EP2188399B1 - Controlled method and device for oxidation/reduction of the surface of a steel strip running continuously through a radiant tube oven for galvanisation thereof - Google Patents

Description

The invention relates to the continuous galvanizing of steel strips including AHSS high levels of silicon, manganese, aluminum and, in particular, facilities comprising a radiant tube furnace without direct flame heating zone.

The evolution of the materials used in the automotive industry has successively led to the galvanization of the steel strips prior to their implementation by the car manufacturers to improve the corrosion resistance of the frame and steel body components. Then, in order to lighten the structures while improving the resistance to crash collapse (crash) of vehicles, new grades of very high yield strength steels with a high elongation capacity have been developed. Such materials, named AHSS (Advanced High Strength Steels) use specific chemical compositions and implementation procedures that singularize certain families of steel such as "DP" or Dual Phase steels, "TRIP" steels or TRansformation Induced Plasticity ... These steels are, in particular, described in the "Advanced High Strength Steel (AHSS) application guidelines" prepared by the "Committee on Automotive Applications" of the International Iron & Steel Institute.

These steels have opened new perspectives in the design of automobiles but pose a number of problems for steel producers. Indeed, some of their alloying elements such as manganese, silicon, aluminum, chromium ... form on the surface of steel strips a thin layer of oxides during the annealing operation preceding the immersion in the galvanizing bath. This selective oxidation damages the "wettability" of zinc and therefore the quality of the coating. These phenomena are due to diffusion processes of highly oxidizable alloying elements towards the surface of the strip where they can oxidize even in the zones with radiant tubes of the furnaces where the atmosphere is nevertheless reducing for the iron oxides.

Numerous studies have been conducted to understand the kinetics of these oxidation phenomena and to provide solutions to the problems posed during galvanizing. The summary document ECSC steel workshop Galvanazing of steel strip, Luxembourg, February 27-28, 2002 The ECSC (ESCC) provides a list of reference documents, most of which come from work carried out under the auspices of the European Community.

Among the solutions proposed for ensuring quality galvanization are surface pretreatments of the steel strips before their use in continuous galvanizing plants (chemical treatments, electrodeposition or vapor phase coating by a very thin iron film , nickel, copper ...), operations of mechanical or chemical removal of the oxides after annealing and before entering the zinc bath.

Another way has been particularly studied which consists in subjecting, in the annealing furnace, the surface of the strips at conditions of temperature and atmosphere suitable for rapidly and deeply oxidizing the alloying elements and thus avoiding their subsequent migration into area. During this operation a layer of oxides is formed which will subsequently be removed in the following areas of the annealing furnace under a reducing atmosphere. Such controlled oxidation / reduction techniques have been the subject of numerous studies and experiments. The document "Enhancing the wettability of High Strength Steels during Hot-Dip Galvanizing" presented at the "Galvatech 2004" conference describes the physical principles that govern the controlled formation and reduction of this oxide layer. The patent JP 02-285057 describes an oxidation phase between 400 and 700 ° C in a slightly oxidizing atmosphere and then a reduction phase between 600 and 800 ° C in a reducing atmosphere, it indicates ranges of temperature and the composition of the gases (contents of O ₂ , N ₂ and H ₂ ). The patent EP 1 285 972 describes the same principle. These two patents, however, remain very general and do not clearly reveal the practical means of controlling reactions.

The patent EP 1 457 580 describes an installation for performing the oxidation phase in a specific chamber where the strip is heated by induction or combustion of a gas, in an oxidizing atmosphere, between 100 and 400 ° C.

The patent US 3,936,543 describes an annealing furnace duct not aimed at the specific coating of AHSS steels but making it possible to avoid the use of cleaning flux during galvanization by the oxidation followed by the superficial reduction of carbon steel strips. The oven Annealing prior to the galvanizing bath is a conventional furnace having a direct flame heating zone (FDF) and a radiant tube temperature holding zone (RTF). The surface oxidation is obtained in the DFF zone by controlling the combustion under super-stoichiometric conditions so that the flue gases have a controlled excess of oxygen. The reduction is obtained in the RTF zone which comprises at least 5% hydrogen, the remainder being nitrogen. The principle laid down by this patent can be implemented for the controlled oxidation / reduction of AHSS steels. It has the advantage of not requiring any additional oxidation installations and of using the mixed galvanization furnaces DFF / RTF without major modifications.

However, not all galvanizing furnaces have the FDF zone necessary to easily practice oxidation and many are those who only use radiant tubes. However, these furnaces, despite their controlled atmosphere, do not avoid the selective oxidation of the alloying elements. The patent WO 2005/017214 proposes two possibilities to solve the problem. The first is to use a direct flame combustion chamber separate from the RTF annealing furnace from which the flue gas has been collected for injection into the furnace. The second is to install a direct flame burner in an area of the oven enclosure. In both cases, the flue gases provide the necessary oxidizing atmosphere under conditions of composition obviously dependent on the temperature of the strip and that of the gases. The reduction is then conventionally obtained by passing through a mixture of nitrogen and hydrogen. These two possibilities require a modification of the existing installations (additional combustion chamber and furnace gas supply ducts, installation of a burner in the oven). In addition, they freeze the position in the annealing furnace of the oxidation zone and thereby freeze the temperature of the oxidation zone, which does not allow a great flexibility of use.

The method and the device for its implementation objects of the present invention provide the solution to these two problems.

In general, the invention consists in injecting an oxidizing medium into a section of a radiant tube furnace, particularly in a nitrogen / hydrogen atmosphere, using one or more tubes, in particular specially modified and capable of being installed in place of any of the existing tubes. Depending on the temperature range chosen for the oxidation, this injection can be carried out in any section of the furnace, preferably in the preheating section.

The medium must have, depending on the temperature of the band and the chemical composition of the latter, a dew point such that the alloying elements such as silicon, manganese, aluminum, chromium are deeply oxidized. and no longer have the ability to migrate to the surface. As a general rule, this dew point is above -20 ° C.

To achieve this objective, the injected medium may be water vapor or air or a mixture rich in oxygen. It can also be the product resulting from the combustion of an over-stoichiometric mixture of air or oxygen-rich air or oxygen / fuel in a burner.

• l'installation dans au moins un endroit de la section de chauffage du four et/ou dans au moins un endroit de la section de maintien du four, d'au moins un tube modifié capable d'injecter un médium oxydant ; et
• l'injection du médium oxydant par l'intermédiaire du (ou des) tube(s) modifiés(s) ;
• le médium oxydant ayant une composition telle que, dans les conditions de température du médium oxydant et de la bande d'acier, et en fonction de la composition chimique de la bande, il possède un point de rosée assurant l'oxydation en profondeur des éléments d'alliage de la bande d'acier.

Thus, the invention as defined in claim 1 relates in particular to a method providing, in a continuous galvanizing annealing furnace of steel strips comprising a preheating section and a holding section and equipped only with radiant tubes, the oxidation of the strip intended to prevent the selective oxidation of the alloying elements of the steel, characterized in that it comprises the following steps:

• installing in at least one location of the heating section of the oven and / or in at least one location of the holding section of the oven, at least one modified tube capable of injecting an oxidizing medium; and
• injecting the oxidizing medium via the modified tube (s);
• the oxidizing medium having a composition such that, under the temperature conditions of the oxidizing medium and the steel strip, and depending on the chemical composition of the strip, it has a dew point ensuring the deep oxidation of the elements alloy of the steel strip.

The control of this selective oxidation preferably makes use of the measurement of the dew point in the zone (s) of installation of the (or) tube (s) modified (s). This measurement can be carried out by dew point transmitters installed in a fixed position and operating in a closed loop with the medium flow control members. oxidant injected by the injectors of oxidizing medium and / or, adjustment of the burners.

The invention also relates to a device according to claim 5 ensuring the organization, in a preheating section and / or a maintenance section of a continuous galvanizing annealing furnace of steel strips equipped only with radiant tubes. at least one oxidation zone for preventing the selective oxidation of the alloying elements of the steel, by injecting an oxidizing medium in the oxidation zone, characterized in that it comprises at least a tube comprising at least one branch provided with calibrated holes allowing the oxidizing medium to pass through the oxidation zone.

The means for introducing the oxidizing medium may be either an injector ensuring the supply of the tube in a hot oxidizing medium such as water vapor, air or a gas rich in oxygen, or a burner feeding the tube into a product resulting from the combustion of an over-stoichiometric air / fuel mixture, a stoichiometric air / oxygen / fuel mixture or a stoichiometric air / oxygen fuel mixture within the non-explosive limit.

The modified tube (s) intended to provide the oxidizing medium necessary for the oxidation of the strip is (are), for example, a U-shaped tube whose inlet branch is equipped at its end with a device for injecting steam or air preheated or not, oxygenated or not oxygen or oxygen and whose branch opposite the inlet branch is closed at its end, at least one of the branches preferably the opposite branch to the input branch, is pierced with calibrated holes allowing said medium to pass. The U-tube can be replaced by any conventional tube shape such as, for example, a P-shape, a double-P shape, a W shape or a thermowell shape.

According to another characteristic of the invention, the radiant tube intended to supply the oxidizing medium is a P-tube having an inlet branch equipped at its end with a burner and at least one of the branches, preferably the opposite branch. the input branch is pierced with calibrated holes allowing flue gases to pass through the furnace enclosure. The branch opposite to the input branch comprising the burner may allow some of the flue gases to escape outside the furnace through a calibrated orifice or comprise a heat exchanger device for preheating the air of combustion with flue gases. The P-tube can be replaced by any conventional tube shape such as, for example, a U-shape, a W-shape, a P-shape or a thermowell. The burner (s) are supplied with an over-stoichiometric air / fuel mixture or with a stoichiometric mixture of oxygen-enriched air / fuel or with a stoichiometric mixture of oxygenated air / fuel within the non-explosive limits.

The tubes equipped with burner or injector, whatever their type, are directly interchangeable with existing ones. They can be installed on demand depending on the temperature chosen for the oxidation or be installed permanently at different points of the oven. In this case, they are activated according to the choice of the temperature at which it is desired to oxidize the strip, therefore the position of the tube in the oven.

Another advantage of the method is to locate the injection of oxidizing medium exactly where it is needed, that is to say very close to the two faces of the steel strip and to be able to take advantage of the local effect of turbulence in contact with the band which favors the reactions between the medium and the band.

• Figure 1, une ligne de galvanisation équipée d'un four à tubes radiants,
• Figure 2, le cheminement de la bande d'acier depuis son entrée dans le four jusqu'à sa sortie du bain de zinc ainsi que la variation de sa température,
• Figures 3 à 6, des tubes radiants selon l'invention équipés de brûleurs,
• Figures 7 et 8, des tubes radiants selon l'invention équipés d'injecteurs.

The remainder of the description refers to the appended figures which represent, respectively:

• Figure 1 , a galvanizing line equipped with a radiant tube furnace,
• Figure 2 the path of the steel strip from its entry into the oven to its exit from the zinc bath and the variation of its temperature,
• Figures 3 to 6 , radiant tubes according to the invention equipped with burners,
• Figures 7 and 8 , radiant tubes according to the invention equipped with injectors.

• Une section d'entrée avec une ou deux dérouleuses de bande 1 une cisaille guillotine 2 une soudeuse de raboutage 3 permettant de raccorder la queue d'une bande issue d'une des dérouleuses à la tête de la prochaine bande issue de l'autre dérouleuse et assurant ainsi un fonctionnement continu de la ligne, un accumulateur de bande 4 qui restitue à son aval de la bande préalablement accumulée lorsqu'on stoppe le déroulement en amont de l'accumulateur pour réaliser la soudure de raboutage.
• Une section 5 de dégraissage des bandes laminées à froid ou de décapage acide des bandes laminées à chaud.
• Un four de recuit 6 assurant le chauffage, le maintien à la température de recuit, le refroidissement, le vieillissement lorsque nécessaire et la mise à température contrôlée de la bande avant son entrée dans le bain de zinc en fusion.
• Une section de galvanisation proprement dite avec le bain de zinc 7 dans lequel est immergée la bande, un dispositif d'essorage du zinc liquide 8 éventuellement un four de galvanealing à induction 9, un refroidissement 10 et un bac de trempe 11.
• Une section de sortie avec un ensemble de Skin-Pass 12 une section de passivation 13 un accumulateur de sortie 14 une cisaille 15 et une ou deux enrouleuses 16 travaillant alternativement.

The coating of the steel strips with zinc or zinc-based alloys is carried out on continuous galvanizing lines as schematized in FIG. figure 1 and which typically include:

• An entrance section with one or two belt unrollers 1 a guillotine shear 2 a splice welder 3 allowing to connect the tail of a strip from one of the unrollers to the head of the next band from the other unroller and thus ensuring continuous operation of the line, a tape accumulator 4 which restores on its downstream band previously accumulated when stopping the flow upstream of the accumulator to perform the solder splice.
• A section 5 for degreasing cold-rolled strip or acid pickling of hot-rolled strip.
• An annealing furnace 6 providing heating, maintaining the annealing temperature, cooling, aging when necessary and the controlled temperature setting of the strip before entering the molten zinc bath.
• A galvanizing section proper with the zinc bath 7 in which the strip is immersed, a liquid zinc spinner 8 possibly an induction galvanizing furnace 9, a cooling 10 and a quenching tank 11.
• An output section with a set of Skin-Pass 12 a passivation section 13 an output accumulator 14 a shear 15 and one or two winders 16 working alternately.

The figure 2 describes the arrangement of the different sections of a radiant tube annealing furnace and, superimposed, the evolution of the temperature of the band B as it travels through the furnace (curve T). Said strip B enters the furnace 6 by a preheating section 61 followed by a temperature holding section 62, a cooling section 63 with slow cooling means 631 and fasting 632, an aging section 64 and a heating section 65 required for immersion in the zinc bath 7.

As is known per se, the heating in particular in the preheating sections 61 and holding 62 of the furnace 6 is obtained by means of radiant tubes.

According to a first embodiment of the invention shown in figure 3 a P-shaped radiant tube 2 is installed in the enclosure 1 of a galvanic annealing furnace, for example a preheating or holding section. It is assembled by a support 5 and a plate 4. A burner 3 fueled and combustion air is disposed at the end of the inlet branch 2a of the tube 2 and delivers in the tube of the flue gas at high temperature. These burnt gases are essentially diffused in the chamber 1 by means of calibrated holes 6 formed in the branch 2b of the tube, opposite to the inlet branch 2a. This branch 2b is closed at its end so that partly the burnt gases recirculate in the tube.

Alternatively, as shown in figure 4 , the branch 2b of the tube 2 in P opposite the burner 3 is equipped with a calibrated or adjustable device 7 to evacuate to the outside of the furnace part of the burnt gases.

In another variant represented in figure 5 the branch 2b of the P-tube opposite the burner 3 is equipped with a heating device 8, 9 of the combustion air by the flue gases.

Finally, the radiant tube may be of the double P type as shown in FIG. figure 6 . In this case, as shown in figure 6 , the burner 3 is disposed in the open end of the central input branch 2a of the tube 2. The holes 6 are then preferably formed in each of the opposite branches 2b located on either side of the central branch 2a.

According to a second embodiment of the invention shown in figure 7 a U-tube 2 is installed in the enclosure 1 of a galvanizing annealing furnace. It is assembled by a support 5 and a plate 4. An injector 10 supplied with oxidizing gas under pressure such as water vapor, air or a mixture rich in oxygen delivers in the tube 2 a mixture of oxidizing gases. and mixing HNx at high temperature present in the oven enclosure. This mixture is diffused in the chamber 1 by means of calibrated holes 6 formed in the branch 2b opposite to the input branch 2a. The end of the branch 2b opposite to the input branch 2a comprising the injector is closed by a plug 11.

In a variant described in figure 8 , the radiant tube 2 may be of the double-P type similar to that described in FIG. figure 6 , the burner being replaced by an injector 10.

The injectors are static devices that do not require any other energy than that of the fluid, which are moreover always available in the metallurgical sites, the water vapor at pressures of 8 to 10 bars. On the other hand, the relaxation energy in the furnace enclosure leads to an effect of brewing and circulation that avoids the use of fans. The energy cost of the process is therefore very limited.

Claims (10)

1. In a continuous galvanising annealing furnace for steel strips including a pre-heating section and a holding section and equipped with radiant tubes without a direct flame zone, method for oxidising the strip intended to prevent selective oxidation of steel alloy elements, characterised in that it includes the following steps:

   installation of at least one modified tube capable of injecting an oxidising medium in at least one place in the furnace pre-heating section and/or in at least one place in the furnace holding section; and injection of the oxidising medium by means of one or more modified tube(s) in that it includes at least one tube including at least one leg provided with calibrated holes allowing the oxidising medium to pass into an oxidation zone and replacing at least one existing radiant heating tube;

   the oxidising medium having a composition such that, in the temperature conditions of the oxidising medium and of the steel strip, and depending on the chemical composition of the strip, it has a dew point ensuring deep oxidation of the alloy elements of the steel strip, such as silicon, manganese, aluminium, chromium, in that the composition of the medium is such that, in the temperature conditions of the medium and of the strip, and depending on the chemical composition of said strip, the dew point of the medium is above -20°C.
2. Method according to claim 1, characterised in that the oxidising medium is water vapour, air, or an oxygen-rich gas injected by means of an injector.
3. Method according to claim 1 or 2, characterised in that the medium injected results from the combustion, thanks to a burner, of an over-stoichiometric air/fuel mixture, a stoichiometric oxygen-enriched air/fuel mixture or a stoichiometric air/fuel mixture oxygenated within the limits of non-explosibility.
4. Method according to any one of claims 1 to 3, characterised in that it includes a step to measure the dew point of the oxidising medium in the furnace sections where the modified tubes are installed and a step to regulate the flow rate of the oxidising medium in said tubes in a closed loop with dew point measurement.
5. Device for implementing the method according to one of the preceding claims providing for the organisation, in a pre-heating section and/or a holding section of a continuous galvanising annealing furnace for steel strips equipped with radiant tubes without a direct flame zone, of at least one oxidation zone intended to prevent selective oxidation of alloy elements of the steel through injection of an oxidising medium into the oxidation zone, characterised in that it includes at least one tube including at least one leg provided with calibrated holes allowing the oxidising medium to pass into the oxidation zone and replacing at least one existing radiant heating tube.
6. Device according to claim 5, characterised in that the tube is a U-shaped or W-shaped or P-shaped or double P-shaped or glove-finger-shaped tube comprising an input leg provided with a burner the combustion gases of which constitute the oxidising medium and a leg opposite the input leg provided with the burner.
7. Device according to claim 6, characterised in that the opposite leg of the U-shaped or W-shaped or P-shaped or double P-shaped tube is sealed at its end.
8. Device according to claim 6, characterised in that the opposite leg of the U-shaped or W-shaped or P-shaped or double P-shaped tube comprises, at its end, a calibrated orifice to evacuate a part of the combustion gases.
9. Device according to claim 6, characterised in that the opposite leg of the U-shaped or W-shaped or P-shaped or double P-shaped tube comprises a heat exchanger device to pre-heat a gas supplying the burner by means of the combustion gases.
10. Device according to claim 5, characterised in that the tube is a U-shaped or W-shaped or P-shaped or double P-shaped or glove-finger-shaped tube, having an input leg provided with an injector intended to introduce the oxidising medium and a leg opposite the input leg comprising the injector, the end of which is sealed.

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