EP0594520B1 - Galvanized steel products and method of making - Google Patents

Abstract

Process for galvanising iron and steel products according to which coating of the said products is performed by dipping in a molten bath of zinc alloy, characterised in that the said alloy contains, in percentages by weight, 1 to 3.5 % of magnesium, 0.5 to 1.5 % of aluminium and 0.0010 to 0.0060 % of silicon. The invention also relates to an iron or steel product (5) coated using this process. <IMAGE>

Description

The invention relates to the coating of steel products, such as steel strips, by immersion of these products in a zinc-based molten metal bath.

The continuous galvanizing treatments of the steel strips consist of passing the strips through a container containing a molten metal alloy essentially consisting of zinc, to which variable amounts of aluminum are generally added. A layer of alloy is thus deposited on the strip. On leaving the bath, the thickness of the coating is standardized by a wringing device comprising rollers or, more often, nozzles projecting a gas jet onto the coated strip, while the zinc alloy layer does not has not yet completed its solidification.

Various chemical elements can be added to the zinc bath. First, mention should be made of aluminum which makes it possible to avoid the formation at the strip-coating interface of Fe-Zn alloys if its content is greater than 0.15% by weight. Magnesium is also added in some cases, because it has a beneficial effect on the corrosion resistance of the finished products, and increases the fluidity of the bath, which allows the galvanization to be carried out at a lower temperature.

The document FR 1446872 recommends, in particular, to use a galvanizing bath comprising from 1 to 4% by weight of magnesium and from 0.05 to 5% by weight of aluminum to obtain a coating which is strongly adherent to the product, and it giving good resistance to corrosion. Likewise, the products marketed under the name GALFAN ® comprising a coating containing from 4 to 5% of aluminum can be improved by incorporating up to 1.8% of magnesium into the coating.

Document WO-A-89 09 844 describes a process with a molten bath of Zn alloy comprising from 0.3 - 3.5% Al by weight, 100 ppm or less of Pb, 1/100 - 1 part of Si per part of A1 and optionally comprising 0.01 - 1.5% by weight of one or more of metals selected from the group comprising Mg, Mn and Cu.

These coatings are conventionally applied to the steel substrate with a thickness of the order of a few tens of »m. It has also been found that coatings of zinc containing between 1 and 3.5% by weight of magnesium and approximately 1% by weight of aluminum exhibited very good anti-corrosion properties and very good adhesion to the substrate. On the other hand, the wetting of the substrate by the liquid bath is weak, because of the large presence of magnesium, and this makes it possible to deposit on the substrate thanks to a bath maintained at a temperature of 400 to 500 ° C. a uniform layer of very thin (10 ”m) without the need to use a device to regulate this thickness.

The coatings thus obtained, however, have serious defects, in this case the presence on the coated product of areas comprising "lack of coating" of several »m wide, that is to say areas where the substrate remains bare. . It is likely that the presence of these gaps is due to a phenomenon of partial dewetting of the steel surface during cooling. This is the counterpart of the low wettability of the steel strip by the liquid bath rich in magnesium and of obtaining a thin coating of regular thickness which it promotes.

The object of the invention is to galvanize the steel strips with a thin and evenly distributed coating, having good anti-corrosion properties and the thickness of which does not require uniformity, after deposition, by an additional device. .

To this end, the subject of the invention is a process for galvanizing steel products according to which the coating of said products is carried out by soaking in a molten bath of zinc alloy, characterized in that said alloy comprises in weight percentages 1 to 3 , 5% magnesium, 0.5 to 1.5% aluminum and 0.0010 to 0.0060% silicon.

The subject of the invention is also a steel product coated according to this process.

As will have been understood, the invention consists in adding a small amount of silicon to a zinc alloy bath of the composition indicated above, having favorable employment properties. This resolves the coating quality problems mentioned above, while retaining the possibility of not using a thickness regulating device.

• la figure 1 qui présente une coupe micrographique d'un échantillon d'acier galvanisé par trempage dans un bain contenant 2,90 % de magnésium et 1 % d'aluminium ;
• la figure 2 qui présente une coupe micrographique d'un échantillon d'acier galvanisé par trempage dans un bain contenant 2,55 % de magnésium, 0,93 % d'aluminium et 60 ppm de silicium.

The invention will be better understood on reading the description which follows, referring to the following figures:

• Figure 1 which shows a micrographic section of a sample of galvanized steel by dipping in a bath containing 2.90% magnesium and 1% aluminum;
• FIG. 2 which presents a micrographic section of a sample of galvanized steel by dipping in a bath containing 2.55% of magnesium, 0.93% of aluminum and 60 ppm of silicon.

The experiments which will be described were carried out on an extra-soft titanium steel without interstitials of composition: C = 0.010%, Mn = 0.120%, S = 0.006%, Si = 0.010%, Al = 0.035%, Ni = 0.020 %, Cr = 0.010%, P = 0.010%, Ti = 0.050%. However, it goes without saying that the invention is in no way limited to the coating of this type of shade.

• dimensions de l'échantillon : lames de 15 x 50 x 0,7 mm ;
• vitesse d'immersion de l'échantillon dans le bain 20 mm/s ;
• durée de l'immersion dans le bain : 3 s ;
• composition du bain de galvanisation : Mg = 2,90 % - Al = 1 %, le reste étant constitué par du zinc et des impuretés inévitables ;
• température de l'échantillon avant immersion : 425°C.

Figure 1 shows a micrographic section of a sample of this steel which has been galvanized under the following conditions:

• sample dimensions: 15 x 50 x 0.7 mm slides;
• rate of immersion of the sample in the bath 20 mm / s;
• duration of immersion in the bath: 3 s;
• composition of the galvanizing bath: Mg = 2.90% - Al = 1%, the remainder consisting of zinc and unavoidable impurities;
• temperature of the sample before immersion: 425 ° C.

On this micrographic section, a distinction is made between the steel substrate 1 and the layer 2 of galvanizing coating, the maximum thickness of which, under the processing conditions specified above, is approximately 10 μm. The low wettability of the steel by the bath, due to the high content of the magnesium bath, makes that from a point of view macroscopically, this thickness seems to be of satisfactory regularity, and this would make it possible, under industrial conditions, not to use an installation for regulating the thickness of this coating at the outlet of the galvanizing bath. However, micrographic observation of the sample shows that in fact, the coating 2 is distributed over the substrate 1 irregularly. On ranges 3 whose width can vary from 10 to 100 μm, it can be seen that the coating has only a very small thickness, or is even completely absent. These areas are said to be "lack of coating", and their presence is probably attributable as has been said to local dewetting of the surface of the steel substrate. These lack of coating would render an industrially manufactured product unacceptable, since the coating could not provide protection of the entire surface of the product against corrosion.

Increasing the bath temperature and / or the temperature of the sample before immersion in the bath does not improve these results. It only leads to an overall increase in the thickness of the coating, without eliminating the gaps. Even a considerable increase (up to 90 s) in the duration of the immersion of the sample is also ineffective.

• durée de l'immersion dans le bain de galvanisation : 3 s ;
• composition du bain de galvanisation : Mg = 2,55 % - Al = 0,93 % - Si = 60 ppm - reste = Zn et impuretés inévitables ; les teneurs en silicium et aluminium sont ajustées par l'addition d'un alliage Al-Si à 10 % de silicium.
• température de l'échantillon avant immersion : 450°C.

The sample, the micrographic section of which is shown in FIG. 2, was coated under the same conditions as that of FIG. 1, with the following differences:

• duration of immersion in the galvanizing bath: 3 s;
• composition of the galvanizing bath: Mg = 2.55% - Al = 0.93% - Si = 60 ppm - rest = Zn and unavoidable impurities; the silicon and aluminum contents are adjusted by the addition of an Al-Si alloy with 10% silicon.
• temperature of the sample before immersion: 450 ° C.

FIG. 2 clearly shows that the addition of silicon to the galvanizing bath results in a spectacular improvement in the quality of the coating 4 deposited on the steel substrate 5 compared to the previous case. Thickness of the covering 4 is always about 10 "m, but this time it has an extremely regular external surface, completely free of the gaps which affected the sample of FIG. 1. On the other hand the external surface of the covering 4 is not disturbed by the irregularities of the surface of the substrate 5. In addition, this coating 4 has remarkable qualities of adhesion to the substrate 5, and very good resistance to folding. Finally, it is checked that it gives a completely satisfactory corrosion resistance to the sample: a salt spray test shows a formation of white rust uniformly distributed over the entire surface of the sample. If a scratch is made on the coating 4 exposing the steel, it does not form red rust but white rust: zinc plays its role of sacrificial protection.

The explanation for this spectacular improvement in the quality of the coating by the introduction of silicon into the galvanizing bath is undoubtedly to be sought in the adsorption of silicon at the interface between the coating 4 in the liquid state and the substrate. 5, which slows the phenomenon of dewetting of the substrate 5.

However, there is a relatively rapid decrease in the concentrations of silicon and magnesium in the bath during the experiment. It is therefore necessary to frequently check the composition of the bath and add the elements consumed when it becomes necessary.

• température du bain de galvanisation : 400 à 450°C ;
• température du substrat d'acier avant son immersion : 350 à 600°C ;
• teneur en magnésium du bain : 1 à 3,5 % ;
• teneur en aluminium du bain : 0,5 à 1,5 % ;
• teneur en silicium du bain : 10 à 60 ppm (soit 0,0010 à 0,0060 %).

Satisfactory galvanizing results are obtained under the following operating conditions:

• temperature of the galvanizing bath: 400 to 450 ° C;
• temperature of the steel substrate before immersion: 350 to 600 ° C;
• magnesium content of the bath: 1 to 3.5%;
• aluminum content of the bath: 0.5 to 1.5%;
• silicon content of the bath: 10 to 60 ppm (i.e. 0.0010 to 0.0060%).

Of course, without departing from the spirit of the invention, it is possible to make improvements to it, in particular by incorporating other galvanizing baths additives which could give the coating even more advantageous physicochemical properties without deteriorating the ability of the bath to distribute itself uniformly over the surface of the substrate.

Claims (3)

1. Process for dip-galvanizing ferrometallurgical products, in which the coating of the said products is carried out in a molten bath of zinc alloy, characterized in that the said alloy comprises, in weight %, 1 to 3.5% of magnesium, 0.5 to 1.5% of aluminium and 0.0010 to 0.0060% of silicon.
2. Process according to Claim 1, characterized in that the molten bath is held at a temperature of 400 to 450°C and in that the product is dipped into the said bath at a temperature of 350 to 600°C.
3. Ferrometallurgical product (5) coated with a layer of zinc alloy, characterized in that the said alloy comprises, by weight %, 1 to 3.5% of magnesium, 0.5 to 1.5% of aluminium and 0.0010 to 0.0060% of silicon.

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