The invention relates to a device and a method for controlling the surface reaction on steel sheets transported in a continuous galvanizing or annealing line.
High strength steel grades generally comprise high contents of elements like silicon, manganese and chromium (respectively typically between 0.5 and 2%; 1.5 and 6%, 0.3 and 1% in wt) making them difficult to coat because an oxide layer of those elements is formed during the annealing preceding the dipping in the galvanizing bath. This oxide layer harms the wetting ability of the steel surface when submerged in the bath. As a result, uncoated areas and a poor adhesion of the coating are obtained.
A well-known method to improve the wetting of these steel grades consists in fully oxidizing the steel surface in a specific chamber when the steel has a temperature typically between 600 and 750° C. The resulting oxide layer comprises a high amount of iron oxides which are then reduced during the end of heating and holding section of the annealing furnace and the following thermal treatment. The target is to obtain an oxide thickness between around 50 and 300 nm, what corresponds to an iron oxide below 2 gr/m2.
There are different ways to oxidize the steel surface before the reduction step. For example, this oxidation can be performed in a direct fired furnace running the combustion with air excess. Another way consists in making this oxidation in a dedicated chamber located in the middle of the annealing furnace and supplied with a mixture of nitrogen and an oxidant. Such implementation is described in the patent EP 2 010 690 B1 and in
The formation of the oxide layer must be carefully controlled to avoid the formation of too thick or too thin layers. In the first case, the reduction in the final part of the furnace can be incomplete due to lack of time. It is also known that, in that case, the oxide can stick to the furnace rolls and generate defects. In the second case, the oxide layer is not efficient enough since the oxidation of the alloying elements cannot be inhibited sufficiently and thereby the wetting in the liquid metal bath is not sufficiently improved.
The formation of the oxide layer is guided by three main parameters: strip temperature, oxygen concentration in the atmosphere of the chamber and the transport of that oxygen to the steel surface. Because the edges of the sheet have not the same boundary conditions and turbulence as the center of the sheet, the transport of the oxidant to the edge is different. Similarly to higher edge cooling in the processing line, the oxidation of the edge used to be higher. The width impacted by this over oxidation is in the range from 1 to 10 cm, depending on the design of the oxidizing chamber and on the process parameters used.
To obtain an uniform oxide thickness, it is therefore needed to have a controllable system which can also accommodate the frequent strip width change in a continuous galvanizing line (typically from 900 to 2000 cm).
Mechanical systems can be designed with variable injection sections but this method is not industrially reliable because of the high temperature of the strip and the induced thermal expansion of the material. This becomes a real problem, knowing also that the oxidation chamber can only be used occasionally since all the steel sheets do not need such an oxidation process.
An aspect of the invention provides a continuous annealing furnace for annealing steel strips, the continuous annealing furnace comprising: a reaction chamber in which the steel strips can be transported vertically, the reaction chamber including reactant openings supplied with a reactant, the reactant openings being located at a top or bottom of the reaction chamber, wherein the reaction chamber further includes inert gas openings supplied with an inert gas, the inert gas openings being located on lateral sides of the reaction chamber.
The present invention will be described in even greater detail below based on the exemplary figures. The invention is not limited to the exemplary embodiments. All features described and/or illustrated herein can be used alone or combined in different combinations in embodiments of the invention. The features and advantages of various embodiments of the present invention will become apparent by reading the following detailed description with reference to the attached drawings which illustrate the following:
An aspect of the present invention provides a continuous annealing furnace for annealing steel strips comprising a reaction chamber wherein the steel strips are transported vertically, said chamber comprising openings supplied with a reactant, also called reactant openings, located at the top or at the bottom of the reaction chamber, wherein the reaction chamber further comprises other openings supplied with an inert gas, also called inert gas openings, said inert gas openings being located on the lateral sides of the reaction chamber.
According to particular preferred embodiments, the furnace according to the invention further discloses at least one or a suitable combination of the following features:
An aspect of the invention also provides a method for controlling a surface reaction on a steel strip running vertically through the reaction chamber of the furnace as described above, comprising a step of injecting laterally an inert gas in the reaction chamber and a step of injecting a reactant upstream of the inert gas flow in said chamber.
According to particular preferred embodiments, the method according to the invention further discloses at least one or a suitable combination of the following features:
Finally, an aspect of the invention also provides a steel strip obtained by the method as described above wherein the steel strip has at the exit of the oxidation chamber an oxide layer with an increase of the mass per surface area between the value at the center of the strip and the maximum value at the edge of the strip inferior to 15% and preferably inferior to 10%.
An aspect of the invention aims to provide a device and a method to control the surface reaction of the edges of a sheet without mechanical system. The surface reaction can be any reaction that can occur in a section of an annealing furnace like a reduction reaction or a nitriding reaction, the section being supplied with the appropriate reactant. Indeed, the problem of formation of layers with a different thickness on the edges of the sheet exists regardless of the type of reactant. As an example, the method and the device are hereafter illustrated for a surface reaction occurring in an oxidation chamber supplied with an oxidant.
The annealing furnace comprises an oxidation chamber provided with means for modulating the oxygen concentration of the atmosphere in the regions close to the edges of the sheet. The oxidation chamber according to the invention can be used in a continuous galvanizing line and in a continuous annealing line without hot-dip galvanizing facilities. In this latter case, the uncoated steel sheet can be further pickled to remove the oxide layer formed during annealing.
The method according to an aspect of the invention consists in injecting an inert gas with a defined flow and temperature through the sides of the oxidation chamber. To this end and as shown in
According to an embodiment of the invention, the lateral openings of the chamber can be in the form of holes and one, two or more than two holes can be provided in each lateral side of the chamber. According to other embodiments, the openings can be in the form of slots or any form appropriate for injecting a gas.
In addition, the oxidation chamber can be provided with means for separately controlling the flow of inert gas on each lateral side.
The transversal openings for injecting the oxidant gas through the chamber are preferably located at the top of the chamber for reasons explained below. An opening is located on each side of the sheet. According to an embodiment of the invention shown in
On the opposite side of the transversal openings, i.e. at the bottom of the oxidation chamber if the oxidant injection is carried out at the top, the chamber comprises extraction openings 7 to reduce the pressure inside the chamber when the fluid is not recycled. They can be in the form of slots on each side of the sheet as shown in
The chamber further comprises rolls or similar sealing system at its entry and exit to separate the atmosphere of this chamber from the rest of the annealing furnace and so to minimize the flow of the oxidant in the other parts of the furnace. For sake of simplicity, only half of the rolls 8 being closest to the chamber are represented in
As an example, typical dimensions of the oxidation chamber are the following. It is between 3 and 5 m long with a width that it is about 150 mm wider than the maximal strip width to run. A typical design is 2 m wide for a maximal strip width of 1850 mm. The minimal distance between the casing of the oxidation chamber and the strip is from 75 to 220 mm, preferably from 100 to 200 mm and more preferably of 100 mm.
As shown in
The following simulation illustrates the efficiency of the method and device according to the invention to evenly distribute the oxide layer through the width of the sheet.
Typical FeO formation on a 1050 mm wide strip of specific composition at 700° C. running at 120 mpm in an oxidation chamber being three meter long and two meters wide, with an oxidant flow of 160 Nm3/h per side at 600° C. and comprising 1% O2 is represented in
For similar conditions but with an injection of 40 Nm3/h of inert gas at 600° C. on each lateral side of the chamber, the oxide uniformity is improved as shown in
As already mentioned, for correct efficiency, the right flow and temperature of the main oxidant and of the inert gas need to be adjusted with the strip width and quality processed.
Each flow is controlled by control valves and flow meters. There is a temperature sensor and the temperature is reached by means of a heat exchanger using gas, electricity or other. The total gas injected (oxidant and inert) may be recycled or not. The pressure inside the chamber is controlled by means of fluid extraction in the sealing devices but can also be done by the extraction slots when the fluid is not recycled. This allows avoiding an overpressure in the chamber as well as a flow of the oxidant in the other parts of the furnace. The extraction flow is adjusted by control of the pressure inside the chamber versus that in the other parts of the furnace. A typical flow control may be done in agreement with the PID principle represented in
The invention has been illustrated and described for an oxidation chamber with transversal openings located at the top of the chamber, the oxidant and the inert gas moving down because their temperatures are inferior to that of the strip. The description also covers the configuration with the transversal openings located at the bottom of the oxidation chamber. In this case, the extraction zones must be disposed at the top of the chamber and the inert gas and the main oxidant must be heated at a temperature superior to that of the strip in order to move up. The lateral openings are similarly disposed downstream of the oxidant flow.
While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. It will be understood that changes and modifications may be made by those of ordinary skill within the scope of the following claims. In particular, the present invention covers further embodiments with any combination of features from different embodiments described above and below. Additionally, statements made herein characterizing the invention refer to an embodiment of the invention and not necessarily all embodiments.
The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B, and C” should be interpreted as one or more of a group of elements consisting of A, B, and C, and should not be interpreted as requiring at least one of each of the listed elements A, B, and C, regardless of whether A, B, and C are related as categories or otherwise. Moreover, the recitation of “A, B, and/or C” or “at least one of A, B, or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B, and C.
Number | Date | Country | Kind |
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15162341.0 | Apr 2015 | EP | regional |
15183169.0 | Aug 2015 | EP | regional |
This application is a U.S. national stage application under 35 U.S.C. § 371 of International Application No. PCT/EP2016/056305, filed on Mar. 23, 2016, and claims benefit to European Patent Applications No. 15 162 341.0 and 15 183 169.0, respectively filed on Apr. 2, 2015, and Aug. 31, 2015. The International Application was published in English on Oct. 6, 2016, as WO 2016/156125 A1 under PCT Article 21(2).
Filing Document | Filing Date | Country | Kind |
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PCT/EP2016/056305 | 3/23/2016 | WO | 00 |