Steel sheet for stain-free phosphate coating process

Abstract

The invention relates to a hot-dip coated and temper-rolled steel sheet, to a method for producing same, and to a use thereof.

Description

[0001] Steel sheet for spotless phosphating

[0002] The invention relates to a hot-dip coated and skin-passed steel sheet, comprising a steel substrate made of a multiphase alloy according to DIN EN 10346 and a metallic coating applied to one or both sides of the steel substrate, which, in addition to zinc and unavoidable impurities, contains additional elements such as aluminum with a content of 0.5 to 8.0 wt.% and magnesium with a content of 0.5 to 8.0 wt.% in the coating. Furthermore, the invention relates to a method for producing a hot-dip coated and skin-passed steel sheet and to a use thereof.

[0003] During hot-dip plating with Al-containing zinc melts, an Al-containing oxide layer forms during the cooling process. With the additional addition of Mg to the melt, a layered oxide layer consisting of an essentially closed Mg oxide layer and underlying Al oxides develops during the cooling process. Due to the higher dipole moment, corresponding to the electronegativity difference of Mg oxide (AEN=2.27) compared to Al oxide (AEN=2.03) or zinc oxide (AEN=1.84), polarizable or dipolar compounds applied to the Mg oxide layer are more strongly bound to the surface. Typically, a corrosion protection system, such as a corrosion protection oil, is applied to the oxide layer after hot-dip plating. Since this oil adheres more strongly to the Mg oxide due to the high dipole moment, the cleaning properties of the surface deteriorate.This negatively impacts pre- and post-treatment processes, which require an oil- and dirt-free surface. In particular, it leads to undesirable staining in the phosphating process.

[0004] Coatings made of zinc, aluminum, and magnesium oxidize in air and form a covering, predominantly Mg-containing oxide layer on the surface. This oxide layer has different chemical properties than established pure zinc or zinc-aluminum coatings. Further processing processes are geared towards established coatings. Changes in the chemical composition of the surface also affect the further processing properties. It is known from typical automotive processing processes that Mg-containing surfaces are more difficult to join, clean, and phosphate than established zinc-containing coatings. This limits the willingness of automobile manufacturers to use them in bodywork. In addition, extensive tests must be completed and passed. Textured skin-pass rollers transfer their texture to the surface of the steel sheets to be processed as a negative during a skin-pass process, i.e.Elevations on the roll surface result in depressions in the steel sheet surface, and vice versa. The skin-pass impressions (depressions) created in this way in the steel sheet surface, so-called (closed) voids, serve as lubricant pockets that retain a lubricant applied to the steel sheet surface and can carry it along during the forming process. From the prior art, skin-passed steel sheets with a stochastic surface structure are known, for example, from patent EP 2 006 037 B1, and steel sheets with a deterministic surface structure are known, for example, from patent EP 2 892 663 B1.

[0005] The contact between the shaping elements of a skin-pass roll and a steel sheet surface during the skin-passing process can alter the surface chemistry of the contact surface. In terms of chemistry, hot-dip coated coatings are structured in such a way that a layer of alloying elements with a higher affinity for oxygen forms on the zinc primarily present in the coating. The mechanical stress during skin-passing can cause the zinc to be exposed at the contact points between the skin-pass roll and the steel sheet instead of the alloying elements magnesium and / or aluminum. Hot-dip coated steel sheets that have been skin-passed with a stochastic surface structure exhibit a different surface chemistry in the skin-pass impressions of the coated steel sheet than on the elevations of the coated steel sheet.While the chemical composition in the skin pass impressions is richer in zinc, the elevations show high proportions of the oxygen-affine alloying elements (Mg and Al), cf. DE 10 2019 215 051 Al.

[0006] Furthermore, it is known from EP 2 841 614 B1 to condition or change the surface chemistry of hot-dip coated steel sheets by mechanical forces, such as brushing or blasting, in order to obtain better adhesive properties, in particular to substantially remove the magnesium-rich (native) oxide layer.

[0007] EP 3 416 760 B1 discloses that typical specific rolling forces during skin passing are in the range of 1.9 kN / mm.

[0008] Multiphase steels according to DIN EN 10346 are also known, which are used in the automotive sector in bodywork. Examples of steels of this type are available under the standard designations HCT490X, HCT590X, or HCT780X. These are cold-rolled steels.

[0009] The task is therefore to modify the surface of hot-dip coated steel sheets in such a way that the product can be processed like existing established products.

[0010] The problem is solved with the features of claim 1.

[0011] The inventors have discovered that the rolling force during skin passing after hot-dip coating influences the surface and thus also the surface chemistry, such that increasing the specific rolling force during skin passing can create a truly enlarged surface compared to a perfectly flat surface. During skin passing, the contact of shaping elements of a skin passing roll with a surface of the hot-dip coated steel sheet causes mechanical stress, through which the elements zinc and aluminum (oxide) lying immediately beneath the magnesium-rich oxide layer (native oxide layer: can have a thickness of > 0 up to 200 nm, in particular up to 100 nm, preferably up to 50 nm, at the surface or near the surface within the coating and is thus to be understood as part of the coating) can reach the surface of the coating.The specific rolling force during skin-passing, particularly its increase compared to the standard process, can be used to increase the surface area. This increase results from the Sdr value determined according to ISO 25178 being at least 2.0%. The Sdr value determined according to ISO 25178 corresponds to the percentage by which the actual surface is larger than a completely flat surface due to the surface structure formed by skin-passing.

[0012] ISO 25178 takes measurements and specifications of three-dimensional surface textures (viewed on a defined area) into account by defining three-dimensional texture parameters and the operators for their determination. ISO 25178 also allows other characteristic quantities, such as the mean arithmetic height Sa (arithmetic mean of the absolute ordinate values), to be recorded three-dimensionally, which was previously only possible two-dimensionally by specifying the mean arithmetic roughness Ra on a line across ISO 4288, particularly along or across the rolling direction. Alternatively, the Sdr value is determined using confocal microscopy. An area of ​​at least 0.5 mm x 0.5 mm is observed. For larger areas, such as 0.8 mm x 0.8 mm, shaped components of the sheet in the observed area are separated using mathematical filters.The maximum area to be observed should not be larger than 5.0 mm x 5.0 mm, preferably not larger than 3.0 mm x 3.0 mm, particularly preferably not larger than 2.0 mm x 2.0 mm, in particular not larger than 1.0 mm x 1.0 mm.

[0013] It is self-explanatory that the term surface and thus also the values ​​determined according to ISO 25178 in this context refer to the entire surface that has been three-dimensionally textured, i.e. skin-passed, by the skin-pass roll. To determine the Sdr and / or Sa values, random samples of individual surface areas are preferably sufficient. In particular, the values ​​determined according to ISO 25178 (the Sdr and / or Sa value) in this context relate to the entire textured area, i.e. the sheet surface with valley and peak areas and including the flank areas that connect the valley and peak areas. Thus, in one alternative, the Sdr and / or Sa values ​​are determined, for example, at a resolution with an area of ​​at least 0.5 mm x 0.5 mm or 0.8 mm x 0.8 mm up to a maximum of no larger than 3.0 mm x 3.0 mm, particularly preferably no larger than 2.0 mm x 2.0 mm, in particular no larger than 1.0 mm x 1.0 mm.

[0014] At an Sdr value of less than 2.0%, the influence on the surface chemistry is too small and would not subsequently lead to spot-free phosphating. High Sdr values ​​above 8.0% are possible, but only with high equipment expenditure and thus are complex to achieve. Therefore, optimal spot-free phosphating is achieved with an Sdr value between 2.00 and 10.0%, in particular of at least 2.10%, 2.20%, 2.30%, 2.40%, preferably of at least 2.50%, 2.70%, 2.80%, 3.0%, preferably of at least 3.10%, 3.20%, 3.30%, 3.40%, 3.50%, and in particular of a maximum of 8.0%, preferably a maximum of 7.0%.

[0015] For the purposes of the invention, spots are defined as apparently dark areas (on the surface). Preferably, a dark area is delimited by dark points which are characterized by the fact that they are darker than other, hence brighter points in the immediate vicinity. In this sense, a point is not to be understood as a mathematical point, which has no extent, but rather, for example, as a pixel or group of pixels. Such a dark point only has a common border with brighter points in a partial area of ​​its circumference. In the remaining partial area of ​​its circumference, it has a common border with dark points which have essentially the same brightness as this delimiting dark point. An above-mentioned dark area thus essentially consists of the latter-mentioned dark points and the former-mentioned delimiting dark points.Since improved corrosion protection is intended, the coating contains, in addition to zinc and unavoidable impurities, magnesium in a content of at least 0.5 wt. %, in particular at least 0.8 wt. %, preferably at least 1.1 wt. %. In addition, aluminum is also present in a content of at least 0.5 wt. %, in particular at least 0.8 wt. %, preferably at least 1.1 wt. % in order to improve the bonding of the coating to the steel sheet and, in particular, to substantially prevent the diffusion of iron from the steel sheet into the coating during heat treatment of the coated steel sheet, so that the positive corrosion properties are retained. The thickness of the coating can be between 2 and 15 pm, in particular between 3 and 10 pm, preferably between 4 and 9 pm per side.

[0016] The mean arithmetic height Sa may be at least 0.50 pm, in particular at least 0.60 pm, preferably at least 0.70 pm. It may be limited to a maximum of 2.0 pm, in particular to a maximum of 1.50 pm, preferably to a maximum of 1.10 pm.

[0017] The multiphase alloy of the steel substrate contains or consists of the following elements in wt.%:

[0018] C: 0.050 to 0.250%, in particular 0.060 to 0.250%, preferably 0.070 to 0.250%;

[0019] Si: 0.020 to 0.50%, in particular 0.020 to 0.40%, preferably 0.020 to 0.30%;

[0020] Mn: 1.30 to 2.0%, in particular 1.40 to 1.90%, preferably 1.50 to 1.80%;

[0021] P: up to 0.10%, in particular up to 0.080%, preferably up to 0.060%;

[0022] S: up to 0.050%, in particular up to 0.040%, preferably up to 0.030%;

[0023] N: up to 0.10%, in particular up to 0.050%, preferably up to 0.030%;

[0024] AI: 0.010 to 0.150%, in particular 0.010 to 0.10%, preferably 0.010 to 0.090%; optionally one or more of the following elements:

[0025] Cu up to 0.80% and / or Cr up to 0.70% and / or Nb up to 0.10% and / or Ti up to 0.20%; remainder iron and unavoidable impurities.

[0026] The surface of the steel sheet can have a stochastic surface structure. This is created using skin-pass rolls whose surfaces are textured using an EDT process. Alternatively, the surface of the steel sheet can have a deterministic surface structure. This is created using skin-pass rolls whose surfaces are textured using a laser.

[0027] In an alternative, skin-pass rolls with an Ra value (the arithmetic mean roughness according to DIN EN ISO 4287:2010) of at least 1.0 pm, preferably at least 1.2 pm, particularly preferably at least 1.3 pm, in particular at least 1.6 pm and a maximum of 11.0 pm, preferably a maximum of 4.5 pm, particularly preferably a maximum of 3.5 pm, in particular 3.0 pm are used. Alternatively or additionally, the skin-pass rolls have an Sdr value of at least 1.5% or 1.8%, preferably at least 2.5%, particularly preferably at least 3.0%, in particular at least 3.5% and a maximum of 55.0%, preferably a maximum of 40.0%, particularly preferably a maximum of 20.0%, in particular 10.0%.

[0028] A surface with a pseudo-stochastic surface structure would also be conceivable. These surface structures have a (guasi-)stochastic appearance, composed of stochastic elements with a recurring structure.

[0029] With an increase in the specific rolling force during skin-passing and the associated increase in surface area, it was found that the standardized Mg content on the surface decreases, so that the Mg content is a maximum of 55%, in particular a maximum of 50%, preferably a maximum of 45%, more preferably a maximum of 40%. The specified standardized content corresponds in particular to the determined mean value, although fluctuations within the scope of measurement tolerances (standard deviation) may occur. A value below 5% is not permitted. The Mg content on the surface can in particular be at least 7%, preferably at least 8%, more preferably at least 10%.

[0030] Furthermore, it was also observed that with an increase in the specific rolling force during skin passing and the associated increase in surface area, the standardized Zn content on the surface increases, so that the Zn content is at least 20%, in particular at least 22%, preferably at least 25%, preferably at least 30%. The specified standardized content corresponds in particular to the determined mean value, although fluctuations within the scope of measurement tolerances (standard deviation) may occur. Exceeding 80% is not possible. The Zn content on the surface can in particular be a maximum of 75%, preferably a maximum of 70%, more preferably a maximum of 65%. The sum of the standardized contents of magnesium, aluminum and zinc is always 100%.

[0031] The relative concentration of zinc, magnesium and aluminum is determined by determining the absolute concentration of these elements and then standardizing them to 100%. The sum of the concentrations of zinc, magnesium and aluminum is set equal to 100 and the proportion of the respective element in this 100% is evaluated or weighted as the relative concentration, i.e. based on 100%. The relative concentration of an element (Al, Mg, Zn) therefore refers to the sum of the concentrations of the elements Mg, Zn and Al, as this sum represents 100%. Since the absolute concentration of the elements Zn, Mg and Al can vary from coating to coating, the information is given as a relative concentration in percentage points in order to define changes precisely. The occurrence of the elements zinc, magnesium and aluminum is recorded for the purposes of the invention regardless of the form in which they are present.It therefore does not matter whether these elements exist as neutral atoms or ions, in a compound, such as an alloy or intermetallic phase, or in a compound such as complex oxides, salts, hydroxides, or the like. Thus, the terms "zinc," "aluminum," and "magnesium" within the meaning of the invention can encompass not only the elements in their pure form, but also oxidic and / or hydroxidic forms, or any form of compounds containing these elements.

[0032] The tendency of stain formation in the phosphating decreases with decreasing standardized Mg content and with increasing standardized Zn content on the surface.

[0033] The relative concentration differences of magnesium, aluminum, and zinc at the coating surface, i.e., on the "native" (magnesium-rich) oxide layer, can be determined by recording the spatial distribution of the signals for these alloying elements using a time-of-flight secondary ion mass spectrometer (ToF-SIMS) in imaging mode or, similarly, using Auger electron or photoelectron spectroscopy. ToF-SIMS is an analytical method for determining the chemical surface composition of the top 1-3 monolayers.

[0034] Using ToF-SIMS, certain relative concentration differences are measured by scanning the surface to be analyzed within a representative measurement area. A spectrum with positive polarity is recorded at each position on the grid, and the raw signals for the main components (alloying elements) are recorded. The relative concentration of element X, which in this case represents one of the alloying elements in the hot-dip coated and temper-passed coating, is calculated from the quotient [X raw signal integral / (Zn raw signal integral + Mg raw signal integral + Al raw signal integral)], with the denominator of the quotient being the sum of the raw signal integrals of all alloying elements in the coating. The “raw signal” of element X in this definition is the intensity or peak area of ​​element X in the mass spectrum.The "raw signal integral" of element X is the integrated intensity, which is represented over a defined area of ​​grid positions and assigned to the respective element X. The ToF-SIMS characterization can be performed in a measurement area of ​​200x200 pm. 2 or 500x500 pm 2 The internal ToF-SIMS measurements were performed using a TOF.SIMS 5 device from ION-TOF GmbH.

[0035] The near-surface chemical composition is determined, for example, using X-ray photoelectron spectroscopy (XPS), whereby the procedure for determining the individual chemical compositions is familiar from the prior art. For the purposes of the invention, the XPS-typical information depth corresponds to a layer with a thickness of essentially 5 nm. The measurement can be carried out, for example, using the Phi Quantera II SXM Scanning XPS Microprobe from Physical Electronics GmbH. The element concentrations measured by XPS can be taken from overview spectra, which are recorded at, for example, a transmission energy of 280 eV over the course of at least 7 cycles and cover, for example, a measurement area of ​​100 x 100 pm. 2 As described above, the values ​​are normalized to 100% to indicate the relative concentrations.

[0036] In the sense of the invention, the term essentially means with respect to a feature or process that this feature or process is almost completely fulfilled, but there remains a difference of a maximum of 50%, 45%, 40%, preferably 30%, 25%, particularly preferably 20%, 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11%, in particular 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1% or 0.5%, 0.1% up to a 100% match.

[0037] Steel sheet is generally understood to mean a cold-rolled flat steel product which can be provided in sheet form, in blank form or in strip form. The thickness of the steel sheet can be between 0.45 and 6.0 mm, in particular at least 0.5 mm, preferably at least 0.6 mm and in particular a maximum of 4.0 mm, preferably a maximum of 2.0 mm. According to a second aspect, the invention relates to a method for producing a hot-dip coated and skin-passed steel sheet, comprising the following steps: - providing a steel substrate made of a multi-phase alloy according to DIN EN 10346 - hot-dip coating the steel substrate on one or both sides with a metallic coating which, in addition to zinc and unavoidable impurities, contains additional elements such as aluminum with a content of 0.5 to 8.0 wt.% and magnesium with a content of 0.5 to 8.0 wt.%-% in the coating, - skin passing of the hot-dip coated steel sheet, whereby a skin passing force during the skin passing is set such that the surface Sdr value on the surface of the hot-dip coated and skin passed steel sheet is at least 2.0%, determined in accordance with ISO 25178.

[0038] The surface (positive form) of the skin-passing roll forms a surface structure through the application of force to the surface of the steel sheet, which defines depressions (negative form) and essentially corresponds to the surface with elevations (positive form) of the skin-passing roll. The set specific rolling force can have a positive influence on the surface chemistry and, in particular, the oxygen-affine alloying elements such as magnesium and aluminum, can be essentially displaced by the application of force during skin-passing on the surface of the coating and the surface area can also be increased. Since magnesium has a higher affinity for oxygen than aluminum, a magnesium-rich oxide layer forms on the surface in the coating or close to the surface, particularly during hot-dip coating or after solidification and cooling.The application of force can essentially displace disruptive layers, such as magnesium-rich oxide layers, so that the relative concentration of zinc and optionally aluminum on the surface increases, which in turn can lead to a spot-free surface during post-treatment and thus during phosphating.

[0039] To avoid repetition, reference is made to the statements on the hot-dip coated and tempered steel sheet according to the invention.

[0040] In order to particularly reduce the relative concentration of magnesium on the surface of the coating in the valley areas or to displace the magnesium-rich oxide layer, according to one embodiment of the method according to the invention, a specific rolling force of at least 2.2 kN / mm is set during skin passing, so that the surface area can be increased. A further increase in the surface area is possible if a specific rolling force of at least 2.5 kN / mm, preferably at least 3 kN / mm, and more preferably at least 3.5 kN / mm is set during skin passing. Skin passing forces above 10 kN / mm offer no advantage and only increase labor and equipment requirements. In addition, the abrasion or wear generated during skin passing due to the shear forces between the sheet and skin passing roll surfaces outside the flow shear zone increases with the specific rolling force.

[0041] In a further embodiment, the hot-dip coated and skin-passed steel sheet described above is oiled with a mineral oil-based corrosion inhibitor. Mineral oils and mineral oil-based corrosion inhibitors are known to those skilled in the art. Mineral oils are produced from coal, peat, wood, petroleum, or natural gas and, unlike oils derived from organisms, contain essentially no fatty acid triglycerides. Mineral oil-based corrosion inhibitors contain or consist of more than 50%, preferably more than 70%, particularly preferably more than 90% mineral oils, as well as optionally other additives and / or so-called synthetic oils, which have a special molecular structure not found in this form in the starting material (e.g., crude oil).

[0042] Experts know what is meant by specific rolling force in skin-passing. The specific rolling force is the absolute rolling force in N divided by the strip width in mm.

[0043] According to a third aspect, the invention relates to a use of a hot-dip coated and tempered steel sheet according to the invention, which has been produced in particular by the method according to the invention, for parts in motor vehicle construction.

[0044] Samples were cut from a 0.6 mm thick hot-rolled steel substrate of grade HCT490X. These samples were hot-dip coated on a laboratory scale with different metallic coatings and skin-passed using different skin-pass parameters. They were then subjected to further testing. The results are summarized in Table 1. The coating thickness (including the oxide layer) was 7 μm per side. Samples 1, 2, V8, and 10 were skin-passed using a pair of skin-pass rolls with a stochastic surface texture, while samples 5 to 7 were skin-passed using a pair of skin-pass rolls with a deterministic surface texture. Samples V3, V4, and V9 were conventionally skin-passed using a pair of skin-pass rolls with a stochastic surface texture. The determination and determination of the normalized concentrations using ToF-SIMS have already been described.

[0045] Table 1

[0046] It is clearly evident that the skin-passing process essentially influences the surface chemistry of a steel sheet hot-dip coated with a Mg-Al-Zn coating, such that mechanical and optical properties can be adjusted while the oxide layer is damaged as little as possible.

[0047] All samples then underwent the following processing steps: oiling with a mineral oil-based corrosion inhibitor, degreasing, cleaning, rinsing, activation, rinsing, phosphating, rinsing, and drying. These processing steps were carried out conventionally using agents familiar to the expert. The phosphated samples were visually inspected, revealing that samples V3 and V8 exhibited very prominent and noticeable stains. Further investigations had shown that in the dark areas, the average crystal size of the phosphate-containing crystals (so-called phosphate crystals) significantly exceeded (5+ / -2) pm in both the mean and standard deviation.

Claims

Patent claims 1. Steel sheet which is hot-dip coated and skin-passed, the steel sheet comprising a steel substrate made of a multi-phase alloy according to DIN EN 10346 and a metallic coating arranged on one or both sides of the steel substrate, which coating contains, in addition to zinc and unavoidable impurities, additional elements such as aluminium with a content of 0.5 to 8.0 wt.% and magnesium with a content of 0.5 to 8.0 wt.% in the coating, characterized in that the hot-dip coated and skin-passed steel sheet has a surface with an Sdr value determined according to ISO 25178 of at least 2.0%.

2. Steel sheet according to claim 1, wherein the alloy of the steel substrate contains or consists of the following elements in wt.%: C: 0.020 to 0.250%; Si: 0.02 to 0.5%; Mn: 1.30 to 2.00%; P: up to 0.10%; S: up to 0.050%; N: up to 0.10%; AI: 0.010 to 1.50%; optionally one or more of the following elements: Cu: up to 0.80% and / or Cr: up to 0.80% and / or Nb: up to 0.10% and / or Ti: up to 0.20%; Rest iron and smelting-related impurities 3. Steel sheet according to claim 1, wherein the surface has a stochastic surface structure. Steel sheet according to claim 1, wherein the surface has a deterministic surface structure. Steel sheet according to claim 1, wherein the surface has a pseudo-deterministic surface structure. Steel sheet according to one of the preceding claims, wherein the surface has a relative Mg content of at most 55%, wherein the sum of the relative contents of magnesium, aluminum, and zinc is 100%, determined by means of XPS. Steel sheet according to one of the preceding claims, wherein the surface has a relative Zn content of at least 10%, wherein the sum of the relative contents of magnesium, aluminum, and zinc is 100%, determined by means of XPS. Steel sheet according to one of the preceding claims, wherein the thickness of the metallic coating is between 1 and 25 pm per side. Steel sheet according to one of the preceding claims, wherein the mean arithmetic height Sa is at least 0.70 pm and at most 2.0 pm.Steel sheet oiled with a mineral oil-based corrosion inhibitor according to one of the preceding claims. A method for producing a hot-dip coated and temper-rolled steel sheet, comprising the following steps: Providing a steel substrate made of a multi-phase alloy according to DIN EN 10346, hot-dip coating of the steel substrate on one or both sides with a metallic coating which, in addition to zinc and unavoidable impurities, contains additional elements such as aluminium with a content of 0.5 to 8.0 wt.% and magnesium with a content of 0.5 to 8.0 wt.% in the coating, Tempering of the hot-dip coated steel sheet, characterized in that a skin-passing force is set during skin-passing such that a surface Sdr value of at least 2.0% results on the surface of the hot-dip coated and skin-passed steel sheet, determined according to ISO 25178. Method according to claim 11, wherein the specific rolling force during skin-passing is set at at least 2.2 kN / mm. Method according to claim 11 or 12, wherein the hot-dip coated and skin-passed steel sheet is oiled with a mineral oil-based corrosion inhibitor. Use of a hot-dip coated and skin-passed steel sheet according to one of claims 1 to 10, in particular produced according to one of claims 11 to 13, for parts in motor vehicle construction. Use according to claim 14 for outer skin parts on a vehicle.

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