EP0401727A2 - Method for improving the quality of the surface of materials containing iron with a coating of aluminium-zinc alloys - Google Patents

Abstract

The invention relates to a process for improving the surface characteristics of steel sheets which have a corrosion coating of an aluminium/zinc alloy. Experience has shown that the surface of these sheets is nonuniform, so that subsequent even coating is not directly possible. However, in order to render the surface sufficiently uniform to permit coating to be carried out, zinc dust is blown into the coating before it has solidified, the zinc dust suppressing the formation of a structure. <IMAGE>

Description

The invention relates to a method for improving the surface quality of steel sheets and strips, which are coated with a corrosion protection. This corrosion protection is achieved by a coating with an aluminum-zinc alloy, which provides the continuous strip with an intermetallic layer in an immersion bath.

Adhesive and corrosion-resistant aluminum-zinc coatings on ferrous materials have become known from DE-AS 15 21 148.

US Pat. No. 3,148,081 describes a method for improving the coated steel strips on their surface. Without changing the chemical composition, the zinc flower formation is to be achieved by sudden solidification of the molten coating, the surface then being further treated. However, the melt is a zinc alloy with the addition of lead, cadmium and aluminum.

Coatings that contain a high proportion of zinc as an alloy component usually produce large-flowered surface structures, the so-called zinc flowers. These structures can be so pronounced that the possible uses, in particular in the case of sheets and strips, are restricted. There is always a restriction if the tapes are subsequently to be coated or varnished. In this case there is a non-uniform surface with the structures of the zinc flowers shining through.

In order to standardize the surface of the zinc coating, there is the possibility of redressing the sheets and strips. This is mainly done in line and with smooth or matt rollers. As a result of this rolling process, the surface is largely smoothed and the coating is compressed and thus solidified.

Nevertheless, this appearance does not always satisfy the user. Even small-flowered flat structures can still interfere with the end product.

An approximately uniform surface is obtained in particular by spraying finely divided particles of zinc against the zinc coating before its crystallization has taken place.

A method for spraying the surface has become known from DE-PS 28 12 370. Powdered zinc is generally used in this process. The zinc particles come into contact with the liquid zinc coating of the sheet by spraying and form an intimate connection. Numerous crystalline nuclei are generated. These germs determine and favor the formation and growth of crystals in large numbers. Because there are many germs, the crystals can only take a very small form.

However, this has the consequence that no large zinc flowers can arise. At the same time, these small zinc flowers solidify the surface of the zinc coating.

EP-PS 0122 856 has found a process for improving the surface of coated steel strips by using an oxygen-free gas to transport the substance to be sprayed on. Since the substances to be sprayed on generally have a great affinity for oxygen, this is intended to prevent oxidation because the oxides formed act as defects on the strip material.

In addition, AT-E 30 602 B describes a process for optimizing the solidification of zinc on a steel strip. The subject of this application is a temperature control in order to spray on the zinc dust just before the solidification point of the zinc coating. However, this is very difficult and very difficult to carry out in a large process with many influencing parameters. In this case too, the steel strips are galvanized.

Another way of preventing the formation of large-area zinc flowers is to produce the above-described microflowers by spraying water or water vapor onto the as yet unconsolidated zinc surface with simultaneous strong cooling.

In the case of steel sheets and steel strips which have an aluminum-zinc coating, the formation of these "zinc flowers" is very pronounced. Correctly, these coatings, which have a higher aluminum content than zinc, can no longer be called a zinc flower. However, since the present invention relates exclusively to aluminum-zinc coatings bearing the trade name Galvalum, surface structures will be referred to below.

The object of the invention is to improve the surface structures of aluminum-zinc coatings on steel sheets or strips and to standardize them so that the sheets and strips have a uniform surface during a subsequent surface coating. This would represent a decisive improvement in quality that could not be achieved with previously known processes, even with dressing.

The object is achieved in that zinc dust is blown onto the strip coming from the melt, onto the surface of the aluminum-zinc coating which has not yet solidified. This zinc dust penetrates the surface and forms a connection with the coating, which prevents the formation of structures. After the zinc dust has been inflated, the coated strip must be cooled at a high cooling rate of at least 25 ° C / s. At the same time, the required properties of the tape are retained with this treatment.

In the following, the invention will be explained in more detail with reference to the illustrations shown.

• Fig. 1 Schematische Bandverzinkungsanlage
• Fig. 2 Oberfläche eines Aluminium-Zink-Überzuges, nachdressiert
• Fig. 3 Wie Fig. 2, jedoch 100fach vergrößert, nachdressiert
• Fig. 4 Oberfläche eines Aluminium-Zink-Überzuges mit anschlie­ßender Zinkstaubaufstäubung, nachdressiert
• Fig. 5 Wie Fig. 4, jedoch 100fach vergrößert, nachdressiert
• Fig. 6 normale Struktur eines Aluminium-Zink-Überzuges als Ober­flächenschliff 50fach vergrößert
• Fig. 7 a Wie Fig. 6, jedoch als Kantenschliff in 500facher Vergrö­ßerung
• Fig. 7 b Wie Fig. 7 a mit unterschiedlicher Ätzung
• Fig. 7 c Wie Fig. 7 a, jedoch auf Mikrostruktur geätzt
• Fig. 8 Aluminium-Zink-Überzug mit relativ geringer Zinkstaubauf­stäubung, als Oberflächenschliff, Sofach vergrößert
• Fig. 8 a Wie Fig. 8, jedoch als Kantenschliff in 500facher Vergrö­ßerung
• Fig. 8 b Wie Fig. 8 a mit unterschiedlicher Ätzung
• Fig. 8 c Wie Fig. 8 a, jedoch auf Mikrostruktur geätzt
• Fig. 9 Aluminium-Zink-Überzug mit relativ viel aufgestäubtem Zinkstaub, als Oberflächenschliff, 50fach vergroßert
• Fig. 9 a Wie Fig. 9, jedoch als Kantenschliff in 500facher Vergrö­ßerung
• Fig. 9 b Wie Fig. 9 a mit unterschiedlicher Ätzung
• Fig. 9 c Wie Fig. 9 a, jedoch auf Mikrostruktur geätzt

Show it:

• Fig. 1 Schematic strip galvanizing plant
• Fig. 2 surface of an aluminum-zinc coating, redressed
• Fig. 3 Like Fig. 2, but enlarged 100 times, redressed
• Fig. 4 surface of an aluminum-zinc coating with subsequent zinc dusting, redressed
• Fig. 5 Like Fig. 4, but enlarged 100 times, redressed
• Fig. 6 normal structure of an aluminum-zinc coating as a surface grinding magnified 50 times
• Fig. 7 a As Fig. 6, but as an edge grinding in 500 times magnification
• Fig. 7 b as Fig. 7 a with different etching
• Fig. 7 c As Fig. 7 a, but etched on microstructure
• Fig. 8 aluminum-zinc coating with relatively low zinc dust, as a surface finish, sofach enlarged
• Fig. 8 a As Fig. 8, but as an edge grinding in 500 times magnification
• Fig. 8 b as Fig. 8 a with different etching
• Fig. 8 c Like Fig. 8 a, but etched on microstructure
• Fig. 9 aluminum-zinc coating with relatively much dusted zinc dust, as a surface grinding, 50 times enlarged
• Fig. 9 a As Fig. 9, but as edge grinding in 500 times magnification
• Fig. 9 b as Fig. 9 a with different etching
• Fig. 9 c As Fig. 9 a, but etched on microstructure

The pretreated steel strip (1) runs over a deflection roller (5) through the furnace nose (3) into the liquid melt (2) of the aluminum-zinc alloy. The strip steel (1) is guided out of the melt (2) again via the deflection rollers (4). So that the aluminum-zinc coating receives a defined thickness, is by Blown off the excess nozzles (6). The steel strip (1) then enters the pollination system (7). Here the still liquid strip coating is additionally blown with zinc dust or a zinc dust mixture. The inflated dust crystallizes with the alloy coating and thus forms an intimate bond with the steel strip. The amount of zinc dust inflated determines the surface of the steel strip. Immediately after the pollination system, the continuous surface-treated steel strip is cooled. This cooling has to happen very quickly. The cooling rate must therefore be at least 25 ° C / s. The surface is further standardized in a subsequent roll pass. Rapid cooling of the steel strip (12) after dusting causes the alloy to solidify and a structureless surface is obtained.

The area of application for the above-described strip steels is primarily in the construction sector, in household appliances and in the automotive industry, i. H. a unified surface is required, which can then be easily used with e.g. B. paint can be coated. This standardized surface could not be produced with aluminum-zinc coatings by previously known processes.

Fig. 2 shows the surface of a normal aluminum-zinc coating. The formation of flowers is clearly visible. Fig. 3 shows the same surface in 100 times magnification. Despite this surface being redesigned, there is no uniform surface. 4 shows a surface of an aluminum-zinc coating, which was additionally blown with zinc dust before solidification. The area is much more uniform, as the enlargement in FIG. 5 shows.

The influence of zinc dust on the surface can be made even clearer in the following figures, which are surface cuts in 50x magnification.

Fig. 6 shows very clearly the crystalline structure of a conventional aluminum-zinc coating. FIGS. 7 a, 7 b, 7 c show the same sample, but in this case as an edge grinding with a 500-fold magnification. The aluminum-rich dendrites (8) in the alloy layer are clearly visible here. The zinc-rich interdendrite areas, which are still very small in this sample, are located between the dendrites (8) due to the low proportion of alloys. Fig. 7 c also makes it clear that the formation of the Si particles (11) in the coating and in the intermetallic alloy layer (10) at the coating / steel interface is not influenced by the zinc dusting. 8 shows a microstructure at the same magnification ratios, which is achieved by dusting with zinc dust. The zinc-rich intendendrite area (9) has become significantly larger, which results in a unification of the surface.

This is extremely evident in FIG. 9, where an even larger proportion of zinc dust has been inflated. The flower structure on the surface has disappeared, a unified surface is available for a subsequent easy coating.

Reference symbol list

• 1 steel strip
• 2 melt
• 3 furnace muzzle
• 4 pulleys
• 5 pulley
• 6 scraper nozzles
• 7 pollination system
• 8 Al-rich dendrites
• 9 Zinc-rich interdendrite areas
• 10 intermetallic alloy layer
• 11 silicon particles
• 12 cooling system

Claims (1)

Process for improving the surface quality of ferrous materials with an aluminum-zinc alloy coating, which consists of 25 to 70 percent by weight aluminum and a remainder made of zinc and silicon, and which is applied to steel strips or steel sheets by fire metallization in a hot-dip coating plant, this being from the alloy melt Leaving, coated steel strip is sprayed with finely divided particles into a suspension in air before the coating crystallizes, cooled and then treated, characterized in that the finely divided particles consist of pure zinc and in the subsequent cooling zone the coated steel band has a cooling rate of subjected to at least 25 ° C / s.

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