Abstract:
A process for galvanizing a metal strip having a given area in a continuous galvanizing line through which the strip is run at a given speed comprising, in series, the steps of: 
     (iii) pre-heating the metal strip; 
     (iv) annealing the metal strip; 
     (v) cooling the metal strip; 
     (vi) dipping the metal strip into a bath comprising liquid zinc or a liquid zinc alloy, the steps (i)-(iv) being maintained in closed fluid contact with each other; 
     (vii) circulating a reducing atmosphere comprising an inert gas and hydrogen to the steps (i)-(iv) in the galvanizing line and exposing the metal strip, before the step (iv) of dipping the metal strip into the bath, to the reducing atmosphere in order to remove oxides present on its surface, 
     (viii) replenishing the reducing atmosphere in the galvanizing line by injecting the reducing atmosphere into the line and adjusting the hydrogen flow rate depending on the area of the metal strip to be treated per unit time.

Description:
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 furnace, the flow rate of inert gas injected into the annealing furnace being 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. 
     Further characteristics and advantages of the invention will emerge from the following description, given by way of nonlimiting example, with regard to the appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic representation of a continuous galvanizing line operating using a process according to the invention; 
     FIG. 2 shows a curve representing the variation in the logarithm of the ratio of the water content to the hydrogen content of the atmosphere at a point in the annealing furnace plotted as a function of the area of metal strip treated, for a given atmosphere setting. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention is also described in French Application Serial No. 98 10392, filed Aug. 13, 1998, the disclosure of which is hereby incorporated by reference. 
     FIG. 1 shows diagrammatically a line  1  for galvanizing a metal strip  3 , for example a steel strip. 
     The galvanizing line  1  comprises, placed in series, a preheat furnace  5 , an annealing furnace  7 , a cooling station  9  and a dipping station  11  which comprises a bath  13  of liquid zinc or of liquid alloy. 
     The preheat furnace  5  is, for example, fitted with naked-flame burners  15  serving, on the one hand, to rapidly reheat the metal strip  3  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. 
     The annealing furnace  7  is, for example, fitted with electrical resistance elements or with radiant tubes, these being shown diagrammatically at  8 . 
     The cooling station  9  serves to cool the metal strip  3 , as it leaves the annealing furnace  7 , to a value close to 470° C., for example. 
     Moreover, the preheat furnace  5 , the annealing furnace  7 , the cooling station  9  and the dipping station  11 , each of which is in the form of a tunnel, are joined together by ducts  17 ,  19  and  21  in order to form, with these, a continuous ducting  23  for circulating a reducing atmosphere composed essentially of nitrogen and hydrogen. 
     In addition, the duct  21  joining the outlet end of the cooling station  9  to the dipping station  11  is inclined downward and its end  25  dips into the liquid bath  13 . This duct  21  is often called a snout or nozzle. 
     Furthermore, the galvanizing line  1  according to the invention comprises, on the one hand, an injector  30  for injecting an inert gas, for example nitrogen, placed in the wall of the snout  21  at a first location  30 A lying near the end  25  of the snout dipped into the liquid bath  13 , above the latter, and, on the other hand, an injector  31  for injecting hydrogen (or a mixture of hydrogen and an inert gas) placed in the wall of the snout  21  at a second location  31 A lying near that end  33  of the latter which is joined to the cooling station  9 . 
     This advantageous arrangement, in which the nitrogen is injected at the first location  30 A lying near the end  25  of the snout  21  dipped into the liquid bath  13 , makes it possible to form, in the lower part of the snout  21 , a buffer which prevents the hydrogen, injected some distance away at the location  31 A, from dissolving in the bath of liquid zinc  13 . 
     As may be seen by looking at the figure, the injector  30  is connected to a feed pipe  32  in which a flow regulator  34  is placed and the injector  31  is connected to a feed pipe  36  in which a flow regulator  38  is placed. 
     Furthermore, the line  1  comprises an adjustor  40  for determining and adjusting the run speed of the metal strip  3 . 
     Moreover, a gas-sampling tap  42  inside the annealing furnace, for example in the middle of the furnace or in the last ⅓ of the furnace, makes it possible to send atmosphere samples, for analyzing, for example as shown in the figure, to an analyzer  47  which analyzes the hydrogen content of the sample and to an analyzer  46  which analyzes the water vapor content of the sample. 
     Of course, instead of this ex situ analysis an oxygen probe in the furnace could also, without departing from the scope of the present invention, be used, which probe delivers a voltage correlated with the H 2 /H 2 O ratio. 
     The adjustor  40  and the analyzers  46  and  47  are connected to a data-processing unit  50  (for example a programmable controller), which unit is able in turn to control the operation of the two flow regulators  34  and  38 . 
     During its treatment, the metal strip  3  guided by rollers  27  passes in succession through the preheat furnace  5 , in order to be brought to a temperature here of between 400° C. and 700° C., then through the annealing furnace  7 , in order to ensure it metallurgical characteristics, through the cooling station  9 , in order to bring it to a temperature close to 470° C., and finally through the dipping station  11 , so as to be coated with zinc. 
     At the same time, the unit  50  measures, as described above, the run speed of the metal strip  3 , the dew point and the hydrogen content of the atmosphere, at least one point ( 42 ) in the annealing furnace  7 , and, by means of the regulators  34  and  38 , controls the flow rates of nitrogen and/or hydrogen injected into the snout  21 , in accordance with one of the embodiments of the invention described above in the present description. 
     The unit  50  adjusts these nitrogen and hydrogen flow rates depending on the area of the metal strip to be treated per unit time. 
     Advantageously, in order to determine the area of the metal strip to be treated per unit time, the speed at which the strip runs through the line, provided by the adjustor  40 , and the width of the strip  3  are taken into account. 
     In order to illustrate more clearly this notion of adjusting the flow rates depending on the area of the metal strip to be treated per unit time (which factor is evaluated by considering only one side of the strip), reference is made to the curve shown in FIG. 2, which was obtained for a given steel having application in the construction industry, under the following conditions: 
     injection at  30 A: nitrogen of cryogenic origin, at a flow rate of 50 Sm 3 /h; injection at  31 A: cracked ammonia, at a flow rate of 70 Sm 3 /h (such conditions therefore give overall a mixture flow rate of 120 Sm 3 /h, a hydrogen flow rate of 52.5 Sm 3 /h and a hydrogen concentration in the mixture of 43.8%); 
     the atmosphere sampling point ( 42 ) was located approximately 1 m from the end of the annealing furnace (taking the direction of movement of the strip into consideration); 
     the speed of the line was between 25 and 80 m/min., for a strip width always within the range going from 1 m to 1.20 m. 
     This figure therefore clearly shows the increase in the H 2 O/H 2  ratio in the annealing furnace when the area treated per unit time (A/t) increases, a sign that water vapor production is increasing. It may therefore be seen on this curve that it is possible to define, for this galvanizing line and this average gas setting employed, two large ranges of area treated per unit time, namely a range in which A/t is less than approximately 50 m 2 /min, and a range in which A/t lies between approximately 50 m 2 /min. and 90 m 2 /min. 
     It therefore seems advantageous, for production conditions corresponding to the first range, to reduce the flow rate of hydrogen injected at  31  and/or the hydrogen content of the mixture, thus allowing the H 2 O/H 2  ratio to degrade slightly while bringing the dew point of the atmosphere back to around −15° C., whereas, for production conditions corresponding to the second range, it would be advantageous to improve the dew point of the atmosphere (reduction in the H 2 O/H 2  ratio) by increasing the flow rate of hydrogen injected at  31  and/or the hydrogen content of the mixture, and thus allow the dew point of the atmosphere to fall to about −15° to −20° C., taking the steel treated in this line into account. 
     It is then possible to propose, for each of the ranges, the following conditions: 
     for the range in which A/t is less than approximately 50 m 2 /min. ″ an N 2 /H 2  mixture flow rate of 130 Sm 3 /h, for a hydrogen flow rate of 27 Sm 3 /h and a hydrogen concentration in the mixture of 20.7%; 
     for the range in which A/t lies between approximately 50 m 2 /min. and 90 m 2 /min., an N 2 /H 2  mixture flow rate of 150 Sm 3 /h, for a hydrogen flow rate of 48.5 Sm 3 /h and a hydrogen concentration in the mixture of 32.3%. 
     Altogether this allows the hydrogen flow rate with respect to the ratio A/t to be kept constant at a value close to 0.009 m 3  of hydrogen per m 2  of strip. 
     FIG. 2 therefore illustrates an example of the plots that can be produced on a given line, for one or more steels treated, by adopting an average gas setting and by covering a typical range of variation of the area treated per unit time (which takes into account the line speed range normally used and the width range of products treated in the line in question), examination of these plots making it possible to determine the gas-feed setting modifications that it is advantageous to make in each case. 
     In the foregoing, an embodiment was described in which the flow rate of nitrogen injected by the injector  30  is maintained constant during the operation of the galvanizing line and only the hydrogen flow rate at  31  is adjusted and modified depending on the area of metal strip to be treated. 
     However, it may be envisaged that, depending on the case in question (improvement or degradation of the dew point of the atmosphere), it is also possible, or as a replacement, to vary the injection of inert gas at the point  30 A. 
     According to another alternative embodiment of the process according to the invention, hydrogen zoning of the cooling zone may be created by injecting, apart from nitrogen into the ducting  23  at the first location  30 A at a substantially constant flow rate and hydrogen at the location  31 A, nitrogen into the annealing furnace  7 , preferably in the final outlet portion of the latter. In this situation, the flow rate of nitrogen injected into the annealing furnace  7  may be adjusted depending on a set-point value of the dew point in this furnace. 
     This arrangement makes it possible, on the one hand, to raise the local hydrogen concentration in the cooling station  9 , thus protecting the surface of the strip from oxidation before it is dipped into the zinc bath  13 , and, on the other hand, to help cool the strip  3 . 
     It may therefore be seen that, depending on the many methods of implementation, the process of the invention makes it possible not only to reduce the consumption of hydrogen, and thus the running cost for regenerating the reducing atmosphere, but also to keep the characteristics of the products, which leave the galvanizing line, constant more reliably and under economic conditions which do not entail simply establishing an excessively high quality of the atmosphere.