US20230017287A1

US20230017287A1 - Device and method for heat treatment of steels, including a wet cooling

Info

Publication number: US20230017287A1
Application number: US17/783,341
Authority: US
Inventors: Sébastien LEMAIRE; Patrice Sedmak; Florent Code
Original assignee: Fives Stein SA
Current assignee: Fives Stein SA
Priority date: 2019-12-09
Filing date: 2020-12-08
Publication date: 2023-01-19
Also published as: WO2021116582A1; EP4073292A1; KR20220110813A; MX2022007100A; CN114929941A; US20230014843A1; FR3104178B1; MX2022007102A; CN114929940A; EP4073291A1; WO2021116594A1; FR3104178A1; KR20220113973A

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.

Description

The invention relates to annealing lines and to the hot dip galvanizing lines for flat products, and more particularly to continuous lines equipped with a non-oxidizing and stripping liquid cooling section. It is associated with the field of metallurgy, and relates both to heat treatment and to the chemistry of steels.

Technical Problems Addressed by the Invention

The lines equipped with gas cooling do not make it possible to cover all steels having a high elastic limit, on account of an insufficient cooling gradient. Indeed, the gas cooling, typically performed by high-speed blowing, onto the product, a mixture of nitrogen and hydrogen, makes it possible to achieve cooling speeds of up to 200° C./s for steel strips of 1 mm thickness. This is insufficient with respect to the gradients sought for obtaining the desired metallurgical structure of new generation steels having a high elastic limit, in particular martensitic steels, which typically require cooling speeds of between 500° C./s and 1000° C./s, indeed 2000° C./s, for steel strips of 1 mm thickness.
In order to achieve a cooling gradient that is sufficient for the thermal cycles of new generation steels, it is necessary to pass through a step of liquid quenching, by spraying a liquid, or a mixture of a liquid and a gas, for example nitrogen or a mixture of nitrogen and hydrogen, onto the strip. The flow rates and pressures to be used in the cooling section depend on the type of the steels to be treated and the cooling gradients to be complied with. The temperature of the strip at the output of the cooling section is typically between 50° C. and 500° C.
However, the water cooling causes surface oxidation which is often incompatible with the subsequent coating when it is present in a large quantity. Said oxidation takes the form of FeO, Fe₂O₃, and Fe₃O₄for a temperature of said product of above 575° C., and it takes the form of Fe₂O₃and Fe₃O₄for a product temperature of below 575° C. The implementation of an intermediate chemical stripping section between the cooling and the coating becomes necessary from then on. Said chemical stripping section is costly in terms of investment and operation, and it increases the footprint of the installation.
FR3014447 and FR3064279 by the applicant describe methods for liquid quenching, the cooling liquid of which is non-oxidizing for the strip and is stripping with respect to the oxides present at the surface of the strip, in particular those formed from addition elements contained in the steel to be treated. This liquid is, for example, made up of a mixture of demineralized water and formic acid. It may be sprayed onto the strip by means of nozzles, alone or together with a gas, for example nitrogen or a mixture of nitrogen and hydrogen. These methods have proven their effectiveness for preventing or reducing the presence of oxides at the surface of the product, by limiting their formation and/or by directly stripping those which have been able to form at the surface of the strip. It is therefore no longer necessary for an intermediate stripping section to be implemented.
However, the applicant has noted that the use of a non-oxidizing and stripping cooling liquid leads to the formation of residues, for example formate salts or iron hydroxides, which remain present on the strip. On an annealing line, said residues provide coloring to the strip at the line output. They may pose problems for the subsequent use of the sheet metal, in particular for the phosphating treatment for the products intended for the automotive market. The phosphating is the first step of a painting process. The sheet metal undergoes this treatment in order for the layers of paint applied to subsequently adhere correctly and over a long duration. If the phosphating has faults, there is a risk of subsequent detachment/peeling/corrosion during the use of the vehicle. The condition for good phosphating is that the sheet metal should be perfectly clean, without pollution of any kind. On a dip galvanizing line, said residues may be the origin of adhesion faults of the coating, and make this incompatible with the quality level sought, in particular in accordance with the requirements of sheet metal for motor vehicles. Said residues of non-oxidizing and stripping cooling liquid must thus be removed in order for the surface quality of the strip at the output of the line to be in accordance with the clients' expectations.
Furthermore, the addition elements present in the new generations of steel can oxidize very easily, compared with iron, and pollute the surface of the product, making it incompatible with galvanization because they prevent the good adhesion of the coating. It is thus possible to find the presence of MnO_x, SiO_x, BO_X, Mn₂SiO₄, MnAl₂O₄and MnB₂O₄at the surface of the product, even when the furnace atmosphere has a very low dew point, for example of −40° C. Unlike iron oxide, these oxides are not reduced under the atmosphere present in the furnace. This is made up of a mixture of nitrogen and hydrogen, typically having 4% hydrogen. The addition of pre-oxidation during the heating phase makes it possible to limit the presence of these oxides at the surface of the strip. Said pre-oxidation is, for example, performed in a direct flame preheating section (DFF—Direct Firing Furnace), by means of excess air-driven burners. It may also be performed in a radiant tube heating section (RTF—Radiant Tube Furnace), for example in a dedicated chamber having an oxidizing atmosphere made up of a mixture of N₂and O₂or indeed N₂and H₂O, or by another oxidizing atmosphere. During said pre-oxidation, a barrier of iron oxide forms at the surface, preventing the migration of addition elements towards the surface; the oxygen diffuses in the matrix and oxidizes these elements and thus blocks them in the steel. The iron oxide is then annealed in the sections downstream of the furnace, under a reductive atmosphere. Only the oxides of addition elements initially present at the surface, and a limited portion of those which have been able to migrate towards the surface, are thus present at the surface of the product.
For some steel grades, it is advantageous to perform selective internal oxidation in a preheating or heating section. It is distinguished from the pre-oxidation in that it targets only the addition elements. It is obtained by combining, at depth, oxygen atoms originating from the surface, with some atoms of addition elements, leading to the formation of oxide precipitates. The selective internal oxidation is generally performed in a dedicated chamber which is sufficiently oxidizing to oxidize the addition elements but not the iron.
Said pre-oxidation or said selective internal oxidation, associated with water cooling, is not of great interest because, even if it limits the formation of oxides at the surface from addition elements during the heating, the liquid water cooling which follows will generate iron oxides at the surface which are incompatible with the dip galvanizing method. In this case, it would be necessary to add a chemical stripping section before the coating.
In summary, the combination of a pre-oxidation of the product, or selective internal oxidation, and humid non-oxidizing and stripping cooling makes it possible to prevent the main disadvantages mentioned above. However, it may lead to the formation of residues at the surface of the product, and cause a lower surface quality, for example adhesion faults of a subsequent coating. The invention makes it possible to limit the formation of said residues and to treat/eliminate those which have been formed and which are present at the surface of the strip after the liquid cooling.

TECHNICAL BACKGROUND

The applicant does not know of any solution according to the prior art which addresses the reduction of the formation of said residues or their removal, in particular because the cooling of the strip by a non-oxidizing and stripping liquid is not yet made use of on industrial lines, in production.
Following the cooling, steel grades require overaging. This has the role of making the steel undergo aging in order to make it pass from a state of metallurgical disequilibrium at the cooling outlet into a stable state. It is obtained by keeping the strip at a given temperature for a sufficient time. The overaging temperature, generally between 300° C. and 600° C. depending on the steel grade, is an important process parameter to be adhered to. The time for maintenance at the overaging temperature is typically between 15 seconds and 90 seconds, depending on the steel grade.
Following the cooling, it is expedient not to exceed a temperature, above the overaging temperature, which would bring about an undesired metallurgical transformation of the steel, which would risk cancelling out the metallurgical effect of the dipping, and deterioration of the mechanical properties of the strip. An overaging chamber may comprise a heating means for the strip, in order to bring said strip to the overaging temperature. In a variant, said heating means may be positioned upstream of the overaging temperature. Said heating means may be an inductor for rapidly bringing the strip to the required temperature. The overaging chamber furthermore comprises radiant heating elements (spark plugs, radiant tubes, or electrical tapes) which ensure temperature maintenance of the strip. An overaging chamber is kept under a hydrogenated atmosphere made up of a mixture of nitrogen and hydrogen, traditionally comprising approximately 4% hydrogen, by volume. Said hydrogen content may not be sufficient for reducing the residues present on the strip at the typical overaging conditions, or it may require the overaging temperature to be increased or the dwell time at the overaging temperature to be lengthened.
In an annealing line, at the output of the overaging section, the strip is cooled to ambient temperature. In a galvanizing line, at the output of the overaging section, the strip may be heated or cooled in order to bring it to a coating temperature, depending on whether the overaging temperature is lower or higher than the coating temperature. This may be performed by dipping, i.e. by immersing the strip into a bath containing the metal, or the metal alloy, forming the coating to be applied to the strip, or by any other means. The coating may be zinc, an alloy containing zinc, or of any other kind. For a dip coating, the coating temperature is close to that of the bath of metal liquid in which it is immersed.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, a method for rapid wet cooling a metal strip, making it possible to limit the amount of residues present on a metal strip at the output of a rapid cooling section of a continuous line, is proposed, said residues being formed during a cooling of said metal strip by a liquid solution that is non-oxidizing for the metal strip and stripping for the oxides present at the surface of the strip, or by a mixture of this liquid solution and a gas. The method making it possible to limit the amount of residue present on the strip is characterized in that it comprises a first step comprising rapid water cooling, followed by a second step of rapid wet cooling, by a solution that is non-oxidizing for the strip.
Since the residues are formed during the rapid cooling by the solution that is non-oxidizing for the strip, it is advantageous to reduce the significance thereof while performing the first step of water cooling. Moreover, during the first step of water cooling, the contact between the water and the strip is limited on account of the presence of a vapor film at the surface of the strip, which has the effect of limiting the oxidation of the strip.
According to a second aspect of the invention, a method for removing residues present on a metal strip at the output of a cooling section of a continuous line is proposed, the residues being formed during cooling of said metal strip by a liquid solution that is non-oxidizing for the metal 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, in order to make the surface of the strip compatible with the subsequent methods (galvanization, phosphating, electro-galvanizing, etc.).
The method according to the invention comprises a step of removing residues obtained by reduction of the residues using hydrogen.
By “step for removing the residues,” the present invention means a reduction step of an oxidoreduction reaction between an oxidant and a reducing agent.
Preferably, the first step of cooling cools the strip to a temperature of greater than or substantially equal to the Leidenfrost temperature. For example, the temperature is between the Leidenfrost temperature and said temperature increased by 50° C.
Preferably, the second step of cooling cools the strip from a temperature of less than or equal to the Leidenfrost temperature. This feature allows for effective stripping of the surface of the strip, in combination with the end of the cooling.
The step of reducing the residues may be of a duration of between 15 seconds and 300 seconds for a strip temperature of between 50° C. and 600° C.
The step of reducing the residues may be performed under the hydrogen content present in the furnace atmosphere, i.e. without increasing it. Thus, for example, the hydrogen content may remain at 4%, which is a content often used on the existing lines. In order to achieve effective reduction of residues, it is thus necessary either to lengthen the step of reduction of the residues, or to increase the temperature of the strip during said step, or a combination of the two, compared with a reduction of oxides which would be performed under a higher hydrogen content.
Advantageously, the step of reduction of residues is performed when the metal strip is in an atmosphere of which the hydrogen content is between 5% and 100%, and preferably greater than or equal to 10%, by volume.
For the step of reduction of residues, hydrogen, or a hydrogenated atmosphere of which the hydrogen content is between 5% and 100%, and preferably greater than or equal to 10%, by volume, may be blown onto the metal strip.
The hydrogen, or the hydrogenated atmosphere, blown onto the metal strip, may be of a temperature of between 500° C. and 800° C. Said high temperature of the blown gas allows for greater effectiveness of the step of reducing residues compared with that obtained by blowing the hydrogen, or the hydrogenated atmosphere, at a lower temperature. The duration of the reduction step, and/or the temperature of the strip during said step, can thus be reduced.
According to one possibility, the speed of blowing of the hydrogen, or of the hydrogenated atmosphere, is between 10 m/s and 160 m/s upon contact with the metal strip.
The method for removing residues according to the first aspect of the invention may further comprise a step of pre-oxidation, or of selective internal oxidation, of the surface of the strip, performed in a preheating, heating or temperature maintenance section of the metal strip, arranged in front of the cooling section.
The method for removing residues according to the first aspect of the invention may be implemented on a continuous line having a section for dip coating of the metal strip in a molten bath, and may further comprise, after the step of reduction of residues, a step of heating or cooling of the metal strip to a temperature close to the temperature of the bath.
The hydrogenated atmosphere is for example made up of a mixture of nitrogen and hydrogen.
According to a third aspect of the invention, a use of a continuous treatment line for a metal strip is proposed, comprising a first step of rapid cooling of said metal strip by means of water, or a mixture of water and a gas, and a second step of rapid cooling of the strip by means of a liquid solution which is non-oxidizing for the metal strip and stripping for the oxides present at the surface of the strip, or using a mixture of said liquid solution and a gas, the use further comprising a step of reducing residues by means of hydrogen, said residues being formed during the rapid cooling of the strip.
According to a fourth aspect of the invention, a continuous treatment line for a metal strip is proposed, comprising a section for rapid cooling of the metal strip by projecting thereon a liquid solution, or a mixture of a liquid solution and a gas, comprising a first zone in which the liquid solution is water, followed downstream, in the direction of travel of the strip, by a second zone in which the liquid solution is non-oxidizing for the metal strip and stripping for the oxides present at the surface of the metal strip, the line further comprising, downstream of said rapid cooling section in the direction of travel of the strip, a section for reduction of residues formed during the cooling and present on the strip, said reduction section being designed to implement a removal method according to an aspect of the invention or one or more of the developments thereof.
The reduction section may comprise means for reducing residues by means of hydrogen, comprising a means for blowing hydrogen, or a hydrogenated atmosphere, onto the metal strip in order to expose the metal strip to an atmosphere of which the hydrogen content is between 5% and 100% by volume, and to a temperature of between 500° C. and 600° C.
The section for reducing residues may comprise, 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.
Said rapid heating device is necessary when the power of the means for heating the strip, present in this section, does not make it possible to rapidly reach this temperature. Indeed, if the power density of the heating means is low, the time required for reaching the temperature necessary for the reduction step has to be added to the dwell time of the strip in the section. On a new line, this would lead to extending the length of the section, and on an existing line this would lead to reducing the speed of the strip.
The rapid heating means also makes it possible to limit the time during which the strip is brought to a temperature higher than that required for obtaining the sought metallurgy, thus limiting the undesired metallurgical transformations.
According to one possibility, the section for reducing residues forms part of an overaging section.
The section for reduction of residues may comprise a means for blowing hydrogen, or a hydrogenated atmosphere, onto the metal strip.
According to one embodiment, the line according to the fourth aspect of the invention may further comprise 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 rapid cooling section, in the direction of travel of the strip.
According to another aspect of the invention, a computer program product is proposed, comprising instructions which cause a line according to the fourth aspect of the invention, or one or more of the developments thereof, to execute the steps of a method according to the first aspect of the invention, or one or more of the developments thereof.
Advantageously, the step of reduction of residues is performed when the strip is in an atmosphere of which the hydrogen content is greater than or equal to 6%, advantageously greater than or equal to 7%, advantageously greater than or equal to 8%, advantageously greater than or equal to 9%, advantageously greater than or equal to 10%, by volume.
According to an embodiment of the invention, the thermal and metallurgical cycle of the strip may comprise one or more of the following steps:
Preheating and pre-oxidation of the strip, performed in a direct flame section DFF, the pre-oxidation being intended to form a layer of iron oxide at the surface of the strip, and thus to limit the quantity of oxides of addition elements present at the surface of the strip upstream of the rapid cooling.
Heating and temperature maintenance in two radiant tube sections RTF, under a reductive atmosphere of nitrogen and hydrogen, in order to obtain the desired metallurgy prior to the rapid cooling, in particular the desired austenite proportion. During said heating and said maintenance, the iron oxides present at the surface of the strip are progressively reduced by the hydrogen. As long as the layer of iron oxide is not completely reduced, it prevents the migration of addition elements towards the surface of the strip. It is therefore advantageous for the layer of iron oxides to not be totally removed until the end of the maintenance, before the start of the rapid cooling. If iron oxides remain at the end of the maintenance, these will be stripped during the cooling. However, the presence thereof at the end of maintenance is not desired, because the stripping products would pollute the solution sprayed for the cooling. It would thus be necessary to replace it more often, leading to increased consumption of acid and demineralized water. The oxides formed from addition elements are not, themselves, reduced in the RTF.
Dipping using a non-oxidizing and stripping fluid, in a cooling section, in order to obtain the desired metallurgy, in particular the transformation of a portion of the austentite into martensite. Said non-oxidizing dipping strips, at the surface, the oxides which could be damaging to the quality of the galvanization, but leaves residues on the strip.
Reheating the strip to a temperature close to the starting temperature of the reactions for reducing residues present on the strip, in an induction heating section.
At the inlet of the overaging section, blowing of hydrogen, or of a hydrogenated atmosphere, onto the strip in order to bring its temperature to that required for starting the reactions for reducing residues.
Cooling of the strip to the overaging temperature.
Maintenance at the overaging temperature in order to set the metallurgical structure, during which the surface of the strip is cleaned (the residues formed during the dipping and present at the surface of the strip at the output of dipping are eliminated).
For an annealing line, the thermal cycle then comprises cooling of the strip to the ambient temperature.
For a dip galvanizing line, the thermal cycle then comprises:
Reheating or cooling the strip to a coating temperature in an induction heating section or a gaseous cooling section.
Coating of the strip by hot immersion in a zinc bath.
Final cooling of the strip in a cooling tower.
Examples of reactions between the hydrogen and the residues implemented according to the method of the invention are set out below, for the case of manganese:
(HCOO)₂Mn+H₂→CO+CO₂+H₂O+Mn
(HCOO)₂Mn+H₂→2HCOOH+Mn
HCOOH+H₂→H₂CO+H₂O (formaldehyde)
H₂CO+H₂→CH₃OH (methanol)
Similar reactions result for iron and other addition elements.
The effectiveness of the reaction is determined in particular by the film temperature at the surface of the strip, the hydrogen content, the dew point of the atmosphere, the duration of contact between the reagents, and the flow speed of the hydrogen, or of the hydrogenated atmosphere, at the surface of the strip.
In order to simplify the description of the invention, in the following it will be considered that the removal of residues is performed in an overaging section. It may also be performed in a section dedicated to this function, in particular if the line does not comprise an overaging section.
According to an embodiment of the invention on a line having a large overaging section, and/or when the overaging is performed at a high temperature, the method according to the invention is implemented in said overaging section by placing this under a hydrogen-enriched atmosphere. Said atmosphere has a volumetric hydrogen content of between 5% and 100%, depending on the dwell time and the temperature of the strip in the overaging section. The hydrogen content is preferably greater than or equal to 10%. The entirety of the overaging section may be kept at this higher concentration of hydrogen, or just a portion thereof may be, depending on the dwell time of the strip required at this higher concentration for eliminating residues. In this configuration, it is not necessary to provide a particular device for injecting hydrogen, other than those typically present on this section. This solution is possible when the overaging temperature is sufficient for starting the chemical reactions of residue reduction, and when the dwell time in the section is sufficient for having available the time required for eliminating the residues. This embodiment of the invention is thus limited to lines comprising a large overaging section and/or to steel grades which allow for a high overaging temperature. It is equally applicable when the temperature at the end of cooling is equal to the overaging temperature, as shown in FIG. 4 , or when said temperature is below the overaging temperature, as shown in FIG. 5 . When it is lower, the strip is first brought to the overaging temperature, for example by induction heating.
According to another embodiment of the invention, in the case where the overaging temperature is not sufficient for starting the chemical reactions for residue reduction, the strip, or the film at the surface of the strip, is brought to a temperature sufficient for starting the chemical reactions, before bringing it, or returning it, to the overaging temperature. Advantageously, the strip is brought to a temperature that is sufficient for starting the chemical reactions at the inlet of the overaging section. Said higher temperature and the possible dwell time at this temperature are limited to those required for starting the chemical reactions, so as not to influence the metallurgy and the mechanical properties of the strip. As for the previous embodiment, the hydrogen content is increased in the overaging section in order to promote the reduction of the residues.
As shown in FIGS. 4 and 6 , when the metallurgy of the steel does not require cooling of the strip to below the temperature required for starting the chemical reactions for reduction of residues, the cooling of the strip is advantageously stopped at this temperature. As shown in FIGS. 5 and 7 to 9 , when the metallurgy of the steel requires cooling of the strip to below the temperature required for starting the chemical reactions for reduction of residues, it is first necessary to bring the strip, or the film at the surface of the strip, to this temperature. As will be seen in the following, this return to the temperature necessary for starting the chemical reactions may be achieved using equipment according to the prior art or by dedicated equipment according to the invention. This can be achieved progressively or in stages. It may be faster or slower, depending on the type of heating applied.
In the remainder of the description of the invention, two examples of heating means for bringing the strip, or the film at the surface of the strip, to the temperature required for starting the chemical reactions of residue reduction will be described. Other means which are not described may be used.
A first heating means for bringing the strip to the temperature required for starting the chemical reactions is induction heating. It has the advantage of allowing for a high power density for a rapid increase in temperature.
A second heating means for bringing the strip to the temperature required for starting the chemical reactions is convection heating. It consists in blowing, onto the strip, hydrogen, or a hydrogenated atmosphere having a hydrogen content of between 5% and 100%, and preferably greater than 10%, and at a high temperature, for example 800° C. This solution makes it possible to rapidly reach a temperature of the film at the surface of the strip that is sufficient for starting the chemical reactions, without it being necessary to bring the entire thickness of the strip to this temperature. The mixing of the atmosphere close to the surface of the strip also makes it possible to accelerate the chemical reactions.
In a variant, the return to the temperature required for starting the chemical reactions is performed in two steps, the first for example by heating by induction, the second by heating by blowing, as described above.
The blowing device may, for example, comprise nozzles, slits, tubes, or plates comprising holes. In order to simplify the description of the invention, in the following only the case of nozzles will be discussed, without this being in any way restrictive.
The blowing device is advantageously positioned at the inlet of the overaging section. It can nonetheless be positioned at a point of the section for which the remaining length downstream in the overaging section allows for a sufficient dwell time for eliminating the residues. The blowing device may comprise a plurality of gas jets over the width of the strip and on each of the two large surfaces of the strip. The jets may be placed so as to face one another, or so as to be offset over the width and/or the length of the strip.
The pitch between two nozzles of the same row may be selected according to the opening angle of the jet and the distance between the nozzles and the strip, so as to cover the entire width of the strip while limiting the coverage between the jets. It may typically be between 50 mm and 200 mm.
Depending on the maximum speed of travel of the strip, a plurality of rows of nozzles may be placed on each face of the strip. The pitch between two rows of nozzles is defined according to the maximum speed of travel of the strip. It may typically be between 50 and 200 mm.
The jets may be substantially perpendicular to the strip, or they are inclined at an angle which may be between 1° and 45° in the flow direction of the strip (downstream) or in the direction contrary to the travel of the strip (upstream).
Preferably, the distance from the blowing orifices to the strip is typically between 40 mm and 200 mm.
The gas feed rate of the nozzles may be controlled individually per row of nozzles, per pair of rows of nozzles (the pair comprising two rows positioned so as to be substantially facing one another on either side of the strip), or according to any other configuration.
Preferably, the speed of escape of gas from the nozzles is between 10 m/s and 160 m/s, and preferably between 80 m/s and 130 m/s.
The amount of gas blown onto the strip may be controlled in particular according to the temperature thereof, the hydrogen content thereof, the speed of travel of the strip, and the film temperature of the strip. It may typically be between 0.5 and 15 kg/m²of strip, for example 1 and 3 kg/m²of strip for a gas of 10% hydrogen, a mixing temperature of 800° C., a strip of width 1 m travelling at 100 m/min, and a dwell time in the overaging section of 30 s.
The nozzles may be supplied with a gas of which the temperature is typically between 500° C. and 800° C. The concentration of hydrogen in the gas blown onto the strip may in particular be associated with the temperature thereof and the blowing speed thereof, as well as with the film temperature sought on the strip. It will be lower, the higher the temperature levels of the gas and the blowing speed.
The temperature of the gas blown onto the strip brings about a heat exchange therewith, mainly by convection. The blowing parameters, for example the temperature of the gas, may be controlled such that the input of heat to the strip by the blown gas is limited to the amount of heat necessary to bring and/or maintain the strip, or the film at the surface of the strip, to/at the desired temperature. The heating power of the heating devices at the input of the overaging section, or upstream thereof, is controlled such that they supply the additional amount of heat which may be necessary for bringing, or maintaining, the strip to or at the desired temperature.
The gas blown onto the strip can be recirculated, using a recirculation fan, the suction side of which is connected to the overaging section, and the discharge side of which is connected to the supply of the nozzles. The recirculation circuit may comprise means for controlling the temperature of the gas (heating or cooling) in order to bring the gas to the temperature desired at the exhaust of the nozzles. A partial renewal of the atmosphere of the overaging section may be performed continuously in order to preserve the desired concentration of hydrogen at the exhaust of the nozzles. The device may also comprise at least one injection point for new gas into the section, it being possible for said gas to have the hydrogen concentration desired at the exhaust of the nozzles, or a higher concentration which may be up to 100% hydrogen.
The hydrogen content of the overaging section, and/or that of the gas optionally blown onto the strip, is also selected according to the type of steel to be treated and the coating quality sought. The hydrogen content may be reduced depending on the amount of residual residues tolerated.
Depending on the type and the content of additional elements present in the steel to be treated, it may be necessary to add a pre-oxidation step, or selective internal oxidation, in one of the heating sections upstream of the cooling, as described above.

BRIEF DESCRIPTION OF THE FIGURES

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:

FIG. 1 is a longitudinal schematic partial view of a shaft furnace galvanizing line according to an embodiment of the invention;

FIG. 2 is an enlarged view of the rapid cooling section of FIG. 1 ;

FIG. 3 is an enlarged view of the rapid cooling section according to a variant of the invention;

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;

FIG. 5 is an enlarged view of the sections 105 for induction heating and 106 overaging of FIG. 1 ;

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;

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;

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;

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;

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;

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;

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

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.
In the description which follows, elements having an identical structure or analogous functions are denoted by the same reference signs.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
The overaging section also comprises a means 9 for cooling the strip, making it possible to rapidly return the strip to the overaging temperature.
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.
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.
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.
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.
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.
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.
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.
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.
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.

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.