EP2824213A1 - Method for improving adherence to a steel sheet with a protective coating - Google Patents

Abstract

There is shown a method of improving adhesiveness on a protective coated steel sheet (2) by applying a Zn-Al-Mg based protective coating on the steel sheet (2) in a continuous process and subjecting it to surface treatment (6) in a further step in which, with the application of an aqueous composition (7), the natural oxide layer (9) having Al 2 O 3 and MgO, without thereby deboning, is modified. In order to markedly increase the adhesiveness to the protective-coated steel sheet, it is proposed that the protective-coated steel sheet (2) be tempered, and then the natural oxide layer (9) be reacted with an aqueous fluoride-containing composition (7) to reduce its MgO content to thereby form the natural oxide layer (9) to modify.

Description

The invention relates to a method for improving the adhesion on a protective coated steel sheet, in which a Zn-Al-Mg-based protective coating is applied to the steel sheet in a continuous process and subjected to a surface treatment in a further step, wherein the application of an aqueous composition the natural, Al2O3 and MgO oxide layer is modified without depleting it.

Methods for passivation of protective coated steel sheets are well known. For example, chromating or phosphating ( EP2092090B1 ). However, all these methods have in common to remove the natural or native oxide layer and to replace it with another passivation layer. Among other things, such passivation layers can also contribute to improving the adhesion with an organic coating, for example paints. A disadvantage of a subsequent processing of the protective-coated steel sheet, a partial removal of the passivation layer can not be avoided. In addition to an increased need for cleaning in subsequent processing zones, this can also lead to changed process parameters, which can impair the reproducibility of the subsequent processing.

Alternatively, the suggests WO2006045570A1 To increase this adhesion to the protective coated steel strip by modifying the natural oxide layer, without depleting this natural oxide layer. Thus, in the continuous process of the protective coating of the steel strip, a cooling of the steel sheet with an aqueous composition or a cooling medium which is to improve the natural oxide layer of the protective coating, for example comprising Zn, Mg and Al. Soluble salts for protecting the natural oxide layer or phosphates for stabilizing the sheet surface may be added to the aqueous composition. However, such a process can not lead to a marked increase in adhesiveness.

The object of the invention is therefore to provide, starting from the initially described prior art, a method with which the surface of the protective coating can be modified with as little effort as possible in such a way that the adhesion to the protective-coated steel sheet is significantly increased.

The invention solves this problem by the fact that the protective coated steel sheet trained and then reacts the natural oxide layer with an aqueous fluoride-containing composition with reduction of their MgO content in order to modify the natural oxide layer.

If the protective-coated steel sheet is dressed and the natural oxide layer then reacts with an aqueous fluoride-containing composition, it has surprisingly been found that the proportion of MgO in the natural oxide layer of the protective coating can be reduced in a gentle manner. This modification of the oxide layer can result in a considerable increase in the adhesive strength, in particular with regard to the ready-to-wear and / or recoatability of a protective-coated steel sheet. For example, thus alos the connection of an adhesive can be improved, so as to preclude an adhesive failure at splices. But in particular, the invention can stand out from the prior art in that this improved adhesion can be achieved without picking the oxide layer. Namely, the oxide film can be activated by the skin-pass coating of the present invention for fluoride-responsive depletion of MgO. Al, which has a relatively high oxygen affinity, can therefore form primarily in the oxide layer or the due to the MgO reduction, vacancies occupy the oxide layer. The latter can in particular contribute to the fact that a diffusion of magnesium into the oxide layer or a magnesium breakthrough can decrease. The oxide layer naturally forming on a Zn-Al-Mg protective coating can thus be shifted in terms of process engineering in an easy to handle manner in the direction of increased proportions of Al 2 O 3 and / or ZnO and reduced amounts of MgO. According to the invention, a particularly well reproducible method is thus created.

In general it is mentioned that the unit of measurement is ppm ppm by weight. In addition, it is generally stated that the invention may be particularly suitable for improving the adhesion of an organic coating to the protective coated steel sheet.

Easily controllable process conditions can be created when the fluoride MgO of the oxide layer is dissolved out and transferred to the aqueous composition. In addition, the growth of a passivation layer, in particular of MgF 2, can thus be retained, as a result of which the natural character of the oxide layer can be obtained.

By adjusting the amount of fluoride in the aqueous composition to extract Mg from the oxide layer, an easy-to-handle procedure for reproducibly modifying the oxide layer can be proposed.

For particularly advantageous process conditions in the targeted attack on the MgO of the oxide layer, the aqueous composition 20 to 3500 ppm F, optionally 0 to 3500 ppm Na, 0 to 20 ppm P, 0 to 10 ppm Fe, 0 to 10 ppm Ni, 0 to 10 ppm Mn and / or 0 to 10 ppm Si and the remainder H2O and production-related unavoidable impurities, for which purpose the aqueous composition may have a pH of 4 to 8 and a temperature of 30 to 95 ° C (degrees Celsius). In addition, it may be sufficient with the adjustment of the pH From 5 to 7.5, the reaction rate of the aqueous composition with the Zn-Al-Mg protective coating can be adjusted relatively easily to a belt running speed of the continuous process. In addition, with an acidic adjustment of the pH, an increased reduction of the MgO content in the oxide layer can be ascertained. A temperature of the aqueous composition of 45 to 90 ° C may be sufficient to further increase their reaction rate with the native oxide layer. Further, Fe, Ni, Mn, P and / or Si may be useful for initiating MgO reduction or stabilizing the modified oxide layer. The unavoidable impurities may be less than 50 ppm in total.

Advantageously, a concentration of fluoride from 20 to 3500 ppm in the aqueous composition can be found to be directed to MgO of the oxide layer or Mg leaching. However, a concentration of fluoride of from 30 to 1500 ppm, preferably from 30 to 300 ppm, may already be sufficient.

For a sufficient reduction in MgO, the protective coating can be surface-treated with the aqueous composition for 0.5 to 20 seconds (seconds), in particular 1.5 to 15 seconds (seconds). In addition, such a short treatment can be particularly well suited for a continuous process.

Generally, it is mentioned that, depending on the level of fluoride in the aqueous composition, the duration of treatment may be shorter. For example, at 1500ppm fluoride with a treatment time of 1.5 seconds, the expense can be found, while with 20ppm fluoride a 20 second treatment time should be sought to reduce the MgO content of the natural oxide layer without depleting it.

The preparation of the aqueous composition can be simplified by using NaF.

The process according to the invention can be distinguished, in particular, by a protective coating which comprises 0.1 to 7% by weight of aluminum, 0.2 to 5% by weight of magnesium and the balance zinc and unavoidable impurities due to the production. Such Zn-Al-Mg protective coatings can be particularly well reduced an oxide layer with respect to unmodified oxide layers of the same alloy composition in their MgO content, which can be used for a significant increase in the adhesive strength.

Preferably, the protective coating specified above may contain 1 to 4% by weight of aluminum and 1 to 3% by weight of magnesium, in order to increase not only an improvement in the adhesive strength but also the reproducibility of the process.

The activation of the oxide layer for a subsequent surface treatment can be improved if, when the steel sheet is applied by casting, dressing impressions are introduced into the protective coating. In addition, these dressing impressions, preferably in their edge regions, form an improved attack surface for fluoride in order to increasingly dissolve MgO from the natural oxide layer. Furthermore, the formation of magnesium fluoride (MgF 2) could be observed here or in this peripheral region, which can further improve the adhesion. In addition, after the surface treatment according to the invention in the field of dressing impressions, more Zn5 (OH) 6 (CO3) 2 (zinc hydroxide carbonate) can be found instead of ZnO, which can additionally improve the adhesion.

The fluoride-containing aqueous composition can be easily removed from the surface of the protective coating if the protective coating is rinsed with another liquid immediately after the surface treatment with the first fluoride-containing aqueous composition. In addition, this aftertreatment with such a liquid can additionally increase the removal of MgO, with it being possible in particular for H2O to be distinguished as the liquid.

If the second liquid has up to 20 ppm of P and / or Si, as well as the remainder of H 2 O and unavoidable impurities, then the native oxide layer reduced in MgO can be further stabilized. With P it is to be expected that this occurs as phosphate in the liquid.

The rinsing action of the second liquid can be significantly improved if the liquid has a temperature of 20 to 90 ° C, in particular from 35 to 85 ° C.

As sufficient rinsing time may turn out when the protective coating is rinsed with the second liquid for 1 to 10 sec. Long.

Simple process conditions can occur when the aqueous composition and / or the liquid is applied to the protective-coated steel sheet in a spraying, dipping or rolling process.

In particular, the invention may be distinguished from the known one if an aqueous fluoride-containing composition is used to reduce the MgO content of the natural oxide layer of a Zn-Al-Mg protective coating on a dressed steel sheet, without thereby dekapieren the natural oxide layer.

Fig. 1

eine schematisch dargestellte Vorrichtung zur Modifizierung der Oxidschicht eines Stahlblechs mit Zn-Al-Mg Schutzbeschichtung und

Fig. 2 und 3

Draufsichten auf die nativen Oxidschichten zweier schutzbeschichteter Stahlbleche.

In the figure, for example, the subject invention is illustrated in more detail with reference to a variant embodiment. Show it

Fig. 1

a schematically illustrated apparatus for modifying the oxide layer of a steel sheet with Zn-Al-Mg protective coating and

FIGS. 2 and 3

Top views of the native oxide layers of two protective coated steel sheets.

To Fig. 1 For example, an apparatus 1 is shown, with which a continuous process for improving the adhesion on a protective-coated Steel sheet 2 is made possible. Thus, in a continuous process, a Zn-Al-Mg-based protective coating is first applied to a moving steel sheet 2 by means of a hot-dip process 3. Hot-dip galvanizing (strip galvanizing), in particular, is envisaged as hot-dip process 3, but other coating methods are conceivable. To illustrate the hot dip method 3, the representation of the relevant plant parts of the device 1 for clarity was limited to a continuous furnace 18, a molten bath 3, a scraper 19 for adjusting the coating layer and a cooling 20. After carrying out the hot-dip process 3, the steel sheet 2 has a Zn-Al-Mg protective coating which forms a natural oxide layer 9. This native oxide layer 9 is known to comprise Al 2 O 3 10, MgO 11, and also, albeit to a minor extent, ZnO 12. The proportion of MgO 11 in the oxide layer 9 is comparatively high, as after Fig. 2 can be recognized.

According to Fig. 2 MgO 11 is seen on the bright surface, Al 2 O 3 10 on the dark surface and ZnO 12 on a mixture of light and dark surfaces. Due to a predominantly bright MgO surface on the surface of the Zn-Al-Mg protective coating, a considerable reduced adhesion is to be expected.

According to the invention, such dominant MgO accumulations in the oxide layer 9 are avoided by passing the steel sheet 2 provided with a Zn-Al-Mg protective coating through a skin pass mill 5 and thus preparing it to modify its native oxide layer 9 - prepared for surface treatment 6 Application of an aqueous fluoride-containing composition 7 in order to reduce their MgO content without picking the natural oxide layer 9. According to Fig. 1 this process step is realized with spray bars 8 arranged on both sides of the steel sheet 2, which apply or spray on the aqueous fluoride-containing composition 7 onto the steel sheet 2. Instead of the spraying method 13, of course, an application with a rolling or dipping method not shown is conceivable.

The fluoride of the aqueous composition subsequently releases MgO 11 from the oxide layer 9 and converts it into the aqueous composition 7. For this purpose, the amount of fluoride, measured with a fluoride-sensitive electrode, in the aqueous composition 7 is adjusted to a dissolution of Mg of the oxide layer 9. The proportion of MgO 11 in the native oxide layer 9 is thus reduced, so that due to the high oxygen affinity of Al increased Al2O3 10 can be expressed on the modified native or native oxide layer 9.

This circumstance is clearly behind Fig. 3 to recognize. Fig. 3 Although also shows MgO 11 on light surfaces, compared to Fig. 2 however, the MgO 11 content is extremely low. For example, Al2O3 10 (dark area) and ZnO 12 or Zn5 (OH) 6 (CO3) 2 (mixture of light and dark areas) clearly outweigh each other. The modified natural oxide layer 9 after Fig. 3 essentially comprises Al 2 O 3 10, thus forming a barrier layer which not only reduces the breakdown of Mg into the oxide layer 9 to form MgO 11, but also the diffusion of O through the oxide layer. Even with comparatively long storage times of the steel sheet 2, this modified natural oxide layer 9 still exhibits a comparatively high adhesiveness.

To increase the reaction rate, the pH may be adjusted to slightly acid in a range of 4 to 8.

Particularly advantageous process conditions in the targeted attack on the MgO of the oxide layer could be ascertained if the aqueous composition contains 20 to 3500 ppm F, optionally 0 to 3500 ppm Na, 0 to 20 ppm P, 0 to 10 ppm Fe, 0 to 10 ppm Ni, 0 to 10 ppm Mn and / or 0 to 10 ppm Si and the remainder H2O and production-related unavoidable impurities, for which purpose the aqueous composition has a pH of 4 to 8 and a temperature of 30 to 95 ° C (degrees Celsius) , However, one has already been sufficient Concentration of fluoride from 30 to 1500 ppm, preferably from 30 to 300 ppm shown.

The fluoride-containing aqueous composition 7, which has been applied to the steel strip 2 via the spray bars 8, is removed from the steel strip 2 by means of a sink, which performs a spraying process 14. For this purpose, the protective coating is surface treated immediately after treatment via spray bar 17 with a second liquid 15. This second liquid 15 consists of H 2 O, but may also have P or Si less than 20 mg / l and unavoidable impurities, P being optionally present as the phosphate in the liquid 15. A treatment time of 1 to 10 seconds has been found to be sufficient.

In addition, in the Zn-Al-Mg protective coating Dressing Impressions 16 are present, which are introduced from the skin pass 5. After the FIGS. 2 and 3 the edges of the Dressiereindrücke 16 emerge as a closed contour particularly. In contrast to Fig. 2 are at the edge of the dressing impression 16 after Fig. 3 To detect MgF2 compounds that result from the surface treatment of the steel sheet 2 according to the invention and increase the bonding of organic layers.

To demonstrate the inventively increased adhesion four steel sheets were examined. Table 1: the examined steel sheets in comparison sheet steel coating Tensile shear strength [MPa] fracture pattern A DX53D ZnAl2,5Mg1,5 20.4 100% SCF B DX53D ZnAl2,5Mg1,5 19.6 20% SCF and 80% AF C DX56D ZnAl2,4Mg2,2 19.6 100% SCF D DX56D ZnAl2,4Mg2,2 18.1 20% SCF and 80% AF

The two hot-dip galvanized steel sheets A and B have a deep-drawing quality DX53D and a sheet thickness of 0.75 mm. As a protective coating, ZnAl2.5Mg1.5 (96 wt% Zn, 2.5 wt% Al, 1.5 wt% Mg) was applied.

Both hot-dip galvanized steel sheets C and D have a deep-drawing quality DX56D and a sheet thickness of 0.7 mm. As a protective coating, ZnAl2.4Mg2.2 (95.4 wt% Zn, 2.4 wt% Al, 2.2 wt% Mg) was applied.

The steel sheets A and C were as in Fig. 1 represented the modification according to the invention subjected to their oxide layers. This involved coating the steel sheets A and B and applying an aqueous composition 7 having a fluoride concentration of 30 to 70 ppm by weight, the temperature of the aqueous composition 7 being set at 70 degrees Celsius. This aqueous composition 7 consists of fluoride, H 2 O and unavoidable impurities of less than 10 ppm. Fluoride was added to the aqueous composition 7 with the aid of NaF. The steel sheets A and C were treated with the aqueous composition 7 for 20 seconds. Subsequently, the steel sheets A and C were rinsed with H2O for 10 seconds. In this liquid 15, a temperature of 35 degrees Celsius was set.

On the other hand, the steel sheets B and D were not subjected to any surface treatment, and thus substantially deteriorated Fig. 2 shown oxide layer.

All the steel sheets A, B, C and D were then provided with an organic coating, namely a one-part epoxy resin adhesive (e.g., BM1496), and the adhesive strength of the adhesive on the protective coating was determined by a tensile shear test.

Investigations on the protective coated steel sheets A, B, C and D showed that only at the steel sheets A and C a break at the interface between oxide layer and adhesive can be avoided. This fraction is almost 100% SCF ("substrate close cohesive failure"), which corresponds to the fracture scenario required in the automotive industry. As expected, the steel sheets B and D show a mixed fracture consisting of 80% AF ("adhesive failure") and 20% SCF, making these protective coated steel sheets B and D unsuitable for the automotive sector. In addition, it can be seen by the inventive method on the steel sheets A and C with an improved connection, evidenced by an increased Zuscherfestigkeit, the adhesive to the protective coating.

It is thus shown that the treatment method according to the invention can modify the oxide layer of the Zn-Al-Mg protective coating in such a way that the adhesiveness to an adhesive on the protective-coated steel sheet A or C is significantly improved compared to a prior art steel sheet B or D.

Claims (16)

Process for improving the adhesion to a protective coated steel sheet (2), wherein a Zn-Al-Mg based protective coating is applied to the steel sheet (2) in a continuous process and subjected to a surface treatment (6) in a further step, wherein with the application of an aqueous composition (7), the natural, Al2O3 and MgO having oxide layer (9), without thereby decapieren, is modified, characterized in that the protective coated steel sheet (2) trained and then the natural oxide layer (9) with a aqueous fluoride-containing composition (7) reacts with reduction of their MgO content, thereby to modify the natural oxide layer (9). A method according to claim 1, characterized in that the fluoride MgO (11) of the oxide layer (9) dissolves and transferred into the aqueous composition (7). A method according to claim 2, characterized in that the amount of fluoride in the aqueous composition (7) is adjusted to the dissolution of Mg (11) from the oxide layer (9). Method according to one of claims 1 to 3, characterized in that the aqueous composition (7) 20 to 3500 ppm F, optional 0 to 3500 ppm N / A, 0 to 20 ppm P, 0 to 10 ppm Fe, 0 to 10 ppm Ni, 0 to 10 ppm Mn and / or 0 to 10 ppm Si and as the remainder H2O and production-related unavoidable impurities, wherein the aqueous composition has a pH of 4 to 8, in particular a pH of 5 to 7.5, and a temperature of 30 to 95 ° C, in particular a temperature of 45 to 90 ° C, has. Method according to one of claims 1 to 4, characterized in that in the aqueous composition (7) a concentration of fluoride of 20 to 3500 ppm, in particular from 30 to 1500 ppm, preferably from 30 to 300 ppm is present. Method according to one of claims 1 to 5, characterized in that the protective coating with the aqueous composition (7) 0.5 to 20 sec., In particular 1.5 to 15 sec., Surface treatment. Method according to one of claims 1 to 6, characterized in that for the preparation of the aqueous composition (7) NaF is used. Method according to one of claims 1 to 7, characterized in that the protective coating comprises 0.1 to 7 wt .-% aluminum, 0.2 to 5 wt .-% magnesium and the remainder zinc and production-related unavoidable impurities. A method according to claim 8, characterized in that the protective coating comprises 1 to 4 wt .-% aluminum and 1 to 3 wt .-% magnesium. Method according to one of claims 1 to 9, characterized in that during the tempering of the steel sheet (2), skin-pressure impressions (16) are introduced into the protective coating. Method according to one of claims 1 to 10, characterized in that the protective coating immediately after the surface treatment with the first, fluoride-containing aqueous composition (7) with a further liquid (15), in particular with H2O, rinsed. A method according to claim 11, characterized in that the liquid (15) to 20 ppm P and / or Si, as well as the remainder H2O and unavoidable impurities. A method according to claim 11 or 12, characterized in that the liquid (15) has a temperature of 20 to 90 ° C, in particular from 35 to 85 ° C. A method according to claim 11, 12 or 13, characterized in that the protective coating is rinsed with the liquid (15) for 1 to 10 seconds. Method according to one of claims 1 to 14, characterized in that the aqueous composition (7) and / or the liquid (15) by spray, dip or rolling method (13, 14) on the protective coated steel sheet (2) is applied / become. Use of an aqueous fluoride-containing composition (7) for reducing the MgO content of the natural oxide layer (9) of a Zn-Al-Mg protective coating on a dressed steel sheet (2), without depleting the natural oxide layer (9).

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