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 steel flat product coated and dressed with a hot-dip coating and to a method for producing a steel flat product coated and dressed with a hot-dip coating.

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

[0003] Conventionally, steel flat products coated with a hot-dip coating are tempered using a textured tempering roller to give the steel flat product the desired final mechanical properties and to impart a certain roughness to the coating surface. The required textures on the tempering rollers can be applied using, among other methods, SBT (Shot Blasting Texturing), EDT (Electro Discharge Texturing), EBT (Electron Beam Texturing), grinding structures, or the TOPO-CROM® process (reactor coating). These processes involve applying a stochastic surface topography to tempering rollers. EP 1 368 140 B1 discloses a corresponding example for EDT. The EBT process also allows for the application of a non-stochastic, i.e., geometrically defined and therefore deterministic, texture.A completely different method for applying stochastic as well as deterministic surface topographies is the laser beam texturing (LT) process, particularly via single or multiple shots, which leads to ablation on the surface of the temper roller material and thus to the creation 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 temper rollers can be fine-ground in a grinding process after texturing. Following this process, depending on the requirements for the temper rollers, they are equipped with a metallic protective layer before being (re)used in the tempering machine.

[0004] Recent developments show rollers with a base structure on their surface, which is partially thermally spray-coated; see, for example, EP 3 394 311 B1. This patent proposes applying a spray coating comprising, among other things, tungsten-containing carbides only to an area between 10 and 90% of the total surface. Specifically, Figure 1c) reveals a non-smooth upper surface of a roller with a thermally spray-coated coating along a thermally spray-coated first area, with the remainder of the upper surface, the second area, remaining uncoated. The spray coating is applied locally as a so-called "pepper spray" and, due to the high degree of 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 with a surface structure having stochastically distributed and stochastically large depressions, wherein the depressions have a depth in the range of 15 to 90 µm, and a roller, in particular a tempering work roller, with a surface structure having stochastically distributed and stochastically large structural elements, wherein the structural elements have a height in the range of 20 to 300 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 protrusions are also known on the surfaces of tempering rollers, cf. https: / / www.topocrom.com / beschichtungen / rollstructuring.

[0008] A key characteristic of high-quality thin steel sheets for applications in automotive body panels, as well as in other applications with similarly high quality requirements, lies in their good formability and paintability. A conflicting objective here is that the thin steel sheets must not be too rough, while at the same time 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 dressed steel sheet with a surface that meets the stated requirements, in particular a hot-dip coated steel flat product 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 hot-dip coated and tempered steel flat product is solved by the features of claim 1. The problem relating to a method for producing a hot-dip coated and tempered steel flat product is solved by the features of claim 5.

[0011] A first teaching of the invention relates to a steel flat product coated and dressed 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 sectionally within the valleys subtextures are present, wherein the subtextures each occupy an area between 1 µm² to 20000 µm², wherein within the subtextures several dome-shaped indentations are contained, each having a diameter between 20 nm and 8000 nm.

[0012] The area of ​​the respective subtextures can be, in particular, at least 2 µm², 5 µm², 10 µm², 15 µm², 20 µm², preferably at least 25 µm², 30 µm², 40 µm², 50 µm², and in particular a maximum of 15,000 µm², 12,000 µm², 8,000 µm², 6,000 µm², preferably a maximum of 5,000 µm², 4,000 µm², 3,000 µm², 2,000 µm², 1,000 µm² 2< .

[0013] The diameter of the respective dome-shaped indentations 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 a maximum of 7500 nm, 7200 nm, 6500 nm, 6000 nm, preferably a maximum of 5500 nm, 5200 nm, 4900 nm, 4700 nm, 4500 nm. The smaller the diameter, the more advantageously the interlocking mechanism can be designed and thus the adhesion properties improved.

[0014] The multiple dome-shaped indentations within the subtextures can have the same or different diameters. For the purposes of the invention, multiple dome-shaped indentations within a subtexture means that at least five or more indentations are present per subtexture. An upper limit can be mathematically defined, for example, depending on the size of the substructures' surface area and the diameter of the dome-shaped indentations. Based on the theoretically largest surface area and the theoretically smallest diameter, an upper limit of a maximum of 500,000 dome-shaped indentations would thus be possible. The upper limit can, in particular, be 200,000, 100,000, 50,000, 20,000, 10,000, preferably 5,000, 2,000, 1,000, 500, 300. The number of impressions per subtexture can 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 advantageously the interlocking mechanism can be designed and thus the adhesion properties improved.

[0015] A second teaching of the invention relates to a method for producing a hot-dip coated and dressed steel flat product comprising the following steps: Providing a hot-dip coated steel flat product; tempering the hot-dip coated steel flat product with a tempering roller and a tempering degree between 0.4 and 2.5%, wherein the tempering roller has a structure comprising stochastically distributed peaks and depressions. The tempering roller has substructures on at least some of the peaks and at least partially on the peaks, each occupying an area between 1 µm² and 20,000 µm², wherein the substructures contain several dome-shaped protrusions, 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 in spacing, shape and size.

[0017] According to one embodiment, the dome-shaped indentations 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, in particular, at least 7%, 9%, 10%, 12%, 15%, preferably at least 20%, 23%, 27%, 30%, preferably at least 35%, 40%, 50%, 60%, and especially a maximum of 99%, 97%, 96%, preferably a maximum of 94%, 93%, or 92%. The higher the proportion of the area, the more advantageous this is for the interlocking mechanism.

[0018] In a particularly preferred embodiment, all subtextures comprise dome-shaped indentations that substantially fill the entire surface of the subtextures. "Substantially" means that up to 5%, particularly up to 3%, preferably up to 2% of the subtexture surface does not have dome-shaped indentations, especially at the edges of the indentations.

[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%, 23%, 27%, 30%, preferably at least 35%, 40%, 50%, 60%, and in particular a maximum of 99%, 97%, 96%, preferably a maximum of 94%, 93%, or 92%.

[0020] In a particularly preferred embodiment, all substructures comprise dome-shaped protrusions that substantially fill the entire surface of the substructures. "Substantially" means that up to 5%, particularly up to 3%, preferably up to 2% of the surface of the substructures do not have dome-shaped protrusions, especially at the edges of the protrusions.

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

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

[0023] According to one embodiment, 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.%, besides zinc and unavoidable impurities. Steel flat products with a zinc-based coating exhibit very good cathodic corrosion protection and have been used in automotive manufacturing for many years. If improved corrosion protection is required, the hot-dip coating additionally contains magnesium with a content of at least 0.3 wt.%, in particular at least 0.6 wt.%, preferably at least 0.9 wt.%. Alternatively or additionally to magnesium with a content of at least 0.1 wt.%, in particular at least 0.3 wt.%, aluminum can be included.-% must be present, for example to improve the bonding of the hot-dip coating to the steel flat product and, in particular, to essentially prevent the diffusion of iron from the steel flat product into the hot-dip coating during any heat treatment of the steel flat product coated with a hot-dip coating.

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

[0025] In a preferred variant, the aluminium 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.%, especially preferably in combination with an aluminium content in the aforementioned preferred variant.

[0027] The hot-dip coating can also consist solely of zinc with small amounts of aluminum, in particular > 0 to 0.5 wt.%, preferably up to 0.4 wt.%, preferably up to 0.3 wt.%, in addition to unavoidable impurities; this is also known in the trade as "Z". The small aluminum content can promote the bonding of the hot-dip coating to the flat steel product, for example, by quasi-in-situ formation during the coating process of an intermetallic intermediate layer containing or consisting of Fe₂Al₅.

[0028] It is also possible to perform heat treatment after the application of the hot-dip coating, whereby iron from the steel flat product can diffuse into the hot-dip coating to create a so-called galvannealed coating.

[0029] Unavoidable impurities, such as elements from the group consisting of silicon, antimony, lead, titanium, calcium, manganese, tin, lanthanum, cerium, and chromium, may be present individually or in combination in the hot-dip coating at a total of up to 0.5 wt.%, in particular up to 0.3 wt.%, preferably up to 0.2 wt.%, and preferably up to 0.1 wt.%. Avoiding impurities is technically difficult or impossible, so they always occur within the aforementioned limits, meaning their content is virtually > 0 wt.%.

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

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

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

[0033] The steel flat product or cold-rolled strip can preferably be a cold-rolled steel according to DIN EN 10346.

[0034] A particularly preferred application of the hot-dip coated and shaped steel flat product is in the outer skin of a vehicle. This concerns components that are visibly installed on a vehicle, such as fenders, side panels, roofs, doors, hoods, and flaps. Especially in this area, very high demands are placed on the coating and its adhesion. The shaped steel flat products according to the invention are ideally suited for this purpose. After cold forming the components from the steel flat product according to the invention, a conventional automotive paint finish is applied.

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

[0036] The drawing shows in Figure 1) shows an exemplary embodiment of a tempering roller in partial top view using SEM, and Figure 2) shows in the left-hand view a tempered surface of a steel flat product coated with a zinc melt dipping coating in partial top view using SEM according to the prior art and in the right-hand view a tempered surface of a steel flat product coated with a zinc melt dipping coating in partial top view using SEM according to an embodiment of the invention.

[0037] In Figure 1An exemplary design of a tempering roller is shown in a partial top view using scanning electron microscopy (SEM) from Zeiss, type Leo 1530. The dark stochastic base structure with peaks and depressions is clearly visible. This stochastic base structure on a tempering roller can typically be produced using EDT (electrode deposition) processes. The white areas represent locally applied tungsten-containing carbides, which may also contain cobalt up to 15 wt%, applied by thermal spraying. These carbides were applied in a spray-like manner, thus resembling the "pepper spray" effect described as disadvantageous in EP 3 394 311 B1. A WCCo powder containing tungsten-containing carbides with an FSSS (Fisher Sub Sieve Size) particle size between 0.9 and 1.3 µm was used, containing approximately 12 wt% cobalt.During thermal spraying, agglomerates were formed which, as substructures, comprised of stochastically distributed peaks and depressions of the dressing roller, preferably at least on some of the peaks and at least sectionally on the peaks, and which vary particularly preferably in diameter and spacing, wherein the substructures contain several dome-shaped protrusions, each having a diameter between 20 nm and 8000 nm.

[0038] The captured image area was 0.65 mm². Imagic Bildverarbeitung AG's "ImageAccess IMS V23H2" software was used for evaluation 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 coating covered approximately 15% of the captured image area. This thermally applied coating thus comprised substructures, each with a dome-shaped protrusion covering between 5% and 100% of the total area of ​​the substructures.

[0039] The substructures applied particularly to the mountains of the structure are imprinted into the surface of the melt-dip coating during the coating process, thus forming subtextures at least in some of the valleys and at least partially within the valleys, whereby, for example, the subtextures within the valleys can each fill an area between 5 and 90% of the total area of ​​the valleys, and several dome-shaped impressions are contained within the subtextures, each with a diameter between 20 nm and 8000 nm.

[0040] Figure 2 The left image shows a partially top-down view of a tempered steel flat product coated with a zinc alloy hot-dip coating, taken using a reflected-light microscope according to the state of the art. The tempering roller used for tempering had a stochastic structure applied by EDT, comparable to the purely dark representation in Figure 1In the right-hand representation of the Figure 2 A striking difference can be seen compared to the left illustration, which shows a dressing roller, exemplified in Figure 1 The structure shown, which also exhibited a white structure in addition to the dark one, was generated. At least in some of the valleys, and at least partially within the valleys, subtextures are present, each occupying an area between 1 µm² and 20,000 µm². Within the subtextures are several dome-shaped indentations, each with a diameter between 20 nm and 8,000 nm. This is shown in the specific implementation of the right-hand illustration in Figure 2The subtexture area is 1872 µm² with 179 dome-shaped indentations with varying diameters between 153 nm and 4130 nm. The subtexture area, number of indentations, and diameters were determined by image analysis of scanning electron microscopy using the software "ImageAccess IMS V23H2" from Imagic Bildverarbeitung AG.

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

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

Claims

1. A hot-dip coated and skin-rolled flat steel product, wherein the surface of the hot-dip coating has a texture comprising stochastically distributed valleys and peaks, wherein at least some of the valleys and at least some sections within the valleys have subtextures, wherein the subtextures each have an area between 1 µm2 and 20000 µm2, characterized in that within the subtextures there are contained a plurality of dome-shaped indentations, each having a diameter between 20 nm and 8000 nm.

2. Flat steel product according to claim 1, wherein the dome-shaped indentations within the subtextures each fill an area between 5 and 100% of the total area of the subtextures.

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

5. Method for producing a hot-dip coated and skin-rolled flat steel product, comprising the following steps: - providing a hot-dip coated flat steel product; - levelling the hot-dip coated steel flat product with a levelling roll and a levelling degree between 0.4 and 2.5%, wherein the skin pass roll has a structure comprising stochastically distributed peaks and valleys, wherein the skin pass roll has substructures on at least some of the peaks and at least in sections on the peaks, each of which has an area between 1 µm2 to 20000 µm2, wherein the substructures contain a plurality of dome-shaped bulges, each having a diameter between 20 nm and 8000 nm.

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

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