EP0235067B1 - Method for the protective coating of metallurgical products - Google Patents

Abstract

A process of protective coating of iron and steel products is described, wherein an adherent and rough layer of scale is formed, on which is directly deposited a layer of a protective material. The qualities of the layer of scale result from the fact that the product is shaped hot and the speed of its cooling is controlled so that the resultant thickness of scale is less than a threshold of adherence characteristic of the material and of the temperature at the end of shaping. The layer of protective material is obtained by metallization with zinc or an alloy based on zinc. The invention is principally applicable to long iron and steel products, and in particular to reinforcing bars for concrete.

Description

The invention relates to the protection of the surface of steel products formed at a relatively high temperature.

A preferred application of the invention relates to long products and, typically but not limited to, the protection of concrete reinforcing bars.

The temperature at the end of production of the concrete rods at the exit from the finishing stands of the rolling mills is usually of the order of 900 to 1000 ° C. The concrete reinforcing bars are then cooled, put on hold, then resumed to be cut to length.

In common practice, concrete reinforcing bars are not considered as noble products and do not undergo any protective treatment.

However, there is currently an increasing need for concrete rods protected against corrosion. Thus, for certain markets where storage in the open air lasts a long time (e.g. six months) and takes place in a relatively corrosive atmosphere such as that of a port, protection, at least temporarily, of concrete reinforcing bars is desirable. In other cases, better resistance to corrosion in service is requested from rounds used in structures made of a particular concrete, some of the constituents of which are more particularly prone to corrosion, or else produced in a relatively aggressive medium. Here, long protection is desirable.

To meet this growing need; it is necessary to find an effective and economically acceptable protection method. This last criterion is all the more severe as the market value of the concrete reinforcing bars in themselves is very low.

Thus, the solution which has been proposed, to produce the rounds in stainless steel, in whole or in plating, cannot be generalized because of its cost.

The processes known to a person skilled in the art for the protection of iron have not, up to now, found a commercially advantageous application for concrete reinforcing bars, since they all require a stage of prior preparation of the product to rid it scale and other oxides developed on the surface during the cooling of the product during its manufacture.

Thus, hot dip galvanizing requires, barring exceptions (JP-A-54/133438), a prior pickling of the round with acid, because the scale is a barrier to the galvanizing reaction. Calamine also opposes electrolytic deposition, or the apposition of a film-forming material, preceded by the formation of a primer layer, either by heat treatment (US-A-3085 034), or by reaction. of the bare surface with an acid solution of metallic sulphates (GB-A-1153 202) or by steam (Review of Current Litterature on the paint and allied Industries, vol. 22, n ° 129, May-June 1949, p. 265).

Likewise, metallization by projection (known as "schooping"), in particular metallization with zinc, is known to a person skilled in the art for requiring stripping, by sandblasting or shot blasting, of the surface to be treated (cf. "Techniques of l 'Engineer ", M1641-4, paragraph 3.6;" Zinc metallization ", p. 5, paragraph 5, and 13, paragraph 9, published by the" Zinc Technical Center "). Systematically therefore, it is sought to obtain on the raw product, the least adherent scale possible to facilitate removal. However, this pickling requires an individual recovery or by very small groups, of the concrete reinforcing bars produced, and therefore considerably increases the cost price of the treated circles.

Similar disadvantages arise from the process disclosed in document FR-A 029 285, according to which a metal borate is applied to the surface to be protected, which is still very hot, which causes a dissolution reaction of the scale, which gives rise to a brittle layer which 'We must detach later by mechanical means.

The object of the invention is to propose a protective coating process for steel products which is effective and economical, and free from the abovementioned drawbacks.

The invention achieves its object by the fact that the product is produced hot and the speed of its accelerated cooling is controlled from a temperature of the order of 900 to 1000 ° C. up to a temperature of 600 to 550 ° C. approximately, so that the thickness of the resulting scale which forms naturally on the surface is less than an adhesion threshold, characteristic of the metal from which the product is made and of its temperature at the end of production, and then deposited, directly on the layer of scale thus formed, a protective material.

In many applications (in particular for concrete reinforcing bars), the cooling of the product after its shaping is controlled, so that the average thickness of the scale layer formed during cooling does not exceed approximately 8 μm .

In its most general sense, the expression "hot" refers to the temperatures, known to those skilled in the art, which result in the rapid formation of a layer of scale.

The invention goes against the prejudice firmly established for the skilled person, prejudice according to which the scale which naturally forms during the production of the hot product, is absolutely incompatible with the adhesion of a surface layer of material protective. Indeed, it has been surprisingly discovered that, according to the invention, when the formation of scale is controlled, this layer may have characteristics (of adhesion and roughness in particular) which make it possible to apply it directly (ie without pickling, but without excessive waiting which would degrade the surface) a perfectly adherent protective coating.

It is understood that, according to the invention, the protective coating is a layer of a material intended to form itself a barrier between the metal surface, covered with scale, and the atmosphere (without the intervention of a chemical reaction mique between said material and the scale, unlike the document FR-A-2 029 285 cited above).

Controlling the formation of scale, according to the invention, involves controlling the cooling: rapid cooling (by soaking in water, for example) is particularly favorable because it avoids maintaining the metal surface for too long at a relatively high temperature, conducive to the rapid development of scale.

In current practice, the cooling control is often easier with long products, especially of relatively small section, than with flat products. In this respect, the invention finds a particularly advantageous application in the context of installations for the production of concrete reinforcing bars where an accelerated cooling device exists. Thus, in the system known as TORSID ^[ _R ^] , the concrete rod is quenched by a sheath of water, while passing through a cooling tube. Subsequent natural heating from the core leaves the product surface at 400-500 ° C at the outlet of the rolling train. The bars are hot cut and placed laterally on an air-cooling table, allowing the temperature to drop to appreciably ambient temperature, with a view to resuming them, for exact lengthening and leaving the installation. . On the cooling table, the fresh scale covering the concrete rods (with an average thickness of 2 to 3 µm) allows direct coating with a layer of protective material.

It is advantageous that the coating is a zinc metallization by spraying, because this technique can take place over a wide range of temperatures, which makes it possible to treat the bars at any stage of their cooling on the cooling table.

It will be understood that thus implemented, the method according to the invention is all the more economical since not only does it avoid the descaling step, but it is inserted locally in a generally uncluttered zone of the installation and is inserted in a passive phase of the round cooling process.

The average thickness of the zinc coating is advantageously of the order of 20 to 40 μm, to provide sufficient protection. A much greater thickness can lead, in the case of folding of the coated round, to cracking of the coating. However, it should be noted that, even when cracked, the zinc coating provides protection (sacrificial type this time).

On concrete reinforcing bars, zinc and aluminum alloys can be used instead of zinc (the latter entering in proportion up to 20%), easier to apply and having better resistance. The preferred alloy is the Dunois alloy (90% Zn - 10% AI).

Without wishing to limit the invention by theoretical explanations, it seems however that it is based on. the following considerations.

The holding of the protective coating directly applied to the scale is conditioned by the adhesion of the scale and the quality of its surface.

The notion of adhesion covers two aspects: the adhesion before application of the protective coating, which is linked to the adhesion of the oxide layer during its growth, during the accelerated cooling of the raw product, and the adhesion to the during the forming of the coated product, which is linked to the deformation ability of the oxides during mechanical deformation of the coated product (for example, by bending the ends of the concrete bars).

As for the first aspect, it turns out that the oxide formation takes place with an increase in volume (x 1.7 to 2) compared to the metal which gave it birth. Therefore, at the metal / scale interface, the oxide is in compression and the metal in tension. All of these accumulated stresses are resolved without breaking for thin calamines (a few micrometers) and all the better since the structure of the oxides is said to be decomposed (accelerated cooling to approximately 600-550 ° C. then slowed down to room temperature - but in a generally weak oxidizing atmosphere). For thick calamines (greater than about 10 μm), the stresses are absorbed by oxides ruptures and detachments at the interface. So, without external mechanical stresses for simple reasons of growth, a scale, to be non-degraded, must be thin (a few µm). Generally, until about 6-8 pm, there is no break.

• de la température de formation de l'oxyde: à 900°C, l'épaisseur limite se situe entre 8 et 13 pm suivant l'état de surface au départ, alors qu'à 750°C, l'épaisseur limite se situe entre 18 et 30 µm;
• de la rugosité de la surface avant oxydation: à 800°C en surface polie, l'épaisseur limite est de 10 pm environ, alors qu'en surface brute (rugueuse), cette épaisseur limite s'élève à 18 pm environ;

With regard to the second aspect, there are two failure modes for calamines: a rupture by cracking perpendicular to the metal / oxide interface without loss of adhesion, therefore acceptable, and a rupture by detachment of oxide scales which is inadmissible. The border between the two modes is a limit oxide thickness which depends on:

• of the oxide formation temperature: at 900 ° C, the limit thickness is between 8 and 13 pm depending on the surface condition at the start, while at 750 ° C, the limit thickness is between 18 and 30 µm;
• the roughness of the surface before oxidation: at 800 ° C. on the polished surface, the limit thickness is approximately 10 μm, while in the rough (rough) surface, this limit thickness amounts to approximately 18 μm;

and also of the structure of the oxides, because for the decomposed oxides, the limiting thickness increases.

So that the metallic coating does not come off with the oxide during a deformation, its thickness must be less than a limit value, which has never been quantified, during experiments on concrete rods, in below 8 µm.

To be able to be coated with the protective material, the surface of the scale must be rough, clean and regular. These qualities most often depend on the stages of prior shaping of the product. In the case of long products (and particularly concrete reinforcing bars), the surface roughness after shaping in rolling trains, is generally important (very often very superior to that of flat products which require a more careful surface quality). The cleanliness and regularity of the surface are then ensured if the installation of the coating takes place, in accordance with the invention, directly on the oxide which has just been formed, without unnecessary waiting.

Furthermore, these two aspects which the notion of scale adhesion covers conveniently are characterized by a "threshold of adhesion" representative of the nature of the metal from which the product is made and of its temperature at the end of production at hot (generally end of rolling) corresponding to the start of the formation of the scale that we want to keep on the product. Thus, this adhesion threshold is defined as the maximum value of the calamine thickness which simultaneously satisfies these two aspects, therefore the more severe of the two. In this case, it is that relating to the adhesion of scale during formation on a straight rectilinear product of manufacture and whose said maximum value reaches approximately 8 μm.

• la figure 1 est une coupe micrographique partielle grossie 500 fois, d'un rond à béton revêtu selon l'invention;
• les figures 2 a et 2 b sont des vues de ronds à béton respectivement nu, c'est à dire recouvert de calamine formée selon l'invention, et revêtu par une matière protectrice (zinc);
• les figures 3 a et 3 b sont des vues correspondantes aux figures 2.a et 2.b, montrant les mêmes ronds à béton, mais après exposition de 400 h à une atmosphère de brouillard salin.

By way of illustration, the examinations and tests have been carried out which are described below with reference to the attached photographic representation plate, on which:

• Figure 1 is a partial micrographic section magnified 500 times, of a coated concrete ring according to the invention;
• Figures 2 a and 2 b are views of concrete bars respectively bare, that is to say covered with scale formed according to the invention, and coated with a protective material (zinc);
• Figures 3a and 3b are views corresponding to Figures 2.a and 2.b, showing the same concrete bars, but after exposure of 400 h to an atmosphere of salt spray.

EXAMINATION No. 1 (fig. 1 and 2 a)

The thickness of scale "C" formed in accordance with the invention was measured on concrete bars "M" obtained by the "TORSID" process. The average thickness is 1 to 2 μm, with, very locally, extra thicknesses of up to 12 μm. Micrographs have confirmed that the oxide layer is very adherent and that the appearance of the external surface is tormented, with significant roughness.

TEST No. 1 (fig. 1 and 2 b)

The concrete bars previously examined (diameter 8mm) were zinc-plated by spraying with a gun. A series of samples received an "R" coating with an average thickness of 60 μm. A second series received a coating with an average thickness of 200 µm. We noted some irregularities in thickness due to the presence of the locks present in relief on the surface of the circles.

The appearance of the products obtained shows that the adhesion of zinc is very good.

EXAMINATION No. 2 (fig. 1)

Micrographs of the rounds of test 1 made it possible to note an excellent penetration of zinc R into all the crevices of the scale 'C' and of the surface of the metal M; very good adhesion of the coating to the scale; a very good compactness of the coating and a very rough external appearance, suggesting good bonding with the concrete.

TEST No. 2 (fig. 3 a and 3 b)

The rounds of test 1 were tested in a salt spray under the following standard conditions: temperature of 35 ° C and water at 5% NaCl (50 g / I).

It was found that the first rust did not appear until about 8 days for thin coatings (60 µm) and 15 days for thick coatings (200 µm).

TEST No. 3 (fig. 2)

Rounds of 10 mm in diameter were coated with zinc for some (fig. 2 b) and with alloy "Dunois" for others, in average thicknesses of 75, 90, 100 and 150 µm. Uncoated witness rings were also kept (Fig. 2 a). We tested their ability to be shaped by folding, by slowly bending them at 90 ° around a mandrel having a diameter three times that of the rounds.

A significant cracking of the coating layer was noted, probably due to an excessive thickness.

TEST No. 4 (fig. 3)

The circles of the previous test were tested for 400 h in a salt spray enclosure. It was found that uncoated witness circles deteriorated almost immediately (Fig. 3 a).

For the same coating thickness, the rounds coated with the Dunois alloy held better than those coated with pure zinc (fig. 3 b).

Shaped circles and rectilinear circles (unshaped) have similar resistance to salt spray for a coating of the same type: in particular, there is no appearance of rust in the chipped zone of the shaped circles surface protection ).

The embodiment described above is the preferred mode. However, as an alternative, other types of coating can be provided. Thus, we can use an epoxy paint and take advantage of the heat of the circles not yet completely cooled (temperature of 200-250 ° C approximately) to achieve rapid self-crosslinking of the paint (sprayed in thermosetting powder by means of 'a gun). Such a coating, for example in a 200 μm layer, makes it possible to obtain excellent protection against salt spray for straight circles. On the other hand, if cracks occur during the folding of these coated rounds, no sacrificial protection is to be expected.

It is worth remembering that you should not confuse calamine, which forms naturally ment during the hot production process of a metallurgical product, and rust which, on the other hand, forms cold in a humid atmosphere on a product in use. One, calamine, dark in color, is essentially formed of iron oxide in a rous iron state, while the other, rust, of reddish color, is essentially formed of iron hydroxides, iron therefore being in the ferric state, and constitutes on the product a friable layer which has no significant mechanical consistency.

Claims (8)

1. Method for the protective coating of a hot-worked metallurgical product, according to which the formation is caused of an adherent and rough layer of scale on completion of working of the product by controlled accelerated cooling from a temperature of the order of 900 to 1000°C to a temperature of approximately 600 to 550°C so that the resulting thickness of scale is less than an adherence threshold characteristic of the metal from which the product is made and its temperature on completion of working, and a protective material is then deposited directly on the layer of scale thus formed.

2. Method according to Claim 1, characterized in that the deposit of protective material is produced by metallization using zinc or using a zinc- based alloy.

3. Method according to Claim 1, characterized in that the metallurgical product is a concrete round cooled by quenching, then in the free air on a cooling table, and in that the coating of the layer of protective material is performed during the phase of cooling in the air.

4. Hot-worked metallurgical product, characterized in that it comprises an adherent and rough layer of scale with a mean thickness of less than approximately 8um directly coated with a layer of protective material.

5. Product according to Claim 4, characterized in that the protective material is zinc or a zinc alloy.

6. Product according to Claim 5, characterized in that the thickness of the layer of zinc or zinc alloy is between 20 and 40um.

7. Product according to Claim 5 or 6, characterized in that the layer of scale has a mean thickness of approximately 2 to 3um.

8. Product according to anyone of Claims 4 to 7, characterized in that a concrete round is involved.

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