Abstract
The invention relates to a method and a device for rapidly cooling a metal strip and removing residues present on the strip after this cooling, wherein the residues are formed during a cooling of said metal strip by a non-oxidizing liquid solution for the metal strip and a stripping liquid solution for the oxides present on the surface of the strip, or by a mixture of this liquid solution and a gas.
Claims
1. Method for rapid cooling of a metal strip travelling in a continuous line, performed in a section of said line, and for removing residues formed during said rapid cooling in a section of said line, characterized in that it comprises a first step of water cooling, or cooling using a mixture of water and a gas, followed by a second step of cooling using a liquid solution that is non-oxidizing for the strip and is stripping for the oxides present at the surface of the strip, or using a mixture of said liquid solution and a gas, said second step leading to the presence of residues at the surface of the strip, followed by a step of removing said residues obtained by reduction of said residues by means of hydrogen.
2. Method according to claim 1, wherein the first step of cooling cools the strip to a temperature of greater than or equal to the Leidenfrost temperature.
3. Method according to claim 1, wherein the second step of cooling cools the strip from a temperature of less than or equal to the Leidenfrost temperature.
4. Method according to claim 1, wherein the step of removing residues is performed when the metal strip is at a temperature of between 50° C. and 600° C., and for a period of between 15 seconds and 300 seconds.
5. Method according to claim 1, wherein the step of removing residues is performed when the metal strip is in an atmosphere of which the hydrogen content is between 5% and 100% by volume, and preferably greater than or equal to 10%.
6. Method according to claim 1, further comprising a step of pre-oxidation, or of selective internal oxidation, of the surface of the metal strip, performed in a preheating section of the strip or a heating section of the strip, or a temperature maintenance section of the strip, said section being arranged upstream of the section of rapid cooling of the strip, according to the direction of travel of the strip.
7. Method according to claim 1, implemented on a continuous line having a section for dip coating of a metal strip in a molten bath, further comprising, after the step of removing residues, a step of heating of the strip or a step of cooling of the strip, in order to bring the strip to a temperature close to the temperature of the bath.
8. Continuous treatment line for a metal strip, comprising a section for rapid cooling of the strip and a section for removing residues formed during the cooling of the strip using a liquid solution that is non-oxidizing for the strip and is stripping for the oxides present at the surface of the strip, or using a mixture of said liquid solution and a gas, said rapid cooling and residue removal sections being capable of implementing a method for cooling and for removing residues according to claim 1.
9. Line according to claim 8, wherein the section for removing residues comprises, at the inlet in the direction of travel of the strip, a rapid heating device for bringing the strip to a temperature close or equal to a predetermined temperature at which chemical reactions for reducing residues start.
10. Line according to claim 8, wherein the section for removing residues forms part of an overaging section.
11. Line according to claim 8, wherein the section for removing residues comprises a means for blowing hydrogen, or a hydrogenated atmosphere, onto the metal strip.
12. Line according to claim 11, further comprising a chamber for pre-oxidation, or selective internal oxidation, of the surface of the strip arranged in a preheating section, a heating section, or a temperature maintenance section, of the metal strip, said section being positioned upstream of the section of rapid cooling, in the direction of travel of the strip.
13. Computer program comprising instructions which lead a continuous treatment line to execute the steps of a method according to claim 1; wherein the continuous treatment line comprises a section for rapid cooling of the strip and a section for removing residues formed during the cooling of the strip using a liquid solution that is non-oxidizing for the strip and is stripping for the oxides present at the surface of the strip, or using a mixture of said liquid solution and a gas, said rapid cooling and residue removal sections being capable of implementing a method for cooling and for removing residues.
Description
BRIEF DESCRIPTION OF THE FIGURES
[0084] Further features and advantages of the invention will become clear from reading the following description, for the understanding of which reference will be made to the accompanying drawings, in which:
[0085] FIG. 1 is a longitudinal schematic partial view of a shaft furnace galvanizing line according to an embodiment of the invention;
[0086] FIG. 2 is an enlarged view of the rapid cooling section of FIG. 1;
[0087] FIG. 3 is an enlarged view of the rapid cooling section according to a variant of the invention;
[0088] FIG. 4 is a graph schematically showing the variation in the exchange coefficient at the surface of the strip, as a function of the temperature of the strip, during the rapid cooling thereof;
[0089] FIG. 5 is an enlarged view of the sections 105 for induction heating and 106 overaging of FIG. 1;
[0090] FIG. 6 is a graph showing the temperature of the strip as a function of time according to a first embodiment of the method according to the invention;
[0091] FIG. 7 is a graph showing the temperature of the strip as a function of time according to a second embodiment of the method according to the invention;
[0092] FIG. 8 is a graph showing the temperature of the strip as a function of time according to a third embodiment of the method according to the invention;
[0093] FIG. 9 is a graph showing the temperature of the strip as a function of time according to a fourth embodiment of the method according to the invention;
[0094] FIG. 10 is a graph showing the temperature of the strip as a function of time according to a fifth embodiment of the method according to the invention;
[0095] FIG. 11 is a graph showing the temperature of the strip as a function of time according to a sixth embodiment of the method according to the invention;
[0096] FIG. 12 is a graph showing, for a galvanizing line, the temperature of the strip as a function of time for 3 different overaging temperatures according to the first embodiment of the method according to the invention shown in FIG. 8.
DETAILED DESCRIPTION OF THE INVENTION
[0097] Since the embodiments described in the following are in no way limiting, it is in particular possible to envisage variants of the invention that comprise only a selection of features described in the following, in a manner isolated from the other features described, if this selection of features is sufficient for providing a technical advantage or for distinguishing the invention from the prior art. This selection comprises at least one feature, preferably functional and without structural details, or having some of the structural details if this part alone is sufficient for providing a technical advantage or for distinguishing the invention from the prior art.
[0098] In the description which follows, elements having an identical structure or analogous functions are denoted by the same reference signs.
[0099] With reference to the schematic view of FIG. 1 of the accompanying drawings, a longitudinal schematic partial view of a shaft furnace galvanizing line according to an embodiment of the invention, in which a metal strip 1 circulates, is visible. It comprises, successively and in the direction of travel of the strip, a section 100 for direct flame preheating in which pre-oxidation of the strip is carried out, a heating section 101, a maintenance section 102, a section 103 for slow gaseous cooling, a section 104 for rapid cooling using an aqueous liquid solution, a section 105 for induction heating, an overaging section 106, a furnace outlet section 107, and a dip galvanizing section 108.
[0100] With reference to the schematic drawing of FIG. 4 of the accompanying drawings, a graph schematically showing the variation in the exchange coefficient at the surface of the strip, as a function of the temperature of the strip, during the rapid cooling thereof in the section 104, is visible. The X-axis shows the temperature of the strip, and the Y-axis shows the exchange coefficient. In this graph, the development of the exchange coefficient during the cooling of the strip is read from right to left. Until reaching the temperature denoted by the letter L on the X-axis, the cooling of the strip is performed in stable mode, on account of the presence of a vapor film at the surface of the strip. Said temperature L is the temperature referred to as Leidenfrost. From said temperature L and until the strip reaches the temperature marked by the letter N on the X-axis, the cooling of the strip takes place in a transition mode having an unstable vapor film. The exchange coefficient thus increases significantly on account of the rupture of the layer of insulating vapor. Subsequently, from the temperature N until the end of the cooling, this is performed in a nucleate boiling regime.
[0101] The Leidenfrost temperature is a critical point which depends on numerous parameters, in particular the features of spraying, such as the surface density of water projected, the speed and the diameter of the drops, the mesh size of the nozzles, the distance of the nozzles from the strip, the temperature and the type of the fluid. Said parameters may be determined experimentally for different types of spraying nozzles in order to form tables which are applicable to cases of industrial production. The typical values of Leidenfrost temperature are between 200° C. and 700° C., depending on the effectiveness of the cooling. An experimental database makes it possible to know the determination of the Leidenfrost temperature associated with each case of production of the line.
[0102] With reference to the diagram of FIG. 2 of the accompanying drawings, an enlarged view of the section 104 for rapid cooling by means of an aqueous liquid solution, of FIG. 1, is visible. It comprises, at the inlet, a means 5 for atmosphere separation, making it possible to prevent the reducing atmosphere of the section 103 of slow gaseous cooling, located upstream, from being polluted by the water vapor originating from the section 104.
[0103] In said section 104, the cooling of the strip is performed by projection of a liquid thereon, or of a mixture of a liquid and a gas, by means of nozzles 3 arranged on either side of the strip. Said cooling section comprises two zones 109, 110 which are located on two different strands of the strip. In the example shown, the first strand is falling and the second is rising. In a variant, the first strand could be rising, and the second falling.
[0104] On the falling strand of the zone 109, the cooling of the strip is performed using water, or using a mixture of water and a gas. Said cooling in the zone 109 makes it possible to bring the strip to a temperature substantially equal to the Leidenfrost temperature. On the rising strand of the zone 110, the cooling is performed by a liquid solution which is non-oxidizing for the strip and is stripping for the oxides present at the surface of the strip, or by a mixture of said liquid solution and a gas.
[0105] An atmosphere separating airlock 5 is arranged on the horizontal strand positioned between the falling strand of the zone 109 and the rising strand of the zone 110. Said airlock prevents the vapors of the non-oxidizing liquid of the zone 110 from entering the zone 109 and from polluting the water vapor present in said zone. The vapors extracted in said airlock may be condensed, and the liquid obtained may be returned into the circuit for recirculation of the cooling liquid of the zone 110.
[0106] Upstream of the airlock 5, gas knives 15 make it possible to limit the amount of water brought in by the strip from the zone 109 into the zone 110. Said gas knives 15 blow a gas onto the strip at high speed, in order to expel the water present thereon. By limiting the entry of water from the zone 109 into the zone 110, the dilution of the liquid solution used in section 110 is limited.
[0107] Each zone 109, 110 comprises a collecting tank 16 which makes it possible to collect the stream of water from the zone before returning it towards the nozzles of the zone using means which are not shown, in particular a pump.
[0108] A means 5 for atmosphere separation preceded by gas knives 15 is positioned at the output of the liquid cooling section 104. These make it possible to prevent the reducing atmosphere of the downstream induction heating section 105 from being polluted by the vapor originating from the section 104, or by the water carried along by the strip.
[0109] With reference to the diagram of FIG. 3 of the accompanying drawings, a representation of the section 104 for rapid cooling according to a variant of the invention is visible. Therein, the two zones 109, 110 are arranged on the same strip strand. Said strand is falling in the example shown, but it could also be rising.
[0110] The induction heating section 105 comprises an inductor 2 intended for reheating the strip. The overaging section 106 comprises other means for atmosphere separation 5, arranged at the inlet and at the outlet of said section.
[0111] The means for atmosphere separation make it possible to have different atmospheres in each section. Thus, for example, the atmosphere of the overaging section 106 may contain 20% hydrogen, while the atmospheres of the sections arranged upstream and downstream contain just 4%. The means for atmosphere separation may be of the roller type, having one single pair of rollers positioned face-to-face on either side of the strip or further. Advantageously, they comprise two pairs of rollers, and tapping is performed between the two pairs of rollers in order to increase the effectiveness of the atmosphere separation.
[0112] The furnace outlet section 107 comprises a gaseous cooling chamber in the rising strand and an inductor 6 in the falling strand. Depending on whether the overaging temperature is higher or lower than the temperature of immersion of the strip into the coating bath 7, the strip is either cooled in the cooling chamber or heated by the inductor 6.
[0113] With reference to the diagram of FIG. 5 of the accompanying drawings, a schematic enlarged view of FIG. 1 showing the sections 105 for induction heating and 106 overaging in greater detail can be seen. Said two sections comprise injection points 10, 12 and exhaust points 11, 13 for the gaseous mixture forming the atmosphere of said sections. The overaging section comprises means 8, 14 for heating the strip, which means are intended to bring the strip, or the film at the surface of the strip, to a temperature sufficient for starting the chemical reactions of residue reduction, in particular when the overaging temperature is not sufficient for this. The heating means 8 is for example radiative or by induction. It is selected from those which allow for significant transfer of heat to the strip over a short length. Indeed, it must make it possible to rapidly bring the strip to the temperature necessary for starting the chemical reactions in such a way as to limit the dwell time of the strip at a temperature above the overaging temperature. The heating means 14 is convective. It consists in blowing a gas, at a high temperature, onto the strip, for example at 800° C. The overaging section may comprise just one single means 8, 14 for heating the strip. If it comprises both, the heating means 8 may be positioned downstream of the heating means 14, in the direction of travel of the strip, as shown in FIG. 5, or upstream thereof.
[0114] The overaging section also comprises a means 9 for cooling the strip, making it possible to rapidly return the strip to the overaging temperature.
[0115] With reference to the graphs of FIGS. 6 to 12 of the accompanying drawings, it is possible to see examples of thermal cycles of the strip as a function of time, according to examples of applications of the method according to the invention, shown schematically. In these graphs, the temperature of the strip is on the Y-axis, and the time is on the X-axis. For all these examples, the same strip format and the same speed of travel of the strip will be considered. The curves of these graphs start with a stage illustrating the end of the maintenance M, at a temperature TM, in the section 102, followed by a slow gaseous cooling RL, to a temperature TRL, in the section 103, then rapid liquid cooling RR in the section 104, to a temperature TRR, an overaging O, at a temperature TO, in section 105.
[0116] In the example of FIG. 6, the steel grade and the metallurgical structure sought do not require the strip to be cooled to below the overaging temperature. In the same way, they lead to an overaging temperature TO which is sufficient for starting the chemical reactions of residue reduction, and the length of the overaging section is such that the dwell time of the strip in the overaging section is sufficient for eliminating the residues. The strip is cooled in the section 104 to the overaging temperature TO, and it is kept at said temperature in the overaging section 106 by the heating means of the section, for example radiant tubes. The induction heating section 105 is not involved. The atmosphere of the overaging section has a hydrogen content suitable for said steel and for the operating conditions. It is for example 10% for 4% in the upstream 105 and downstream 107 sections.
[0117] In the example of FIG. 7, the steel grade and the metallurgical structure sought require the strip to be cooled to a temperature TRR of less than the overaging temperature. The overaging temperature TO is still sufficient for starting the chemical reactions of residue reduction, and the length of the overaging section is such that the dwell time of the strip in the overaging section makes it possible to eliminate the residues. The strip is cooled in the section 104 to the temperature TRR. The inductor of the heating section 105 makes it possible to raise the temperature of the strip back to the overaging temperature TO, and it is kept at said temperature in the overaging section 106 by the heating means of the section.
[0118] In the example of FIG. 8, the steel grade and the metallurgical structure sought lead to an overaging temperature which is insufficient for starting the chemical reactions of residue reduction. However, they do not require the strip to be cooled to below the temperature TE necessary for starting the chemical reactions. The strip is cooled in the section 104 to the temperature TE, for example 400° C. The induction heating section 105 is not involved. The stage E at the temperature TE is limited to the period required for starting the chemical reactions, for example one minute. Depending on the speed of travel of the strip, said stage may be obtained during the passage of the strip into the induction heating section 105, the thermal insulation of which prevents cooling of the strip. If the dwell time of the strip in the induction heating section is not sufficient, the stage E ends at the start of the overaging section. Cooling RE is then performed, in order to bring the strip to the overaging temperature TO. Depending in particular on the format of the strip, the speed of movement thereof, and the temperature difference between TE and TO, the cooling may be obtained simply by controlling, in particular stopping, the heating equipment arranged at the inlet of the overaging section. If this is not sufficient, a cooling means 9 makes it possible to cool the strip to the temperature TO. Said means consists for example in blowing, onto the strip, a gas at an appropriate temperature. The strip is then kept at the overaging temperature by the heating means of the section.
[0119] In the example of FIG. 9, the steel grade and the metallurgical structure sought lead to an overaging temperature which is still insufficient for starting the chemical reactions of reduction. Moreover, they require the strip to be cooled to below the temperature TE necessary for starting the chemical reactions. The strip is thus cooled in the section 104 to the temperature TRR. The induction heating section 105 is involved for reheating the strip to the temperature TE. Once again, the stage E at the temperature TE is limited to the period required for starting the chemical reactions.
[0120] In the example of FIG. 10, the steel grade and the metallurgical structure sought require the strip to be cooled to below the overaging temperature TO. Moreover, they lead to an overaging temperature which is insufficient for starting the chemical reactions of reduction when said temperature TO is reached by the induction heating of the section 105. The strip is cooled in the section 104 to the temperature TRR. The inductor of the heating section 105 makes it possible to raise the temperature of the strip back to a temperature TI which is less than the overaging temperature TO. After said first heating CI, a second heating CC makes it possible to bring the strip to the overaging temperature. Said heating CC is performed by blowing a hot gas onto the strip, for example at 800° C., by the means 14 visible in FIG. 2. This leads to a temperature of the film at the surface of the strip that is at least equal to the temperature TE necessary for starting the chemical reactions of residue reduction, while the core of the strip may remain at a lower temperature. It is thus not necessary to bring the strip to a temperature above the overaging temperature in order to start these reactions.
[0121] The example of FIG. 11 is close to that of FIG. 10. It is distinguished therefrom in that the overaging temperature TO is substantially lower. The second heating CC is also performed by blowing a hot gas onto the strip by the means 14. This leads to a temperature of the film at the surface of the strip that is at least equal to the temperature TE necessary for starting the chemical reactions of residue reduction, while the core of the strip only reaches a temperature TS lower than TE, but in this case said temperature is greater than the overaging temperature TO. As in the example of FIG. 8, cooling RE is then performed, in order to bring the strip to the overaging temperature.
[0122] FIG. 12 shows three examples of thermal cycles according to the overaging temperature for a galvanizing line. The first example shown in a solid line corresponds to the case of FIG. 6, i.e. that the overaging temperature T01 is lower than the temperature TI at which the strip must be immersed in the coating bath. The strip is heated from T01 to TI in the furnace outlet section 107 by means of the inductor 6. For these 3 examples, after leaving the bath, cooling FC brings the strip to ambient temperature. The second example shown in a broken line corresponds to the case where the overaging temperature T02 is equal to the temperature TI at which the strip must be immersed in the coating bath. The strip simply passes through the furnace outlet section 107 without being either heated or cooled. The third example shown by a succession of crosses corresponds to the case where the overaging temperature T03 is higher than the temperature TI at which the strip must be immersed in the coating bath. The strip is cooled from T03 to TI in the furnace outlet section 107. This cooling is performed by blowing a gas onto the strip, for example a mixture of nitrogen and hydrogen.
[0123] Of course, the invention is not limited to the embodiments described above, and a number of developments can be made to said embodiments, without departing from the scope of the invention. Moreover, the various features, types, variants, and embodiments of the invention may be associated with one another, in accordance with various combinations, insofar as they are not mutually incompatible or exclusive.
