EP0026757B1 - Process for hot galvanizing iron and steel articles - Google Patents

Description

The invention relates to a method for hot-dip galvanizing iron or steel pipes in automatic galvanizing plants, in which the pipes to be galvanized are immersed in a zinc bath after degreasing, pickling, rinsing, application of fluxing agents and optionally drying and after removal from the zinc bath blown off and quenched.

The required zinc coating in qlm ^{2 is} prescribed especially for use in drinking water pipes. In addition to the pure zinc layer, the zinc coating also includes all iron-zinc alloys that occur due to diffusion processes. In the known methods for hot-dip galvanizing iron or steel objects, the surfaces of the objects to be galvanized are prepared by pickling, applying fluxes and optionally drying in a drying oven, after which they are introduced into the zinc bath. In the known processes, the zinc bath temperature is at least 450 to 465 ° C., and in the case of steel tubes a dip in the zinc bath of approximately 120 s is usually maintained. After the pipes have been removed from the zinc bath, they are blown off with compressed air and blown out with steam. Usually, due to the required handling time and in order to achieve a smooth and shiny surface, a time of about 10 s is required before immersion in the water quenching bath. The water quench bath usually has a temperature of about 50 to 60 ° C.

For the production of corrosion-resistant, tempered prestressing steel, it has already been proposed to carry out the tempering directly in a zinc bath, whereby the patenting of the wire is combined with the galvanizing. In this way, further heat treatments are avoided, the duration of exposure to zinc in the bath being determined solely by the desired heat treatment. From DE-OS 2 711 041 it has become known to use a salt-zinc bath for quenching from the recrystallization temperature to the tempering temperature, whereupon a heat treatment, in particular a tempering and control treatment at temperatures between 300 ° C. and 400 ° C. by heating is provided for the formation of the desired structure in the sheet. This is intended to increase the ductility of the metal.

The present invention now aims to keep the zinc consumption, that is to say the amount of zinc taken up from the zinc bath, in order to achieve a specific zinc coating in g / m ² as low as possible without sacrificing corrosion resistance.

The invention further aims to keep the risk of diffusion of iron from the objects to be galvanized into the zinc of the bath as low as possible and also to reduce the zinc consumption in order to achieve a certain zinc coating. To achieve this object, the invention consists in that the pipes to be galvanized are immersed in the zinc bath until a quantity of zinc / m ^{2 is} absorbed, which corresponds to a weight per unit area of the zinc coating / m ² after blowing off with air and / or water vapor, which is less than the desired weight per unit area of the zinc coating / M ² and at most 95% of the desired weight per unit area of the zinc coating / M ² corresponds to the fact that the galvanized pipes after they have been removed from the zinc bath at temperatures of over 250 ° C. until the zinc coating has grown to the desired weight per unit area / m ^{z are} maintained, wherein an intermetallic zinc-iron alloy layer is achieved over a partial area of at least 60% of the layer thickness of the zinc coating by diffusion of iron into the zinc and an outermost layer of the zinc coating is formed from pure zinc, and that the galvanized pipes in be deterred in a manner known per se.

The fact that the immersion time in the zinc bath is chosen so that after blowing off a zinc amount / m ² remains on the object to be galvanized, which is less than the desired weight per unit area of the zinc coating / m ² and at most 95% of the desired weight per unit area of the zinc coating / m ² corresponds, it is achieved that less zinc is applied from the bath. The dipping time is thus shortened compared to the known methods, and strong blowing off or stripping of the zinc adhering to the surface of the objects to be galvanized can also be achieved by increased blower outputs. In order to achieve the desired zinc coating / m ² , the objects are then kept at temperatures above 250 ° C. At temperatures of over 250 ° C., diffusion processes take place, in which iron diffuses into the zinc layer and an alloy layer is formed at the expense of the pure zinc layer. This iron-zinc alloy layer formed by diffusion now leads to higher values for the zinc coating when the total zinc coating is determined. Due to the growth of the iron-zinc alloy layers after the galvanized objects have been lifted out of the bath, iron is prevented from diffusing into the zinc bath and at the same time the required zinc coating is achieved by choosing the appropriate holding time at temperatures of over 250 ° C. through diffusion processes and alloy formation achieved. Cooling to temperatures below 250 ° C only stops the diffusion processes and thus further alloy formation. The quality of the galvanizing is not affected, since iron-zinc alloy layers also provide very good corrosion protection. The post-alloying time should only be limited by the fact that the diffusion processes do not go so far that no pure zinc layer remains.

The objects are preferably immersed in the zinc bath until a quantity of zinc / m ^{2 is} absorbed which, after being blown off with air and / or water vapor, is about 85% of the desired Basis weight of the zinc coating / m ^z corresponds. In this way, savings of up to 15% in zinc are readily possible, although this consideration does not take into account that the lower contamination of the zinc bath by iron also results in a significant improvement in economy.

The holding time at temperatures above 250 ° C. is preferably such that the zinc coating / m ² increases by at least 10%, preferably at least 15%, by alloy formation. The galvanized objects can preferably be kept at temperatures above 250 ° C. after they have been removed from the zinc bath until an intermetallic zinc-iron alloy layer has formed, for at least 75% of the layer thickness of the zinc coating. The formation of the iron-zinc alloy layer can preferably extend over at least 80%, in particular 90%, of the layer thickness of the zinc coating, the upper limit being given only by the fact that an outermost layer of the zinc coating, preferably with a layer thickness of at most 5% of the The total thickness of the zinc coating, which is to be formed from pure zinc, is to be retained. For this purpose, the articles are preferably kept at temperatures of above 250 ° C., preferably above 300 ° C., for 10 to 120 s, preferably at least 20 s, in particular 60 to 90 s, after they have been removed from the zinc bath.

According to the invention, diving times of 20 to 180 s, preferably 20 to 120 s, are possible, after which the galvanized objects are kept in heated, still air or in a water vapor atmosphere. It has proven to be particularly advantageous if the holding time at temperatures above 250 ° C. after the objects have been removed from the zinc bath is longer than the immersion time of the objects in the zinc bath. This ensures minimal zinc consumption.

The method is advantageously carried out as wet galvanizing. In wet galvanizing, for example, pipes with significantly lower temperatures enter the zinc bath than after. a drying oven would be the case. Since diffusion processes only start at 250 ° C, the time until which diffusion processes in the zinc bath cannot yet take place is extended in this way. The diffusion processes in the bath should be kept as low as possible and only take place outside the zinc bath over the holding time. It is therefore advantageous according to the invention to use a zinc bath with an addition of aluminum in the amount of 0.08 to 0.5, preferably 0. 2% by weight, since such an addition of aluminum largely suppresses the diffusion processes and thus the formation of alloys in the bath.

The process according to the invention thus controls the ratio of the alloy layer to the pure zinc layer when the zinc coating is applied, and the proportion of the alloy layer in the total zinc coating which is substantially higher than known processes saves zinc during the galvanizing process.

The invention is explained in more detail below on the basis of comparative experiments.

Pipes of various dimensions were galvanized once without extended alloying times and once with extended holding times after they had been removed from the zinc bath. The increase in the zinc coating in percent was then determined when the holding time was extended outside and inside. The distribution of the zinc coating on the outside and inside of pipes was also determined, and surprisingly, with larger pipe dimensions, the distribution on the outside and inside was evened out. The results are summarized in Table 1 below.

The invention is further illustrated by the diagrams shown in the figures of the drawing explained in more detail, in which, in addition to the zinc coating determined, the distribution of the zinc coatings over the length of the tube can also be seen at different holding times at temperatures above 250 ° C.

. 2-inch pipe, in Fig for a 3/4-inch pipe, in Figure 3 for a 6/4-inch pipe, in Fig 4 for a 3-inch tube and in _- in Figure 1 are the ratios for a 1/2... Fig. 5 again shown for a 1/2 inch tube.

1 to 5, the zinc coating in g / m ^{2 is} plotted on the ordinate and the length of the pipe is plotted on the abscissa, 1 corresponding to the pipe start and 12 to the pipe end.

1 to 3, a uniform diving time of 110 s was observed for better comparability. In Fig. 1 the pull-out speed from the bath was uniformly 0.7 m / s. A stripping pressure of 1.2 bar and a blow-out pressure of 5 bar were observed with a blow-out time of 0.8 s. The curves labeled 1 represent the values for the zinc coating, which were achieved by quenching 12 s after removal from the bath. The solid line shows the values measured on the outside of the pipe, while the corresponding dashed curve shows the values inside the pipe. The sample length was measured uniformly at 500 mm.

The values measured on the outside are shown continuously with 2 and the values measured on the inside are shown in broken lines after a holding time of 60 s. Curves 3 show the corresponding values after a holding time of 90 s. The tubes were kept at temperatures above 250 ° C during the hold time.

2 shows the conditions for a 3/4-inch pipe, work being carried out with a pull-out speed reduced to 0.6 m / s or a blow-out pressure of 6 bar compared to the representation according to FIG. 1. In analogy to the illustration according to FIG. 1, curves 4 again show the values measured outside and inside after quenching 13 s after being removed from the bath. Curves 5 correspond to the values after a holding time of 60 s, the solid lines in turn corresponding to the values measured on the outside and the broken line corresponding to the values measured on the inside.

3 shows the conditions for a 6/4 inch pipe, 0.7 m / s again being selected as the pull-out speed, 1.1 bar as the stripping pressure and 4 bar as the blow-out pressure with a blow-out time of 1.2 s. The sample length here was 400 mm. Curves 6 represent the external and internal values of the zinc coating in the manner already described after quenching 10 s after being removed from the bath. Curves 7 show the conditions after a holding time of 75 s.

4 is based on 3-inch pipes which are pulled out after a diving time of 140 s at a speed of 0.5 m / s. 1 bar was used as stripping pressure and 9 bar as blow-out pressure with a blow-out time of 1.7 s. The sample length was 150 mm. Curves 8 in turn show the values when quenching 10 s after being pulled out, while curves 9 were measured after a holding time of 60 s.

5, 1/2 inch tubes were immersed for 60 s at a zinc bath temperature of 460 ° C. and excess zinc was stripped off under a pressure of 2 bar. Curves 10 illustrate the values which were measured after quenching 10 s after removal, while curves 11 represent the values obtained after a holding time of 90 s.

The corrosion resistance of a prolonged holding time at temperatures subjected to pipes subjected to temperatures above 250 ° C. has proven to be at least equivalent to the corrosion resistance of pipes which have been coated with zinc in accordance with the known methods. Folded samples in accordance with DIN 2444 also show no defects in the coating according to the invention.

Claims (12)

1. Process for galvanizing iron or steel tubes in automatic galvanizing plants, in which the tubes to be galvanized are, after degreasing, pickling, rinsing, applying fluxes and, if desired, drying, dipped into a bath of molten zinc and after removal from the zinc bath, are gas wiped and subsequently quenched, characterised in that the tubes to be galvanized are dipped into the zinc bath until having taken up a zinc coating weightlm² which, after gas wiping with air and/or steam, corresponds to a zinc coating weight/m² which is less than the desired zinc coating weight/m² and at maximum corresponds to 95 % of the desired zinc coating weight/m², that after removal from the zinc bath the galvanized tubes are kept at temperatures above 250 °C until the zinc deposit has increased to the desired weight/m², thereby obtaining an intermetallic layer of zinc-iron-alloy over a part of at least 60 % of the thickness of the zinc coating by the diffusion of iron into the zinc and providing an outermost layer of the zinc coating, consisting of pure zinc, and that subsequently the galvanized tubes are quenched in a manner known per se.

2. Process as claimed in claim 1, characterised in that the tubes are dipped into the zinc bath until they have taken up an amount of zinc/m² which corresponds, after gas wiping with air and/or steam, to approximately 85 % of the desired zinc coating weight/m².

3. Process as claimed in claim 1 or 2, characterized in that after removal from the zinc bath the galvanized tubes are kept at temperatures above 250 °C until the zinc deposit/m² has increased by at least 10 %, preferably at least 15 %.

4. Process as claimed in claim 1, 2 or 3, characterised in that after removal from the zinc bath the galvanized tubes are kept at temperatures above 250 °C until an intermetallic layer of zinc-iron-alloy has been formed over at least 75 % of the thickness of the zinc deposit.

5. Process as claimed in any one of claims 1 to 4, characterised in that the time interval during which the galvanized tubes are, after removal from the bath, kept at a temperature above 250 °C is selected such that a thickness of the layer of the intermetallic alloy of at least 80 %, preferably at least 90 %, of the thickness of the zinc deposit is achieved.

6. Process as claimed in any one of claims 1 to 5, characterised in that the maximum time interval during which the tubes are, after removal from the zinc bath, kept at a temperatur above 250 °C is selected such that the outermost layer of the zinc deposit consists of pure zinc and at maximum comprises 5 % of the total thickness of the zinc deposit.

7. Process as claimed in any one of claims 1 to 6, characterised in that the tubes are, after removal from the zinc bath, kept at temperatures above 250 °C, preferably above 300 °C, for 10 to 120 s, preferably for at least 20 s, particularly for 60 to 90 s.

8. Process as claimed in any one of claims 1 to 7, characterised in that the tubes are dipped into the zinc bath for 20 to 180 s, preferably for 20 to 120 s, and are then, prior to quenching, kept at temperatures above 250 °C outside of the zinc bath.

9. Process as claimed in any one of claims 1 to 8, characterised in that the tubes are kept at elevated temperatures in static air or in a steam atmosphere.

10. Process as claimed in any one of claims 1 to 9, characterised in that the time interval during which the tubes are kept at temperatures above 250 °C after removal from the zinc bath is selected longer than the dipping time interval of the articles in the zinc bath.

11. Process as claimed in any one of claims 1 to 10, characterised in that the process is a wet- galvanizing process.

12. Process as claimed in any one of claims 1 to 11, characterised in that a zinc bath is used to which aluminum has been added in an amount of 0,08 to 0,5, preferably 0,2 % by weight.

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