METHOD FOR PRODUCE A MELTSIDE COATED STEEL SHEET AND MELTSIDE COATED STEEL SHEET

Description

[0001] The invention relates to a method for producing a hot-dip coated steel sheet and to a hot-dip coated steel sheet.

[0002] Increasingly complex forming processes in the production of sheet metal components lead to ever higher material stresses and are sometimes accompanied by cracking within the coating of hot-dip coated sheet metal, especially steel sheet metal. This is particularly evident in areas with tight radii and in (steel) sheet metal component areas that undergo a high degree of deformation during the manufacturing process. Examples include sheet metal components for the automotive industry, whose shape is characterized by locally high forming stress, or sheet metal in industrial applications, such as trapezoidal sheets.

[0003] Conventionally, hot-dip coated steel sheets with a zinc-based coating are used to ensure cathodic corrosion protection. While cracks in the zinc-based coating do not necessarily impair the cathodic corrosion protection, the risk of corrosion attack increases with the possibility of atmospheric moisture penetrating down to the steel (substrate). In the automotive industry, the coated surface of the steel sheet is treated with a conversion chemical, such as phosphating, only after forming or shaping, so that any areas of the steel exposed during forming or shaping are sealed again.

[0004] The presence of cracks in the coating and / or on its surface can lead to these cracks becoming filled with a process medium, such as an (alkaline) cleaner, an (alkaline) activation agent, or an (acidic) phosphating agent, and thus not being completely cleaned / dried. This can, among other things, result in the outgassing of these components (of the process medium) during heat treatment, for example, during cathodic dip coating (e-coating). Alternatively or additionally, it is conceivable that the components (of the process medium) remain trapped in the cracks, so that upon subsequent contact with water, for example, through diffusion through an applied coating layer, an alkaline or acidic solution can form within the cracks, which can attack the coating and thus adversely affect the cathodic corrosion protection.

[0005] Even pre- or post-treatment applied via so-called coil coating can cause local cracking of the coating, leading to a local loss of its passivating effect. In these applications, for example, moisture can penetrate the conversion coating, potentially resulting in corrosion and, in particular, paint delamination.

[0006] It is therefore known that cracking can damage the zinc-based coating to such an extent that the layer and / or at least parts of the layer can detach.

[0007] Crack propagation in the coating is promoted by its structure. In the zinc-based coating, which is applied to a steel substrate (steel sheet) in a hot-dip process while still liquid and subsequently cooled and solidified, a few large zinc grains with the same orientation form, extending through the entire coating thickness. When these crystals are subjected to stress, fracture occurs in a preferred direction, depending on the crystal's orientation. With varying crystal sizes, larger crystals are less resistant to mechanical stress than smaller crystals.To influence and thus vary the crystal sizes, either the process conditions would have to be changed, for example, the cooling rate required to solidify the liquid coating (e.g., by accelerating cooling to achieve small zinc crystals in the coating), or the zinc-based melt would have to be enriched with additional elements, for example, by adding lead as a nucleating agent. Changes to the process conditions in conventionally operated continuous strip galvanizing processes are only feasible to a very limited extent; for example, cooling rates are technically limited.High cooling rates are possible with additional costs (high investment), and the addition of further elements as grain refiners / germination agents can negatively impact the already complex coating system, not only economically, for example through the additional effort of addition and monitoring, but also through negative influences on secondary metallurgy and process control, and also on the environment.

[0008] The object of the invention is therefore to provide not only a method for producing a hot-dip coated steel sheet, but also a hot-dip coated steel sheet with which the disadvantages of the prior art can be eliminated.

[0009] The problem relating to the method for producing a hot-dip coated steel sheet is solved by the features of claim 1. The problem relating to the hot-dip coated steel sheet is solved by the features of claim 8.

[0010] A first teaching of the invention relates to a method for producing a hot-dip coated steel sheet, wherein a steel substrate, in particular a cold-rolled one, is provided which is coated on one or both sides with a zinc-based coating by hot-dip coating to obtain a hot-dip coated steel sheet, wherein the steel substrate provided has a deterministic surface texture on one or both sides before hot-dip coating.

[0011] Without having to implement the already known and sometimes complex measures for influencing the zinc bloom size through cooling rate and the addition of grain refiners / nucleating agents, the inventors have discovered that the crystallization behavior of zinc-based hot-dip coatings can also be controlled using simple means. According to the invention, this targeted control is achieved by imprinting a deterministic texture on the surface of the steel substrate, either on one or both sides, prior to hot-dip coating. A deterministic surface texture is understood to be a recurring texture with a defined shape and / or configuration; see, for example, EP 2 892 663 B1. The surface texture thus preferably has defined indentations and / or elevations on the surface of the steel substrate, for example, when viewed in cross-section.EP 2 892 663 B1 describes a process in which cold-rolled steel substrates with a deterministic texture are dressed, and a cathodic corrosion protection coating is subsequently electrolytically deposited. Electrolytic deposition is not comparable to a hot-dip coating process. Document DE10 2019 14136 A1 discloses a generic process in which the dressing of the steel sheet takes place after the hot-dip coating.

[0012] In particular, the freedom of shape and the deterministic distribution of the texture on the surface of the steel substrate allow for surprising influence on the distribution and / or size of the zinc grain crystallization nuclei in the zinc-based coating. The zinc grain crystallization nuclei form at the interface between the liquid zinc-based melt and the steel substrate, so that during solidification, the crystals grow laterally within the coating until they abut their neighboring crystals. The deterministic surface texture thus provides zones, preferably defined as indentations and / or elevations, in which crystallization is initiated and promoted. Depending on the specific design of the deterministic texture, a preferably high density of crystallization nuclei can grow until they encounter their neighbors.Regarding crack behavior, studies have shown that cracks within a crystal always run parallel to each other. Thus, cracks can propagate undisturbed within the crystal and are only stopped upon reaching the grain boundary and the adjacent, differently oriented crystals. Therefore, cracks can propagate faster and further in a few large crystals than in many small crystals. A high density of crystal nuclei leads to small crystals, so a fine-grained coating is more resistant to the macroscopic formation of cracks.

[0013] The deterministic texture can be applied to one or both sides of a steel substrate, either during a cold rolling process or in a separate rolling process, using appropriately deterministically structured rolls. Preferably, the deterministic texture is imprinted into the surface of the steel substrate during a cold rolling process, wherein the last rolling stand of a cold rolling mill is equipped with at least one roll, preferably a pair of rolls, which has a corresponding deterministic structure. Methods and devices for producing the structuring of rolls are prior art; see, inter alia, EP 2 892 663 B1. Preferably, a deterministic topography is introduced into the surface of the roll by means of material removal using a laser.

[0014] A steel substrate is understood to be a flat steel product, which can be in the form of strip, blank, or sheet. A cold-rolled steel substrate is preferably used. The production of cold-rolled steel substrates is also state of the art.

[0015] Hot-dip coating of a steel substrate with a zinc-based coating is also state of the art.

[0016] According to one embodiment, the deterministic surface texture has a closed texture with indentations. A closed texture on the surface of the steel substrate means that individual indentations are provided which extend into the depth of the substrate, but are essentially not (all) interconnected and are therefore considered closed. Thus, there is essentially only one continuous elevation, which is oriented essentially in the plane of the surface of the steel substrate.

[0017] According to an alternative embodiment, the deterministic surface texture has an open texture with raised areas. An open texture on the surface of the steel substrate means that individual raised areas are present, which protrude from the plane of the substrate, but are essentially not (all) connected to one another and are therefore considered open. Thus, there is essentially only one continuous indentation, which is oriented primarily in the plane of the lowest area of ​​the raised areas on the steel substrate.

[0018] According to one embodiment, the deterministic surface texture has at least one indentation or at least one raised area, which occupies a surface area between 100 and 25,000 µm². The area can be, in particular, at least 200 µm², preferably at least 400 µm², and especially a maximum of 20,000 µm², preferably a maximum of 18,000 µm². Preferably, two or more indentations or two or more raised areas are present, each having a surface with a centroid, wherein the distance between at least two adjacent centroids is between 10 and 1,000 µm. The smaller the areas and the smaller the distances between the respective centroids, the higher the number of crystallization nuclei that form at the interface between the liquid zinc-based melt and the steel substrate.

[0019] According to one embodiment, the zinc-based coating contains, in addition to zinc and unavoidable impurities, additional elements such as aluminum with a content of up to 10.0 wt.% and / or magnesium with a content of up to 10.0 wt.% in the coating. Impurities from the group consisting of silicon (Si), sab (Sb), lead (Pb), titanium (Ti), calcium (Ca), manganese (Mn), tin (Sn), lacid (La), ce, and chromium (Cr) may be present individually or in combination, totaling up to 0.5 wt.% in the coating. If improved corrosion protection is desired, the 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 may be present.-% may be present, for example, to improve the bonding of the coating to the steel substrate and, in particular, to essentially prevent the diffusion of iron from the steel sheet into the coating during heat treatment of the coated steel sheet, thus ensuring, for example, good adhesion. Preferably, the coating contains magnesium with a content of at least 1.0 wt.% and aluminum with a content of at least 1.0 wt.%. If aluminum and magnesium are present in the coating, it is referred to below as a Zn-Al-Mg coating. Aluminum or magnesium may each be present in the coating in amounts particularly up to a maximum of 8.0 wt.%, preferably up to a maximum of 6.0 wt.%, and more preferably up to a maximum of 5.0 wt.%. The coating thickness may be between 1.5 and 15 µm, particularly between 2 and 12 µm, and preferably between 3 and 10 µm.

[0020] According to one embodiment, the hot-dip coated steel sheet is skin-treated to imprint a desired surface texture onto the coating and / or to adjust the final mechanical properties of the steel sheet. Skin-treating can be carried out with a deterministically structured skin-treating roller, see for example EP 2 892 663 B1, or alternatively with a stochastically structured skin-treating roller, see for example EP 2 006 037 B1.

[0021] The steel substrate can consist of a steel material with the following chemical composition in wt.%: C up to 0.1%, in particular between 0.0002% and 0.1%; Mn up to 2.0%, in particular between 0.01% and 2.0%; Si up to 0.3%, in particular between 0.0002% and 0.3%; P up to 0.1%, in particular up to 0.05%; S up to 0.1%, in particular up to 0.05%; N up to 0.1%, in particular up to 0.01%; and optionally one or more alloying elements from the group (Al, Cr, Cu, Nb, Mo, Ti, V, Ni, B, Sn, Ca): Al up to 0.2%, in particular between 0.001% and 0.1%; Cr up to 1.0%, in particular up to 0.8%; Cu up to 0.2%, in particular up to 0.18%; Nb up to 0.1%, in particular up to 0.05%; Mo up to 0.2%, in particular up to 0.1%; Ti up to 0.2%, in particular up to 0.15%; V up to 0.2%, in particular up to 0.1% Ni up to 0.2%, in particular up to 0.18% B up to 0.005%, in particular up to 0.004% Sn up to 0.1%, in particular up to 0.05% Ca up to 0.1%, in particular up to 0.01% balance Fe and unavoidable impurities.

[0022] According to a second teaching, the invention relates to a hot-dip coated steel sheet comprising a steel substrate with a zinc-based coating applied to one or both sides, wherein the steel substrate has a deterministic surface texture on one or both sides.

[0023] A higher density of zinc grains in the coating generally corresponds to a smaller, finer grain size, thus increasing the density of grain boundaries on the surface. This results in higher reactivity compared to lower density and therefore smaller grain boundaries on the surface, even with subsequent pre- or post-treatments of the hot-dip coated steel sheet. The zinc grain sizes are typically between 20 and 250 µm, particularly up to a maximum of 220 µm, preferably up to a maximum of 170 µm, more preferably up to a maximum of 130 µm, and further preferably up to a maximum of 105 µm. The size of a zinc grain is defined, for example, by scanning electron microscopy, as the greatest possible distance between two points within a continuous grain with the same orientation.

[0024] To avoid repetition, reference is made to the advantageous embodiments of the method according to the invention.

[0025] The only Figure 1 Figure 1 shows scanning electron microscope (SEM) images of two surface-textured steel substrates before and after hot-dip coating with a zinc-based coating, viewed from above. The deterministic surface texture was created by indentations in the surface of the steel substrate, exemplified by a double-I texture based on EP 2 892 663 B1. The deterministic surface texture was thus a closed texture in both cases. The texture differed only in the dimensions of the indented "I" texture: the top left image shows 80 µm x 25 µm, resulting in an area of ​​2000 µm² per indented "I", while the top right image shows 210 µm x 70 µm, resulting in an area of ​​14700 µm² per indented "I". Figure 1 .For the imprinted "I's," the centroid of each area can be easily determined. The distance between at least two adjacent centroids was, for example, 50–70 µm for the left image and 120–150 µm for the right image. The grain sizes in the left image are up to a maximum of 200 µm and in the right image up to a maximum of 900 µm. The lower left image, compared to the lower right image, illustrates different configurations and dimensions of the deterministic surface texture of a steel substrate. Figure 1 , that smaller zinc grains (bottom left) are formed by finer textures (top left) on the surface of the steel substrate. The crystallization nuclei preferentially form at the transitions where the indentations on the surface of the steel substrate meet.

[0026] The density of the crystallization nuclei is therefore dependent on the dimensions of the texture, or rather, on the number of textures per area. In all experiments, it was found that those steel substrates with a higher number of structures relative to a constant area consistently exhibited smaller zinc grains after hot-dip coating. Comparable results, not shown here, were also obtained for Zn-Al-Mg coatings. When considering hot-dip coated structures with eutectic phases (Zn-Al-Mg coating), a finer grain structure on the surface of the steel substrate is advantageous for corrosion resistance. With regard to the Zn-Al-Mg coating, the MgZn₂ in the eutectic phase acts as a sacrificial anode for the zinc grains. This means that under corrosive stress on the coating, the MgZn₂-, for example, during acidic pre- or post-treatments of the hot-dip coated steel sheets (pickling), are the first to be destroyed.The aluminum phases, for example during alkaline pre- or post-treatments of hot-dip coated steel sheets (degreasing and / or cleaning, especially before pickling), are attacked in the eutectic, and subsequently the zinc grains are attacked. In air, such a protective effect (anodic sacrificial mechanism) depends essentially on the distance between the more noble and less noble metallic phases. If the phases are too far apart, cathodic corrosion protection can no longer be guaranteed. Accordingly, smaller zinc grains surrounded by eutectic are advantageous for corrosion resistance, since the distance from the eutectic to the center of the zinc grain is smaller, thus enabling better anodic sacrifice of the phases in the eutectic to protect the zinc.

Claims

1. Method for producing a hot-dip coated steel sheet, wherein in particular a cold-rolled steel substrate is provided, which is coated on one or both sides with a zinc-based coating by hot-dip coating to obtain a hot-dip coated steel sheet, characterized in that the steel substrate provided has a deterministic surface texture on one or both sides prior to hot-dip coating.

2. Method according to claim 1, wherein the deterministic surface texture has a closed texture with indentations.

3. Method according to claim 1, wherein the deterministic surface texture has an open texture with elevations.

4. Method according to one of the preceding claims, wherein the deterministic surface texture has at least one indentation or at least one elevation, which occupies an area between 100 and 25,000 µm2.

5. Method according to claim 4, wherein two or more indentations or two or more elevations are present, each of which has an area with a respective area center of gravity, wherein the distance between at least two adjacent area centers of gravity is between 10 and 1000 µm.

6. Method according to one of the preceding claims, wherein the zinc-based coating contains, in addition to zinc and unavoidable impurities, additional elements such as aluminum with a content of up to 10.0 wt% and / or magnesium with a content of up to 10.0 wt% in the coating.

7. Method according to one of the preceding claims, wherein the hot-dip coated steel sheet is skin passed.

8. Hot-dip coated steel sheet comprising a steel substrate with a zinc-based coating applied to one or both sides, in particular manufactured according to one of the preceding claims, characterized in that the steel substrate has a deterministic surface texture on one or both sides, wherein the zinc grain sizes are between 20 and 250 µm.

9. Steel sheet according to claim 8, wherein, in addition to zinc and unavoidable impurities, the coating may contain additional elements such as aluminum with a content of up to 10.0 wt% and / or magnesium with a content of up to 10.0 wt%.

10. Steel sheet according to claim 8 or 9, wherein the hot-dip coated steel sheet is skin passed with a stochastic or deterministic surface texture.