EP3405600B1 - Method for producing a steel product with a zn coating and a tribologically active layer deposited on the coating, and a steel product produced according to said method - Google Patents

Description

The invention relates to a process for producing a steel product which has a protective coating based on zinc and a tribologically active layer applied to the protective coating.

Likewise, the invention relates to a provided with such a layer structure steel product, wherein this steel product is in particular a flat steel product.

When the term "flat steel products" is used, it refers to rolled products that are available as strip, sheet metal or blanks and blanks derived therefrom. The term "thin sheet" here refers to flat steel products with a sheet thickness of typically up to 3 mm.

In order to optimize their surface properties, galvanized steel flat products usually undergo temper rolling after galvanizing, in which they are deformed with low degrees of deformation. By temper rolling the texture of the respective flat steel product is imprinted, which increases the roughness of the substrate and as a result improves the adhesion and the appearance of the subsequently applied organic coatings. In addition, it is known that the temper rolling process has positive effects on the mechanical properties of the flat steel product. In-depth information on the tempering process and Their effects on a modern sheet are described in the publications titled "Skin-pass rolling I - Studies on roughness transfer and elongation under normal loading" and "Skin-pass rolling II - Studies of roughness transfer under combined normal and tangential loading " written by Hideo Kijima and Niels Bay, published in International Journal of Machine Tools & Manufacture, Part I 48 (2008) 1313-1317 & Part II 48 (2008) 1308 - 1312 ,

Hot-dip galvanized flat steel products are increasingly replacing electrolytically galvanized flat steel products in the field of automobile body construction.

Regardless of how the Zn coating is applied, the steel flat product to be formed in each case or an already preformed steel component for forming into a component is inserted into a forming machine and then shaped by the machine to the respective component. The transformation can be carried out as cold forming, that is to say as forming at temperatures below the recrystallization temperature of the respective steel of the flat steel products, or as hot forming, ie as forming at working temperatures which are above the recrystallization temperature.

A typical example of such a forming process is deep-drawing, in which the flat steel product to be formed is pressed by means of a punch into a die. The shape of the die and die here determine the shape that the flat steel product receives through the forming process.

With each forming operation, relative movements occur between the forming tool used for the shaping and the product to be formed. At the same time there is contact between the surfaces of the product and their associated surfaces of the forming tool. The case between the tool and the The resulting tribological system is determined by the material properties of the product to be formed and the tool and by the media present between the product to be formed and the tool. As a result of the relative movement between the forming tool and the forming product touching the reshaping formed product friction.

This friction can be very different locally, especially during the forming of flat steel products, because the material of the flat steel product is deformed differently in sections as part of the forming and thus the material of the flat steel product flows locally as differently at the deformation. It is therefore precisely in the production of complex-shaped components by deep drawing or comparable cold forming processes, which are usually achieved in large degrees of deformation and complex shapes are displayed, dynamically changing frictional conditions in which static and sliding friction can occur alternately.

These frictional forces occurring in particular during cold forming can be so high that they disturb the continuous course of the shaping process and can cause a faulty impression of the component to be molded in each case. At the same time, the unavoidable friction causes considerable tool wear.

In this context, flat steel products, in which a zinc-based protective coating protecting against corrosion or other environmental influences is applied to the actual flat steel product, prove to be particularly critical.

In order to optimize the tribological properties of electrolytically galvanized sheet metal for automotive typical forming processes, usually a phosphate layer is built up on the Zn coating. In the classical Tri-cation phosphating takes place first a pickling attack on the Zn-coated base substrate, in which first metal cations go under hydrogen evolution in solution. In the next process step, poorly soluble phosphates precipitate near the surface due to the pH change and form a firmly adhering conversion layer.

Modern phosphatizations belong to the so-called layer-forming phosphatizations. There, the layer build-up is carried out by metal cations from the phosphating solution (e.g., zinc, manganese). However, because of the acid pickling process, cations from the base substrate can also be incorporated into the phosphate layer.

The good sliding properties of the phosphate layer are based z.T. on easy shearing of phosphate crystals. In addition to the tribological aspect, phosphating improves the corrosion protection of the electrolytically galvanized sheet. For procedural reasons, a line phosphating of hot-dip galvanized flat steel products, however, is not economically feasible.

The phosphate coating applied in the typical automotive phosphating process does not belong to the classic dry lubricants (such as graphite, MoS ₂ ). The lubricating effect of the phosphate layer is due to the effect of interacting with the anticorrosive oils or pre-lubes with the phosphate layer and the underlying hot-dip galvanized substrate for added protection to the flat steel products.

Subsequently, such coated thin sheets are applied with a corrosion protection oil or a pre-lube to guarantee sufficient corrosion protection during transport and subsequent storage. Furthermore, lubrication ensures additional pre-lubrication during the forming process.

From an ecological point of view, a classic tri-cation phosphating process should at least be regarded as critical, since the disposal of the resulting process media (phosphate sludge, wastewater) is complex. Also, high energy costs incurred during phosphating, since the process baths operate at elevated temperatures.

From the EP 2 851 452 A1 It is known that carbonate-based conversion layers improve the tribological properties of galvanized sheet. The carbonate supplier is for example selected from ammonium hydrogencarbonate, ammonium carbonate etc. and a hydroxide supplier, selected from alkali metal hydroxides, alkali metal oxides etc. This layer is applied according to invention by means of Chem Coater. The layer weight of the dry substance is 25 to 200 mg / m ² . The pH of the aqueous solution according to the invention is preferably in the range of 9 ± 0.5. Due to the basic environment, purposeful technical and personal protective measures (eg protective gloves, safety goggles) must be taken.

In the DE 699 06 555 T2 describes a coating consisting of zinc hydroxysulfate. For this purpose, a galvanized steel substrate is coated with an aqueous solution containing a sulphate ion concentration of more than 0.07 mol / l. The layer thus applied is partially water-insoluble and improves the tribological properties of the coated thin sheet. Example 7 of this publication illustrates the relationship between the water-insoluble and water-soluble portion of the zinc hydroxysulfate formed. From this example it can be seen that the improvement of the lubricating effect is provided by the water-insoluble part. In addition, it is shown that the proportion of the water-insoluble fraction increases with increasing coating time. It can therefore be assumed that the pre-lubricating effect of the applied layer increases with coating times of> 20 seconds. However, due to the water-insoluble content of the zinc hydroxysulfate layer special Requirements (eg high / low pH values) are placed on the inline cleaning baths of the car manufacturers, which can be economically and ecologically disadvantageous.

From the US 6,194,357 B1 It is known to use various emulsions which improve the cold working of metals. The emulsions consist of the following components: (A) water-soluble inorganic salt (eg borax, potassium tetraborate, sodium sulphate etc.), (B) solid lubricant (eg phyllosilicates, metal soaps etc.), (C) natural (eg mineral oil etc.) and synthetic oils, (D) surfactant and (E) water. The ratio between (B) and (A) is in the range of 0.05: 1 to 2: 1. The ratio between (C) and (B) + (A) is between 0.05: 1 and 1: 1. The dry layer weight of the coating described is given in a range between 1 and 50 g / m ² . The layer according to the invention unfolds its positive tribological properties on metals only by means of all stated components ((A) - (E)). Individual components of the layer according to the invention, such as potassium tetraborate, are classified as hazardous to health.

In addition, from the US 4,168,241 a solid lubricant based on a sulfate (calcium or barium sulfate), an already known lubricant from the group of graphite, graphite fluoride, molybdenum disulfide, etc. and an organic lubricant (eg fatty acids, metal soaps, etc.) described. The sulfate-based solid lubricant exhibits its intended effect when the particle size is <100 μm. This requires a more complex manufacturing process. The dry layer weight of the coating is in the range of 5 to 15 g / m ² .

From the GB 739,313 In addition, an oxalate-containing coating is known which is intended to improve the tribological properties of the metallic substrate coated with it. In doing so, the improved tribological Properties of the coating attributed to the contained iron oxalate. However, iron oxalate is hazardous to health.

In the US 2004/0255818 A1 a coating based on aluminum sulfate and aluminum sulfate precursors, boric acid and boric acid precursors and polycarboxylate and polycarboxylate precursors is described. The applied coating weights are 0.2 to 1.2 g / ft ² . The improvement of the forming behavior is achieved only by the interaction of the mentioned components. Boric acid, however, is now classified as particularly harmful to the environment.

WO-A-2015 197430 discloses a method of coating zinc coated steel substrates. In this case, a solution containing alkali metal sulfate, and alkali metal carbonate having a pH of 9-12 is used.

Against the background of the prior art described above, the object has arisen of naming a method which, with simple products that are harmless with regard to environmental pollution, allows to produce a coating having the optimum tribological effect on a galvanized surface of a steel product.

Likewise, a steel product should be specified which, in addition to optimized corrosion protection, has optimum suitability for forming into a component, in particular a body component.

With regard to the method, this object has been achieved according to the invention by the method specified in claim 1.

With regard to the product, the solution according to the invention is that a steel product has the features mentioned in claim 9.

Advantageous embodiments of the invention are mentioned in the dependent claims and are explained below as the general inventive concept in detail.

• Auftragen einer wässrigen Lösung bestehend aus Ammoniumsulfat und vollentsalztem Wasser auf die Schutzbeschichtung des Stahlprodukts,
  • wobei die Konzentration des Ammoniumsulfates bezogen auf die SO₄ ^2--Ionen 0,01 - 5,7 mol/l beträgt,
  • wobei der pH-Wert der wässrigen Lösung 4 - 6 beträgt
    und
  • wobei die oberflächennahe Reaktionszeit zwischen der wässrigen Lösung und der Schutzbeschichtung mehr als 0 Sekunden und höchstens 5 Sekunden beträgt,
  • so dass nach einem ohne vorgeschaltetes Spülen durchgeführten Trocknen auf dem Schutzüberzug eine tribologisch aktive Schicht vorliegt, die aus Ammoniumzinksulfat besteht.

The method according to the invention for producing a steel product which has a protective coating based on zinc and a tribologically active layer applied to the protective coating comprises the following steps:

• Applying an aqueous solution consisting of ammonium sulfate and demineralized water to the protective coating of the steel product,
  • wherein the concentration of ammonium sulfate with respect to the SO ₄ ^2- ions 0.01-5.7 mol / l,
  • wherein the pH of the aqueous solution is 4-6
    and
  • wherein the near-surface reaction time between the aqueous solution and the protective coating is more than 0 seconds and at most 5 seconds,
  • so that after drying carried out without prior rinsing on the protective coating, a tribologically active layer is present, which consists of ammonium zinc sulfate.

The invention thus provides, in a no-rinse method (ie in a method in which rinsing after application of the aqueous solution consisting of ammonium sulfate and demineralized water is dispensed with the protective coating) on the Zn protective coating of each processed Steel product to apply an aqueous solution consisting of ammonium sulfate and demineralized water.

The concentration of ammonium sulfate in relation to the total volume is in the range of 0.01 to 5.7 mol / l.

For the application of the present invention to be applied to the Zn coating solution, a conventional chemical or coil coater can be used. Such chem- or coil coater are for example in the book " Coil Coating - Coil Coating: Processes, Products and Markets "by P. Meuthen, Almuth-Sigrun Jandel, Friedr. Vieweg & Sohn Verlag / GWV Fachverlage GmbH, 1st edition 2005, ISBN: 3-528-03975-2 described. In this case, the aqueous solution is applied to at least one side of the zinc alloy coating of the steel substrate. Alternatively, it is also possible to apply the aqueous solution by means of spraying, wherein the spraying is followed by squeezing to adjust the thickness of the film formed from the solution remaining on the respective substrate. This procedure is typically used in coating systems that are completed in continuous operation of the respective steel product.

The dry layer weight of the tribologically active layer produced according to the invention and accordingly present on a steel product according to the invention is typically 1-100 mg / m ^2, based on the sulfur content, with dry layer weights of at most 20 mg / m ² , in particular from 10 to 20, particularly with regard to weldability mg / m ² , in each case based on the S-content of the coating have proven to be particularly favorable. Practical dry film thicknesses are 10 to 15 mg / m ^2, also based on the S content.

Another surface chemical characteristic of the tribologically active layer produced according to the invention is that the double sulfate (NH ₄ ) ₂ Zn (SO ₄ ) ₂ shows a high adhesion to the zinc alloy coating due to the Zn mixed crystal formed.

The tribologically active coating applied according to the invention simultaneously offers an exceptionally high lubricating effect, in particular Automotive typical cold forming processes and is thus optimally suitable for forming into a component in a forming tool. Examinations have proven that the tribologically active layer produced according to the invention does not adversely affect the subsequent processes typical for the production of automobile body parts, such as gluing, welding, phosphating or electrophoretic painting. Flat steel products coated according to the invention have significantly improved tribological properties compared to only oiled thin sheets. Furthermore, the coating produced and obtained according to the invention offers excellent sequence process compatibility in the automobility-typical production process (joining, phosphatability, KTL-capability, etc.).

The inventively provided and generated, tribologically active layer is also easily removable, for example, with water, if any influence on the succession processes should be excluded by them safely. Due to their chemical composition, the inventively produced and provided on a steel product Ammoniumzinksulfat coating is extremely environmentally friendly and harmless to health.

The invention is based on the findings, in a general form already in the non-prepublished European patent application 14 18 44 15.9 ( EP-A-2 995 674 In addition, however, provides information on the parameters, such as in particular the reaction time between the applied, according to the invention ammonium sulfate aqueous solution and the respective steel substrate, which are crucial to the inventively recognized as favorable constitution of the tribologically active layer receive. The content of the European patent application 14 18 44 15.9 is therefore included to explain the technical context in which the invention stands, and the possible practical implementation of the method according to the invention by reference in the present patent application.

The steel substrates to be provided for the process according to the invention and forming the basis of the steel products designed according to the invention are coated with a protective coating based on zinc in order to protect against corrosion. The Zn-based coating may be applied conventionally as a pure zinc layer or as a zinc alloy layer and may have levels of Mg, Al, Fe or Si to improve or adjust its properties. Alloy prescriptions which characterize typical practice-proven compositions of such Zn-based anticorrosive coatings are disclosed in e.g. 0.5-5% by weight of aluminum and / or up to 5% by weight of magnesium and balance zinc and unavoidable impurities. For coating with such a Zn coating, for example, a flat steel product can be cooled to the respective bath inlet temperature by a conventional pretreatment and then immersed in an immersion time of 0.1-10 s with an iron saturated 420-520 ° C Zn Melt bath containing, in addition to the main component zinc and unavoidable impurities 0.05 to 5 wt .-% Al and / or up to 5 wt .-% Mg.

The Zn protective coating of the steel product provided according to the invention may have been applied electrolytically, for example. From a practical and economic point of view, however, it proves to be particularly advantageous if the Zn protective coating has been applied to the respective steel substrate of the flat steel product by application of methods which are likewise known per se by hot-dip coating.

The respective steel substrate of the flat steel product to be coated by the method according to the invention may have any composition known from the prior art as long as it permits a coating with a Zn-based protective coating and is suitable for the respective subsequent process. Typical examples of steels which make up the steel substrate of flat steel products coated according to the invention are IF steels, microalloyed steels, bake hardening steels, TRIP steels, Dual-phase steels and deep-drawing steels such as, for example, the steels known under the designation DX51D to DX58D (material numbers 1.0226, 1.0350, 1.0355, 1.0306, 1.0309, 1.0322, 10.0853).

The aqueous solution applied according to the invention to the Zn-based protective coating contains, in addition to the main components "demineralized water" and "ammonium sulfate", no additional constituents. In particular, the presence of other organic components, especially those that might be environmentally hazardous, is excluded.

The concentration of the ammonium sulfate in the aqueous solution is based on the SO ₄ ^2- ions in the range of 0.01 to 5.7 mol / l chosen so that the inventively provided, from the double sulfate (NH ₄ ) ₂ Zn ( SO ₄ ) _{2 forms} existing coating securely on the Zn coating. In this regard, it may be useful if the concentration of ammonium sulfate with respect to the SO ₄ ^2- ions is 0.1-3 mol / l. Especially in this case, and especially when the concentration of ammonium sulfate in relation to the SO ₄ ^2- ions is 0.4-0.7 mol / l, the optimum for the layer formation according to the invention is obtained without further addition of acids or bases pH value natively, without the need for additional aids to adjust the pH of the solution must be added. In addition, this concentration range is optimal from an ecological and economic point of view, since only the amount of ammonium sulfate is used, which is necessary to form an inventive Ammoniumzinksulfatschicht under the application conditions described on galvanized sheet.

In order to achieve a formation of the coating to improve the forming properties of the thin sheet, the pH of the solution used is between 4 and 6, wherein the inventively provided, tribologically active (NH ₄ ) ₂ Zn (SO ₄ ) ₂ layer particularly reliable especially when the pH of the aqueous solution is 4.2 to 5.7.

Table 1 shows the relationship between the pH and the ammonium zinc sulfate layer formed according to the invention

The invention is effective regardless of the particular composition of the protective coating, as long as the base of the protective coating is zinc, so zinc is the predominant component of the protective coating. For the experiments reported here, a hot-dip galvanized sheet was used.

Prerequisite for the present invention desired formation of the double sulfate (ammonium zinc sulfate) is the pickling reaction, i. the zinc dissolution due to the reaction between the solution applied according to the invention and the surface of the Zn protective layer wetted with the solution. The invention here is based on the recognition that the pickling process is only effective at acidic pH's, i. at pH values less than 7 expires.

The adjusting pH of the solution is, as described above, dependent on the concentration of ammonium sulfate and must be within the range specified in the invention. If the ammonium sulfate concentration of the solution is too low, the pH of the solution is too high for the reaction desired according to the invention, the Zn of the coating does not dissolve and no ammonium zinc sulfate is formed.

In order to achieve the not according to the invention high pH values mentioned in Table 1, the respective solution was additionally added to a base (eg NaOH). For solutions with such high pH values, only a passivation of the Zn coating takes place and ammonium sulfate forms on the surface or the dissolved ammonium sulfate dries. However, the ammonium zinc sulfate to be formed according to the invention does not arise.

The coating temperature plays no role in the application of the solution.

For the experiments, the results of which are summarized in Table 1, the following solutions have been prepared: pH value: 4.6: 0.7 mol / l ammonium sulfate dissolved in demineralized water pH value: 5.1: 0.1 mol / l ammonium sulfate dissolved in demineralized water pH value: 8.8 - 9.6: 0.7 mol / l ammonium sulfate dissolved in demineralized water + base (eg NaOH) Table 1: Relationship between the pH of the application solution and the formed (NH <sub> 4 </ sub>) <sub> 2 </ sub> Zn (SO <sub> 4 </ sub>) <sub> 2 < / sub> layer PH value (NH ₄ ) ₂ Zn (SO ₄ ) ₂ layer formation 4.6 Yes (according to the invention) 5.1 Yes (according to the invention) 8.8 No (not according to the invention) 9.1 No (not according to the invention) 9.6 No (not according to the invention)

With the aqueous solution adjusted in accordance with the invention and applied without rinsing ("no-rinse method"), it is possible to ensure that the zinc dissolution process used for the formation of the (NH ₄ ) ₂ Zn (SO ₄ ) ₂ layer is required to reliably run off at the interface between the Zn protective coating and the aqueous solution.

The near-surface reaction time between the aqueous solution and the protective coating present on the steel substrate of the respective steel product is of particular importance according to the findings of the invention. Thus, according to the invention, the reaction close to the surface after the application of the aqueous solution must take sufficiently long time to allow the formation of the double sulfate (NH ₄ ) ₂ Zn (SO ₄ ) ₂ on the Zn coating enable. However, the reaction time must not last too long, otherwise there is the formation of undesirable, poorly soluble zinc sulfate. These two conditions are met if, as specified by the invention, the near-surface reaction time is more than 0.1 seconds, but at most 5 seconds. At reaction times of longer than 5 s, the first portions of the unwanted zinc sulfate form. Already by small amounts of zinc sulfate, the subsequent process compatibility, such as the adhesiveness of the coating is impaired.

The reaction time can be controlled over the period of time which elapses between the application of the respective solution to the Zn protective coating and the drying of the solution. Using the example of a coating plant, which is completed in continuous operation of a steel product, this means that the available reaction time is defined by the line application process. If, for example, the length of the application zone in which the solution is applied to the respective steel product by spraying with subsequent squeezing is 2 to 3 m and this application zone is run through at a belt speed of 2-3 m / s, then the application time and concomitantly the reaction time is about 1s. Directly behind the application zone, the steel product then enters a dryer, which dries the remaining wet film formed from the solution at a temperature of 70-90 ° C. Drying stops the reaction.

The same applies to a chem- or coli-coater application, although there is dried directly after the coater of the wet film by means of a dryer.

In order to show that, when the inventively given reaction times between the applied ammonium sulfate solution and the galvanized thin sheet, the ammonium zinc sulfate layer provided according to the invention reliably sets, a steel substrate is provided which has a Zn protective coating containing 1 wt .-% aluminum and the balance contained zinc and unavoidable impurities, hot-dip galvanized, in an ammonium sulfate solution with a concentration of ammonium sulfate based on the SO ₄ ^2- ions of 0.1 mol / l and one with it associated pH of 5.1. This coating was carried out at room temperature.

In parallel, the forming coating was measured continuously by means of confocal Raman spectroscopy. The intensity of the v ₁ -ZnSO ₄ vibrational band at 962 wavenumbers served as a direct measure of the formation of the unwanted zinc sulphate.

The results are shown in Table 2. Table 2: Proportion of ZnSO <sub> 4 </ sub> formed as a function of reaction time Reaction time [s] Proportion of ZnSO ₄ vibrational band (962 cm ^-1 ) [%] 0 0 (according to the invention) 5 0 (according to the invention) 6 5 (not according to the invention) 10 12 (not according to the invention) 30 32 (not according to the invention) 60 58 (not according to the invention) 90 91 (not according to the invention) 180 100 (not according to the invention) 300 100 (not according to the invention)

X-ray diffractometry is a suitable analytical method for characterizing the ammonium zinc sulfate layer applied according to the invention. In Fig. 1 the X-ray diffractogram of a Ammoniumzinksulfatschicht prepared according to the invention is shown on a hot-dip galvanized steel substrate. The X-ray diffractogram shows typical reflections of a layer of ammonium zinc sulfate and was confirmed by reference spectra. No zinc sulfate was formed under the application parameters according to the invention.

In addition, confocal Raman spectroscopy is suitable as a vibrational spectroscopic method, especially for the characterization of thinnest coatings. In Fig. 2 the Raman spectrum of an ammonium zinc sulfate layer produced according to the invention is shown

In order to produce the tribologically active layer provided according to the invention, the solids-free aqueous solution, which is composed appropriately according to the invention, can be applied to at least one side of the galvanized steel sheet by means of a chemical or coil coater or any other suitable method. A chemical reaction then takes place between the ammonium sulfate completely dissociated in the solution and the zinc surface. As in the essays of RE Lobnig et. al. "Atmospheric corrosion of zinc in the presence of ammonium sulfate particles" published in Journal of the Electrochemical Society 143 (1996) pp. 1539-1546 , and from C. Cachet et. al. "EIS investigation of zinc dissolution in aerated sulphate medium - Part II: zinc coatings", published in Electrochimica Acta 47 (2002) pp. 3409 - 3422 , it can be seen, there is first a dissolution of the oxidized zinc coating or the metallic zinc (ZnO + 2 H ₃ O ⁺ → Zn ²⁺ + 3H ₂ O; 2Zn + 8NH ₄ ⁺ + O ₂ → 2Zn (NH ₃ ) ₄ ^{2 +} + 2H ₂ O + 4H ⁺ ). After acid dissolution of the galvanized base substrate, the layer formation of the tribologically active substance, consisting of ammonium zinc sulfate (4 Zn (NH ₃ ) ₄ ²⁺ + 2 (NH ₄ ) ₂ SO ₄ + 6H ₂ O → (NH ₄ ) ₂ Zn (SO _{4 2} x 6H ₂ O + 6NH ₃ + 2H ⁺ ).

It is the ammonium zinc sulfate formed according to the invention is a specific double sulfate. This double sulphate is decisive for the improved tribological properties as well as for the automotive-typical subsequent process compatibility. Investigations have shown that with the method according to the invention such a layer can be produced reliably and in a manner suitable for large-scale production.

The Ammoniumzinksulfatschicht produced according to the invention is water-soluble and in this point has no special requirements for Cleaning processes, such as are typically performed in the manufacture of automobile bodies and the like typically after the shaping of the steel product and before its further processing. The tribologically active layer formed according to the invention can thus be easily removed, for example, before a phosphating process and subsequent KTL application.

The formation of the ammonium zinc sulfate layer according to the invention as a tribologically active layer can be easily checked by means of X-ray diffractometry and Raman spectroscopy. Characteristic of the tribologically active layer produced according to the invention is that it consists entirely of the double sulfate ammonium zinc sulfate and that it is free from sparingly soluble zinc hydroxysulfate.

It goes without saying in this context that the terms "complete", "exclusive", "free from" and the like used here in connection with the formation of the tribologically active layer according to the invention are to be understood in the technical sense, that is to say, for example in the case of a "completely" or "exclusively" consisting of ammonium zinc sulfate according to the invention formed tribologically active layer may be that technically unavoidable other ingredients are present in her, but have no effect on the effect of this layer.

After application of the aqueous solution, it can be dried in air at ambient temperatures. In industrial use, however, it can be expedient to accelerate the process sequence to force drying in an oven, in particular a continuous furnace. For this purpose, the respective steel product can be kept at a temperature of 70-90 ° C over a period of 1 - 3 s in each oven.

Regardless of how the drying is carried out, the ammonium zinc sulfate layer of the present invention is formed. The layer weight of dry matter related to the sulfur content per m ² is 0.1 - 100 mg / m ^2, preferably 10 - 50 mg / m ^2, wherein based on the sulfur coating weights from 10 - to be particularly useful 20 mg / m ² to have. The "sulfur content per square meter" given in milligrams is also briefly referred to herein as "mgS / m ² ".

The inventive method for producing the double sulfate on the surface of the hot-dip galvanized sheet requires no special safety measures, since ammonium sulfate is not a hazardous substance.

The application of the inventively provided tribologically active layer can be integrated easily into a conventional, the current state of the art fire-coating system.

To control the layer thickness of the inventively provided and produced Ammoniumzinksulfatschicht is for example the well-known for this purpose method of X-ray fluorescence analysis. This method is based on the use of X-rays for material characterization. In this process, nucleated electrons are knocked out of the inner shells of the atom. This allows electrons from higher energy levels to return to their ground state. The released energy is characteristic for the respective element. For the layer applied according to the invention, sulfur is evaluated as the identification element. An addition of further tracer elements is not necessary.

In addition, the tribologically active layer can be analyzed by means of the likewise known glow discharge spectroscopy. In this process, the metallic workpiece is switched as a cathode and removed with argon ions. The ablated atoms are excited in the plasma and emit photons of characteristic wavelength.

In principle, it is possible to treat preformed steel products intended for finish-forming in pieces according to the invention in order to ensure optimum conditions for the finish-forming. However, the invention proves to be particularly advantageous when the steel product to be processed according to the invention is a flat steel product. In the case of such flat steel products, the work steps to be carried out according to the invention can be incorporated into a coating installation completed in the course, whereby a particularly economical large-scale implementation of the method according to the invention is possible. This applies in particular when the flat steel product according to the invention is a steel strip.

After the respective coating process, a corrosion protection oil or a pre-lube can be applied to the coated and dried steel product according to the invention in a manner known per se in order to avoid surface corrosion on the transport path to the respective forming plant and to further improve the forming behavior during the forming process.

In order to ensure optimum adhesion of the coating composition according to the invention for use on the respective surface, the surface in question can be cleaned alkaline prior to application of the coating composition. In a continuous process in which the process of the invention is carried out immediately after a zinc coating, can be dispensed with an alkaline cleaning. There, the coating is then applied directly after galvanizing.

Fig. 1

ein Diffraktogramm einer (NH₄)₂Zn(SO₄)₂-Schicht, die erfindungsgemäß auf ein mit einer durch Feuerverzinken erzeugten Zn-Beschichtung versehenes Stahlflachprodukt appliziert worden ist;

Fig. 2

ein Ramanspektrum einer (NH₄)₂Zn(SO₄)₂-Schicht, die erfindungsgemäß auf ein mit einer durch Feuerverzinken erzeugten Zn-Beschichtung versehenes Stahlflachprodukt appliziert worden ist;

Fig. 3

schematisch einen Versuchsaufbau für einen Streifenziehversuch;

Fig. 4

ein Diagramm, das die Zugscherfestigkeit und den Schälwiderstand einer nur Zn-beschichteten Referenz-Probe und einer erfindungsgemäß mit einer Ammoniumzinksulfatschicht belegten Probe wiedergibt.

The invention will be explained in more detail by means of exemplary embodiments. The figures show:

Fig. 1

a diffractogram of a (NH ₄ ) ₂ Zn (SO ₄ ) ₂ layer, which has been applied according to the invention on a steel flat product provided with a Zn coating produced by hot-dip galvanizing;

Fig. 2

a Raman spectrum of a (NH ₄ ) ₂ Zn (SO ₄ ) ₂ layer which has been applied according to the invention to a steel flat product provided with a Zn coating produced by hot-dip galvanizing;

Fig. 3

schematically a test setup for a strip pulling test;

Fig. 4

a diagram showing the tensile shear strength and the peel resistance of a Zn-coated reference sample and a sample coated with an ammonium zinc sulfate layer according to the invention.

The effectiveness of the invention was tested by means of test series which a) were carried out under laboratory conditions and b) on an industrial scale on a coating line.

In the following, the common features of the experiments and the examination methods used for checking the test results are explained in a general form. This is followed by an illustration of the experimental conditions actually used in the experiments and of the experimental results obtained in detail.

For the coating tests, a cold-rolled steel strip consisting of a typical DX51D automotive grade (material number 1.0226) was used, coated by conventional hot-dip galvanizing with a 7 μm zinc layer consisting of 1% by weight aluminum, balance Zn and technical grade unavoidable impurities existed.

On the thus Zn-coated steel substrate was applied an application solution consisting of ammonium sulfate dissolved in deionized water (Conductivity <0.05 μS / cm). The ammonium sulfate was completely in solution. No further substances were added. The application solution was thus a completely aqueous, solids-free solution.

After application of the aqueous solution to the Zn-coated steel flat product samples, the samples were dried.

Subsequently, various tests were carried out with the samples coated in accordance with the invention in order to check the effect of the ammonium zinc sulfate layer produced according to the invention.

To determine the frictional behavior of the steel flat product samples coated in accordance with the invention with a tribologically active ammonium zinc sulfate layer, a strip pulling test system was used. The strip tension test carried out with this system greatly simplifies the frictional conditions during a deep-drawing process on the hold-down device. The demolition criterion of the Streifenzieh-An attempt represents the occurrence of the stick-slip effect. This effect refers to the adhesion sliding of mutually moving solids. There is a rapid sequence of adhesion, tension, separation and sliding mechanisms between the surfaces in relative motion and occurs with insufficient separation of the surfaces (eg by a lubricant). Before the start of the strip-pulling experiment, the substrate coated according to the invention is oiled with a pre-lube. The tests used were, for example, those offered under the trade name Anticorit PL 3802-39S by FUCHS Europe Schmierstoffe GmbH (see catalog "Lubricants for Exterior and Car Body Parts in the Automotive Industry", bloesch-partner.de 07/2008 1.0) Pre-Lube. The oil layer was 1.5 g / m ² . The sample geometry of the coated flat steel products was 700 x 50 mm ² , while the tool area was 660 mm ² . The test speed was 60 mm / min.

The surface pressure increased linearly from 1 MPa to 100 MPa over the entire test area. The measuring section was 500 mm. The result of the investigation of the strip pulling test is shown as the dependence of the coefficient of friction μ on the surface pressure [Mpa]. The experimental setup is schematic in Fig. 3 shown.

In order to be able to assess the effect on the adhesion behavior of a body shell, angle peel tests according to DIN EN 1465-2009 were carried out. To this end, an adhesive sold under the trade name Betamate 1480.V203 by Dow Automotive was used.

The fracture surface and the fracture pattern were then visually evaluated according to the specification of EN ISO 10365: 1995. The break occurred either in the adhesive itself or in the joining part material. It was distinguished in fractures in the adhesive between a cohesive failure, in which the separation takes place in the adhesive, and an adhesion failure, in which the break occurs at the interface between the adherend and the adhesive. In addition, the material of the sample failed itself while the adhesive remained intact. Here, a distinction was made between a split part break and a break caused by delamination.

In welding tests, the weldability of the steel flat products coated according to the invention in the manner explained above was subsequently tested. For this purpose, resistance spot welds were carried out, in which, in addition to the determination of the electrode level, the welding current range of two superimposed flat steel products was checked. For the electrode level, it was checked how many spot welds can be made with one electrode pair, without falling below the spot diameter of 3.6 mm. The diameter was examined after 100 points. The specifications regarding the electrode quantity depend on the manufacturer. However, at least 400 spot welds should be realized with one pair of electrodes can, since then a re-milling of the electrodes is necessary. In addition, a sufficiently large welding area in the automotive industry is of essential importance with regard to the weldability of the material. It is necessary that this area is at least 1 kA in size. The lower limit is given by the minimum lens diameter, the upper limit by spattering.

a) Experiment under laboratory conditions

An aqueous coating solution was prepared. To this was dissolved 92.5 g of ammonium sulfate in one liter of demineralized water. No special measures were taken to adjust the pH of the coating solution, but the native pH of the solution was about 5. In particular, there was no addition of bases or acids to adjust the pH.

The hot dip galvanized steel flat product samples were alkaline cleaned prior to application of the coating.

The treatment solution was uniformly distributed on the hot-dip galvanized sheet by means of a conventional coil coater.

Subsequently, the applied wet film is dried in a continuous oven at a drying temperature of 50-90 ° C.

By appropriate adjustment of the coil coater, the applied amount of aqueous coating solution was adjusted so that the dry layer weight of the (NH ₄ ) ₂ Zn (SO ₄ ) ₂ layer obtained on the samples corresponded to the specifications according to the invention.

The measurement of the layer coverage in mgS / m ^{2 was} carried out by means of mobile X-ray fluorescence analysis (RFA).

Before the stripping test, the test samples were coated with the Pre-Lube (Anticorit PL 3802-39S). The oil layer was 1.5 g / m ² .

In order to show the correlation between the applied dry layer weight in mgS / m ² and the resulting friction coefficients as a function of the surface pressure in MPa, various dry layer weights were applied by means of coater and tested by means of strip pulling test.

Table 3 summarizes the results of these experiments. It can be clearly seen that the forming capacity is significantly improved with increasing coating weight. Table 3: Occurrence of the stick-slip effect as a function of (NH <sub> 4 </ sub>) <sub> 2 </ sub> Zn (SO <sub> 4 </ sub>) <sub> 2 </ sub> dry weight (laboratory application) Coating weight [mgS / m ² ] Surface pressure up to which a stick-slip effect occurs [MPa] 0 2 10 35 20 > 100 30 > 100

b) Experiment under large-scale technical conditions (line application)

First, a coating solution having a concentration of 51 g / L ammonium sulfate in demineralized water was prepared. The unchanged native pH of the resulting aqueous solution was about 5.

The aqueous ammonium sulfate solution was applied by means of an application device arranged inline behind a conventional hot-dip galvanizing plant, in which the solution was sprayed onto the galvanized flat steel product and then squeezed off in a conventional manner to adjust the layer thickness.

The drying took place in a continuous furnace at 80 ° C.

The measurement of the layer coverage in mgS / m ^{2 was} carried out by means of mobile X-ray fluorescence analysis (RFA).

The resulting flat steel product samples coated in the manner according to the invention were oiled with an oiling amount of about 1 g / m ² with a thixotropic and barium-free corrosion protection oil sold under the trade name RP4107S by FUCHS Europe Schmierstoffe GmbH.

The friction-reducing effect of the ammonium zinc sulfate layer produced according to the invention was characterized by means of the strip-pulling experiment.

Table 4 shows the surface pressure achieved in MPa before the stick-slip effect occurred and the test had to be stopped. Again, there was a significant improvement in the tribological properties of ammonium zinc sulfate coated substrates. Table 4: Occurrence of the stick-slip effect as a function of (NH <sub> 4 </ sub>) <sub> 2 </ sub> Zn (SO <sub> 4 </ sub>) <sub> 2 </ sub> layer weight (line application) Coating weight [mgS / m ² ] Surface pressure up to which a stick-slip effect occurs [MPa] 0 7 20 65 25 > 100 36 > 100 70 > 100

In addition to the forming-promoting properties of the tribological coating, the subsequent process compatibility of such a coating plays an important role in the applicability of such coatings in the automotive sector.

Therefore, in the steel flat product samples obtained in the above-described manner, the angle peel test already explained in general above was carried out to check the adhesiveness with use of commercial structural adhesives.

In Fig. 4 the tensile shear strength and the peel resistance of an uncoated reference (Z) and of an ammonium zinc sulfate layer according to the invention having a coating weight of 20 mg S / m ² are shown.

The reduction in tensile shear strength and peel resistance compared to the uncoated reference is acceptable and, surprisingly, does not limit the use of this coating in the automotive body sector. Furthermore, the fracture pattern is close to the substrate cohesive.

Welding tests according to Stahl-Eisen-Prüfblatt 1220-2 showed that the resistance spot welding of coated sheets according to the invention was not impaired by the ammonium zinc sulfate layer applied according to the invention if the dry layer weight of 25 mg S / m ² , in particular 20 mg S / m ² , was not exceeded , At coating weights of more than 25 mg S / m ² , in the investigated flat steel product samples coated according to the invention, there were increased surface splashes which would make use in the automotive sector more difficult. When dry layer weights in the range of 10-15 mg S / m ^{2 were} observed, both the weld area Δl determined according to Steel Iron Test Sheet 1220-2 and the electrode stand amount proved to be sufficiently large. Table 5 summarizes the suitability of the coated steel flat product samples produced according to the invention for various subsequent processes.

A dry coating weight of, for example, 100 mgS / m ² would severely adversely affect resistance spot welding and subsequent sticking. However, the phosphating would not be a problem and continue to be unproblematic because shortly before the phosphating several rinsing and Purification cascades take place and the ammonium zinc sulfate layer according to the invention is thereby removed. Table 5: Suitability of the thin plates coated according to the invention for various automobility-typical subsequent processes Coating weight [mg S / m ² ] Fitness to Resistance spot welding stick phosphating 10 Very well Very well Very well 20 Very well Very well Very well 30 medium medium Very well 40 Bad Bad Well 80 Very bad Very bad Well 100 Very bad Very bad Well

In order to demonstrate the adhesive suitability of the ammonium zinc sulfate layer on a hot-dip galvanized steel sheet (99% by weight zinc, 1% by weight aluminum) compared to a zinc sulfate layer on a hot-dip galvanized steel sheet (99% by weight zinc, 1% by weight). Aluminum), an angle peeling test based on DIN EN 1464-2010 was carried out with the adhesive Elastosol M 105 (Bostik GmbH) at a test temperature of 130 ° C. The fracture surface and the fracture pattern were then visually evaluated according to the specification of EN ISO 10365: 1995. Here, the advantageous properties of the ammonium zinc sulfate coating in comparison to a zinc sulfate coating are clear: Table 6: Evaluation of the fracture surface according to the angle peel test based on DIN EN 1464-2010 at a test temperature of 130 ° C. Variant 1 is a hot-dip galvanized steel sheet (99% zinc, 1% aluminum) coated with ammonium zinc sulphate with a dry film thickness of 20 mg S / m <sup> 2 </ sup>. Variant 2 is a hot-dip galvanized steel sheet (99% zinc, 1% aluminum) coated with zinc sulphate with a dry-film coating of 20 mg S / m <sup> 2 </ sup> variant Delaminated area (visually assessed according to EN ISO 10365: 1995) 1 <1% 2 > 80%

Claims (12)

1. Process for producing a steel product having a protective coating based on zinc and a tribologically active layer applied to the protective coating, comprising the following steps:

   - providing a flat steel product provided with the protective coating,

   - applying an aqueous solution comprising ammonium sulfate and demineralized water to the protective coating of the steel product,

   - where the concentration of the ammonium sulfate based on the SO₄ ^2- ions is 0.01-5.7 mol/L,

   - where the pH of the aqueous solution is 4-6
   and

   - where the near-surface reaction time between the aqueous solution and the protective coating is more than 0 seconds and not more than 5 seconds,

   - such that, after a drying operation conducted without preceding rinsing, there is a tribologically active layer consisting of ammonium zinc sulfate atop the protective coating.
2. Process according to Claim 1, characterized in that the concentration of the ammonium sulfate in the aqueous solution is 0.1-3 mol/L.
3. Process according to Claim 2, characterized in that the concentration of the ammonium sulfate in the aqueous solution is 0.4-0.7 mol/L.
4. Process according to any of the preceding claims, characterized in that the pH of the aqueous solution is 4.2-5.7.
5. Process according to any of the preceding claims, characterized in that the steel product is a flat steel product.
6. Process according to any of the preceding claims, characterized in that the aqueous solution is applied to the protective coating by means of a chemcoater or coil coater.
7. Process according to any of the preceding claims, characterized in that the steel product, after the aqueous solution has been applied, is dried at a temperature of 70-90°C.
8. Process according to any of the preceding claims, characterized in that the surface of the steel product to be coated is subjected to alkaline cleaning prior to the coating operation.
9. Steel product comprising a steel substrate and a protective coating based on zinc borne by the steel substrate, wherein a tribologically active layer consisting of ammonium zinc sulfate has been formed atop the protective coating.
10. Steel product according to Claim 9, characterized in that the dry coat weight of the tribologically active layer is 1-100 mg/m² based on the sulfur content.
11. Steel product according to Claim 10, characterized in that the dry coat weight of the tribologically active layer is 10-50 mg/m² based on the sulfur content.
12. Steel product according to Claim 11, characterized in that the dry coat weight of the tribologically active layer is 10-20 mg/m² based on the sulfur content.

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