Method for improving adherence

Abstract

A method for improving the adhesive capacity of a protectively coated steel sheet is proposed, in which, in a continuous process, a protective coating based on ZnAlMg is applied to the steel sheet and, in a further step, the protective coating undergoes a surface treatment in which an aqueous composition is applied in order to modify the natural oxide layer, which contains Al.sub.2O.sub.3 and MgO, without pickling this natural oxide layer as a result. In order to significantly increase adhesive capacity of the protectively coated steel sheet, the invention proposes skin-pass rolling the protectively coated steel sheet and then reacting the natural oxide layer with an aqueous fluoride-containing composition, reducing its MgO content in order to thus modify the natural oxide layer.

Claims

1. A method for improving the adhesive capacity of an organic coating to a protectively coated steel sheet, using a continuous process, comprising: applying a protective coating based on ZnAlMg to the steel sheet and, in a further step, applying a surface treatment to the protective coating that includes applying an aqueous fluoride-containing composition to the protective coating in order to modify a natural oxide layer, which contains Al.sub.2O.sub.3 and MgO, without pickling this natural oxide layer as a result, and skin-pass rolling the protectively coated steel sheet and then reacting the natural oxide layer with the aqueous fluoride-containing composition, reducing its MgO content in order to thus modify the natural oxide layer.

2. The method according to claim 1, wherein the fluoride dissolves MgO out of the oxide layer and transfers the MgO into the aqueous composition; and in order to accomplish this, the quantity of fluoride in the aqueous composition is correspondingly set to dissolve Mg out of the oxide layer.

3. The method according to claim 1, wherein the aqueous composition comprises: TABLE-US-00002 20 to 3500 ppm F, optionally 0 to 35000 ppm Na, 0 to 4000 ppm Al, 0 to 4000 ppm Mn, 0 to 20 ppm P, 0 to 10 ppm Fe, 0 to 10 ppm Ni, and/or 0 to 10 ppm Si, and a remainder of H.sub.2O as well as inevitable impurities due to the, manufacturing process.

4. The method according to claim 1, wherein the aqueous composition contains a concentration of F of 5 to 3500 ppm.

5. The method according to claim 1, wherein the aqueous composition contains Al and/or Mn.

6. The method according to claim 5, wherein the aqueous composition contains a concentration of Al and/or Mn of 5 to 4000 ppm.

7. The method according to claim 1, comprising surface treating the protective coating with the aqueous composition for 0.5 to 20 seconds.

8. The method according to claim 1, wherein the aqueous composition has a pH value of 4 to 8.

9. The method according to claim 1, wherein the aqueous composition has a temperature of 30 to 95 C.

10. The method according to claim 1, comprising using NaF and/or NaHF.sub.2 when manufacturing the aqueous fluoride-containing composition.

11. The method according to claim 1, comprising using Na.sub.3[AlF.sub.6] when manufacturing the aqueous fluoride-containing composition.

12. The method according to claim 1, wherein the protective coating contains 0.1 to 7 wt % aluminum, 0.2 to 5 wt % magnesium, and a remainder of zinc as well as inevitable impurities due to die manufacturing process.

13. The method according to claim 12, wherein the protective coating contains 1 to 4 wt % aluminum and 1 to 3 wt % magnesium.

14. The method according to claim 1, wherein during the skin-pass rolling of the steel sheet, skin-pass rolling pressures are introduced into the protective coating.

15. The method according to claim 1, wherein immediately after the surface treatment with the fluoride-containing aqueous composition, the method further comprises rinsing the protective coating with a second liquid.

16. The method according to claim 15, wherein the second liquid contains up to 20 ppm P and/or Si as well as a remainder of H.sub.2O and inevitable impurities.

17. The method according to claim 15, wherein the second liquid has a temperature of 20 to 90 C.

18. The method according to claim 15, comprising rinsing the protective coating with the second liquid for 1 to 10 seconds.

19. The method according to claim 15, comprising applying the aqueous composition and/or the second liquid to the protectively coated steel sheet using a spraying, dipping, or rolling method.

20. The method according to claim 1, further comprising, after the surface treatment of the protectively coated steel sheet, providing an organic layer on the protective coating.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The subject of the invention is shown in greater detail in an embodiment variant by way of example. In the drawings:

(2) FIG. 1 shows a schematically depicted device for modifying the oxide layer of a steel sheet with a ZnAlMg protective coating; and

(3) FIGS. 2 & 3 show top views of the native oxide layers of two coated steel sheets.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) FIG. 1 shows an example of an apparatus I that makes it possible to carry out a continuous process for improving the adhesive capacity on a protectively coated steel sheet 2. In a continuous process, first a protective coating based on ZnAlMg is applied to a moving steel sheet with the aid of a hot dip process 3. A hot dip process 3 is particularly taken to mean continuous hot-dip galvanizing (strip galvanizing)but other coating processes are also conceivable. To illustrate the hot dip process 3, the depiction of the system parts of the apparatus 1 that relate to this has been restricted for the sake of clarity to a continuous furnace 18, a molten bath 3, a stripper 19 for adjusting the coating deposition, and a cooling unit 20. After the hot dip process 3 is carried out, the steel sheet 2 has a ZnAlMg protective coating, which forms a natural oxide layer 9. As is known, this native oxide layer 9 contains Al.sub.2O.sub.3 10, MgO 11, and alsoalbeit in a low quantityZnO 12. The percentage of MgO 11 in the oxide layer 9 is comparatively high, as can be seen in FIG. 2,

(5) According to FIG. 2, MgO 11 is visible as the light areas, Al.sub.2O.sub.3 10 is visible as the dark areas, and ZnO 12 is visible as a mixture of light and dark areas. In a predominantly light MgO area on the surface of the ZnAlMg protective coating, a significantly reduced adhesive capacity can be expected.

(6) According to the invention, such predominantly MgO accumulations in the oxide layer 9 are avoided in that the steel sheet 2 that is provided with a ZnAlMg protective coating is conveyed through a skin-pass rolling unit 5 and is thus prepared for the modification of its native oxide layer 9i.e. prepared for a surface treatment 6 through the application of an aqueous fluoride-containing composition 7 in order to reduce the MgO percentage of the natural oxide layer 9, without pickling it. According to FIG. 1, this process step is implemented by means of spray bars 8 situated on both sides of the steel sheet 2, which apply or spray the aqueous fluoride-containing composition 7 onto the steel sheet 2. Instead of the spraying process 13, it is naturally also conceivable to carry out an application with a rolling or dipping process that is now shown in greater detail.

(7) The aqueous composition subsequently dissolves MgO 11 out of the oxide layer 9 and conveys it into the aqueous composition 7. For this purpose, the quantity of fluoride in the aqueous composition 7, as measured by a fluoride-sensitive electrode, is adjusted in order to dissolve Mg out from the oxide layer 9. The percentage of MgO 11 in the native oxide layer 9 is thus reduced so that due to the high oxygen affinity of Al, more Al.sub.2O.sub.3 10 can develop on the modified natural or native oxide layer 9.

(8) This situation is clearly visible in FIG. 3. FIG. 3 does in fact also show MgO 11 in light areas, but the percentage of MgO 11 is extremely low in comparison to FIG. 2. As a result, Al.sub.2O.sub.3 10 (dark area) and ZnO 12 or Zn.sub.5(OH).sub.6(CO.sub.3).sub.2 (mixture of light and dark area) clearly predominate. The modified natural oxide layer 9 according to FIG. 3 essentially contains Al.sub.2O.sub.3 10 and thus constitutes a barrier layer, which not only reduces a breakthrough of Mg into the oxide layer 9 to form MgO 11, but also reduces 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 demonstrates a comparatively high adhesive capacity.

(9) In order to increase the reaction speed, the pH value is set within a range from 4 to 8, particularly in the weakly acidic range; the aqueous composition should also have a temperature of 30 to 95 C. (degrees Celsius).

(10) It was possible to establish particularly advantageous process conditions in the targeted attack on the MgO in the oxide layer if the aqueous composition contained 20 to 3500 ppm F (fluoride), optionally 0 to 35000 ppm Na (sodium), 0 to 4000 ppm Al (aluminum), 0 to 4000 ppm Mn (manganese), 0 to 20 ppm P (phosphorus), 0 to 10 ppm Fe (iron), 0 to 10 ppm Ni (nickel), and/or 0 to 10 ppm Si (silicon), and a remainder of H.sub.2O (water) as well as inevitable impurities due to the manufacturing process. But even a concentration of F of 5 to 3500 ppm, 5 to 1500 ppm. 5 to 1500 ppm, 10 to 500 ppm, 20 to 150 ppm, 30 to 1500 ppm, or 30 to 300 ppm can be sufficient.

(11) In addition, even a presence of Al and/or Mn in the aqueous composition can turn out to be helpful for the method. In general, it should be noted that Al in the aqueous composition can improve the oxide layer by shifting it in the direction of elevated percentages of Al.sub.2O.sub.3 and reduced percentages of MgO. Specifically, Al in the aqueous composition 7 preferably settles in the reduced-Mg locations of the oxide layer. Such locations can be produced during treatment of the oxide layer with the aqueous composition 7, for example through the dissolution of MgO out of the oxide layer through conversion to MgOHF. A similar effect can also be achieved with Mn. It should also be noted that it can generally be conceivable for the protectively coated steel sheet 2 to be reacted with the aqueous fluoride-containing composition 7 in a way that reduces its MgO percentage, in that Mg and/or a magnesium compound (e.g. MgO 11) is dissolved out from the oxide layer 9 by means of fluoride and/or a fluoride compound (e.g. HF) and is replaced by Al and/or Mn in order to thus modify the natural oxide layer toward a reduced percentage of MgO.

(12) The fluoride-containing aqueous composition 7, which has been applied to the steel sheet 2 by means of the spray bars 8, is removed from the steel sheet 2 with the aid of a rinsing unit that carries out a spraying process 14. To this end, immediately after the treatment by means of spray bars 17, the protective coating is surface treated with a second liquid 15. This second liquid 15 is composed of H.sub.2O, but can also contain less than 20 mg/l of P or Si as well as inevitable impurities; the P may possibly be present in the form of phosphate in the liquid 15. A treatment duration of 1 to 10 seconds has been determined to be sufficient.

(13) In addition, skin-pass indentations 16 that are produced by the skin-pass rolling unit 5 are present in the ZnAlMg protective coating. According to FIGS. 2 and 3, the edges of the skin-pass indentations 16 are particularly evident in the form of closed contours. By contrast with FIG. 2, at the edge of the skin-pass indentation 16 according to FIG. 3, MgF.sub.2 compounds can be detected, which are produced by means of the steel sheet 2 surface treatment according to the invention and increase the bonding of organic layers.

(14) Six steel sheets were tested in order to prove the increased adhesive capacity according to the invention.

(15) TABLE-US-00001 TABLE 1 Comparison of the steel sheets tested Tensile shear Steel strength sheet Coating [MPa] Fracture pattern A.sub.1 DX53D ZnAl2.5Mg1.5 20.5 100% SCF A.sub.2 DX53D ZnAl2.5Mg1.5 20.4 100% SCF B DX53D ZnAl2.5Mg1.5 19.6 20% SCF and 80% AF C.sub.1 DX56D ZnAl2.4Mg2.2 19.8 100% SCF C.sub.2 DX56D ZnAl2.4Mg2.2 19.6 100% SCF D DX56D ZnAl2.4Mg2.2 18.1 20% SCF and 80% AF

(16) The hot-dip galvanized steel sheets A (A.sub.1 & A.sub.2) and B have a deep-drawing grade of DX53D and a sheet thickness of 0.75 may ZnAl2.5Mg1.5 (96 wt %t Zn, 2.5 wt % Al, and 1.5 wt % Mg) was applied as a protective coating.

(17) The hot-dip galvanized steel sheets C (C.sub.1 & C.sub.2) and D have a deep-drawing grade of DX56D and a sheet thickness of 0.7 mm, ZnAl2.4Mg2.2 (95.4 wt % Zn, 2.4 wt % Al, and 2.2 wt % Mg) was applied as a protective coating.

(18) The steel sheets A (A.sub.1 & A.sub.2) and C (C.sub.1 & C.sub.2) underwent the modification of their oxide layers according to the invention, as shown in FIG. 1. This included a skin-pass rolling of the steel sheets A and B and an application of an aqueous composition 7 with a concentration of fluoride of 30 to 70 ppm by weight; the temperature of the aqueous composition 7 was set to approximately 70 degrees Celsius and its pH value was set in the range between 5 and 7.5. Fluoride in the form of Na.sub.3[AlF.sub.6] was added to the aqueous composition 7 for treating steel sheets A.sub.1 and C.sub.1. Consequently, this aqueous composition 7 is composed of fluoride, Na, Al, H.sub.2O), and inevitable impurities of less than 10 ppm. NaF was added to the aqueous composition for treating steel sheets A.sub.2 and C.sub.2. If need be, this composition can be enriched with Al. Instead of or alternatively to NaF, it is also conceivable to use NaHF.sub.2 (bifluoride). The steel sheets A (A.sub.1 & A.sub.2) and C (.sub.1 & C.sub.2) were treated with the respective aqueous composition for 10 seconds. Then the steel sheets A and C were rinsed with H.sub.2O for 10 seconds. This second liquid 15 was set to a temperature of 35 degrees Celsius.

(19) The steel sheets B and D, however, did not undergo any surface treatment and thus essentially had an oxide layer as shown in FIG. 2.

(20) All of the steel sheets A, B, C, and D were then provided with an organic coating, namely a single-component epoxy resin glue (e.g.: BM1496) and the adhesive capacity of the glue to the protective coating was determined by means of a tensile shear test.

(21) Tests on the protectively coated steel sheets A, B, C, and D showed that only in the steel sheets A (A.sub.1 & A.sub.2) and C (C.sub.1 & C.sub.2) is it possible to avoid a fracture at the boundary surface between the oxide layer and the glue. This fracture is almost 100% SCF (substrate close cohesive failure), which corresponds to the fracture scenario required in the automotive sector. in the steel sheets B and D, as is to be expected, a mixed fracturing composed of 80% AF (adhesive failure) and 20% SCF occurs, making these protectively coated steel sheets B and D unsuitable for the automotive sector. The method according to the invention can also clearly be recognized in the steel sheets A and C by an improved bonding of the glue to the protective coating, as evidenced by an increased tensile shear strength.

(22) It is therefore clear that the method according to the invention is able to modify the oxide layer of the ZnAlMg protective coating in a way that significantly improves the adhesive capacity for a glue on the protectively coated steel sheet A and C as compared to a steel sheet B and D according to the prior art.