EP0401727B1 - 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 belts are then 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 a 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 high 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. This subject of registration is a temperature control in order to spray the zinc dust exactly 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 that carry the trade name Galvalum, we shall speak of surface structures 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 in 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

Der vorbehandelte Bandstahl (1) läuft über eine Umlenkrolle (5) durch die Ofenschnauze (3) in die flüssige Schmelze (2) der Aluminium-Zink-Legierung. Über die Umlenkrollen (4) wird der Bandstahl (1) wieder aus der Schmelze (2) herausgeführt. Damit der Aluminium-Zink-Überzug eine definierte Stärke erhält, wird mittels Abstreifdüsen (6) der überschüssige Anteil abgeblasen. Anschließend gelangt der Bandstahl (1) in die Bestäubungsanlage (7). Hier wird der noch flüssige Bandüberzug zusätzlich mit Zinkstaub oder einem Zinkstaubgemisch angeblasen. Der aufgeblasene Staub kristallisiert mit dem Legierungsüberzug und bildet somit einen innigen Verbund mit dem Bandstahl. Die aufgeblasene Menge Zinkstaub bestimmt die Oberfläche des Bandstahles. Unmittelbar nach der Bestäubungsanlage wird der durchlaufende oberflächenbehandelte Bandstahl abgekühlt. Diese Abkühlung muß sehr schnell geschehen. Die Abkühlungsgeschwindigkeit muß deshalb mindestens 25°C/s betragen. In einem nachfolgenden Walzendurchlauf wird die Oberfläche weiter vereinheitlicht. Durch das schnelle Abkühlen des Bandstahles (12) nach dem Bestäuben wird die Legierung zum Erstarren gebracht und man erhält eine strukturlose Oberfläche.Show it:

Fig. 1

Schematic galvanizing line

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 with a surface grinding, enlarged 50 times

Fig. 7 a

Like Fig. 6, but as a bevelled edge 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 little zinc dust, as a surface finish, sofach enlarged

Fig. 8 a

Like Fig. 8, but as a bevelled edge 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 a fair amount of dusted zinc dust, as a surface grinding, 50 times larger

Fig. 9 a

Like FIG. 9, but as an edge grind in 500 times magnification

Fig. 9 b

As FIG. 9 a with different etching

Fig. 9 c

Like 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 led out of the melt (2) again via the deflection rollers (4). So that the aluminum-zinc coating receives a defined thickness, is by means of Blown off excess nozzles (6). The steel strip (1) then arrives in 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. The alloy is solidified by the rapid cooling of the steel strip (12) after dusting 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 standardized 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 100x 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 can be clearly seen here. The zinc-rich interdendrite areas, which are still very small in this sample, are located between the dendrites (8) due to the low alloy content. 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 affected 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 list

1

Steel strip

2nd

melt

3rd

Oven muzzle

4th

Pulleys

5

Pulley

6

Scraper nozzles

7

Pollination plant

8th

Al-rich dendrites

9

Zinc-rich interdendrite areas

10th

intermetallic alloy layer

11

Silicon particles

12th

Cooling plant

Claims (1)

1. Method of improving the surface condition of iron-containing materials having a coating of an aluminium and zinc alloy, which comprises 25 to 70 percent by weight of aluminium and a remainder comprising zinc and silicon, and which is applied by hot-metallising in a melt-dip refining plant to steel strips or plates, wherein, prior to crystallisation of the coating, the coated steel strip emerging from the alloy melt is sprayed with finely distributed particles in a suspension in air, cooled and subsequently finished, characterised in that the finely distributed particles comprise pure zinc and, in the subsequent cooling zone, the coated steel strip is subjected to a cooling rate of at least 25°C/s.

Images