DE102020208991A1 - Process for producing a hot-dip coated steel sheet and hot-dip coated steel sheet - Google Patents

Abstract

The invention relates to a method for producing a hot-dip coated steel sheet, the method comprising the following steps: - providing a steel sheet, - hot-dip coating the steel sheet with a zinc-based coating, the steel sheet passing through a molten bath containing aluminum between 0.1 and 4.0 wt. -%, optionally magnesium between 0.1 and 4.0% by weight and the remainder zinc and unavoidable impurities,- stripping the steel sheet still coated with liquid melt to set a predetermined thickness of the coating, which is then converted into a solidified state Furthermore, the invention relates to a hot-dip coated steel sheet with a zinc-based coating which has aluminum between 0.1 and 4.0% by weight, optionally magnesium between 0.1 and 4.0% by weight and the remainder zinc and unavoidable impurities .

Description

• - Bereitstellen eines Stahlblechs,
• - Schmelztauchbeschichten des Stahlblechs mit einem zinkbasierten Überzug, wobei das Stahlblech ein Schmelzbad durchläuft, welches Aluminium zwischen 0,1 und 4,0 Gew.-%, optional Magnesium zwischen 0,1 und 4,0 Gew.-% und Rest Zink und unvermeidbare Verunreinigungen umfasst,
• - Abstreifen des noch mit flüssiger Schmelze beschichteten Stahlblechs zur Einstellung einer vorgegebenen Dicke des Überzugs, welche anschließend in einen erstarrten Zustand überführt wird.

The invention relates to a method for producing a hot-dip coated steel sheet, the method comprising the following steps:

• - providing a steel sheet,
• - hot-dip coating the steel sheet with a zinc-based coating, the steel sheet passing through a molten bath containing aluminum between 0.1 and 4.0% by weight, optionally magnesium between 0.1 and 4.0% by weight and the balance zinc and unavoidable includes impurities,
• - Stripping the still coated with liquid melt steel sheet to set a predetermined thickness of the coating, which is then converted into a solidified state.

Furthermore, the invention also relates to a hot-dip coated steel sheet with a zinc-based coating.

From the patent EP 2 297 372 B1 For example, there is known a method of hot-dip coating a steel sheet with the aim of providing a metal-coated steel sheet having a small waviness. On the one hand, the stripping for adjustment of the coating should be carried out in an oxidative atmosphere in order to be able to bring about a reduction in waviness after the coating has solidified on the steel sheet. On the other hand, the steel sheet should be cooled at different cooling rates: after leaving the stripping unit, the steel sheet with the coating still applied in liquid form should be cooled at a cooling rate of less than 5 K/s until the coating begins to solidify; after the start to the end of the solidification of the coating, the cooling of the steel sheet should be at a cooling rate greater than 20 K/s in order to be able to provide better, reduced waviness.

Chemical treatment and modification of the steel sheet surface is required to ensure successful paint bonding to steel sheets. In the automotive sector, a great deal of effort is put into a phosphating process, so that a surface-covering growth of phosphate crystals can be set on a coating that is usually hot-dip coated (hot-dip galvanized), so that sufficient adhesion and a homogeneous appearance of the paint can be achieved. Before crystal formation occurs, the surface of the hot-dip coated steel sheet is "pickled" by the phosphoric acid present in the phosphating solution in order to at least partially remove/dissolve the non-reactive oxide layer from the coating. Successful conversion chemistry can only be developed after this reaction barrier has been removed. Pre-treatments can also be carried out in the coil coating area, which often also include “pickling” components in order to at least partially remove/dissolve the existing oxide layer on hot-dip-coated steel sheets or also on bare (non-coated) steel sheets and thereby prepare the surface for further treatment steps activate.

Depending on the coating, depending on the process and composition, oxide layers of different thicknesses form after application. For example, due to the process, electrolytically galvanized steel sheets have oxide layers (ZnO) that are smaller than 10 nm. With hot-dip finishing (hot-dip galvanizing), significantly thicker oxide layers are formed, which have a different chemical composition depending on the hot-dip composition. Such hot-dip coated coatings continue to react with the surrounding air atmosphere immediately after stripping. Depending on the system, the hot-dip coated steel sheets can also pass through cooling sections in which they are actively cooled with air. The contact of the steel sheet with the air intensifies and accelerates the oxidation processes and ensures that the elements in the coating with an affinity for oxygen diffuse preferentially to the surface. This generally forms a thick oxide layer that extends deep into the coating.

In the automotive as well as in the coil coating area, a certain contact time with the "pickling" medium is required, which should ideally be long enough for the oxide layer on the surface of the steel sheets to be essentially completely removed. If this is not the case, spots can form during phosphating, which can be attributed to locally different crystal growth. The connection of typical automotive adhesion promoters or pre-treatments from the coil coating area, which have been designed for application to metallic coatings, cannot fully and satisfactorily develop their effect due to the existing, in particular thicker, oxide layer. A faulty connection of such systems generally results in poorer adhesive suitability and/or poorer paint adhesion. In the worst case, areas in which the oxide layer has not been completely stripped away can represent predetermined breaking points.

In order to counteract this disadvantage, there is a need for a method for hot-dip finishing a steel sheet, with which an oxide film having a smaller oxide film thickness as compared with the prior art can form on the metallic coating of the steel sheet.

The object of the invention is to specify a method for producing a hot-dip coated steel sheet with which the phosphatability and/or paint finish of hot-dip-coated steel sheets can be improved and to specify a corresponding hot-dip-coated steel sheet.

The object is achieved in relation to the method with the features of patent claim 1 and in relation to the steel sheet with the features of patent claim 10.

The inventors have surprisingly found that a positive influence can be exerted on improving the phosphatability and/or appearance of the paintwork in that the oxide layer can only be thinner than in the prior art after the liquid coating has been applied to the steel sheet, if the stripping is carried out in takes place in an inert atmosphere which contains hydrogen between 0.1 and 10% by volume, the remainder nitrogen and unavoidable impurities and has a dew point between -50°C and +5°C. The targeted setting of the dew point between -50°C and +5°C, in particular between -40°C and 0°C, preferably between -50°C and -10°C, can already prevent the formation of a thick oxide layer, since the lowering of the dew point means that the atmosphere loses moisture, so that fewer water molecules can form, which react with the oxygen-affinity alloying elements on the surface of the coating to form metal oxides or hydroxides, and the formation of thicker oxide layers is therefore prevented. As a result, oxide layers of up to 10 nm can form on the coating, with the thin oxide layer also resulting in a substantially homogeneous thickness on the coating.

The steel sheet is hot-dip coated with a zinc-based coating, the steel sheet passing through a molten bath containing aluminum between 0.1 and 4.0% by weight, optionally magnesium between 0.1 and 4.0% by weight and the balance zinc and unavoidable includes impurities. Elements such as bismuth, zirconium, nickel, chromium, lead, titanium, manganese, silicon, calcium, tin, lanthanum, cerium and iron can be present as impurities in the molten pool in individual or cumulative amounts of up to 0.4% by weight. In order to set a predetermined thickness of the coating, the melt applied to the steel sheet while still in the liquid state is stripped off in that, after leaving the molten pool, the steel sheet coated with liquid melt is guided through a stripping device which has means, for example nozzles, in particular slotted nozzles, which act on the steel sheet on both sides, preferably by means of a gas flow. After setting the specified thickness, the melt, which is still in the liquid state on the steel sheet, is then converted into a solidified state. Stripping devices are known from the prior art.

Depending on the composition of the molten pool, the solidified coating on the steel sheet can be applied in different ways, depending on the requirements and intended use. In addition to zinc and unavoidable impurities, the zinc-based coating can contain additional elements such as aluminum with a content of between 0.1 and 4.0% by weight and optionally magnesium with a content of between 0.1 and 4.0% by weight. Sheet steel with a zinc-based coating has very good cathodic protection against corrosion, which has been used in automobile construction for years. If improved corrosion protection is provided, the coating also has magnesium with a content of at least 0.1% by weight, in particular at least 0.5% by weight, preferably at least 0.5% by weight. Aluminum is present in addition to magnesium in a content of at least 0.1% by weight, in particular at least 0.5% by weight, preferably at least 0.5% by weight. The zinc-based coating particularly preferably has aluminum and magnesium, each with at least 0.5% by weight, in order to be able to provide an improved cathodic protective effect.

Sheet steel is to be understood as meaning a flat steel product in strip form or sheet/plate form. It has a longitudinal extent (length), a transverse extent (width) and a vertical extent (thickness). The steel sheet can be a hot strip (hot-rolled steel strip) or cold-rolled strip (cold-rolled steel strip), or it can be made from a hot strip or from a cold strip.

The thickness of the steel sheet is, for example, 0.5 to 4.0 mm, in particular 0.6 to 3.0 mm, preferably 0.7 to 2.5 mm.

Further advantageous configurations and developments emerge from the following description. One or more features from the claims, the description and the drawing can be combined with one or more other features from them to further refine the invention. One or more features from the independent claims can also be linked by one or more other features.

According to one embodiment of the method according to the invention, the stripping is carried out with active cooling, the active cooling taking place at a cooling rate of at least 3 K/s. The stripping by means of a gas flow is temperature-controlled, which means that the gas, which acts on the liquid melt on the steel sheet for stripping, is temperature-controlled to a certain temperature, which in particular depends on the gas flow (speed, pulse) to a cooling of the liquid melt and thus leads to the initiation of solidification. It is advantageous to allow rapid solidification after the steel sheet has been wetted with the liquid melt in order to minimize the possibility of oxygen diffusing into the coating. In particular, the cooling rate is at least 5 K/s, preferably at least 8 K/s, preferably at least 11 K/s, more preferably at least 14 K/s in order to avoid the formation of a thick oxide layer.

According to one embodiment of the method according to the invention, the cooling is carried out actively with a cooling rate of at least 3 K/s. The liquid melt on the steel sheet can be actively cooled before, during and/or after stripping, for example in the inert atmosphere, which is appropriately temperature-controlled in order to achieve active cooling with a cooling rate of at least 3 K/s, which leads to a cooling of the liquid melt and thus to the initiation of solidification. It is advantageous to allow rapid solidification after the steel sheet has been wetted with the liquid melt in order to minimize the possibility of oxygen diffusing into the coating. In particular, the cooling rate is at least 5 K/s, preferably at least 8 K/s, preferably at least 11 K/s, more preferably at least 14 K/s in order to avoid the formation of a thick oxide layer.

According to one embodiment of the method according to the invention, the zinc-based coating has a thickness of between 2 and 20 μm, in particular between 4 and 15 μm, preferably between 5 and 12 μm. This corresponds to the specified thickness, which can be adjusted in a targeted manner when stripping off the still liquid melt before solidification.

According to one embodiment of the method according to the invention, an oxide layer forms on the coating, which has a thickness of less than 10 nm. Depending on the process parameters described above, an oxide layer thickness can also form which is in particular less than 8 nm, preferably less than 6 nm, preferably less than 5 nm. The lower the thickness of the oxide layer, the contact times with, for example, acidic media can be shortened if necessary and in the case of an area-wide removal of the oxide layer in particular.

According to one embodiment of the method according to the invention, the hot-dip coated steel sheet is skin-passed. Skin-passing imprints a surface structure into the hot-dip coated steel sheet, which can be a deterministic surface structure, for example. A deterministic surface structure is to be understood, in particular, as meaning regularly recurring surface structures which have a defined shape and/or configuration or dimensioning. In particular, this also includes surface structures with a (quasi) stochastic appearance, which are composed of stochastic form elements with a recurring structure. Alternatively, the introduction of a stochastic surface structure is also conceivable.

According to one embodiment of the method according to the invention, the hot-dip coated steel sheet is phosphated. In the typical automotive phosphating process, alkaline cleaning modifies and possibly thins out the (native) oxide layer on the coating that occurs in conventional hot-dip coating. In order to modify or activate the thick oxide layer in the immersion phosphating process, more time is required than with the lower oxide layer thickness that occurs according to the invention, so that the phosphating process has more time available to form a phosphate layer that covers the entire area. Alternatively or additionally, the phosphating process could be accelerated.

According to one embodiment of the method according to the invention, the hot-dip coated steel sheet is painted. The hot-dip coated steel sheet is preferably first phosphated and then painted.

According to a second teaching, a hot-dip coated steel sheet is specified with a zinc-based coating which has aluminum between 0.1 and 4.0% by weight, optionally magnesium between 0.1 and 4.0% by weight and the remainder zinc and unavoidable impurities . According to the invention, an oxide layer is formed on the coating, which has a thickness of less than 10 nm. In particular, the thickness of the oxide layer is substantially homogeneous on the coating. In essence, this means that local fluctuations of up to +/- 1.5 nm are possible, for example. The oxide layer thickness can be measured, for example, by means of X-ray photoelectron spectroscopy scopy can be carried out by means of a depth profile measurement, the removal rate on which the calculation of the oxide layer thickness is based corresponds, for example, to that of silicon dioxide on a silicon wafer.

In order to avoid repetition, reference is made to the statements relating to the method according to the invention.

• Ein konventionelles Stahlblech der Güte DC04, in Form eines Stahlbands mit der Dicke 0,7mm, wurde in einem konventionellen Schmelztauchbeschichtungsprozess mit einem zinkbasierten Überzug beschichtet, wobei das Stahlband durch ein Schmelzbad mit AI = 1,8 Gew.%, Mg = 1,4 Gew.-%, Rest Zink und unvermeidbare Verunreinigungen hindurchgeführt wurde. Die mittlere Bandgeschwindigkeit betrug 90 bis 110m/min. Das Stahlband wurde aus dem Schmelzbad herausgeführt und einer konventionellen Abstreifvorrichtung zugeführt, welche Schlitzdüsen aufwies, welche beidseitig auf die auf dem Stahlband flüssige Schmelze einwirkten und überflüssige Schmelze abstreiften, wobei der Gasstrom aus den Schlitzdüsen derart eingestellt wurde, dass sich nach dem Erstarren des zinkbasierten Überzugs eine Dicke von und 7µm einstellte. Der Überzug wies eine Zusammensetzung von AI = 1,6 Gew.-% und Mg = 1,1 Gew.-%, Rest Zink und unvermeidbare Verunreinigungen auf. Das Abstreifen erfolgte an Luftatmosphäre und auch als das Gas zum Abstreifen wurde Luft verwendet. Auch die anschließende Abkühlung erfolgte mit Umgebungsluft mit einer Kühlrate von 7 K/s. Bedingt durch den forcierten Kontakt mit der Luft wurden die Oxidationsvorgänge verstärkt und beschleunigt und sorgten dafür, dass die sauerstoffaffinen Elemente im Überzug bevorzugt an die Oberfläche diffundierten. Aus dem schmelztauchbeschichteten Stahlband wurde eine Probe abgeschnitten und weiter untersucht. Es wurde festgestellt, dass sich infolge des konventionellen Schmelztauchbeschichtungsprozesses eine dicke, tiefe in den Überzug reichende Oxidschicht ausbildet, welche inhomogen auf der Oberfläche des Überzugs verteilt war. Es wurde an drei Stellen mittels Röntgenphotoelektronenspektroskopie gemessen und Dicken zwischen 17 und 33nm gemessen. Im Mittel betrug die Oxidschichtdicke 24nm mit einer Standardabweichung von 7nm. Eine weitere Probe wurde aus dem schmelztauchbeschichteten Stahlband entnommen und einer Phosphatierung zugeführt, wobei die Probe für 120s eine Phosphatierungslösung ohne vorherige Aktivierung getaucht wurde. 1 zeigt das Phosphatierungsbild, aufgenommen mittels Rasterelektronenmikroskopie. Gut zu erkennen ist, dass Phosphatkristalle aufgrund der vorhandenen relativ dicken Oxidschicht mit unterschiedlichen Größen ungleichmäßig und inhomogen verteilt vorlagen.

Specific configurations of the invention are explained in more detail below:

• A conventional steel sheet of quality DC04, in the form of a steel strip with a thickness of 0.7 mm, was coated with a zinc-based coating in a conventional hot-dip coating process, the steel strip being melted through a molten bath with Al = 1.8% by weight, Mg = 1.4 % by weight, balance zinc and unavoidable impurities. The average belt speed was 90 to 110 m/min. The steel strip was led out of the molten bath and fed to a conventional stripping device, which had slotted nozzles which acted on both sides of the liquid melt on the steel strip and stripped off excess melt, with the gas flow from the slotted nozzles being adjusted in such a way that after the zinc-based coating had solidified set a thickness of and 7µm. The coating had a composition of Al = 1.6% by weight and Mg = 1.1% by weight, balance zinc and unavoidable impurities. The wiping was performed in air atmosphere, and air was also used as the gas for wiping. The subsequent cooling was also carried out with ambient air at a cooling rate of 7 K/s. Due to the forced contact with the air, the oxidation processes were intensified and accelerated and ensured that the elements in the coating with an affinity for oxygen diffused preferentially to the surface. A sample was cut from the hot-dip coated steel strip and further examined. It was found that as a result of the conventional hot-dip coating process, a thick, deep-reaching oxide layer forms, which was inhomogeneously distributed on the surface of the coating. It was measured at three points using X-ray photoelectron spectroscopy and thicknesses between 17 and 33 nm were measured. On average, the oxide layer thickness was 24 nm with a standard deviation of 7 nm. Another sample was taken from the hot-dip coated steel strip and subjected to phosphating, the sample being immersed in a phosphating solution for 120s without prior activation. 1 shows the phosphating image recorded by scanning electron microscopy. It is easy to see that phosphate crystals of different sizes were distributed unevenly and inhomogeneously due to the relatively thick oxide layer present.

A further investigation was carried out with the same hot-dip coating parameters and the same sheet steel (steel strip), but the stripping was carried out according to the invention in an inert atmosphere with H ₂ = 5% by volume, the remainder N ₂ and unavoidable impurities and a dew point of -20°C . Contact with air was prevented between exiting the melt pool and passing through a wiping device. In other words, the area where the steel sheet exits the molten bath up to the stripping device was exposed to the above-mentioned composition, air access being excluded by the targeted arrangement of nozzles for generating the inert (homogeneous) atmosphere, the inert atmosphere being tempered in such a way that so that active cooling could take place at a cooling rate of 7 K/s. After stripping, using nitrogen as the stripping gas to minimize oxidation, with the parameters set analogously to the conventional example, a zinc-based coating was formed on the steel sheet/strip with a thickness of 7µm out. Until the coating had completely solidified, the hot-dip coated steel strip was guided through a tunnel in the absence of air and cooled therein, the cooling rate of 7 K/s being maintained. A sample was cut from the hot-dip coated steel strip and further examined. It was found that as a result of the hot-dip coating process according to the invention, a thin oxide layer had formed, which was distributed essentially homogeneously on the surface of the coating. It was measured in three places by X-ray photoelectron spectroscopy and thicknesses throughout (mean) of 4nm were measured.

With the same hot-dip coating parameters and the same sheet steel (steel strip), a further investigation was carried out analogously to the first exemplary embodiment according to the invention. Until the coating had completely solidified, the hot-dip coated steel strip was guided through a tunnel in the absence of air and cooled therein, the cooling rate of 15 K/s being maintained. A sample was cut from the hot-dip coated steel strip and further examined. It was found that as a result of the hot-dip coating process according to the invention compared to the first according to the invention Embodiment had formed even thinner oxide layer, which was distributed substantially homogeneously on the surface of the coating. It was measured at three locations by X-ray photoelectron spectroscopy and thicknesses throughout (average) of 1nm were measured. Another sample was taken from the hot-dip coated steel strip and subjected to phosphating, the sample being immersed in a phosphating solution for 120s without prior activation. 2 shows the phosphating image recorded by scanning electron microscopy. It is easy to see that phosphate crystals of the same size were distributed evenly and homogeneously due to the very thin oxide layer present.

The cooling rate was determined in such a way that pyrometers recorded the temperature of the steel sheet above the melt and in front of the first deflection roller in the so-called cooling tower of a standard hot-dip coating system and the average cooling rate of the steel sheet/strip was determined as a function of the strip speed.

The method according to the invention can be used to prevent the formation of an excessively thick oxide layer on a hot-dip, zinc-based coating, which can lead to problems and disadvantages in the further processing, for example during phosphating and painting. The adhesive properties of the coatings can also be improved by reducing the thickness of the oxide layer. For example, adhesives are developed in such a way that they prefer to bond to metal surfaces and not necessarily to oxide surfaces. For example, adhesives may contain components to adjust the pH of the adhesive, attacking the components of the oxide layer and exposing the metallic substrate. A reduced oxide layer thickness ensures a more successful exposure of the metallic components of the coating, to which the adhesive can bond better. Due to the rapid initiation of solidification of the coating, oxygen diffusion into the coating is made more difficult.

Alternatively, the area where the steel sheet emerges from the molten bath up to the stripping device can also be provided with a housing and flooded with the atmosphere described above.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• EP 2297372 B1 [0003]

Claims (10)

Method for producing a hot-dip coated steel sheet, the method comprising the following steps: - providing a steel sheet, - hot-dip coating the steel sheet with a zinc-based coating, the steel sheet passing through a molten bath containing aluminum between 0.1 and 4.0% by weight, optionally magnesium between 0.1 and 4.0% by weight and the remainder zinc and unavoidable impurities, - stripping off the steel sheet still coated with molten metal to set a predetermined thickness of the coating, which is then converted into a solidified state, characterized that the stripping takes place in an inert atmosphere which contains hydrogen between 0.1 and 10% by volume, the rest nitrogen and unavoidable impurities and has a dew point between -50°C and +5°C. procedure after claim 1 , wherein the stripping is performed with an active cooling, wherein the active cooling takes place with a cooling rate of at least 3 K/s. Method according to any one of the preceding claims, wherein the cooling is carried out actively with a cooling rate of at least 3 K/s. Method according to one of the preceding claims, wherein the zinc-based coating comprises Al and Mg with at least 0.5% by weight each. A method according to any one of the preceding claims, wherein the zinc-based coating has a thickness of between 2 and 20 µm. Method according to one of the preceding claims, in which an oxide layer which has a thickness of less than 10 nm is formed on the coating. A method according to any one of the preceding claims, wherein the hot-dip coated steel sheet is skin-passed. A method according to any one of the preceding claims, wherein the hot dip coated steel sheet is phosphated. A method according to any one of the preceding claims, wherein the hot dip coated steel sheet is painted. Hot-dip coated steel sheet with a zinc-based coating which contains aluminum between 0.1 and 4.0% by weight, optionally magnesium between 0.1 and 4.0% by weight and the remainder zinc and unavoidable impurities, characterized in that an oxide layer is formed on the coating, which has a thickness of less than 10 nm.

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