EP0496678A1 - Process for continuous galvanizing at high temperature - Google Patents

Abstract

Continuous high temperature galvanising process intended more particularly for galvanising metallurgical products made of steel, by means of which the said products are first of all descaled by a mechanical action of the shot-blasting type, are then heated by induction before being introduced into an enclosure containing a melt bath of pure zinc, containing no or only small amounts of agents which inhibit the iron-zinc reaction, which process is characterised in that at least the surface temperature of the said products is between 530 DEG C and 570 DEG C at the entry of the said enclosure, the temperature of the said zinc bath being maintained at a temperature equal to or less than 30 DEG C lower than the surface temperature of the said products.

Description

The present invention relates to a continuous galvanizing process at high temperature, more particularly intended for galvanizing steels from zinc baths or molten zinc alloys.

We know the so-called "dip" galvanizing techniques by which metallurgical products, generally made of steel, are immersed to order in baths of molten metal alloys, normally based on zinc. When the products are in the form of sheets or strips, rather a continuous galvanizing process is used, always by dipping, the latter running at greater or lesser speeds through successive pickling, fluxing, galvanization and possibly cooling. These dipping techniques have the drawback of requiring very bulky installations and do not always provide a sufficient coating quality for certain applications such as stamping. Furthermore, it is often compulsory to subject the metallurgical products shaped a high temperature thermal reprocessing, of the annealing type, in order to restore to them the intrinsic mechanical qualities which they have lost during their forging. In this regard, it is desirable that the annealing can be carried out during the galvanization, the rise in temperature necessary for the annealing being exploited to condition the metallurgical product treated in a manner suitable for its subsequent coating with a protective layer of zinc. .

A continuous galvanizing process is also known which is particularly suitable for obtaining protective layers of thin zinc on continuous or discontinuous elongated products. In the latter process, the galvanizing operations consist in activating by blasting the reactivity of the surface of the products, which are then rapidly heated by induction before passing into a "bubble" of molten zinc where the metallurgical reaction, all taking place under a slightly reducing inert atmosphere; on leaving this zinc "bubble", the coated products are "wiped" by means capable of eliminating the thickness of excess zinc, before being finally immediately cooled, always in a neutral atmosphere. This process makes it possible to very quickly treat products of the type of concrete reinforcing wire, angles, or even sheets, the other advantage of this process being the economy which it makes it possible to achieve on the consumption of and zinc.

All the previously mentioned galvanizing techniques recommend a treatment temperature of around 450 ° C, both for the treated products and for the bath of zinc or zinc alloy in fusion. However, such a temperature proves to be insufficient for the annealing of forged metallurgical products, the latter then having to undergo an autonomous pre- or post-treatment.

Furthermore, at the chosen temperature of 450 ° C., the balance diagram of the iron-zinc alloy shows that the coating deposited by galvanization has, after cooling, a complex crystallographic structure formed of several phases, known under the names of "delta phase" and "dzeta phase", the surface layer of zinc being, for its part, in a crystallographic phase called "eta". The thicknesses of the layers of stable intermetallic compounds formed by these various phases naturally depend on the galvanization kinetics, in particular on the cooling rate, rapid cooling making it possible to avoid or moderate the thermal diffusion of the phases between them which is responsible for the 'grayish or brown appearance of certain galvanized parts. The presence of the intermetallic phase "dzeta" - whose crystal lattice is monoclinic, with a macroscopic tree structure not very plastic - at the interface of the surface layer of practically pure zinc in phase "eta" and the intermetallic layer "delta" - whose crystal lattice is hexagonal, that is to say very regular and particularly compact - considerably reduces the mechanical resistance of the coating.

In this regard, metallurgical products galvanized with zinc to be subsequently stamped require that this "dzeta" phase be eliminated, since it is not very ductile; it is then common, in "dip" galvanizing techniques, to add a percentage of another chemical element, for example aluminum, in the molten zinc alloy, so that it plays an inhibitory role in the formation of said "dzeta" phase during the galvanization cycle. For example, Japanese patent JP-A-63,447 relates to a process for manufacturing hot-dip galvanized steel parts having high plasticity; this process uses a zinc bath enriched with aluminum (of the order of 20 to 24%) which is maintained at a temperature of between 500 ° and 540 ° C, the immersion time of the products being between 1 and 5 seconds . The primary objective of adding aluminum in a percentage as high as that recommended in this last patent is to inhibit the iron-zinc reaction, and therefore the formation of intermetallic layers between the underlying steel and the layer. zinc. In this regard, we know that beyond a critical percentage of aluminum C0, there appears a period of inhibition of iron-zinc reactions whose value is given by the relationship: t = (C2 - C02) / a with C the percentage of aluminum and has a parameter determining the inhibition; under the conditions of the above-mentioned patent, the inhibition period t is equal to 133 hours, with the consequence of a total inhibition of the reactions for the formation of iron-zinc intermetallic alloys.

It is also known, from the balance diagram of the iron-aluminum reaction, that the melting point of the iron-aluminum alloys is higher the higher the percentage of aluminum; under the conditions of the process described in the aforementioned patent, the melting temperature of the iron-aluminum alloy is thus between 480 ° and 500 ° C. Consequently, if one wants to respect the normal conditions of supercooling of the bath, namely from 20 ° to 40 ° C minimum - this in order to avoid the problems linked to the accidental freezing of molten liquid metal in the enclosures and the galvanizing ovens -, the temperature of the previous iron-aluminum alloy bath should be maintained between 500 ° and 540 ° C.

Although the prior method described above advantageously solves the problems associated with the undesirable growth of the least ductile iron-zinc intermetallic layers, it has been demonstrated that the presence of a certain thickness of intermetallic compounds in the delta phase is desirable under the layer of zinc, normally practically pure, which is deposited on the surface of metallurgical products to be stamped.

This is why, according to the present invention, there is proposed a process of continuous galvanization at high temperature, more particularly intended for the galvanization of steel metallurgical products, by which said products are first stripped by a mechanical action of the type of shot blasting, then heated by induction, before being introduced into an enclosure containing a bath of pure molten zinc, this process being characterized in that at least the surface temperature of said products is between 530 ° C. and 570 ° C at the entrance to said enclosure, the temperature of said bath of pure zinc being maintained at a temperature equal to or less than 30 ° C below the surface temperature of said products.

It will first be observed that, by "pure zinc bath", is meant a zinc bath which may contain a certain percentage of wetting agents, such as lead, which the person skilled in the art knows that they do not exert. that little influence on the growth kinetics of intermetallic iron-zinc compounds.

The invention thus aims to obtain in a single step metallurgical products galvanized with zinc having, at their iron-zinc interface, a layer of a intermetallic alloy in the delta crystallographic phase, without the presence of any other crystallographic phase, the nature and thickness of this same layer being controlled, mainly, by the galvanization temperature.

Since no inhibiting agent such as aluminum is incorporated in the galvanizing bath, the liquidus is reached from 420 ° C .; at a temperature of 550 ° C., the supercooling of the bath thus reaches 130 ° C., which is far too high for normal working conditions. For the ordinary person skilled in the art, it is, moreover, entirely unexpected to "overheat" a bath of pure zinc because, in fact, until now, the advantage that we can never have demonstrated to extract from it the selective growth of iron-zinc intermetallic compounds. In this regard, it has even been proposed, in Japanese patent JP-A-62-4858, to improve the plasticity of steel products previously galvanized with zinc, by subjecting them to a post-treatment at high temperature (between 420 ° and 600 ° C); the objective of this latter treatment is to cause the posterior formation of an intermediate layer of an iron-zinc intermetallic alloy, of suitable crystalline structure, occupying between a quarter and three-quarters of the initial zinc layer. Such a process in two distinct stages is perfectly obsolete when compared to the process according to the invention, which leads to a similar result, if not better, in a single galvanizing operation carried out under properly selected physical conditions. In addition, if we observe the iron-zinc diagram, we see that it is very improbable that by heating a galvanized product already covered with a layer of pure zinc, we can obtain an intermetallic layer in the crystallographic phase alone. "delta"; the appearance of the "dzeta" phase is indeed favored if no special precaution is taken on the cooling rate.

We therefore understand that the galvanizing technique proposed according to the invention is simpler and more economical than prior galvanizing techniques. In addition, this new process makes it possible to obtain, in a single treatment, not only a quality of coating well suited to the subsequent shaping of said products, for example by stamping, but also a simultaneous thermal reprocessing of the steel constituting the products. metallurgical treated.

• le chauffage par induction à une température supérieure de plus de 100°C par rapport à celle qui est normalement employée pour la galvanisation au zinc permet d'améliorer très fortement le coefficient d'allongement des produits métallurgiques forgés par un laminage à froid, sans pour autant modifier sensiblement le coefficient d'allongement des produits métallurgiques forgés à chaud : on obtient donc, dans tous les cas, une amélioration conséquente de la plasticité des produits, ce qui est bien sûr très favorable à leur emboutissage éventuel. De même, le module d'élasticité des produits métallurgiques forgés à chaud est augmenté dans des proportions importantes.
• seule la phase "delta" du composé intermétallique fer-zinc peut se former d'une manière stable à une telle température. Après refroidissement des produits, la phase "dzeta" est donc absente sans recours à aucun inhibiteur.

At the chosen galvanizing temperature, particularly high for zinc containing no inhibiting agents, the objectives of the invention are simultaneously and perfectly fulfilled in a very simple manner:

• induction heating to a temperature over 100 ° C higher than that normally used for zinc galvanizing makes it possible to greatly improve the elongation coefficient of metallurgical products forged by cold rolling, without as much to significantly modify the elongation coefficient of hot forged metallurgical products: we therefore obtain, in all cases, a significant improvement in the plasticity of the products, which is of course very favorable to their possible stamping. Similarly, the modulus of elasticity of hot forged metallurgical products is increased in significant proportions.
• only the "delta" phase of the iron-zinc intermetallic compound can form stably at such a temperature. After cooling the products, the "dzeta" phase is therefore absent without the use of any inhibitor.

According to another characteristic of the invention, the time taken for the metallurgical products to pass through the high-temperature galvanizing bath must be very short, for example less than 5 seconds, the coated metallurgical products leaving the enclosure having to be cooled with about 100 ° C as quickly as possible, for example in less than half a second.

In this way, we could see that, because we very quickly stops diffusion phenomena which are mainly of thermal origin, the intermetallic iron-zinc layer in the "delta" phase has a thickness of less than 10 microns, while the thickness of practically pure surface zinc reaches from a few microns to a few tens of microns depending on the type of "wiping" used at the exit of the galvanizing chamber - "wiping" consists in removing surpluses of non-frozen zinc alloy from the surface of the treated products; this operation, which is done by gravity in the oldest technologies, can be forced by gas blowing means or by electromagnetic means). We are thus able to perfectly control the thicknesses of the deposited layers and we know, for example, depositing a coating having an intermetallic iron-zinc layer in "delta" phase with a thickness equal to 5 microns and a layer of pure zinc in phase " eta "ranging from 10 to 30 microns. The latter case is well suited to the applications of the method of the invention to stamping, rolling, drawing and notching, especially when cold.

Furthermore, it is known that the kinetics of the metallurgical reaction between iron and molten zinc depends exponentially on the contact temperature, according to the well-known Arrhenius law; this last law makes it possible to predict that the speed of formation of the coating is multiplied by a factor at least equal to 30 (for an activation energy corresponding to standard experimental conditions as they will be given later) when the temperature passes from 450 ° C to 550 ° C. This high factor explains why it is possible to obtain a coating of low thickness - but sufficient for protection against corrosion - at high temperature in very short times.

According to the invention, the ratio between the two previously mentioned thicknesses (ie that of the layer of pure zinc over that of the intermetallic layer) then makes it possible to define a limit value, of the order of 3, beyond which the structure of the coating, perpendicular to the surface of the coated metallurgical products, is particularly favorable to their cold plastic transformation. In particular, in the case of stamping and similar processing technologies, the presence of a majority of practically pure zinc on the surface makes it possible to significantly reduce the work required to carry out this operation, since, in fact, the pure zinc is particularly ductile. A common practical case is that where this ratio reaches a value equal to approximately 6.

• un fil lisse tréfilé et cranté à froid, dit Ech.1,
• un fil lisse laminé et cranté à froid, dit Ech.2,
• un fil lisse laminé et cranté à froid, dit Ech.3,
• un fil laminé et crênelé à chaud, en acier trempé au revenu, dit Ech.4,
• un fil laminé et crênelé à chaud, en acier microallié, dit Ech.5.

We will now give a nonlimiting example of the implementation of the process according to the invention in the galvanization of steel wires for concrete in FeE 500 steel, with a length of 24 m and a diameter of 8 mm, of which the commercial references are as follows:

• a smooth cold drawn and notched wire, called Ech.1,
• a smooth cold rolled and notched wire, called Ech.2,
• a smooth cold rolled and notched wire, called Ech.3,
• a hot rolled and crênelé wire, in tempered steel, called Ech.4,
• a hot rolled and crênelé wire, in microalloyed steel, called Ech.5.

The tests consisted in passing the concrete wires through, at a speed of 80 m / mm, in an enclosure containing a bath of molten zinc at a temperature of approximately 560 ° C. after having descaled by shot blasting, then heated by induction at around 550 ° C, all under a slightly reducing inert atmosphere, and finally cooled in water or in air in forced convection at 25 ° C. This cooling lowers the temperature of the wires very quickly, since they lose 100 ° C in 0.03 seconds in water and in 0.62 seconds in air.

avec Ag ^b l'allongement uniformément réparti du fil brut de laminage, Ag ^g l'allongement uniformément réparti du fil galvanisé selon le procédé conforme à l'invention, E _totale l'épaisseur totale du revêtement et E _inter l'épaisseur de la couche de composés intermétalliques Fe/Zn en phase "delta", ces deux épaisseurs étant données en micromètres.The following table lists the characteristics of cold-treated wires, before and after treatment, as well as those of their coating:

with Ag ^b the uniformly distributed elongation of the raw rolling wire, Ag ^g the uniformly distributed elongation of the galvanized wire according to the process according to the invention, E _total the total thickness of the coating and E _inter the thickness of the layer of Fe / Zn intermetallic compounds in the "delta" phase, these two thicknesses being given in micrometers.

Thus, by the sole application of the process according to the invention, a cold forged metallurgical product, originally uncoated and having a given coefficient of elongation, becomes, due to the structure of the coating and the simultaneous and consequent improvement of its elongation, perfectly suitable for being stamped.

avec E _totale et E _inter comme définies précédemment, et Re ^b et Re ^g les modules d'élasticité des fils bruts de laminage et galvanisés respectivement. Le gain sur le module d'élasticité Re est donc très élevé, ce qui est normal étant donné le retraitement thermique à haute température qu'on subit les fils traités lors de la galvanisation.Likewise, we can give the characteristics of the heat treated wires, before and after treatment, as well as those of their coating:

with E _total and E _inter as defined above, and Re ^b and Re ^g the elastic moduli of the raw rolling and galvanized wires respectively. The gain on modulus of elasticity Re is therefore very high, which is normal given the high temperature thermal reprocessing that the wires treated undergo during galvanization.

Claims (8)

1 - Process of continuous galvanization at high temperature, more particularly intended for the galvanization of steel metallurgical products, by which said products are first stripped by a mechanical action of the type of a shot blasting, then heated by induction before d '' be introduced into an enclosure containing a bath of pure molten zinc, containing little or no agents inhibiting the iron-zinc reaction, process characterized in that at least the surface temperature of said products is between 530 ° C and 570 ° C at the entrance to said enclosure, the temperature of said zinc bath being maintained at a temperature equal to or less than 30 ° C below the surface temperature of said products. 2 - Process of continuous high temperature galvanizing according to claim 1, characterized in that the surface temperature of the metallurgical products at the entrance to the galvanizing chamber is equal to 550 ° C. 3 - Metallurgical products capable of being coated by the implementation of the continuous galvanizing process at high temperature according to any one of the preceding claims. 4 - Metallurgical products according to claim 3, characterized in that the coating comprises a layer of an iron-zinc intermetallic compound in its hexagonal crystallographic phase "delta", covered with a layer of practically pure zinc in its crystallographic phase "eta ". 5 - Metallurgical products according to claim 4, characterized in that the intermetallic iron-zinc layer in the "delta" phase has a thickness of less than 10 microns, while the thickness of the layer of practically pure zinc in the "eta" phase is between 10 microns and a few tens of microns. 6 - Metallurgical products according to any one of claims 3 to 5, characterized in that the ratio between the thickness of the practically pure zinc layer in the "eta" phase and the thickness of the iron-zinc intermetallic layer in the "delta" phase is at least equal to 3. 7 - Metallurgical products according to claim 6, characterized in that the ratio between the thickness of the practically pure zinc layer in the "eta" phase and the thickness of the iron-zinc intermetallic layer in the "delta" phase is equal to 6. 8 - Use of metallurgical products according to any one of claims 4 to 7 in cold plastic transformations of the type of stamping, rolling, drawing and notching.