Flat steel product with excellent adhesion properties and method for producing same

Abstract

The invention relates to a flat steel product coated and tempered with a hot-dip coating and to a method for producing a flat steel product coated and tempered with a hot-dip coating.

Description

[0001] The invention relates to a flat steel product coated and tempered with a hot-dip coating and to a method for producing a flat steel product coated and tempered with a hot-dip coating.

[0002] Essentially, various coated steel surfaces experience insufficient bonding or adhesion of a layer applied to them under critical conditions such as temperature, humidity, etc. This typically leads to quality losses or, in the worst case, product failure. This particularly affects the bonding of an organic coating, for example, paint adhesion on a flat steel product coated with a hot-dip coating in coil coating, or the bonding of so-called thin-film coatings on automotive exterior skin substrates, both for e-coating and adhesive bonding.

[0003] Conventionally, flat steel products coated with a hot-dip coating are skin-passed using a textured skin-pass roll to impart the desired final mechanical properties to the flat steel product and impart a certain roughness to the coating surface. The required textures on the skin-pass rolls can be applied using, among other methods, the SBT (shot blasting texturing), EDT (electro discharge texturing), EBT (electron beam texturing), abrasive structures, or the TOPO-CROM ®< process (reactor coating). These processes involve the application of a stochastic surface topography to skin-pass rolls. EP 1 368 140 B1 discloses a corresponding example for EDT. The EBT process also enables the application of a non-stochastic, i.e., a geometrically determined and thus deterministic texture.A completely different method for applying stochastic as well as deterministic surface topographies is the laser beam texturing (LT) process, particularly using a single or multiple shot. This process leads to ablation on the surface of the skin-pass roll material and thus to the establishment of a surface topography, see, for example, EP 0 184 568 B1, JP 56-119 687 A, JP 03-267 319 A, and DE 695 09 883 T2. Depending on the application, the skin-pass rolls can be finely ground in a grinding process after texturing. After this process, depending on the requirements of the skin-pass rolls, they are then coated with a metallic protective layer before being (re)used in the skin-pass mill.

[0004] Recent developments show rollers with a basic structure on their surface, which is partially thermally spray-coated, see, for example, EP 3 394 311 B1. This document proposes providing a coating with a spray layer comprising, among other things, tungsten-containing carbides only in an area between 10 and 90% of the entire surface. Specifically, Figure 1c) discloses a non-smooth upper surface of a roller with a thermally sprayed coating along a thermally spray-coated first area, with the remainder of the upper surface remaining uncoated as a second area. The spray layer is applied locally as a so-called "pepper spray" and, due to the high randomness of the coating, is not in accordance with the teaching of EP 3 394 311 B1.

[0005] Furthermore, WO 2017 / 144407 A1 discloses a flat product made of a metal material having a surface structure with stochastically distributed and stochastically large depressions, wherein the depressions have a depth in the range of 15 to 90 µm.

[0006] Furthermore, a surface with a stochastic basic structure and substructures in the micrometer range is known from the report "Development of an innovative hybrid procedure for combining tension-levelling and skin pass rolling (HYPROCOM)", RFCS 2012, doi:10.2777 / 31818, see Figure 99.

[0007] Furthermore, dome-shaped bulges on the surfaces of skin-pass rolls are also known, cf. https: / / www.topocrom.com / beschichtungen / rollstructuring.

[0008] A key property of high-quality steel sheet for automotive exterior applications, as well as other applications with similarly high quality requirements, is its good formability and paintability. A conflicting objective is that the steel sheet must not be too rough while simultaneously ensuring good paint adhesion, especially with increasingly thinner paint layers.

[0009] The object of the invention is therefore to provide a hot-dip coated and skin-passed steel sheet with a surface which meets the requirements, in particular a hot-dip coated flat steel sheet with excellent adhesion properties, good formability and improved paintability, as well as to provide a method for its production.

[0010] The problem relating to a flat steel product coated with a hot-dip coating and temper-passed is solved by the features of claim 1. The problem relating to a method for producing a flat steel product coated with a hot-dip coating and temper-passed is solved by the features of claim 6.

[0011] A first teaching of the invention relates to a flat steel product coated with a hot-dip coating and tempered, wherein the surface of the hot-dip coating has a texture comprising stochastically distributed valleys and peaks, wherein subtextures are present at least in some of the valleys and at least in sections within the valleys, wherein the subtextures each occupy an area between 1 µm 2< to 20000 µm 2<, wherein within the subtextures several dome-shaped impressions are contained, each having a diameter between 20 nm and 8000 nm.

[0012] The area of ​​the respective subtextures can in particular be at least 2 µm 2< , 5 µm 2< , 10 µm 2< , 15 µm 2< , 20 µm 2< , preferably at least 25 µm 2< , 50 µm 2< , 40 µm 2< , 50 µm 2< , and in particular at most 15000 µm 2< , 12000 µm 2< , 8000 µm 2< , 6000 µm 2< , preferably at most 5000 µm 2< , 4000 µm 2< , 30000 µm 2< , 2000 µm 2< , 1000 µm 2<.

[0013] The diameter of the respective dome-shaped impressions can be, in particular, at least 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, preferably at least 90 nm, 100 nm, 120 nm, 150 nm, and in particular at most 7500 nm, 7200 nm, 6500 nm, 6000 nm, preferably at most 5500 nm, 5200 nm, 4900 nm, 4700 nm, 4500 nm. The smaller the diameter, the more advantageously the toothing mechanism can be designed and thus the adhesion properties can be improved.

[0014] The multiple dome-shaped impressions within the subtextures can have the same or different diameters. Multiple dome-shaped impressions within a subtexture, in the sense of the invention, mean that at least five or more impressions are contained per subtexture. An upper limit can be mathematically determined, for example, depending on the size of the surface area of ​​the substructures and the diameter of the dome-shaped impressions. Based on the theoretically largest surface area and the theoretically smallest diameter, an upper limit of a maximum of 500,000 dome-shaped impressions would thus be possible. The upper limit can be, in particular, 200,000, 100,000, 50,000, 20,000, 10,000, preferably 5,000, 2,000, 1,000, 500, or 300. The number of impressions per subtexture may in particular be at least 10, 15, 20, 25, preferably at least 40, 50, 70, 80, 90, 100.The higher the number of impressions, the more advantageous the toothing mechanism can be designed and thus the adhesion properties can be improved.

[0015] A second teaching of the invention relates to a method for producing a hot-dip coated and temper-rolled flat steel product comprising the following steps: Providing a flat steel product coated with a hot-dip coating; skin-passing a flat steel product coated with a hot-dip coating using a skin-pass roll and a skin-pass degree of between 0.4 and 2.5%, wherein the skin-pass roll has a structure comprising stochastically distributed peaks and depressions. The skin-pass roll has substructures on at least some of the peaks and at least in sections on the peaks, each substructure occupying an area between 1 µm 2< and 20,000 µm 2<, wherein the substructures contain a plurality of dome-shaped bulges, each having a diameter between 20 nm and 8,000 nm.

[0016] Stochastically distributed textures on the surface of a hot-dip coating, as prescribed in this case, are irregular textures characterized by an irregular statistical distribution of design features such as valleys and / or peaks, which in turn vary or can vary from one another in terms of distance, shape and size.

[0017] According to one embodiment, the dome-shaped impressions within the subtextures can each fill an area between 5 and 100% of the total area of ​​the subtextures. The proportion of the area can be at least 7%, 9%, 10%, 12%, 15%, preferably at least 20%, 25%, 27%, 30%, preferably at least 35%, 40%, 50%, 60%, and in particular a maximum of 99%, 97%, 96%, preferably a maximum of 94%, 93%, 92%. The higher the proportion relative to the area, the more advantageous the effect on the gearing mechanism.

[0018] In a particularly preferred embodiment, all subtextures comprise dome-shaped impressions that essentially fill the entire surface of the subtextures. Essentially means that up to 5%, in particular up to 3%, preferably up to 2% of the surface of the subtextures are free of dome-shaped impressions, especially at the edge of the impressions.

[0019] According to one embodiment, the dome-shaped protrusions on the substructures can each occupy an area between 5 and 100% of the total area of ​​the substructures. The proportion of the area can be, in particular, at least 7%, 9%, 10%, 12%, 15%, preferably at least 20%, 25%, 27%, 30%, preferably at least 35%, 40%, 50%, 60%, and in particular a maximum of 99%, 97%, 96%, preferably a maximum of 94%, 93%, 92%.

[0020] In a particularly preferred embodiment, all substructures comprise dome-shaped bulges that essentially fill the entire surface of the substructures. Essentially means that up to 5%, in particular up to 3%, preferably up to 2%, of the surface of the substructures are free of dome-shaped bulges, especially at the edge region of the bulges.

[0021] The area of ​​the respective subtextures and substructures are to be understood and / or determined in plan view, quasi two-dimensional in a plane and not in three dimensions, thus following the contour or as a development of the three-dimensional surface into a plane.

[0022] The hot-dip coating is preferably based on zinc.

[0023] According to one embodiment, in addition to zinc and unavoidable impurities, the hot-dip coating can contain or consist of additional elements such as aluminum with a content of up to 8 wt.%, in particular up to 5 wt.% and / or magnesium with a content of up to 8 wt.%, in particular up to 5 wt.%. Flat steel products with a zinc-based coating have very good cathodic corrosion protection and have been used in automobile construction for years. If improved corrosion protection is required, the hot-dip coating additionally contains magnesium with a content of at least 0.5 wt.%, in particular at least 0.6 wt.%, preferably at least 0.9 wt. Aluminum can be used alternatively or in addition to magnesium with a content of at least 0.1 wt.%, in particular at least 0.5 wt.-% in order, for example, to improve the bonding of the hot-dip coating to the flat steel product and, in particular, to substantially prevent diffusion of iron from the flat steel product into the hot-dip coating during a possible heat treatment of the flat steel product coated with a hot-dip coating.

[0024] If magnesium and aluminum, in particular at least 0.5 wt.% each, are contained in the hot-dip coating in addition to zinc and unavoidable impurities, the hot-dip coating is known in the professional world as zinc-magnesium, ZM or Zn-Al-Mg.

[0025] In a preferred variant, the aluminum content is 1.1 to 8 wt.%, in particular 1.2 to 5 wt.%.

[0026] In a preferred variant, the magnesium content is 1.1 to 8 wt.%, in particular 1.2 to 5 wt.%, particularly preferably in combination with an aluminum content in the aforementioned preferred variant.

[0027] The hot-dip coating may also contain only zinc with low aluminum components, in particular > 0 to 0.5 wt.%, preferably up to 0.4 wt.%, preferably up to 0.5 wt.%, in addition to unavoidable impurities, also known by the designation "Z" in specialist circles. The low aluminum content can promote the bonding of the hot-dip coating to the flat steel product, for example, through the quasi-in-situ formation of an intermetallic intermediate layer containing or consisting of Fe 2 Al 5 during the coating process.

[0028] Heat treatment is also possible after the hot-dip coating has been applied, allowing iron from the flat steel product to diffuse into the hot-dip coating to create a so-called galvannealed coating.

[0029] Unavoidable impurities, for example, may include elements from the group consisting of silicon, antimony, lead, titanium, calcium, manganese, tin, lanthanum, cerium, and chromium, individually or in combination, in a total amount of up to 0.5 wt.%, in particular up to 0.5 wt.%, preferably up to 0.2 wt.%, and more preferably up to 0.1 wt.% in the hot-dip coating. Avoiding impurities is technically difficult or even impossible, so they always occur within the aforementioned limits, meaning their content is essentially > 0 wt.%.

[0030] Alternatively, the hot-dip coating can also be based on an aluminum base.

[0031] The thickness of the hot-dip coating per side can be between 1.5 and 50 µm, in particular between 2 and 35 µm, preferably between 3 and 25 µm, and more preferably between 4 and 18 µm. Below the minimum limit, sufficient cathodic corrosion protection cannot be guaranteed, and above the maximum limit, joining problems can occur when bonding a component made from it to another component. In particular, if the thickness of the hot-dip coating exceeds the specified maximum limit, a stable process cannot be ensured during thermal joining or welding.

[0032] According to one embodiment, the flat steel product can have a thickness between 0.5 and 2.5 mm. The flat steel product is preferably cold-rolled, thus a cold-rolled strip.

[0033] According to one embodiment, the flat steel product or cold-rolled strip may preferably be a cold-rolled steel according to DIN EN 10346.

[0034] A particularly preferred use for the hot-dip coated and skin-passed flat steel product is in the outer skin of a vehicle. These are components that are visibly installed on a vehicle, such as fenders, side panels, roofs, doors, hoods, and lids. This area in particular places very high demands on the coating and adhesion of the coating. The skin-passed flat steel products according to the invention are ideally suited for this purpose. After cold forming, the components made from the inventive flat steel product are subjected to a conventional automotive paint finish.

[0035] The invention is explained in more detail using the following embodiments in conjunction with the drawing.

[0036] The drawing shows Figure 1) shows an exemplary embodiment of a skin pass roll in partial plan view by means of SEM, and Figure 2) shows in the left illustration a skin pass surface of a steel flat product coated with a ZM hot-dip coating in partial plan view by means of SEM according to the prior art and in the right illustration a skin pass surface of a steel flat product coated with a ZM hot-dip coating in partial plan view by means of SEM according to an embodiment of the invention.

[0037] In Figure 1An exemplary design of a skin-pass roll is shown in a partial top view using scanning electron microscopy (SEM) from Zeiss, type Leo 1530. The dark stochastic basic structure with peaks and depressions is clearly visible. The stochastic basic structure on a skin-pass roll can be produced, as is customary in the art, preferably using EDT processes. The white areas show tungsten-containing carbides applied locally by thermal spraying. These carbides may also contain cobalt, for example, up to 15 wt.%. These carbides were applied locally in a spray-like manner, thus effectively applying the "pepper spray" cited as disadvantageous in EP 3 394 311 B1. A WCCo powder with tungsten-containing carbides with an FSSS (Fisher Sub Sieve Size) grain size between 0.9 and 1.5 µm was used, containing a cobalt content of approximately 12 wt.%.During thermal spraying, agglomerates were thus formed which, as substructures, are applied individually in a size range between 1 µm 2< to 20,000 µm 2< on the structure comprising stochastically distributed peaks and depressions of the skin pass roll, preferably at least on some of the peaks and at least in sections on the peaks, and particularly preferably vary in diameter and spacing, wherein the substructures contain several dome-shaped bulges, each having a diameter between 20 nm and 8,000 nm.

[0038] The captured image area was 0.65 mm². Imagic Bildverarbeitung AG's "ImageAccess IMS V23H2" software was used for analysis and display. Depending on the system used, the captured image area can vary, for example, between 0.2 and 2.0 mm². The locally thermally applied spray layer covered approximately 15% of the captured image area. The thermally applied spray layer thus encompasses substructures, which are dome-shaped bulges on the substructures, each occupying an area between 5 and 100% of the total area of ​​the substructures.

[0039] The substructures applied in particular to the mountains of the structure are embossed into the surface of the hot-dip coating during the tempering process and thus form subtextures at least in some of the valleys and at least in sections within the valleys, whereby, for example, the subtextures within the valleys can each fill an area between 5 and 90% based on the total area of ​​the valleys, within the subtextures several dome-shaped impressions are contained, each having a diameter between 20 nm and 8000 nm.

[0040] Figure 2 The left image shows a skin-passed surface of a steel flat product coated with a ZM hot-dip coating in partial plan view using a reflected light microscope according to the state of the art. The skin-pass roller used for skin-passing had a stochastic structure applied by EDT, comparable to the purely dark image in Figure 1. In the right-hand illustration of the Figure 2 a striking difference can be seen compared to the left illustration, which shows a dressing roller, which is used as an example in Figure 1 shown and which also had the white structure in addition to the dark one. At least in some of the valleys and at least in sections within the valleys, subtextures are present, each of which covers an area between 1 µm 2< to 20000 µm 2<. Within the subtextures are several dome-shaped impressions, each of which can have a diameter between 20 nm and 8000 nm. In the specific version shown in the right illustration in Figure 2The area of ​​the subtexture is 1872 µm 2< with a number of 179 dome-shaped impressions with a varying diameter between 153 nm and 4130 nm. The determination of the area of ​​the subtexture, recording of the number of impressions and the diameter was carried out by image analysis of the scanning electron microscopy image using the software "ImageAccess IMS V23H2" Imagic Bildverarbeitung AG.

[0041] In both of the aforementioned cases, the skin-pass ratio was 0.8%. The flat steel product in each case was a cold-rolled strip of grade DC04 with a thickness of 0.7 mm and a ZM hot-dip coating with a thickness of approximately 9 µm.

[0042] Several samples were cut from the aforementioned cold-rolled strips, each coated with a cathodic dip coating on a laboratory scale, and heat-treated at 170 °C for 20 minutes. The adhesion strength of the coating was estimated on the differently coated samples using the cross-cut test according to DIN EN ISO 2409:2020-12. On average, the coated samples according to the invention showed little to no flaking compared to the coated samples according to the state of the art.

Claims

1. A steel flat product coated and tempered with a hot-dip coating, wherein the surface of the hot-dip coating has a texture comprising stochastically distributed valleys and peaks, wherein at least in some of the valleys and at least in sections within the valleys subtextures are present, wherein the subtextures each have an area between 1 µm 2 up to 20000 µm 2 take, characterized in that Within the subtextures there are several dome-shaped impressions, each with a diameter between 20 nm and 8000 nm.

2. Steel flat product according to claim 1, wherein the dome-shaped impressions within the subtextures each fill an area between 5 and 100% of the total area of ​​the subtextures.

3. Steel flat product according to one of the preceding claims, wherein the hot-dip coating contains or consists of, in addition to zinc and unavoidable impurities, additional elements such as aluminum with a content of up to 8 wt.% and / or magnesium with a content of up to 8 wt.%.

4. Steel flat product according to one of the preceding claims, wherein the steel flat product has a thickness between 0.5 and 2.5 mm.

5. Flat steel product according to one of the preceding claims, wherein the flat steel product is a cold-rolled steel according to DIN EN 10346.

6. A method for producing a hot-dip coated and temper-passed flat steel product, comprising the following steps: - providing a hot-dip coated flat steel product; - temper-passing a hot-dip coated flat steel product with a temper-pass roll and a temper-pass degree of between 0.4 and 2.5%, wherein the temper-pass roll has a structure comprising stochastically distributed peaks and depressions; characterized in that the skin pass roll has substructures at least on some of the mountains and at least in sections on the mountains, each of which has an area between 1 µm 2 up to 20000 µm 2 , whereby the substructures contain several dome-shaped bulges, each having a diameter between 20 nm and 8000 nm.

7. The method according to claim 6, wherein the dome-shaped bulges on the substructures each occupy an area between 5 and 100% of the total area of ​​the substructures.

Images