Metal treatment – Process of modifying or maintaining internal physical... – Producing or treating layered – bonded – welded – or...
Reexamination Certificate
1999-08-06
2001-05-01
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Producing or treating layered, bonded, welded, or...
C427S321000, C427S432000, C427S433000
Reexamination Certificate
active
06224692
ABSTRACT:
BACKGROUND OF THE INVENTION:
(i) Field of the Invention
The present invention relates to a process for galvanizing a metal component (strip, plate, etc.) in a continuous galvanizing line, the galvanizing line comprising, placed in series and connected to each other by ducts in order to form ducting for circulating a reducing atmosphere usually composed essentially of an inert gas, such as nitrogen or argon, and, of hydrogen, a preheat furnace, an annealing furnace, a cooling station and a station for dipping the metal component into the bath of liquid zinc or of a zinc alloy, in which process, before the metal component is dipped into the liquid bath, it is exposed to this reducing atmosphere in order to remove oxides present on the surface of the metal components.
The description which follows will refer to metal “strip” in order to be specific, and will refer indiscriminately to a bath of liquid zinc or a bath of liquid zinc alloy, without the reference chosen being regarded as restrictive, because, as is known, the industry uses alloys which are extremely varied, especially in their zinc and/or aluminum content.
In general, then, a continuous galvanizing line comprises at least four zones for treating the metal strip to be galvanized, namely a preheat zone, an annealing zone, a cooling zone and a dipping zone which comprises a zinc bath into which the metal strip to be galvanized is dipped.
(ii) Description of the Related Art
Galvanizing lines are known in which the preheat zone comprises a furnace fitted with naked-flame burners serving, on the one hand, to rapidly reheat the metal strip to be treated to a temperature typically of between 400° C. and 700° C. and, on the other hand, to make the rolling oils present on the surface of the strip undergo pyrolysis.
In order to prevent oxidation of the metal strip thus treated, the burners are operated in air depletion mode in order to provide an atmosphere which is nonoxidizing with respect to iron.
In order to be able to ensure good galvanizing, that is to say, in particular, good adhesion between coating and metal strip, it is absolutely essential to remove any surface oxide layer before the metal strip is dipped into the zinc bath. This is achieved by exposing the metal strip in the annealing furnace to a reducing atmosphere usually consisting of a mixture of nitrogen and hydrogen, the hydrogen content generally being between 15% and 40%.
For this purpose, the various treatment zones of the galvanizing line are connected together by ducts in order to form ducting for circulating the reducing atmosphere.
In order to constantly regenerate this reducing atmosphere in this ducting and thus to preserve its reducing nature, the mixture of nitrogen and hydrogen is injected into a duct also called a spout or nozzle, one end of which dips into the zinc bath and the other end is joined to the outlet end of the cooling station, so that the reducing atmosphere flows in the opposite direction to the direction in which the metal strip to be treated runs.
At the present time, for a given galvanizing line, the flow rate of the mixture of nitrogen and hydrogen and the hydrogen content of this mixture are maintained at the same level, independently of the characteristics and the run speed of the metal strip to be treated.
In practice, in order to make it possible both to treat very wide metal strips and narrow strips and to accommodate low run speeds and high speeds, the flow rate of the mixture of nitrogen and hydrogen and the hydrogen content of the mixture are fixed at a high level so as to allow the treatment even of the most unfavorable cases, i.e. metal strip of large surface dimensions and/or treated at high speeds. However, it may be imagined that this excessive quality represented by a hydrogen-rich mixture injected at a high rate entails a not insignificant cost for this reducing atmosphere. Moreover, since the atmosphere injection conditions are fixed, which the surface to be treated per unit time may vary, the production of water vapor in the enclosure, because of the reduction of the oxides, will well and truly make the reducing nature of the atmosphere vary and therefore cause variations in the quality of the final product.
SUMMARY AND OBJECTS OF THE INVENTION
The object of the invention is to provide a process making it possible to optimize the use of the reducing atmosphere for the purpose of reducing the cost that it entails in running the galvanizing line, as well as to better maintain the level of quality of the products which leave the line.
For this purpose, the subject of the invention is a process for galvanizing a metal strip in a continuous galvanizing line, the galvanizing line comprising, placed in series and connected to each other by ducts in order to form continuous ducting for circulating a reducing atmosphere which comprises an inert gas and hydrogen, a preheat furnace, an annealing furnace, a cooling station and a station for dipping the metal strip into a bath of liquid zinc or of a liquid zinc alloy, in which process, before the metal strip is dipped into the bath, it is exposed to this reducing atmosphere in order to remove oxides present on its surface, characterized in that, in order to replenish the reducing atmosphere in the ducting, the inert gas and the hydrogen are injected into it, with the hydrogen flow rate being adjusted depending on the area of the metal strip to be treated per unit time.
The process according to the invention may also include one or more of the following characteristics:
the area of metal strip to be treated per unit time is determined from the width of the metal strip to be treated and from the speed at which the latter runs through the galvanizing line;
the ratio of the hydrogen concentration to the water vapor concentration of the atmosphere is maintained, at least at one point in the ducting, substantially at a predefined level;
the ratio is maintained at a predefined level at least at one point in the annealing furnace;
the inert gas is injected at a first location into the ducting and hydrogen, or an inert-gas/hydrogen mixture, is injected at a second location a certain distance from the first location and further away from the liquid bath of the dipping station;
the inert gas and the hydrogen, or an inert-gas/hydrogen mixture, are/is injected into the duct which connects the cooling station to said dipping station;
the flow rate of inert gas injected into the ducting at the first location is fixed and the flow rate of hydrogen or of inert-gas/hydrogen mixture injected at second location is adjusted, depending on a set-point value of the water vapor content at a point in the annealing furnace;
the flow rate of inert gas injected into the ducting at the first location is fixed and the flow rate of hydrogen or of the inert-gas/hydrogen mixture injected at the second location is adjusted so as to carry out the operation of maintaining, at least at one point in the annealing furnace, the ratio of the hydrogen concentration to the water vapor concentration of the atmosphere substantially at the predefined level;
the flow rate of inert gas injected into the ducting at the first location is adjusted depending on a set-point value of the water vapor content at a point in the annealing furnace;
the flow rate of inert gas injected into the ducting at the first location is adjusted so as to carry out said operation of maintaining, at least one point in the annealing furnace, the ratio of the hydrogen concentration to the water vapor concentration of the atmosphere substantially at the predefined level;
the inert gas is injected into the ducting at the first location at a substantially constant flow rate and inert gas is also injected into the annealing furnace, the flow rate of inert gas injected into the annealing furnace being adjusted depending on a set-point value of the water vapor content at a point in the annealing furnace;
the inert gas is injected into the ducting at the first location at a substantially constant flow rate and inert gas is also injected into the annealing
Burns Doane Swecker & Mathis L.L.P.
L'Air Liquide Societe Anonyme pour l'Etude et l'E
Wyszomierski George
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