EP2188399B2 - 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, in particular AHSS, with high silicon, manganese and aluminum contents and, in particular, to installations comprising a radiant tube furnace without a direct flame heating zone.

The development of materials used in automobile construction has successively led to the galvanization of steel strips prior to their implementation by automobile manufacturers in order to improve the corrosion resistance of steel chassis and bodywork elements. Then, in order to lighten the structures while improving the resistance to collapse by shock (crash) of the vehicles, new grades of steel with very high elasticity limit presenting a great capacity for elongation were developed. Such materials, called AHSS (Advanced High Strength Steels) call upon specific chemical compositions and implementation procedures which distinguish certain families of steel such as "DP" or Dual Phase steels, "TRIP" 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 up new perspectives in automobile design 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 the steel strips a thin layer of oxides during the annealing operation preceding the immersion in the galvanizing bath. This selective oxidation affects the “wettability” of the zinc and therefore the quality of the coating. These phenomena are due to diffusion processes of the highly oxidizable alloying elements towards the surface of the strip where they can oxidize even in the radiant tube zones of the furnaces where the atmosphere is however reducing for iron oxides.

Numerous studies have been carried out in order to understand the kinetics of these oxidation phenomena and to provide solutions to the problems posed during galvanization. The summary document Meeting report ECSC steel workshop Galvanizing of steel strip, Luxembourg, February 27-28, 2002 » of the ECSC (ESCC) gives a list of reference documents resulting for the most part from work carried out under the aegis of the European Community.

Among the solutions proposed in order to ensure quality galvanizing are surface pre-treatments of the steel strips before their implementation in continuous galvanizing installations (chemical treatments, electrodeposition or vapor phase coating with a very thin iron film , nickel, copper, etc.), mechanical or chemical removal of oxides after annealing and before entry into the zinc bath.

Another way has been particularly studied which consists in subjecting, in the annealing furnace, the surface of the strips to conditions of temperature and atmosphere suitable for oxidizing quickly and in depth the alloying elements and thus avoiding their subsequent migration in surface. During this operation, a layer of oxides is formed which will subsequently be eliminated in the following zones 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 within the framework of the “Galvatech 2004” conference describes the physical principles which govern the controlled formation then the reduction of this layer of oxides. The patent JP 02-285057 describes an oxidation phase between 400 and 700°C in a slightly oxidizing atmosphere then a reduction phase between 600 and 800°C in a reducing atmosphere, it indicates temperature ranges and the composition of the gases (contents of O2, N2 and H2 ). 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 the reactions.

The patent EP 1 457 580 describes an installation making it possible to carry out the oxidation phase in a specific enclosure 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 operation aimed not at the specific coating of AHSS steels but making it possible to avoid the use of cleaning fluxes during galvanizing thanks to the oxidation then the surface reduction of carbon steel strips. The annealing furnace preceding the galvanizing bath is a conventional furnace comprising a direct flame heating zone (DFF) and a radiant tube temperature holding zone (RTF). Surface oxidation is obtained in the DFF zone by adjusting the combustion to over-stoichiometric conditions so that the burnt 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 oxidation/reduction controlled AHSS steels. It has the advantage of not requiring additional oxidation installations and of using the DFF / RTF mixed galvanizing furnaces without major modifications.

However, galvanizing furnaces do not all have the DFF zone necessary to carry out oxidation easily and many are those which 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 offers two options for solving the problem. The first consists in using a direct flame combustion chamber separated from the RTF annealing furnace and from which the burnt gases are collected in order to inject them into the furnace. The second is to install a direct flame burner in an area of the oven enclosure. In both cases, the burnt gases provide the necessary oxidizing atmosphere under composition conditions that obviously depend on the temperature of the strip and that of the gases. The reduction is then conventionally obtained by passage through a mixture of nitrogen and hydrogen. These two possibilities require a modification of the existing installations (additional combustion enclosure and ducts for supplying the burnt gases to the furnace, fitting a burner in the furnace). 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 great flexibility of use.

The method according to the preamble of claim 1 and an associated device for its implementation provide the solution to these two problems.

In general, the invention consists in injecting an oxidizing medium into a section of a furnace with radiant tubes, in particular with a nitrogen/hydrogen atmosphere, using one or more tubes, in particular specially modified and capable of being installed in place and 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 present, depending on the temperature of the strip and the chemical composition of the latter, a dew point such that the alloying elements such as silicon, manganese, aluminum, chromium are oxidized in depth and no longer have the possibility of migrating to the surface. Typically, this dew point is above -20°C.

To achieve this objective, the injected medium can 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 air or super-oxygenated air or oxygen/fuel mixture 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 defined according to claim no.1 relates in particular to a method ensuring, in a continuous galvanizing annealing furnace for steel strips comprising a preheating section and a holding section and equipped only with radiant tubes, strip oxidation aimed at preventing the selective oxidation of alloying elements of steel, characterized in that it comprises the following steps:

• the installation in at least one place of the heating section of the furnace and/or in at least one place of the holding section of the furnace, of at least one modified tube capable of injecting an oxidizing medium; And
• injecting the oxidizing medium through 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 in-depth oxidation of the elements steel strip alloy.

Control of this selective oxidation preferably involves measuring the dew point in the zone(s) in which the modified tube(s) are installed. This measurement can be carried out by dew point transmitters installed in a fixed position and operating in a closed loop with the members for adjusting the flow rate of the oxidizing medium injected by the oxidizing medium injectors and/or adjusting the burners.

The invention also presents an associated device for its implementation of said method and ensuring the organization, in a preheating section and/or a holding section of a continuous galvanizing annealing furnace for steel strips equipped only radiant tubes, with at least one oxidation zone aimed at preventing the selective oxidation of steel alloy elements, by injecting an oxidizing medium into the oxidation zone, characterized in that it comprises at least one tube comprising at least one branch provided with calibrated holes allowing the oxidizing medium to pass into the oxidation zone.

The means for introducing the oxidizing medium can be either an injector supplying the tube with a hot oxidizing medium such as steam, air or an oxygen-rich gas, or a burner supplying the tube into a product resulting from the combustion of an over-stoichimetric air/fuel mixture, a stoichiometric super-oxygenated air/fuel mixture or a stoichiometric air/oxygenated fuel mixture within the non-explosive limit.

The modified tube(s) intended to supply the oxidizing medium necessary for the oxidation of the strip is (are), for example, a U-shaped tube, one inlet branch of which is equipped at its end with a device for injecting water vapor or air, preheated or not, super-oxygenated or not, or oxygen and whose branch opposite the inlet branch is closed at its end, at least one of the branches preferably the branch opposite 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, double-P shape, W-shape or thimble shape.

According to another characteristic of the invention, the radiant tube intended to supply the oxidizing medium is a P-shaped tube having an inlet branch equipped at its end with a burner and of which at least one of the branches, preferably the opposite branch at the inlet branch, is pierced with calibrated holes allowing the burnt gases to pass into the enclosure of the furnace. The branch opposite the inlet branch comprising the burner can allow part of the burnt gases to evacuate outside the furnace through a calibrated orifice or comprise a heat exchanger device making it possible to preheat the combustion air with the burnt gases. The P-tube can be replaced by any conventional tube shape such as, for example, a U-shape, W-shape, double-P shape or glove finger shape. The burner(s) are supplied with an over-stoichiometric air/fuel mixture or a stoichiometric super-oxygenated air/fuel mixture or a stoichiometric air/oxygenated fuel mixture within the non-explosive limits.

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 oxidation or be permanently installed at different points in the furnace. 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 process is to locate the injection of oxidizing medium exactly where it is needed, that is to say very close to the two sides 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 rest 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 progress of the steel strip from its entry into the furnace to its exit from the zinc bath as well as the variation in 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 steel strips with zinc or zinc-based alloys is carried out on continuous galvanizing lines as shown schematically in figure 1 and which typically include:

• An entry section with one or two strip unwinders 1 a guillotine shear 2 a butt welder 3 allowing the tail of a strip coming from one of the unwinders to be connected to the head of the next strip coming from the other unwinder and thus ensuring continuous operation of the line, a strip accumulator 4 which restores downstream of the previously accumulated strip when the unwinding upstream of the accumulator is stopped to perform the butt welding.
• A section 5 for degreasing cold rolled strips or for acid pickling hot rolled strips.
• An annealing furnace 6 ensuring the heating, the maintenance at the annealing temperature, the cooling, the aging when necessary and the controlled temperature setting of the strip before it enters the bath of molten zinc.
• A galvanizing section proper with the zinc bath 7 in which the strip is immersed, a liquid zinc wringing device 8 possibly an induction galvanealing furnace 9, cooling 10 and a quenching tank 11.
• An output section with a Skin-Pass assembly 12 a passivation section 13 an output accumulator 14 a shear 15 and one or two winders 16 working alternately.

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

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

According to a first embodiment of the invention shown in picture 3 , a radiant tube 2, in P, is installed in the enclosure 1 of a galvanizing annealing furnace, for example a preheating or holding section. It is assembled by a support 5 and a plate 4. A burner 3 supplied with fuel and combustion air is arranged at the end of the inlet branch 2a of the tube 2 and delivers high temperature burnt gases into the tube. . These burnt gases are essentially diffused in the enclosure 1 using calibrated holes 6 made in the branch 2b of the tube, opposite the inlet branch 2a. This branch 2b is closed off at its end so that some of 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 making it possible to evacuate part of the burnt gases to the outside of the furnace.

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 burnt gases.

Finally, the radiant tube can be of the double P type as shown in figure 6 . In this case, as shown in figure 6 , the burner 3 is arranged in the open end of the central inlet branch 2a of the tube 2. The holes 6 are then preferably made 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 steam, air or a mixture rich in oxygen delivers into the tube 2 a mixture of oxidizing gases and high temperature HNx mixture present in the furnace enclosure. This mixture is diffused in the enclosure 1 using calibrated holes 6 provided in the branch 2b opposite the inlet branch 2a. The end of branch 2b opposite to inlet branch 2a comprising the injector is closed off by a plug 11.

Alternatively described in figure 8 , the radiant tube 2 can be of the double P type similar to that described in figure 6 , the burner being replaced by an injector 10.

The injectors are static devices requiring no other energy than that of the fluid which are moreover always available in metallurgical sites, water vapor under pressures of 8 to 10 bars. On the other hand, the expansion energy in the enclosure of the oven leads to a stirring and circulation effect which avoids the use of fans. The energy cost of the process is therefore very limited.

Claims (4)

1. Method ensuring, in a continuous galvanizing annealing furnace for steel strips comprising a preheating section and a holding section and equipped with radiant tubes without direct flame zone, the oxidation of the strip with the aim of preventing selective oxidation of the steel alloy elements, characterized in that it comprises the following steps:

   installing, in at least one location of the preheating section of the furnace and/or in at least one location of the holding section of the furnace, of at least one modified tube capable of injecting an oxidizing medium; and

   injecting the oxidizing medium via the one or more modified tubes in that it comprises at least one tube having at least one branch provided with calibrated holes letting the oxidizing medium pass into an oxidation zone and replacing at least one existing radiant heating tube;

   the oxidizing medium having a composition such that, in the temperature conditions of the oxidizing medium and of the steel strip, and as a function of the chemical composition of the strip, it has a dew point ensuring the deep oxidation of the alloy elements such as silicon, manganese, aluminium, chromium of the steel strip, in that the composition of the medium is such that, in the temperature conditions of the medium and of the strip and as a function of the chemical composition of said strip, the dew point of the medium is higher than -20°C.
2. Method according to Claim 1, characterized in that the oxidizing medium is water vapour, air or an oxygen-rich gas, injected by means of an injector.
3. Method according to Claim 1 or 2, characterized in that the injected medium results from the combustion, by virtue of a burner, of a superstoichiometric air/fuel mixture, of a stoichiometric superoxygenated air/fuel mixture or of a stoichiometric mixture of air/fuel oxygenated within the limits of non-explosivity.
4. Method according to any one of Claims 1 to 3, characterized in that it comprises a step of measuring the dew point of the oxidizing medium in the sections of the furnace where the modified tubes are installed and a step of regulating the flow rate of oxidizing medium in said tubes in a closed loop with the measurement of the dew point.

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