NL1022263C2 - Method for improving zinc layers. - Google Patents

Description

METHOD FOR IMPROVING ZINC LAYERS

The present invention relates to the treatment of workpieces comprising steel. Steel workpieces are often provided with a protective layer of a different metal.

Such a process can be carried out by immersing a steel workpiece in a molten zinc bath. The method produces a number of alloy layers 10 as well as a layer of pure zinc on top of these layers. Such layers are shown in Figure 1. Reference numeral 1 here indicates the steel of the workpiece. The γ (gamma) layer 2 located thereon comprises a mixture of iron and zinc. On this is δ (delta) layer 3, which comprises a different composition of iron and zinc. A further layer, the ζ (zeta) layer 4, comprises a further mixing of iron and zinc. Finally, the η (êta) layer comprises almost pure zinc.

The galvanizing yielding the layers described above is carried out at a temperature of approximately 450-465 degrees Celsius. The combined layer thickness is often 50 to 150 micrometers.

Workpieces which obtain a layer composition on the surface according to the above-mentioned galvanizing method are very widely applicable in many technical environments in which the protective action of the zinc layers is important, for example in construction or applications where steel is exposed to weather influences such as, for example, the use of containers. Furthermore, galvanized steel gratings are often used in areas of application such as (petro-chemical) factories or in other environments such as, for example, walking floors.

A disadvantage of such zinc layers is that, for example in the event of a fire or in other incidents in which the temperature rises above the melting point of zinc, the zinc will melt off the surface on which it is applied. Certainly the, generally thickest, outer layer of pure zinc will melt quickly.

In order to obviate such a drawback, the present invention provides a method for galvanizing a workpiece comprising steel, comprising steps for: - contacting the workpiece with liquid zinc with a temperature in a first temperature range of 420-800 degrees Celsius for obtaining a number of alloy layers on the steel surface, - heating the workpiece for a time period of 1 second-24 hours at a temperature in the second temperature range of 420-800 degrees Celsius for obtaining new layer compositions with desired composition obtained during treatment.

The above-described prior art layers arise from the chemical and physical reaction between the surface of the workpiece and the molten zinc during the step of contacting the workpiece with the zinc to obtain a transition zone comprising layers of different compositions of both metals.

During the heating, further chemical and physical reactions are created between the metals. In this case, zinc particles further diffuse into the iron 1 and iron particle further diffuse into the zinc top layer 5. This achieves that iron particles will occur up to the surface of the zinc layer 5. Due to the connections between iron and zinc particles in this modified top layer, it is achieved that this top layer is not subject to melting even at very high temperatures. Experiments have shown that even at temperatures of 1500 degrees, at which the metal emits cherry red light, no run-off or meltdown occurred.

A further advantage of the treatment according to the present invention is that the surface of the workpiece is "clean". That is, no zinc drop residues, thickenings, hard zinc grains or sheets or rough elements occur on the surface that normally occur after removal of a workpiece from a zinc bath.

Preferably the second temperature range is 420-600 degrees Celsius and more preferably 440-560 degrees Celsius. As will be described in more detail below, tests have been carried out in these ranges, which have led to satisfactory results in which the advantages described above have been achieved.

Preferably, a predetermined atmosphere is used during heating of the workpiece. A preferred embodiment of this is that the atmosphere comprises a reducing gas. An alternative embodiment is that the atmosphere comprises oxygen.

The workpiece preferably comprises a number of perforations or openings. As described in the foregoing, the heat treatment changes the layers so that the layers comprising zinc become thicker in total.

Furthermore, the perforations or openings which (partially become clogged during the hot-dip galvanizing are released by the heat treatment. Also, the released surface of the perforations or openings is given the same surface layer as the rest of the surface of the workpiece. be removed from an after-treatment, which is a costly treatment.

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It also occurs that steel in the form of small particles (hard zinc particles) ends up in a zinc bath during hot-dip galvanizing and that such particles adhere to the surface of a workpiece when the workpiece is taken out of the zinc bath. The heat treatment removes these hard zinc particles from the surface, which occurs, for example, by incorporating the metal thereof into the surface of the workpiece. As a result, these particles do not have to be removed mechanically or manually, which is a long-term and (partly because of that) expensive treatment. The same problem is solved in the case of the presence of bumps on drops that remain on the surface of the workpiece during hot-dip galvanizing.

In a further embodiment, the workpiece comprises a fence or grid-shaped element. The hard zinc particles also occur in such workpieces and are also removed by the treatment.

In a further embodiment, the workpiece comprises a building element or building element for construction, wherein the heat treatment provides similar advantages as in the previous embodiments. An example of such a building element is a lintel.

A further aspect of the present invention relates to a galvanized product obtained by a method as described in the foregoing or in the appended claims.

Further advantages, features and details of the present invention will be further described with reference to the accompanying figures, in which: - Fig. 1 is a diagrammatic sectional view of a number of alloy layers that arise during a galvanizing process; 1022, hs. 5 - Fig. 2 is a representation of a workpiece that has been galvanized according to the state of the art (Fig. 2a) and a workpiece that has been galvanized using the present invention (Fig. 2b); Fig. 5 is a perspective view of a further galvanized workpiece.

There are various methods for galvanizing a steel workpiece. Known are, among others, hot-dip galvanizing, centrifuge galvanizing, Sendzimir galvanizing, Sherard 10 galvanizing, spraying of zinc wire or powder, electrolytic galvanizing or mechanical galvanizing. An embodiment of a method according to the present invention comprises subsequent steps to improve galvanized products or workpieces according to the prior art.

Each of the methods according to the prior art, as a result of the galvanizing, provide a number of layers of different alloys on the surface of the steel workpiece.

In the case of, for example, hot-dip galvanizing (Fig. 1), four alloy layers are formed on the steel 1. These layers each have a specific zinc-steel ratio or zinc-steel alloy. The compositions of these layers are known. Layer 1 is the steel of the workpiece. Layer 2, also called the Y (gamma) layer, contains 21-28% Fe. Layer 3, also referred to as the δ (delta) layer, comprises 7-11.5% Fe. Layer 4, also referred to as the ζ (zeta) layer, comprises 6.0-6.2% Fe. Layer 5, also referred to as the η (êta) layer, comprises approximately 0.08% Fe. This top layer is therefore an almost pure zinc layer.

This substantially pure zinc layer 5 melts, for example, in a fire where the temperature rises above 420 degrees Celsius. This makes the workpiece, for example a slatted floor, unusable. If dripping zinc falls on hot stainless steel, the zinc will alloy with the steel. Any nickel present in stainless steel promotes the formation of such alloys. For example, if stainless steel pipes are made, these will be damaged and leak and in such cases both the galvanized workpieces and the damaged pipes must be replaced.

A disadvantage of the top layer 5 of almost pure zinc is therefore the thermal vulnerability.

An embodiment of the present invention is that a galvanized workpiece with the alloy layers described above is heated, for example in an oven, to a temperature of, for example, 420 to 550 degrees Celsius. This is done, for example, for one hour.

In one experiment, two hot-dip galvanized strips were brought to a temperature of 550 degrees Celsius in a closed electric oven for one hour. The surface layers had a thickness of ± 60 µm prior to the start of this operation. The strips had a smooth appearance, which is normal after hot-dip galvanizing. A few zinc drops and sharp protrusions are located on the galvanized surface of the strips before they are introduced into the oven.

After the heat treatment in the oven, the strips were discolored to a gray gray strip. The zinc layers were approximately 100 µm in diameter. The surface had become slightly dull / rough instead of shiny. The drops and rough elements had disappeared. With regard to the composition of the iron-zinc layer, which had become thicker, further analyzes must be carried out.

After this treatment, a test was conducted to test the heat resistance of the new layer. An untreated, i.e., non-oven-treated, strip with the zinc layer of the prior art and a treated strip of an embodiment of the present invention were heated with a burner with a red-yellow flame. The strip of the prior art exhibited melting of the zinc. The strip treated according to an embodiment of the present invention turned red-hot but no zinc dripped off. The red glow of the heated strip was cherry red, which indicates a temperature of around 1500 degrees Celsius.

In a further experiment, a perforated plate was hot-dip galvanized according to the prior art. The result was a galvanized sheet with a shiny surface with zinc droplets and sharp protrusions. Furthermore, a large part of the holes or perforations were silted up with a thin zinc film. In FIG.

2a a graphical representation of the perforated plate 22 can be seen. A few holes or perforations 24 were open after the treatment. A few holes 25 were completely provided with a zinc film. Some holes 26 were partly provided with a zinc film. Furthermore, zinc droplets or sharp protrusions 23 were visible on the surface. The plate was then placed in an oven at 550 degrees Celsius for 10 minutes with a temperature spread of 10 degrees Celsius. The result is shown in Fig. 2b. Plate 20 has a somewhat dull roughened surface. However, the gross irregularities have disappeared. Furthermore, all openings or perforations are nicely evenly provided with a galvanized surface and fully open. This has the advantage that after galvanizing, the perforations do not have to be opened up or cleaned. This open work or cleaning is a very labor-intensive job. The removal of surface roughness 23 is also very laborious.

A similar result can be seen in Fig. 3. Fig. 3a shows a workpiece 32 with surface roughnesses 34 1022263. These surface roughnesses include drops and sharp protrusions. The steel beam 30 of Fig. 3b processed according to an embodiment of the present invention has a somewhat dull, slightly roughened surface, but is otherwise even.

The workpieces of Figs. 2b and 3b have a thicker alloy layer on the surface than the workpieces of Figs.

2a and 3a. This is the result of a further exchange of Fe particles from the workpiece to the zinc layers.

The surfaces of the heat-treated workpieces of Figs. 2b and 3b are very suitable for applying a further coating such as, for example, an organic coating such as a powder coating. The adhesion to such surfaces is very good. In the experiments, a zinc oxide layer was found to form on the workpiece. In order to prevent this, for example, nitrogen can be used as shielding gas in the oven.

A further test involved heating a workpiece for 12 hours at 380 degrees Celsius and a quarter of an hour at 450 degrees Celsius. The surface after this treatment was coarse and showed the characteristics of an Si steel. The subsequent application of a coating resulted in slightly more degassing than normal. However, the adhesion was good. Prior to submitting this application, accurate analyzes had not yet been carried out with regard to the percentages of Fe and Zn in the newly formed top layers. It was found that the alloy layers in total are substantially 20-50%, or about 30% thicker than before the heat treatment in the oven. The top layer before treatment comprises approximately 0.08% Fe. The layer 30 after the treatment preferably comprises at least 5% Fe or the amount of Fe as in the former δ (delta) layer 3 or ζ (zêta) layer 4.

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The present invention is not limited to the preferred embodiment described above; the rights sought are defined by the following claims, within the scope of which many modifications can be envisaged.

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Claims (19)

A method for galvanizing a workpiece comprising steel, comprising the steps of: 5. contacting the workpiece with liquid zinc with a temperature in a first temperature range of 420-800 degrees Celsius for obtaining a number of alloy layers on the steel surface, characterized by heating the workpiece for a period of time of 1 second - 24 hours at a temperature in the second temperature range of 420-800 degrees Celsius to obtain new layer compositions with a desired composition obtained by the treatment. 2. Method for treating a galvanized steel workpiece comprising steps for heating the galvanized workpiece for a time period of 1 second to 24 hours at a temperature in a second temperature range of 420-800 degrees Celsius to obtain new layer compositions with a desired composition obtained by the treatment. 3. Method as claimed in one or more of the foregoing claims, wherein the second temperature range is 420-600 degrees Celsius and more preferably 440-560 degrees Celsius. Method according to one or more of the preceding claims, wherein the first temperature range is 400-550 degrees Celsius and preferably 440-460 degrees Celsius. 5. Method according to one or more of the preceding claims, wherein the time period is in the range of 5 - 1000 minutes, preferably 6 - 300 minutes and more preferably 10 - 60 minutes. 1 J "Ό 3 6. Method as claimed in one or more of the foregoing claims, comprising steps for allowing the workpiece to cool to at least a predetermined temperature between contacting the liquid zinc and heating of the workpiece. Method according to one or more of the preceding claims, wherein during the heating of the work piece a predetermined atmosphere is used. The method of claim 7 wherein the atmosphere 10 comprises a reducing gas. The method of claim 7 wherein the atmosphere comprises oxygen. 10. Method as claimed in one or more of the foregoing claims, wherein the duration of the heating is such that at the temperature applied in the upper surface layer an iron content of 2-14%, preferably 4-12% and more preferably of 6 -11% is present. Method according to one or more of the preceding claims, wherein the heating takes place in an oven. A method for coating a workpiece after undergoing a method according to one or more of the preceding claims. 13. Method as claimed in one or more of the foregoing claims, wherein the workpiece comprises a number of perforations or openings, such as for example in a perfosted steel plate. Method according to one or more of claims 1-12, wherein the workpiece comprises a fence or grid-shaped element. 15. Method as claimed in one or more of the claims 1-12, wherein the workpiece comprises a construction element or construction element for construction. The method of claim 15, wherein the building element is a lintel. 1022263 A galvanized product obtained by a method according to one or more of the preceding claims. 1 Cr; ""