Galvanized sheet steel

Abstract

The invention relates to a steel sheet according to claim 1 coated with a galvanizing coating.

Description

[0001] Electrolytically galvanized sheet steel

[0002] The invention relates to a steel sheet coated with an electrolytic zinc coating having a surface, wherein the surface of the electrolytically galvanized steel sheet has at least a mean roughness value R a and a peak number R Pc The invention further relates to a method for producing a steel sheet coated with an electrolytic zinc coating.

[0003] The mean roughness value R a in pm and the peak number R Pc in 1 / cm can be determined along a defined measuring section, see DIN EN ISO 4287:2010-07.

[0004] Surface-treated steel sheets in areas requiring high quality, such as automotive exterior panels, are often coated using an electrolytic galvanizing process. Unlike hot-dip galvanizing, this offers the advantage of a uniform coating that is essentially free of surface defects and thus meets even the highest visual requirements for the final product.

[0005] The surface finish is particularly important for the subsequent processing of electrogalvanized steel sheets (“ZE”). For a given mean roughness according to DIN EN ISO 4287, a high peak count is advantageous, for example for the intended application of cold forming, because this results in relatively narrow valleys. At the same time, there are many valleys / cavities in which the forming agent required for cold forming can be stored. A higher peak count is also an indicator of lower boredom in the usual production method, for example when texturing the skin-pass rolls with finer craters in the EDT process. The mean roughness must be neither too high nor too low. If it is too high, wear and the coefficient of friction during forming will be high. If it is too low, the cavities will be small and insufficient to accommodate the forming agent.In the automotive sector, especially for some OEMs, this is usually specified.

[0006] EP 3 204 530 B1 describes an electrolytically galvanised cold-rolled sheet and a process for its production.

[0007] The task is therefore to provide a steel sheet coated with an electrolytic zinc coating which, after coating, has an ideal combination of surface characteristic parameters, such as the mean roughness and the peak count, in order to have both optimal cold forming suitability and a visually appealing paint appearance during later use, as well as to specify a corresponding process for its production.

[0008] The object is achieved by a steel sheet coated with an electrolytic zinc coating having the features of claim 1 and by a method for its production having the features of claim 5.

[0009] According to a first teaching, the invention relates to a steel sheet coated with an electrolytic zinc coating having a surface, wherein the surface of the electrolytically galvanized steel sheet is characterized by at least a mean roughness R a and a peak number Rp c is characterized, whereby the mean roughness and the peak number are determined according to DIN EN ISO 4287, whereby the conditions R a * Rp c > 0.85 * IO -2 and R a / Rp c < 1.45 * 10 _ 10 m 2 are fulfilled.

[0010] The inventors have surprisingly found that the first condition R a * R Pc > 0.85 * IO -2 ensures that both the mean roughness and the peak count are sufficiently high so that the surface of the electrolytically galvanized steel sheet can absorb a sufficient amount of forming agent. The second condition R a / R Pc< 1.45 * 10 _ 10 m 2 This ensures that the mean roughness is not too high compared to the peak count. This would have a negative impact on the resulting paint appearance if the steel sheet is intended to be used as a cold-formed component in the automotive exterior skin. By simultaneously fulfilling both conditions, it is ensured that the steel sheet according to the invention has both adequate cold formability and the potential to meet the highest quality requirements for the appearance of a subsequent paint finish.

[0011] Steel sheet refers, among other things, to a flat steel product in the form of strip, sheet, or plate. Steel sheet therefore has a longitudinal dimension (length), a transverse dimension (width), and a vertical dimension (thickness).

[0012] The zinc coating is to be understood as a coating essentially based on zinc, wherein at least 97% by weight, in particular at least 98% by weight, preferably at least 98.5% by weight, preferably at least 99% by weight, more preferably at least 99.5% by weight of zinc is contained in the coating along with unavoidable impurities. The impurities, such as one or more of the elements lead, cadmium, copper, strontium, iron, aluminum, nickel, can be present individually or in combination in a total of up to 3% by weight, in particular up to 2% by weight, preferably up to 1.5% by weight, more preferably up to 1% by weight, more preferably up to 0.5% by weight in the coating.

[0013] The thickness of the electrolytically galvanized steel sheet is, for example, 0.3 to 5.0 mm. The steel sheet can preferably be a cold-rolled steel sheet, whereby the thickness (including coating) can be at least 0.4 mm, preferably at least 0.5 mm, and up to a maximum of 3.0 mm, preferably up to 2.5 mm.

[0014] Before applying an electrolytic zinc coating, the surface of the steel sheet is structured using skin-pass rolling in a (conventional) skin-pass mill or, for example, in the last rolling stand in a cold rolling mill or separately in a (re-)rolling stand. The surface structure on the surface of the steel sheet (negative imprint) is essentially reflected on the roll as a positive imprint, with a cavity (depression) on the surface of the steel sheet corresponding to a peak on the surface of the roll. The roughness setting, or the mean roughness value R, is determined by the surface roughness. a and the peak number R Pcon the surface of the steel sheet depends on the roll roughness or the average roll roughness R a and the peak number R Pc the surface of the roll and on the other hand on the transfer rate, which depends on the degree of rolling and / or the rolling force, and can therefore be controlled.

[0015] The mean roll roughness R a For the structuring (skin-passing process) of the surface of the (uncoated) steel sheet, values ​​between 0.8 and 3.0 pm, in particular 0.9 and 2.0 pm, must be set in order to ensure the required roughness values ​​on the steel sheet.

[0016] The application or deposition of the electrolytic zinc coating takes place in an aqueous electrolyte and essentially follows the embossed surface structure, so that the introduced structuring is essentially retained after coating. Minor, negligible changes in the characteristic parameters before and after coating can certainly occur and are particularly within the tolerance range, for example fluctuations of between + / - 10%. According to one embodiment, the coating can have Sn with a content of between > 0 and 0.010 wt.%, the remainder Zn and unavoidable impurities. The content can in particular be at least 0.0002 wt.%, preferably at least 0.0004 wt. Tin can in particular be present in the coating with a maximum of 0.0050 wt.%, preferably with a maximum of 0.0020 wt.%. The targeted addition orThe presence of tin has a positive effect on the deposition of the coating and thus on the conditions set.

[0017] According to one embodiment, the surface of the electrolytically galvanized steel sheet has a mean roughness R a between 1.0 and 2.0 pm and a peak number R Pc between 70 and 120 1 / cm to meet the requirements. In particular, the mean roughness R a at least 1.05 pm, preferably at least 1.10 pm, in particular 1.12 pm. In particular, the mean roughness R a maximum 1.9 pm, preferably maximum 1.8 pm, particularly preferably 1.5 pm or 1.4 pm, in particular 1.3 pm. In particular, the peak number R Pc at least 72 1 / cm, preferably at least 74 1 / cm, in particular 80 cm 1 / cm. In particular, the peak number R Pcmaximum 100 1 / cm, preferably maximum 90 1 / cm. This requires that the two aforementioned conditions are met. However, it is not the individual absolute values ​​for the mean roughness Ra and peak count RPc mentioned above that are important, but rather their combination according to the conditions R a * R Pc > 0.85 * IO -2 and R a / R Pc < 1.45 * 10 10 m 2 .

[0018] According to one embodiment, the steel sheet consists of a steel material described in DIN EN 10268 or DIN EN 10152. This is preferably a multiphase, IF, BH, or micro-alloyed steel. The steel sheet is particularly preferably a cold-rolled flat product.

[0019] According to a second aspect, the invention relates to a method for producing a steel sheet coated with an electrolytic zinc coating, the method comprising the following steps: - providing a steel sheet, - skin-passing the steel sheet, - coating the skin-passed steel sheet with an electrolytic zinc coating by depositing zinc in an aqueous electrolyte, wherein tin with a content of between 0.40 and 1.60 mg / l is added to the aqueous electrolyte.

[0020] After the surface structure is Z-embossed into the prepared steel sheet using rollers, the steel sheet is coated, on one side or preferably on both sides, with an electrolytic zinc coating in an electrolytic coating system. The electrolytic coating process is well known in the art. The steel sheet to be galvanized passes through or is guided through an aqueous electrolyte, in which the desired coating is deposited depending on the current density and immersion time. Aqueous electrolytes for the electrolytic galvanizing of steel sheet are state-of-the-art and therefore well known in the art. In particular, electrolytic galvanizing, especially of cold-rolled flat steel products, is regulated by the standard DIN EN 10152:2017-06.

[0021] Conventional electrolytes for zinc plating have no tin content or contain tin content in the range of less than or equal to 0.1 mg / l. The determination of zinc, tin, and other (all) chemical elements, such as metals, in an electrolyte is described in DIN ISO EN 11885:2009-09.

[0022] Tin can be added to the electrolyte, for example, as tin chloride and / or tin sulfate. The corresponding chemical compounds and mixtures for achieving the aforementioned tin content range in the electrolyte are known among experts.

[0023] Surprisingly, it was discovered that the relatively low addition of tin has a positive effect on the surface characteristics, particularly on the formation of zinc grains. Exceeding the required 1.60 mg / l leads to coarse-grained deposition of the zinc grains and thus also affects the surface of the deposited zinc layer, negatively affecting the roughness (average roughness) and thus no longer meeting the requirements. Below the required 0.40 mg / l, no positive effect can be observed compared to the standard.

[0024] Adding tin to the aqueous electrolyte with a content of at least 0.45 mg / l, 0.50 mg / l, preferably at least 0.55 mg / l, 0.60 mg / l can be further advantageous. The tin addition can be up to a maximum of 1.55 mg / l, 1.50 mg / l, preferably up to a maximum of 1.45 mg / l, 1.40 mg / l.

[0025] Thus, the invention also relates to the use of tin in the form of cations, in particular tin chloride and / or tin sulfate, in an aqueous electrolyte for producing an electrolytically deposited zinc coating with a precisely defined surface as described above. According to one embodiment, the aqueous electrolyte is electroplated with a current density between 20 and 150 A / dm 2 In particular, the current density can be at least 40 A / dm 2 , preferably at least 55 A / dm 2 In particular, the current density can be a maximum of 120 A / dm 2 , preferably maximum 100 A / dm 2 be.

[0026] According to one embodiment, the aqueous electrolyte is maintained at a temperature between 30 and 85°C. In particular, the temperature can be at least 40°C, preferably at least 45°C. In particular, the temperature can be a maximum of 75°C, preferably a maximum of 65°C.

[0027] For example, depending on the current density and in particular the duration (immersion time) of the galvanizing process, the thickness of the zinc coating can be adjusted between 1 and 20 pm, in particular between 2 and 15 pm, preferably between 3 and 10 pm.

[0028] The surface to be galvanized is first cleaned, degreased, and optionally pickled. After the galvanizing process, the galvanized surface can be passivated, for example, to improve the desired corrosion protection.

[0029] In the following, specific embodiments of the invention are explained in more detail with reference to the drawings. The drawings and the accompanying description of the resulting features are not to be interpreted as limiting the respective embodiments, but serve to illustrate exemplary embodiments. Furthermore, the respective features can be used with each other as well as with features of the above description for possible further developments and improvements of the invention, especially in additional embodiments not shown.

[0030] In practical tests, steel sheets of grade DC04 with a thickness of 0.7 mm were used. These were skin-passed using EDT textured rollers (see EP 2 006037 B1), with a surface roughness between 1.8 and 1.9 pm and peak counts between 90 and 95 1 / cm, with a skin-pass degree of approximately 1.1%. Samples were prepared from these. Different aqueous electrolytes (1-5) with varying tin contents were prepared for further investigation on a laboratory scale (see Table 1), in which the samples were electrolytically coated by passing them through. The pH of the electrolytes was approximately 3.5. The different electrolytes were each coated with a current density of 60 A / dm 2and were each heated to a temperature of 65°C. The influence of the different tin contents on the mean roughness and the number of peaks of the zinc layer deposited on the steel sheet is shown in Figure 1. It can be clearly seen that when tin with a content between 0.40 and 1.60 mg / l is added to an aqueous electrolyte, the two conditions R a * Rpc > 0.85 * IO -2 and R a / Rp c < 1.45 * 10 10 m 2 can be essentially met. The samples were then subjected to a typical automotive electrolytic coating process, also carried out on a laboratory scale. The samples were evaluated visually, and it was determined that the samples galvanized in electrolytes 3 and 4 exhibited a more even coating appearance.

[0031] Table 1

Claims

Claims 1. Steel sheet coated with an electrolytic zinc coating having a surface, the surface being characterized by at least a mean roughness R a and a peak number R Pc is characterized, wherein the mean roughness and the peak number are determined according to DIN EN ISO 4287:2010, characterized in that the surface meets the conditions R a * RPC > 0.85 * IO 2 and R a / RPC < 1.45 * 10 10 m 2 fulfilled.

2. Steel sheet according to claim 1, wherein the coating comprises Sn in a content between > 0 and 0.010 wt.%, the remainder Zn and unavoidable impurities.

3. Steel sheet according to one of the preceding claims, wherein the surface of the electrolytically coated steel sheet has a mean roughness R a between 1.0 and 2.0 pm and a peak number R Pc between 70 and 120 1 / cm.

4. Steel sheet according to one of the preceding claims, wherein the steel sheet corresponds to a steel material according to DIN EN 10268 or DIN EN 10152.

5. A process for producing a steel sheet coated with an electrolytic zinc coating, the process comprising the following steps: - Providing a steel sheet, - Tempering of the steel sheet, - Coating the tempered steel sheet with an electrolytic zinc coating by depositing zinc in an aqueous electrolyte, characterized in that tin with a content of between 0.40 and 1.60 mg / l is added to the aqueous electrolyte.

6. The method according to claim 5, wherein the aqueous electrolyte has a current density between 20 and 150 A / dm 2 is fed.

7. The method according to claim 5 or 6, wherein the aqueous electrolyte is tempered at a temperature between 30 and 85 °C.

8. Use of tin in the form of cations, in particular tin chloride and / or tin sulfate, in an aqueous electrolyte for producing an electrolytically deposited zinc coating.

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