DEVICE AND METHOD FOR CARRYING OUT CONTROLLED OXIDATION OF METAL STRIPS IN A CONTINUOUS FURNACE

Abstract

The invention relates to a chamber (1) for the controlled oxidation of metal strips in a furnace for annealing a continuous production line of strips which are hot-coated, for example by galvanisation, the oxidation chamber allowing the oxidation of the metal strips by means of an oxidising gas injected on at least one of the faces of a strip (15), the oxidation chamber comprising oxidation portions (17) extending over the width and/or length thereof, each portion comprising at least one blow opening (4) and at least one suction opening (5) between which an oxidising gas circulates, each portion being controllable in a different way so as to adjust the oxidation induced on the strip over the width and length of the oxidation chamber.

Claims

1. An oxidation chamber for the controlled oxidation of metal strips in an annealing furnace of a continuous production line for hot-coated strips, the oxidation chamber comprising means of an oxidizing gas injected on at least one of the sides of a strip and oxidizing portions extending over a width and/or length of the oxidation chamber, each oxidizing portion comprising at least one blowing port and at least one suction port between which an oxidizing gas circulates; each portion can be controlled separately to adjust the oxidation induced on the strip over the width and length of the oxidation chamber.

2. An oxidation chamber according to claim 1, wherein the oxidizing gas is injected onto the strip in a direction substantially perpendicular to the strip by means of blowing ports and the oxidizing gas circulates in the chamber to suction ports in a direction substantially parallel to the moving direction of the strip.

3. An oxidation chamber according to claim 1, wherein it is placed downstream, in the moving direction of the strip, a section in which the strip undergoes a first oxidation.

4. An oxidation chamber according to claim 1, wherein the oxidizing gas used is air.

5. An oxidation chamber according to claim 1, wherein the oxidizing gas used is a mixture of air and flue gas.

6. An oxidation chamber according to claim 1, further comprising at least one oxidation sensor situated upstream and/or downstream of the oxidizing portion, information from the oxidation sensor being integrated into the calculation of the oxidizing gas flow leaving the blowing port of the oxidizing portion.

7. A process of controlled oxidation of metal strips implemented in a controlled oxidation chamber according to claim 1, by means of an oxidizing gas injected on at least one of the sides of the strip, wherein the oxidizing gas is air or combustion flue gas, or a mixture of air and combustion flue gas.

8. The process according to claim 7 wherein the characteristics of the oxidizing gas and/or the kinetics of injection and suction of the oxidizing gas in the portions are controlled separately to adjust the oxidation on the strip over the width and length of the oxidation chamber.

9. The process according to claim 7, wherein the dimensions of a portion are controlled by the choice of the blowing ports and the suction ports operating in said portion.

10. The process according to claim 9, wherein the residence time of the oxidizing gas in the controlled oxidation chamber is adjusted by portion according to the length of said portion in the moving direction of the strip.

Description

[0029] In what follows, the invention is explained in detail based on examples of the process with reference to FIGS. 1 to 7 of the drawings.

[0030] FIG. 1 is a partial schematic representation of an oxidation chamber according to an example embodiment of the invention, as seen from one side of the strip, comprising circular blowing and suction ports, distributed over a blowing zone and a suction zone,

[0031] FIG. 2 is a partial schematic representation of an oxidation chamber according to an example embodiment of the invention like that of FIG. 1, as viewed from one side of the strip, the blowing and suction ports being rectangular,

[0032] FIG. 3 is a partial schematic representation of an oxidation chamber according to an example embodiment of the invention like that of FIG. 2, as seen from one side of the strip, the wall of the oxidation chamber comprising four series of ports instead of two,

[0033] FIG. 4 is a partial schematic representation of an oxidation chamber according to an example embodiment of the invention like that of FIG. 3, as seen from one side of the strip, the wall of the oxidation chamber also comprising suction ports placed transversely,

[0034] FIG. 5 is a partial schematic representation of an oxidation chamber in cross-section according to an example embodiment of the invention in which the blowing ports do not project beyond the internal walls of the chamber;

[0035] FIG. 6 is a partial schematic representation of an oxidation chamber in cross-section according to an example embodiment of the invention in which the blowing ports protrude from the internal walls of the chamber, and

[0036] FIG. 7 is a partial schematic representation of a continuous line comprising an oxidation chamber according to an example embodiment of the invention.

[0037] Throughout the following description of various embodiments of the invention, the relative terms such as front, back, upstream and downstream are to be interpreted in view of the strip's moving direction as well as terms such as above, below are to be interpreted in view of the position of the different elements in the figures.

[0038] FIGS. 1 to 4 present in schematic views examples of oxidation chamber architectures according to the invention in which the strip travels in the direction indicated by the arrow 16, in an oxidizing or non-oxidizing furnace zone. These figures show schematically in front view an example of a wall 2 of a controlled oxidation chamber 1 according to the invention, as seen from one side of the strip. The walls of the oxidation chamber here consist of elementary modules 3 juxtaposed, of rectangular shape. For example, it can be a brick made from refractory material. However, this example embodiment is just an illustration; other embodiments may be used. For example, the walls of the oxidation chamber may be in one module. They can be covered with refractory fiber, and possibly covered with a stainless-steel plate.

[0039] As can be seen in these figures, certain elementary modules 3 comprise circular or rectangular ports 4, 5 through which the gas is injected onto the strip or is discharged from the oxidation chamber. The number of injection ports 4 per elementary module and the unit section of these ports are chosen to cover the entire width of the strip with unit gas jets whose shape and kinematics allow to cover a unitary band surface with a speed adapted to ensure the oxidation of the strip.

[0040] In these examples, suction ports 5 are placed above blowing ports 4, but this example is not restrictive, the suction ports can be placed below the injection ports. In these examples, if the strip circulates as represented from bottom to top, the flow of the injected gas is therefore in the direction of flow of the strip. If the strip flows from top to bottom, the flow of the injected gas is therefore in the opposite direction of the flow of the strip. Regarding the use of these references at high and low positions, we thought these figures illustrate a vertical chamber. Obviously, it could also be a horizontal chamber, with a horizontal direction of travel of the strip, or an inclined chamber, for which the position of the ports would then be defined more generally according to the moving direction of the strip.

[0041] In FIG. 1, we can see an example embodiment in which the blowing ports 4 are located on two successive rows of unitary modules 3. The blowing ports are thus aligned on two lines 6, 7 parallel to the width of the strip. In this example, we have three ports per unit module. The position of the ports is shifted to the second row 7 with respect to the first row 6, so as to obtain a greater coverage of the strip surface over its width. The suction ports 5 have a similar distribution and are distributed in two rows 8 and 9. The distribution of the suction ports 5 is symmetrical to that of the blowing ports 4 along an axis of transverse symmetry passing half way between the blowing ports 4 and suction ports 5. The distance between the blowing zone and the suction zone, in the moving direction of the strip, depends on the maximum travel speed of the strip and the kinematics of the oxidizing gas blown on the strip. Here it corresponds to three rows of unitary modules.

[0042] The number of blowing ports 4 and suction ports 5 in operation and their location are adjusted according to the locations on the surface of the strip that require additional oxidation. The suction ports 5 in operation are naturally aligned with the blowing ports 4, in the moving direction of the strip.

[0043] The flow rate of the oxidizing gas may be adjusted by line 6, 7 of blowing ports, by set of blowing ports, or individually by blowing port 4, so as to adjust for each port 4 or set of ports the kinematics of the oxidant gas jets and effect on the strip.

[0044] Moreover, when the oxidizing gas is a mixture of air and flue gas, it is also possible to vary the concentration of oxygen in the oxidizing gas through the blowing port, or through a set of blowing ports, by adjusting the proportions of air and flue gas, and thus adjust the oxidizing power of the gas jets.

[0045] We see that more resources can be used independently or in combination to fine-tune the oxidation of the strip at each point in the process.

[0046] In FIG. 2, we can see a schematic representation of an example embodiment like that shown in FIG. 1 but with rectangular blowing and suction ports. A unitary portion 17 delimited by a blowing port and a suction port is shown in this figure.

[0047] FIG. 3 schematically represents, by way of example, the architecture of an oxidation chamber according to the invention having 8 lines 6 to 13 of ports per strip face. This oxidation chamber longer than those of FIGS. 1 and 2 is especially adapted for high travel speeds of the strip. Furthermore, for the same strip travel speed as that of the chambers shown in FIGS. 1 and 2, the longer length of the oxidation chamber makes it possible to carry out the oxidation with a slower kinematics, which may be advantageous for certain types of steel.

[0048] For example, this chamber can thus have two successive oxidation zones by blowing/suction, the lines of ports 6, 7, 10 and 11 ensuring blowing and lines 8, 9, 12 and 13 suction. It is for example possible to dedicate each to a different type of gas, or to blow the same gas with two different injection kinematics.

[0049] This chamber can also be operated using only the lines of ports 6 and 7 for blowing the oxidizing gas and lines 8 to 13 for suction. Depending on the required exchange length between the oxidizing gas and the strip, the suction ports used will be those of lines 8 and 9, those of lines 10 and 11 or those of lines 12 and 13, the lines 8 and 9 leading to the shortest exchange length and lines 12 and 13 to the longest exchange length.

[0050] FIG. 4 schematically represents, by way of example, the architecture of an oxidation chamber according to the invention in the same principle as that of FIG. 3 but advantageously having transverse suction arranged successively according to the width of the furnace. The presence of these transversal suction ports 14 makes it possible to delimit precisely on the width of the strip, and on the length of the oxidation chamber, zones in which the oxidation can be controlled separately.

[0051] The device according to the invention can thus be composed of a longitudinal blowing system in several independently controlled parts and a suction system arranged alternately to the blowing and arranged at an advantageous distance to control the required oxide value on the strip. The suction and blowing parts of the zone in question are controlled simultaneously, which allows the injected air flow to be discharged after an equivalent residence time at the set distance and not to be spread laterally to other areas of the strip, and thus cause unwanted oxidation on other areas of the strip.

[0052] FIG. 5 schematically represents a sectional view of an oxidation chamber 1 at the level of blowing ports 4, according to one embodiment of the invention. In this example, the blowing ports do not protrude from the unit modules 3 in the direction of the strip 15.

[0053] FIG. 6 schematically represents a cross-sectional view of an oxidation chamber 1 at the level of blowing ports 4, according to another example embodiment of the invention in which the blowing ports protrude from the unit modules 3 in the direction of the strip 15.

[0054] In the two example embodiments in FIGS. 5 and 6, the suction ports are not shown. They may not protrude from the unit modules 3 in the direction of strip 15 or protrude from said modules. In an oxidation chamber according to the invention, the blowing and suction ports may not protrude from the unit modules 3 towards the strip 15, the blowing ports may not protrude while the suction ports protrude, and the blowing ports may protrude while the suction ports do not protrude.

[0055] The distance between the strip and the end of the blowing and suction ports is related to the flow rate and the kinematics of the oxidizing gas jets.

[0056] The inventor states that the minimum air injection rate in the oxidation zone is very low (for example 10 Nm.sup.3/h of air for a flow of oxidizing gas over a length of one meter, measured between blowing and sucking and/or length, in the longitudinal direction of the strip'smovement, corresponding to the required oxidation portion, said length giving an oxide thickness of 70 nm over a 1500 mm wide strip traveling at 100 m/min at a temperature of 650 C.), the control of the oxidation can take place advantageously by opening/closing one or more oxidation zones (blowing/suction) and thus varying the overall flow rate by varying the strip's residence time under oxidizing gas and thus varying the thickness of oxide. If only some of the zones are used in oxidation, and in order not to diffuse the oxidizing gas in other zones, it can be replaced by a nitrogen flow toto create a barrier with the oxidation zone

[0057] This operation can be performed over the entire width of the strip or only a part, thus giving great flexibility in the management of the atmosphere in contact with the strip while keeping the critical speeds of injection on the strip in the required oxidation zone and isolating the other zones by injection of a neutral gas such as nitrogen for example. This operating mode makes it possible to dispense with the travel speed of the strip in the control of the oxide thickness.

[0058] According to an advantageous example embodiment, the device according to the invention is placed downstream of an oxidation section without precise control of oxidation on the width of the strip. This allows, for example, to achieve most of the targeted oxide layer quickly, that is to say over a limited furnace length. The device according to the invention then makes it possible to carry out additional oxidation locally, for example to obtain a homogeneous oxide thickness over the width of the strip or to reinforce it locally.

[0059] The oxidation section without the exact oxidation of the strip's width also make it possible to produce a layer whose oxides will have a morphology or a given composition different from the surface layer, which will then be produced by the device according to the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

[0060] According to an example embodiment of the invention, represented in FIG. 7, the oxidation section 100 without precise control of the oxidation over the width of the strip is a portion of a furnace 110 preheating the strip by direct flame. From thestrip input, this furnace comprises a zone 120 for preheating the strip by exhausting the flue gas followed by a heating zone 130 equipped with direct flame burners. In this example embodiment, in the moving direction of the strip, the first 15 pairs of burners (over 13 m of furnace length) operate under stoichiometry so as to avoid oxidizing the strip. The last 3 pairs of burners (over 4.2 m of furnace length) delimit the section 100 in which the burners operate with a large excess of air to obtain a significant oxidation of the strip. The device 1 according to the invention placed downstream of this oxidizing zone then makes it possible to fine-tune the oxidation over the width of the strip.

[0061] The 1500 mm wide strip circulates with a nominal speed of 100 m/min. The chamber 1 has a length of 475 mm in the moving direction of the strip. The blowing zone has 55 ports arranged on two transverse lines 80 mm apart. The suction zone has 55 ports arranged on two transverse lines 80 mm apart. The distance between the nearest blowing and suction lines is 315 mm. The blowing ports are placed 100 mm from the strip every 58 mm depending on the width of the strip. Their injection diameter is 25 mm. The suction ports are placed 100 mm from the strip every 58 mm according to the strip width. Their suction diameter is 25 mm.

[0062] The oxidizing gas is air. It is injected on the stripat a nominal speed of 3 m/s. The injection speed is modulated by injector, or injector assembly, between 0 and 5 m/s according to the amount of the oxidation required on the surface of the affected strip. The strip is at 650 C. when it enters the oxidation chamber. The oxidizing gas is injected at a temperature of 650 C.