can also be applied to steel sheets or steel blanks. In description the term steel strip is used interchangeable with steel blank or steel sheet or steel substrate. The steel surface in this context can be the surface of a steel strip, steel sheet or blank or steel substrate. The plasma treatment can be in-situ plasma treatment. The term in-situ plasma treatment means that the plasma treatment is performed just before and connected to the post processing step . However the method is not limited to only in-situ but can also be applied as a single standalone unit followed by a transport of the steel strip to the post processing step. The transport can be through a transport unit. The plasma treatment can be performed as an in-line process or as an off-line process.

The plasma treatment of the steel strip by the magnetron plasma sputter unit can be also in an inert atmosphere. The inert atmosphere can be achieved by using a nitrogen atmosphere or an argon gas atmosphere or by using an atmosphere by another inert gas. Advantageously, by using a magnetron plasma sputter unit, an increased electron density can be obtained on the steel strip, which allows for a transfer of the activated steel strip through atmospheric conditions to the post-processing unit without adverse surface effects. The magnetron plasma sputter unit typically comprises a number of magnets that are provided in one or more rows. These magnets can be permanent magnets. To increase the density of the plasma near the steel surface, the magnets are used to capture electrons and thereby increase their ionization efficiency. During operation of the magnetron plasma sputter unit, the positive ions are accelerated towards the steel substrate to be cleaned by a voltage difference maintained between a container of the magnetron plasma sputter unit and the steel substrate, which is grounded. The row of magnets can be placed on the side of the steel sheet that is to be cleaned. This has the major advantage that the thickness of the steel sheet does not affect the sputtering process. It is also possible to place the magnets on the other side of the steel sheet even though it might be of lesser efficiency especially for thicker steel substrates.

Plasma cleaning is performed on steel surfaces with a plasma cleaning apparatus that comprises a magnetron plasma sputter unit. In the description this term plasma cleaning is used interchangeable with plasma sputter cleaning and sputter cleaning. In a plasma cleaning apparatus a gas is fed into an ionisation chamber where the gas is ionised by a voltage difference between the ionisation chamber and the substrate. The gas in the apparatus is inert, preferably Argon gas (Ar) or Argon based gas because of its high atomic mass. The sputter cleaning process is based on the impact of the charged gas atoms, therefore it is obvious to use the heavy Ar gas. However this can be replaced by the cheaper N₂ gas, even though it can be less efficient due to its lower weight and its possible reactiveness with the surface. Furthermore, one can even consider to add hydrogen to the gas mixture to increase the reduction effect. On the other hand hydrogen might induce hydrogen embrittlement so there might be a trade-off and gas composition can be optimised. Typically with the plasma surface cleaning treatment Ar gas or an Ar based gas is supplied to the ionisation chamber where it is ionised to a plasma due to a voltage difference between the container defining the ionisation chamber (anode) and the grounded steel substrate and wherein the Ar ions subsequently bombard the steel surface (grounded) due to the voltage difference. The voltage difference for instance can be in the order of inner unit 300- 3000V. To realise an efficient cleaning operation it is preferred that the Ar gas and subsequently the Ar ions are evenly distributed over the surface area to be cleaned.

The influence of a plasma sputter treatment on the HDG coating quality of a steel strip is investigated. The plasma sputter treatment has been investigated as a surface treatment step before annealing and as an intermediate step between annealing and the HDG process. The plasma sputter intensity, ageing time between sputtering and hot dip galvanization and the time between annealing and sputtering have been investigated. It has been found that the alloying elements from the steel are migrating towards the surface of the steel where they form oxides by selective oxidation. This is one of the main reasons for possible defects in HDG coated HSS. Application of plasma sputter treatment will result in a strong improvement in coating quality by reducing the coating defect percentage significantly. An improvement up to 99 percent point can be obtained by removing the surface oxides and at the same time activating the steel surface.

After the plasma treatment the steel strip can be sent to a hot dip galvanizing unit at atmospheric conditions. The coating is a zinc coating, a zinc based alloy or a multilayer structure..

. Plasma treatment in this case is used for surface activation which is beneficial for a subsequent post processing step. . HDG is a widely used zinc coating method that is typically used by steel plants and hence employing a flexible surface treatment method that can be incorporated within the line is advantageous to improve the efficiency of the plant.

The present invention seeks to provide a reliable solution to transfer the steel strip from a plasma cleaning unit to a post processing unit in atmospheric conditions. Performing the transfer at atmospheric conditions improves the flexibility in performing the post processing step by introducing time delays between the two steps. The transfer can be performed through a transfer unit. Moreover it relaxes the requirement of placing the plasma cleaning unit within the vicinity of the post processing unit. In other words, it allows the plasma cleaning unit to be not placed within the vicinity of a post processing unit.

During annealing and processing of the steel strip, oxides are formed on the steel surface especially for the higher alloyed types of steel. The alloying elements tend to diffuse towards the steel surface where they form oxides. The formation of these oxides can result in a bad wettability, the formation of bare spots and defects during the subsequent post processing step. A plasma cleaning step before the post processing step such as a continuous hot dip galvanizing process reduces these surface oxides and at the same time increases the surface energy and thereby result in an activated surface. This results in a much better coating quality as compared to a non-plasma treated surface. Thus, the invention solves the problem of bad wettability on the steel surface and bare spots on a coated steel and improves the overall quality of the coating especially for the more difficult-to-coat types of steel such as high strength steels. In an embodiment it describes a (short) plasma based surface treatment, which is placed after an annealing section and prior to a post processing step such as an HDG coating step in order to remove the surface oxides/contaminations and activation of the steel surface. This step can be a continuous in-line step directly after the annealing step. After the plasma cleaning step the steel strip enters the HDG bath under atmospheric conditions. The combination of these processes results in a significant improvement of the surface quality of the coatings such as HDG coatings.

The present invention allows to modify the plasma cleaning setup according to the line specifications, it is simple to construct and does not require any additional processes such as laser light irradiation. Most importantly, the present invention has shown that with this type of plasma cleaning apparatus it is not necessary to keep the steel strip surface under a protective atmosphere after plasma treatment, as the steel surface remains activated even under (dry) atmospheric conditions up to at least 30 minutes after the treatment. This makes the plasma cleaning process flexible and can be integrated at different locations in the line or even off-line in a batch oriented coating process.

The present invention further allows to clean the steel surface over large areas without requiring additional equipment. The method provides a surface treatment of the steel surface before the post processing such as an HDG coating and thereby improving the final coating quality.

According to the invention, a method for treating the surface of a steel strip is provided as claimed in claim 1. The method comprises the steps of providing a steel strip, plasma treatment of the steel strip by a magnetron plasma sputter unit. The plasma treatment can be in-situ plasma treatment. The method further comprises transferring of the plasma treated steel strip in atmospheric conditions to a post-processing unit within 30 minutes and post processing the steel strip after the plasma treatment wherein the post processing unit is a continuous hot dip galvanizing unit. The plasma near the steel strip may be densified due to the presence of magnets. An additional advantage of performing the method as described above is that the wettability of a steel surface to a post processing is improved without the requirement of having additional protective environment. This allows to perform the post processing step in a cost effective manner. The plasma environment in the magnetron sputtering unit can be a vacuum based plasma or it can be an atmospheric based plasma. In an embodiment of the present invention, the plasma treatment of the steel strip by the magnetron plasma sputter unit is performed in a protective atmosphere. The protective atmosphere can be an atmosphere that is under vacuum. The vacuum can be within the pressure range of 10⁶ mbar - 1000 mbar. For this purpose, a vacuum pump can be connected to the magnetron plasma sputter unit. For a protective atmosphere, a dry atmosphere or a dry air atmosphere can be used. Alternatively, an argon or nitrogen based atmosphere or a mixture of them can be used.

In a preferred embodiment of the invention, the magnetron plasma sputter unit comprises a plurality of magnets placed on the same side of the magnetron plasma sputter unit for densifying the plasma near the surface of the steel strip. The presence of magnetic field densifies the plasma near the steel strip. Electrons are trapped in the magnetic field and thereby create more Ar ions. Subsequently the Ar ions bombard the steel, thereby increasing the surface energy i.e. activating the surface. This also allows the steel strip to remain activated which helps to do the transfer to a post processing unit in atmospheric conditions. The transfer can be made through a transfer unit. The magnetron plasma sputter unit can be integrated in the process at the previously indicated positions. Electrons expelled from the steel substrate by secondary emission are also trapped in the magnetic field, which prevents them from getting lost in the environment or at an anode. The electrons move in a direction along the surface of the steel sheet. This increased electron density results in a higher Ar ion concentration close to the steel sheet and thus in an increased sputter intensity. According to a further embodiment of the present invention the method comprises cooling the plurality of magnets. Cooling can be provided by having a cooling tube comprising a cooling medium.

In an embodiment of the present invention, the transferring of the plasma treated steel strip from the plasma cleaning unit to the post processing unit is performed in the atmospheric conditions within 20 mins, preferably within 10 mins. This allows transferring of the activated steel strip from the plasma cleaning unit to the post processing unit in the atmospheric conditions within 20 mins of cleaning, preferably within 10 mins of cleaning. For example the activated steel strip can be transferred from the plasma cleaning unit to an HDG unit in the atmospheric conditions within 10 mins of cleaning. As another example, the activated steel strip can be transferred from the plasma cleaning unit to a HDG unit in the atmospheric conditions within 3-4 mins of cleaning.

An even further embodiment of the present invention relates to performing the transferring of the plasma treated steel strip outside the magnetron plasma sputter unit in a reducing atmosphere. In other words, the transferring of the plasma treated steel strip is performed in a reducing atmosphere The reducing atmosphere can be achieved using a HNX mixture with hydrogen concentrations in the range 0-20 %. The reducing mixture or gases can be fed to a transferring path between the outside of the magnetron plasma sputter unit and the post processing unit.

In a preferable embodiment the steel strip is pre-processed before the plasma treatment. The pre-processing step can be an annealing step such as a batch annealing step or a continuous annealing step. The plasma cleaning step can be placed before an annealing step or before a HDG step or in between an annealing and HDG step. The plasma treatment step can be performed after a heat treatment step. . Considering the various possibilities of choosing the pre-possessing steps as well as the post processing steps, several scenarios are envisaged where some of the plausible and non-limiting scenarios are discussed below.

In one scenario, the steel strip can be first passed through a cold rolling unit and then can be annealed either by a batch annealing step or by a continuous annealing step. The plasma cleaning step can be performed following the annealing step. After plasma cleaning step the steel strip can be transferred to a post processing unit such as a HDG unit where the transferring of the plasma treated steel strip is performed in atmospheric conditions to the post-processing unit. In a second scenario, the steel strip can be heat treated to a coat cycle. Followed by the heat treatment, the plasma cleaning step can be performed from where the steel strip can be transferred to a post processing unit such as a HDG unit. The transferring of the plasma treated steel strip to the post-processing unit is performed in atmospheric conditions. The transfer can be performed through a transfer unit. In another scenario, the steel strip can be first passed through cold rolling unit and then send to a plasma cleaning unit to perform the plasma treatment. From the plasma cleaning unit the steel strip is transferred to a post processing unit in atmospheric conditions.. In a different scenario, the steel strip is first annealed by an annealing process. The annealing process can be a batch annealing process or a continuous annealing process. Annealed steel strips can be send to a further processing unit such as a HDG unit. From the further processing unit, the steel strip is sent in atmospheric conditions to a plasma cleaning unit to perform the plasma treatment. The post processing unit can additionally comprise a painting unit which is used for painting the steel strips. The post processing of the steel strip can be performed in a continuous in-line hot dip galvanizing unit.

In a preferable embodiment the steel strip can be a high strength steel. A high strength steel has a tensile strength of at least 500 MPa, preferably at least 800 MPa, more preferably at least 1000 MPa The high strength steel can be steel having a Dual phase, Martensitic etc. An example of a steel strip is DP800HpF or DP1000. The alloying elements of such particular grade of steel can diffuse to the surface to form oxides. These oxides are composed typically of silicon, aluminum and manganese, and can be a mixture of them or can be of other alloying elements.

The present invention in a further aspect relates to a steel strip, sheet or blank, obtained via the method as described above, where the steel strip comprises less than 100% of surface oxides. The low concentration of surface oxides is a concentration that is less than 100% of surface oxides, preferably less than 50% of surface oxides and more preferably less than 25% of surface oxides. The low concentration of surface oxides come from the annealing/reducing section where a 100% surface oxide coverage is when it is not being reduced/treated previously. A non-reduced surface will contain a surface oxide concentration up to 100% where everything on the surface will be oxides. However after an annealing or a reducing step, the Fe will be reduced. Even then, a selective percentage of surface oxidation can be present owing to the presence of the alloying elements in the steel. These oxides attributed by the alloying elements will not be reduced, and it depends thus on the type of steel, the concentration of alloying elements, their diffusion to the surface and oxidation kinetics on how the final percentage of the oxide concentration present on the surface. The present invention in a further aspect relates to a coating unit to coat a steel strip, wherein the coating unit comprises a magnetron plasma sputter unit configured to plasma treat a steel strip and a post-processing unit to coat the steel strip. The post-processing unit is a continuous hot dip galvanizing unit The coating unit is configured to transfer the steel strip from the magnetron plasma sputter unit to the post-processing unit in atmospheric conditions within 30 mins. The transferring can be done through a transfer unit wherein the transfer unit is in atmospheric conditions. The transfer unit can be placed between the plasma cleaning unit and the post processing unit. The transfer unit can be a fixed unit or a temporary unit and it can be placed within a limited vicinity of the plasma cleaning unit. Thus, transferring of the plasma treated steel strip in atmospheric conditions from a magnetron plasma sputter unit to a post-processing unit can be done through a transfer unit. In other words, transferring of the plasma treated steel strip in atmospheric conditions from a magnetron plasma sputter unit to a continuous hot dip galvanizing unit can be done through a transfer unit. This transfer unit can be placed for transferring of the plasma treated steel strip in atmospheric conditions within 30 mins. The plasma treatment can be in-situ plasma treatment by treating in-line directly in front of the follow up processing step. But plasma treatment can be also a stand-alone unit where it is possible with a transport of the steel strip to the next processing step. The post-processing unit is a hot dip galvanizing unit. In a further embodiment, the post-processing unit is a continuous in-line hot dip galvanizing unit The invention solves the problems related with bad wettability and bare spots on HDG coated steel and improves the overall quality of the coating.

Another embodiment of the present invention relates to coating unit where the coating unit comprises an annealing unit. The annealing unit is further arranged before or after the magnetron plasma sputter unit. An even further embodiment of the present invention relates to coating unit where the coating unit is configured to transfer the steel strip from the magnetron plasma sputter unit to the post-processing unit through a transfer unit in atmospheric conditions. The post processing unit can also comprise a painting unit. The painting unit is used for painting the plasma treated and coated steel strip where the adhesion of the paint is substantially improved by the plasma treatment. Another embodiment of the present invention relates to a coating unit where the magnetron plasma sputter unit comprises a plurality of magnets placed on the same side of the magnetron plasma sputter unit for densifying the plasma near the surface of the steel strip. An even further embodiment of the present invention relates to coating unit where a vacuum chamber is placed around the magnetron plasma sputter unit. The vacuum chamber enables a longer mean free-path to the Ar ions to bombard the steel surface and thereby maximizing the energy impact on the surface.. A further embodiment of the present invention relates to a coating unit where the coating unit comprises a shielding of non- magnetic material around the plurality of magnets. The shielding can be of aluminium or copper. The shielding can be rotatable. By providing a shielding of non-magnetic material around the plurality of magnets, the effect of shielding the magnetic field by the iron-debris is strongly reduced.

The present invention embodiments allow to perform the plasma cleaning process over a larger area of the steel surface and do not require additional equipment for this purpose such as usage of an additional laser light. Moreover it allows the plasma treated steel surface to remain activated even up to 30 minutes under atmospheric conditions such as in air. This is advantageous as it provides a method to yield steel strips having better coating quality such as in the case of HDG coating compared to coatings performed without a plasma treatment. Although in some cases, a protective atmosphere is used to minimize the effect of oxidation between the plasma treatment and a post processing step such as a coating step, it is not required. Avoiding a protective atmosphere increases the flexibility in choosing placement of a coating process.

Brief description of the drawings The present invention will now be explained by means of the following, non-limiting figures. fig. 1 shows a schematic of a coating unit according to an embodiment of the present invention. fig. 2 shows a schematic cross-section of a magnetron plasma sputter unit according to an embodiment of the present invention, fig. 3 shows a schematic cross-section of a magnetron plasma sputter unit according to a second embodiment of the present invention, fig. 4 shows an image of a steel substrate that is imaged (b) after a plasma cleaning step and (a) without a plasma cleaning step, and fig. 5 shows a schematic representation describing different scenarios on placing the plasma cleaning unit. fig. 6 shows a graphical representation showing relation between exposure time and improvement.

Detailed description of the drawings As explained in the previous sections, before applying a coating or a paint on a steel strip (5), the steel surface needs to be thoroughly cleaned, de-oxidized and activated in order to obtain good material adhesion. According to the present invention embodiments, a method is provided using a plasma sputter unit which is a magnetron plasma sputter unit. Argon (Ar) gas is fed to this unit which is (partly) ionized by the applied voltage difference between the unit and steel strip. The Ar ions bombard the surface of the steel strip and thereby remove the contaminations, oxides and surface enrichments and at the same time it activates the surface. Subsequently, a post processing step such as an annealing step and/or applying a coating such as a hot dip galvanizing step or a paint can be performed to the cleaned and/or activated steel surface in an efficient manner. Fig. 1 shows a coating unit 30 comprising a magnetron plasma sputter unit 1 or plasma cleaning apparatus 1 according to one embodiment of the present invention. A steel strip 5 is first passed to the magnetron plasma sputter unit 1 through vacuum locks 31 that are placed on either side of the magnetron plasma sputter unit 1. A vacuum pump 32 is connected to the magnetron plasma sputter unit 1 to reduce its pressure. The steel strip 5 is plasma cleaned in the magnetron plasma sputter unit 1 and transferred through atmospheric conditions 33 into a post processing unit 35. The coating unit 30 is configured to transfer the steel strip from the magnetron plasma sputter unit 1 to the post-processing unit 35 in atmospheric conditions within 30 mins The post processing unit 35 in this embodiment is a HDG unit. Optionally electromagnetic brakes 34 can be applied to the steel strip before entering it into the post processing unit 35. The hot dip galvanized steel strip 5 is passed out of the post processing unit 35 after performing the step of hot dip galvanization. The coating unit 30 is configured to transfer the steel strip 5 from the magnetron plasma sputter unit 1 to the post-processing unit 35 through a transfer unit in atmospheric conditions.

In fig. 2, a magnetron sputter unit 1 or plasma cleaning apparatus 1 is shown comprising an ionisation chamber 2 in which an Ar gas is ionized into a plasma. The ionisation chamber 2 comprises a container 3 with an opening 4 at the side where a steel strip 5 is guided over the ionisation chamber 2. The steel sheet 5 is supported by rolls 6 which may also serve as transport rolls to transport the steel strip over the plasma cleaning device 1. The container 3 of the ionisation chamber 2 is positioned in a shielding container 7 which is at a distance from and not in contact with container 3 for instance by non- electrically conductive spacers not shown in the drawing. The shielding container 7 is grounded whereas the container 3 of the ionisation chamber 2 is kept at a voltage with respect to the steel strip 5 for the ionisation and plasma forming in ionisation chamber 2. The voltage is applied to container 3 by means of electric connection 9, which is guided insulated through shielding container 7 and connects to container 3.

Inside the ionisation chamber 2, a number of permanent magnets 10 are provided in one or more rows. In the example of fig.2, the magnets 10 are inside a hollow shielding 11 of a non-magnetic material, wherein the shielding is provided with hollow pivot axis 12 that extend till outside container 7 and are insulated from container 3 of ionisation chamber 2. The pivot axis 12 provide that the hollow shielding 11 can be rotated from outside the containers 3,7. The magnets 10 are mounted inside a separate tube 13 wherein the tube 13 is provided with an inlet and outlet line 14,15 which are coaxially with the pivot axis 12 and extend till outside pivot axis to connect these to a cooling medium system. With this set-up the magnets 10 can be kept in position while the shielding 11 can be rotated depending on the amount of debris on the shielding 11 between the magnets 10 and the substrate to be cleaned. Depending on the orientation of the magnets 10 in the shielding 11 , that is whether or not rotation of the shielding 11 makes a difference to the orientation of the magnetic field, the shielding 11 can be used to hold permanent magnets 10 in position and be used as cooling tube wherein hollow pivot axis 12 are connected to a cooling liquid supply system to cool permanent magnets 10. The shielding container 7 has an opening 19 which is in register with the opening 4 of the container 3 of ionisation chamber 2. A supply line 16 for Argon gas or Argon based gas is provided which connects to a gas tube 22 inside ionisation chamber 2 that extends over at least part of the ionisation chamber 2 and is provided with a number of nozzles 23 to distribute the Ar gas over the ionisation chamber 2. The supply line 16 is insulated from shielding container 7 and container 3. The gas tube 22 is positioned parallel or about parallel to the shielding 11 and as a result the gas flows around shielding 11 in the direction of the substrate 5 to be cleaned. In the outer shielding container 7 outflow openings 17 are provided for Ar gas flowing out of the ionisation chamber 2 taking along debris removed from steel sheet 5 by plasma cleaning. The outflow openings 17 are slit or grid shaped and are provided near and/or adjacent to the substrate 5 to be cleaned along at least part of the circumference of container 7 and typically along most or even all sides of container 7. The outflow openings 17 shown in the drawing are slit shaped wherein the slits run at an angle to the sides of shielding container 7 and parallel or about parallel to the substrate 5 to be cleaned. The slits are defined by parallel strips of material 18, for instance steel strips, that are supported by shielding container 7. The shielding container is grounded and so are the parallel strips 18 if made from electrically conductive material. In the embodiment shown in the drawing the substrate 5 is very near the last, most outward strip 18 and with that also a slit shaped outflow opening is defined between the last strip 18 and the substrate 5. The last strip could also function as a support for a substrate 5 such as a steel sheet 5, since the strips 18 are either grounded like the substrate 5 or are not electrically conductive. However, with steel strip 5 the preferred option would be to use rolls 6 and keep a certain distance between the steel strip 5 and the last strip 18 of the outflow openings 17. To that end the magnetron sputter unit 1 is provided with a distance control system 20 to move the unit 1 to and from the steel strip 5 to keep the unit at a certain distance from the steel strip 5. In operation the positive ions are accelerated towards the steel substrate 5 to be cleaned by a voltage difference between container 3 and the substrate which is grounded. To increase the density of the plasma 21 near the steel surface, the magnets 10 are used to capture electrons and thereby increase their ionization efficiency. The row of magnets 10 is located on the front side of the steel sheet, that is the side of the steel sheet 5 that is to be cleaned, inside the ionisation chamber 2. This has the major advantage that the thickness of the steel sheet 5 does not affect the sputtering process. In contrast to by placing the magnets 10 on the other side of the steel sheet 5, the ferromagnetic steel will short circuit part of the magnetic field thereby reducing its efficiency especially for thicker substrates. However at the front side of the steel sheet 5, the magnets 10 will be contaminated by the surface material removed from the front side of the steel sheet 5.

The contamination problem is solved or diminished by providing two features which can be used individually or combined to improve the operation time of the plasma sputter unit 1. By performing the plasma pre-treatment prior to the coating process, surface material is removed which will subsequently be re-deposited inside the ionisation chamber 2. This causes short-circuit effects, contamination problems and a reduced efficiency of the surface treatment of the steel sheet 5 by lowering the magnetic field. By using and optimizing the Ar feed flow for the plasma, removed surface material can be guided to a large extent to outside of the sputter unit 1. This prevents the build-up of debris to a large degree.

One or more rows of magnets 10 are used to densify the plasma 21 near the surface of the steel sheet 5. The removed and re-deposited surface material (Fe) would arrive at least partly on top of the magnets 10 causing a reduction of the magnetic field therewith lowering the plasma sputtering efficiency. By providing a rotatable shielding 11 of non- magnetic material around the magnets 10, for example of aluminium or copper, the effect of shielding the magnetic field by the iron-debris is strongly reduced.

A third feature is the cooling system provided to cool the magnets 10. Since the magnets 10 are close to the steel strip 5, which heats up as function of sputtering the magnets 10 need to be cooled to maintain their magnetic properties. Since the magnets 10 are inside the ionisation chamber 2 and inside container 3, which is under a high positive voltage (the steel strip is grounded) the cooling system is electrically insulated from container 3.

The plasma surface pre-treatment consist of an Ar based gas which is being fed through supply line 16 and gas tube 22 to inside the ionisation chamber 2. A voltage difference (inner unit 300-3000V) between the container 3 of ionisation chamber 2 and steel strip 5 creates Ar ions which subsequently bombard the steel surface (grounded) due to the voltage difference. The Ar gas flow and the power used are adjusted depending on the required sputter rate of the steel surface. To this end it is further provided that the position of gas tube 22 inside ionisation chamber 2 is adjustable, that is supply line 16 and therewith gas tube 22 can be moved so as to change the distance between gas tube 22, nozzles 23 with respect to the magnets 10 and shielding 11. Typically Ar flows used are in the range of 60-650 seem for a sputter unit having dimensions of 35 cm x 21 cm x 12 cm. The power supply is DC or pulsed DC with a typical frequency between 30 and 250 kHz. This results in a vacuum background pressure in a vacuum chamber typically between 10^-4 and 10² mbar. Typically DC is used for conducting surfaces whereas RF (radio frequency) power supplies are used for non-conducting surfaces. The pressure inside the sputter unit is typically in the order of 10-1 O² mbar. The required sputter energy density of the plasma treatment to obtain good adhesion for PVD coatings ranges from 90 kJ/m2 (mild steel 0.2 mm + IR heating), 100- 200 kJ/m2 (mild steel 0.2 mm - only plasma), 400 kJ/m2 for DP800/CP800 steel (0.2 mm), 1008 kJ/m2 for DP800HpF (1.1 mm) and DP1000 (1.8 mm), 1400 kJ/m2 for high-Si steel (0.2 mm), 1800 kJ/m2 for M1400 (2 mm), up to 2400 kJ/m2 for certain types of DP800 steels. All these measures take care of removing surface material and activating the steel surface prior to a post processing while keeping the plasma cleaning apparatus 1 sufficiently clean from debris for a long time. This is a key step to obtain good coating adhesion properties. By having a well-defined Ar flow into the sputter unit, the removed surface material can be forced/guided out of the critical positions of the sputter unit. The plasma cleaning apparatus 1 with the rows of magnets 10 inside the ionisation chamber 2 allows to have a second cleaning apparatus 1 at the other side of substrate 5 and opposite to the first cleaning apparatus 1. With that configuration both sides of substrate 5 can be plasma cleaned at the same time. Moreover, with this configuration to plasma clean both sides of the substrate 5 at the same time, a relatively compact build of the plasma cleaning installation can be realized. Fig. 3 shows a schematic cross-section of a plasma cleaning apparatus 1100 comprising a first plasma cleaning apparatus 1 and a second plasma cleaning apparatus T positioned at opposite sides of a substrate 5 to be cleaned. With the magnets 10 inside the ionisation chamber 2 of the first plasma cleaning apparatus 1 and the second plasma cleaning apparatus T have the advantage that the substrate 5 can be plasma cleaned at directly opposing sides at the same time. The container 3 and shielding container 7 are adapted to form a receptacle to receive debris dropping of the shielding 11 of the rows of magnets 10. All other parts of the first plasma cleaning apparatuses 1 and the second plasma cleaning apparatus T have the same reference numbers as in fig. 2 as far as appropriate. In an example embodiment a plasma treatment for 8 min at 200W on a 0.05 m² steel surface is performed. In that example, the time taken to open the vacuum chamber and pack the samples was 2-3 minutes in air. T ransfer of the sample to the post processing unit, which is a HDAS (HDG) coater under Ar protective atmosphere is 10 minutes. Preparing to insert in the sample into the HDAS is 5 minutes in air. After the plasma treatment altogether the steel surface was in atmospheric conditions for 18 minutes before inserting into the post processing unit. Even then, the hot galvanization coating showed good adhesion properties onto the steel surface.

Fig. 4 shows an image comprising image (a) and image (b). Image (a) shows an HDG coated DP800HpF steel substrate 50 where the coating is performed without performing a plasma cleaning step. Image (b) shows the same steel substrate 60 where the coating is performed after a plasma cleaning step. The zinc coating in image (b) is much more homogenous compared to the plasma untreated surface in image (a). The non-plasma treated zinc coating of steel substrate 50 shows large patches of uncoated materials. An example plasma treatment set up that was used for this step comprises using an argon based plasma with a power of 200 watts with a sputter time of 8 minutes where total treated surface area is 0.05 m². The experimental set up used a metallic strip coater with a chamber pressure of 1X1 O ⁴ mbar. As clearly seen, introducing a plasma cleaning step prior to the HDG coating step provides better adhesion properties and thereby a better coating.

Fig. 5 shows a schematic representation showing different scenarios on placing the plasma cleaning unit 1 in a typical flow line. Considering various possibilities of pre possessing steps and post processing steps, several scenarios are envisaged. The various steps can be explained as provided below:

In Example 1 of Fig. 5, a steel strip 5 is first passed through cold rolling unit 70 and then sent to an annealing unit. The annealing unit is a batch annealing unit 80. A plasma cleaning step is performed in a magnetron sputter unit 90 following the batch annealing step from where the steel strip 5 is transferred to a post processing unit. The post processing unit is a HDG unit 100. Transferring of the plasma treated steel strip to the HDG unit 100 is performed in atmospheric conditions.

In Example 2 of Fig. 5, a steel strip 5 is first passed through cold rolling unit 70 and then sent to an annealing unit. The annealing unit is continuous annealing unit 110. A plasma cleaning step is performed in a magnetron sputter unit 90 following the annealing step from where the steel strip 5 is transferred to a post processing unit. The post processing unit is a HDG unit 100. Transferring of the plasma treated steel strip to the HDG unit 100 is performed in atmospheric conditions. ln Example 3 of Fig. 5, a steel strip 5 is heat treated to a coat cycle in a heat treatment unit 120. Followed by the heat treatment, a plasma cleaning step is performed. The plasma cleaning step is performed in a magnetron sputter unit 90 from where the steel strip 5 is transferred to a post processing unit. The post processing unit is a HDG unit 100. Transferring of the plasma treated steel strip is performed in atmospheric conditions to the HDG unit 100.

In Example 4 of Fig. 5, a steel strip 5 is first passed through cold rolling unit 70. A plasma cleaning step is performed in a magnetron sputter unit 90 following the cold rolling step. After plasma cleaning, the steel strip 5 is transferred to a post processing unit. The post processing unit is a continuous annealing unit 110 from where the steel strip 5 is transferred to a HDG unit 100. Transferring of the plasma treated steel strip from the magnetron sputter unit 90 to the continuous annealing unit 110 is performed in atmospheric conditions.

In Example 5 of Fig. 5, a steel strip 5 is first passed through cold rolling unit 70. A plasma cleaning step is performed in a magnetron sputter unit 90 following the cold rolling step. After the plasma cleaning, the steel strip 5 is transferred to a post processing unit. The post processing unit is a batch annealing unit 80 from where the steel strip 5 is transferred to a HDG unit 100. The transferring of the plasma treated steel strip from the magnetron sputter unit 90 to the batch annealing unit 80 is performed in atmospheric conditions. In Example 6 of Fig. 5, a steel strip 5 is first annealed in an annealing unit. The annealing unit is a continuous annealing unit 110. Continuously annealed steel strip 5 is send to a processing unit such as a HDG unit 100. Followed by the hot dip galvanization, a plasma treatment of the steel strip is performed by sending it to a magnetron sputter unit 90. From the magnetron sputter unit 90, the steel strip 5 is transferred to a post processing unit in atmospheric conditions. The post processing unit is a painting unit 130 that is used for painting the steel strip 5.

In Example 7 of Fig. 5, a steel strip 5 is first annealed by an annealing unit. The annealing unit is a batch annealing unit 80. Batch annealed steel strip 5 is then sent to a processing unit such as a HDG unit 100. Followed by hot dip galvanization, a plasma treatment is performed by sending the steel strip to a magnetron sputter unit 90. From the magnetron sputter unit 90, the steel strip 5 is transferred to a post processing unit in atmospheric conditions. The post processing unit is a painting unit 130 that is used for painting the steel strip 5.

In an exemplary experiment to obtain a significant reduction of defects, a sample is sputtered on its front side with an intensity of -200 kJ/m². The ageing time, which is the time for surface reoxidation and de-activation, has been investigated by the plasma treatment of the substrate with an intensity of 200 kJ/m², after which the samples have been exposed to the environment (22.8Ό and RH of 49%) for different time intervals before being coated by the HDG process. As shown in figure 6, for an exposure time between 0 and 30 minutes, the improvement as compared to the backside follows a decreasing linear trend (R2 = 0.9344). Here, the backside is a non-sputtered side of the sample. In this way the backside acts as a reference for the sputtered front side of the sample and therefore the difference in defect percentage can be considered as the improvement induced by the plasma sputter treatment. The data point at the Y-axis at 100 is not a measured data point, but it is the theoretical maximum that is achievable improvement without exposure and it is only used for fitting purposes.

The present invention has been described above with reference to several exemplary embodiments as shown in the drawings. The embodiments described by the present invention are not limiting to the examples provided above. Modifications and alternative implementations of some parts or elements are possible, and are included in the scope of protection as defined in the appended claims.

Claims

1. A method for treating the surface of a steel strip, wherein the method comprises the steps of: providing a steel strip (5), plasma treatment of the steel strip by a magnetron plasma sputter unit (1); transferring of the plasma treated steel strip in atmospheric conditions to a post processing unit (35) within 30 mins; and post processing the steel strip after the plasma treatment wherein the post- processing unit (35) is a continuous hot dip galvanizing unit.

2. The method according to claim 1, wherein the plasma treatment of the steel strip by the magnetron plasma sputter unit (1) is performed in a protective atmosphere.

3. The method according to claim 1 or 2, wherein the magnetron plasma sputter unit (1) comprises a plurality of magnets placed on the same side of the magnetron plasma sputter unit (1) for densifying the plasma near the surface of the steel strip.

4. The method according to claim 3, wherein the method further comprises cooling the plurality of magnets.

5. The method according to any of the preceding claims, wherein transferring of the plasma treated steel strip is performed in the atmospheric conditions within 20 mins, preferably within 10 mins.

6. The method according to any of the preceding claims, wherein the transferring of the plasma treated steel strip is performed in a reducing atmosphere.

7. The method according to any of the preceding claims, wherein the steel strip is pre- processed before the plasma treatment.

8. The method according to any of the preceding claims, wherein the post processing of the steel strip is performed in a continuous in-line hot dip galvanizing unit .

9. The method according to any of the preceding claims, wherein the steel strip is a high strength steel.

10. A coating unit (30) to coat a steel strip (5), wherein the coating unit (30) comprises: a magnetron plasma sputter unit (1) configured to plasma treat a steel strip (5) and a post-processing unit (35) to coat the steel strip (5) wherein the post-processing unit (35) is a continuous hot dip galvanizing unit; and wherein the coating unit (30) is configured to transfer the steel strip (5) from the magnetron plasma sputter unit (1) to the post-processing unit (35) in atmospheric conditions within 30 mins.

11. The coating unit (30) according to claim 10 wherein the post-processing unit (35) is a continuous in-line hot dip galvanizing unit.

12. The coating unit (30) according to claim 10 or 11 wherein the coating unit (30) further comprises an annealing unit; wherein the annealing unit is further arranged before or after the magnetron plasma sputter unit (1).

13. The coating unit (30) according to claims 10-12 wherein the coating unit (30) is configured to transfer the steel strip (5) from the magnetron plasma sputter unit (1) to the post-processing unit (35) through a transfer unit in atmospheric conditions .

14. The coating unit (30) according to any of the claims 10-13, wherein the magnetron plasma sputter unit (1) comprises a plurality of magnets placed on the same side of the magnetron plasma sputter unit (1) for densifying the plasma near the surface of the steel strip.

15. The coating unit (30) according to any of the claims 10-14, wherein a vacuum chamber is placed around the magnetron plasma sputter unit (1).