EP4073292A1

EP4073292A1 - Device and method for heat treatment of steels, including a wet cooling

Info

Publication number: EP4073292A1
Application number: EP20842011.7A
Authority: EP
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: 2022-10-19
Also published as: CN114929940A; US20230017287A1; CN114929941A; US20230014843A1; KR20220113973A; WO2021116582A1; EP4073291A1; WO2021116594A1; FR3104178B1; FR3104178A1; KR20220110813A; MX2022007100A; MX2022007102A

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

DESCRIPTION

TITLE: DEVICE AND METHOD FOR THERMAL TREATMENT OF STEELS INCLUDING WET COOLING

Designation of the technical field concerned

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

Technical problems addressed by the invention

Lines equipped with gas cooling cannot cover all high yield strength steels due to insufficient cooling slope. In fact, gas cooling, typically carried out by high speed blowing on the product of a mixture of nitrogen and hydrogen, makes it possible to achieve cooling speeds of up to 200 ° C / s for strips. steel thickness 1 mm. This is insufficient in relation to the slopes targeted to obtain the desired metallurgical structure of the latest generation steels with high elastic limits, in particular martensitic steels, which typically require cooling speeds between 500 ° C / s and 1000 ° C / s, or even 2000 ° C / s, for steel strips 1 mm thick.

To obtain a sufficient cooling slope for the thermal cycles of the latest generation steels, it is necessary to go through a liquid quenching step, by spraying on the strip a liquid, or a mixture of a liquid and d. a gas, for example nitrogen or a mixture of nitrogen and hydrogen. Flow rates and pressures to be implemented in the cooling section depend on the nature of the steels to be treated and the cooling slopes to be observed. The temperature of the strip at the outlet of the cooling section is typically between 50 ° C and 500 ° C. However, water cooling generates surface oxidation which is often incompatible with the subsequent coating when it is found in large quantities. This oxidation takes the form of FeO, Fe ₂ 03 and Fe3O4 to a product temperature above 575 ° C, and it takes the form of Fe2O3 and Fe3O4 to a product temperature below 575 ° C. The installation of an intermediate chemical pickling section between the cooling and the coating is therefore made necessary. This chemical stripping section is costly in investment and operation and increases the footprint of the installation. FR3014447 and FR3064279 of the Applicant describe liquid quenching processes in which the cooling liquid is non-oxidizing for the strip and stripping with regard to the oxides present at the surface of the strip, in particular those formed from the addition elements contained in the steel to be treated. This liquid is, for example, composed of a mixture of demineralized water and formic acid. It can be projected onto the belt by means of nozzles, alone or with a gas, for example nitrogen or a mixture of nitrogen and hydrogen. These processes have proven their effectiveness in preventing or reducing the presence of oxides on the surface of the product, by limiting their formation and / or by directly stripping those which may have formed on the surface of the strip. The installation of an intermediate stripping section is thus no longer necessary.

However, the Applicant has observed 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, these residues impart coloring to the strip leaving the line. They are likely to pose problems for the subsequent use of the sheet, in particular for the treatment of phosphating for products. intended for the automotive market. Phosphating is the first step in a painting process. This treatment is applied to the sheet so that the layers of paint applied subsequently adhere properly and over a long period. If the phosphating has any defects, there is a risk of subsequent peeling / chipping / corrosion when using the vehicle. The condition for having a good phosphating is that the sheet is perfectly clean, without pollution of any kind. On a dip galvanizing line, these residues can be the cause of adhesion defects of the coating and make it incompatible with the desired level of quality, in particular in accordance with the requirements of sheet metal for the automobile. These non-oxidizing and stripping coolant residues must therefore be removed so that the surface quality of the belt at the outlet of the line meets customer expectations. In addition, the addition elements present in the new generations of steel are very easy to oxidize, compared to iron, and to pollute the surface of the product, making it incompatible with galvanization because they prevent good adhesion. of the coating. We can thus find on the surface of the product the presence of MnO _x , SiO _x , BO _x , Mn2Si04, MnAl204 and M ^h B2q4, even when the oven atmosphere has a very low dew point, for example at -40 ° C. . Unlike iron oxide, these oxides are not reduced under the atmosphere present in the furnace. This is composed of a mixture of nitrogen and hydrogen, typically with 4% hydrogen. The addition of a pre-oxidation during the heating phase makes it possible to limit the presence of these oxides at the surface of the strip. This pre-oxidation is, for example, carried out in a direct flame preheating section DFF (for English Direct Firing Furnace) by means of burners operating in excess of air. It can also be carried out in a heating section with RTF radiant tubes (for English Radiant Tube Furnace), for example in a dedicated chamber with an oxidizing atmosphere composed of a mixture of N ₂ and Ü ₂ or else of N ₂ and FI ₂ O, or by another oxidizing atmosphere. During this pre-oxidation, an iron oxide barrier forms on the surface preventing the migration of the addition elements towards the surface, the oxygen diffuses in the matrix and comes to oxidize these elements and thus block them in the steel. The iron oxide is subsequently reduced in the downstream sections of the furnace, under a reducing atmosphere. Only the oxides of the addition elements initially present at the surface, and a limited part of those which may have migrated to the surface, are thus present at the surface of the product.

For certain grades of steel, it is advantageous to carry out an internal selective oxidation in a preheating or heating section. It differs from pre-oxidation in that it targets only the additive elements. It is obtained by combining, at depth, oxygen atoms from the surface with certain atoms of addition elements, leading to the formation of precipitates of oxides. The internal selective oxidation is generally carried out in a dedicated chamber sufficiently oxidizing to oxidize the additives but not the iron.

This pre-oxidation, or this internal selective oxidation, associated with water cooling is of little interest since if it limits the formation of oxides on the surface from the addition elements during heating, liquid cooling to the water which follows will generate iron oxides on the surface incompatible with the dip galvanizing process. In this case, a chemical stripping section should be added before coating.

In summary, the combination of a pre-oxidation of the product, or of an internal selective oxidation, and of a non-oxidizing and pickling wet cooling makes it possible to avoid the main drawbacks mentioned above. However, it can lead to the formation of residues on the surface of the product and cause a lower surface quality, for example adhesion defects of a subsequent coating. The invention makes it possible to limit the formation of these residues and to treat / eliminate those which have been formed and which are present on the surface of the strip after liquid cooling. Technical background

The Applicant is not aware of any solution according to the state of the art which addresses the reduction of the formation of these residues or their elimination, in particular because the cooling of the strip by a non-oxidizing and pickling liquid is not yet exploited. on industrial lines in production.

After cooling, steel grades require overaging. Its role is to age the steel in order to bring it from a state of metallurgical imbalance after cooling to a state of equilibrium. It is obtained by maintaining 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 observed. The overaging temperature hold time is typically between 15 seconds and 90 seconds, depending on the steel grade.

After cooling, it is advisable not to exceed a temperature, beyond the overaging temperature, which would lead to an unwanted metallurgical transformation of the steel, which would risk canceling the metallurgical effects of the quenching and degrading the mechanical properties of the tape. An overaging chamber may include means for heating the strip to bring the latter to the overaging temperature. As a variant, this heating means can be placed upstream of the overaging chamber. This heating means can be an inductor to quickly bring the strip to the required temperature. The overaging chamber also includes radiant heating elements (candles, radiant tubes or electrical ribbons) which ensure that the temperature of the strip is maintained. An overaging chamber is maintained under a hydrogenated atmosphere composed of a mixture of nitrogen and hydrogen conventionally comprising about 4% hydrogen by volume. This hydrogen content may not be sufficient to reduce the residues present on the belt at usual overaging conditions, or it may require increasing the overaging temperature or lengthening the residence time at the overaging temperature.

In an annealing line, leaving the overaging section, the strip is cooled to room temperature. In a galvanizing line, leaving the overaging section, the strip can be heated or cooled to bring it to a coating temperature, depending on whether the overaging temperature is lower or higher than the coating temperature. This can be done by dipping, that is to say by immersing the strip in a bath containing the metal, or the metal alloy, constituting the coating to be applied to the strip, or by any other means. The coating can be zinc, an alloy containing zinc, or any other kind. For a dip coating, the coating temperature is close to that of the liquid metal bath in which it is immersed.

Summary of the invention

According to a first aspect of the invention, there is proposed a rapid wet cooling process making it possible to limit the quantity of residues present on a metal strip at the outlet of a rapid cooling section of a continuous line, said residues being formed during cooling of said metal strip by a non-oxidizing liquid solution for the metal strip and pickling 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 quantity of residues present on the belt is characterized in that it comprises a first step with rapid cooling with water, followed by a second rapid cooling step by wet process, with a non-solution. oxidizing for the tape.

Since the residues are formed during rapid cooling by the non-oxidizing solution for the strip, it is advantageous to limit the size of the latter by carrying out the first cooling step with water. In addition, during the first stage of cooling with water, the contact between the water and the strip is limited due to the presence of a vapor film on the surface of the strip, which has the effect of limiting the oxidation of the strip.

According to a second aspect of the invention, there is provided a method for removing residues present on a metal strip at the outlet of a cooling section of a continuous line, the residues being formed during cooling of said metal strip. by a non-oxidizing liquid solution for the metal strip and pickling for the oxides present on the surface of the strip, or by a mixture of this liquid solution and a gas, in order to make the surface of the strip compatible with subsequent processes ( galvanizing, phosphating, electrogalvanizing ...).

The method according to the invention comprises a step of removing residues obtained by reducing the residues with hydrogen.

By step of removing residues, the present invention relates to a step of reducing an oxidation-reduction reaction between an oxidant and a reducing agent.

Preferably, the first cooling step cools the strip to a temperature greater than or substantially equal to the Leidenfrost temperature. For example, the temperature is between the Leidenfrost temperature and the latter increased by 50 ° C.

Preferably, the second cooling step cools the strip from a temperature less than or equal to the Leidenfrost temperature. This feature allows efficient stripping of the strip surface, in combination with the end of cooling

The residue reduction step can last between 15 seconds and 300 seconds for a strip temperature between 50 ° C and 600 ° C.

The residue reduction step can be carried out below the hydrogen content present in the atmosphere of the furnace, that is to say, without increasing it. Thus, for example, the hydrogen content can remain at 4%, a content often used on existing lines. To obtain an effective reduction of residues, it is then necessary either to lengthen the residue reduction step, or to increase the strip temperature during this, or a combination of the two, compared to a reduction of the oxides which would be achieved under a higher hydrogen content.

Advantageously, the residue reduction step is carried out while the metal strip is in an atmosphere in which the hydrogen content is between 5% and 100%, and preferably greater than or equal to 10% by volume.

For the residue reduction step, hydrogen, or a hydrogenated atmosphere in which the hydrogen content is between 5% and 100%, and preferably greater than or equal to 10% by volume, can be blown onto the belt. metallic.

The hydrogen, or hydrogenated atmosphere, blown onto the metal strip can have a temperature between 500 ° C and 800 ° C. This high temperature of the blown gas allows a greater efficiency of the residue reduction step compared to 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 it, can thus be reduced.

According to one possibility, the blowing speed of the hydrogen, or the hydrogenated atmosphere, is between 10 m / s and 160 m / s in contact with the metal strip.

The method for removing residues according to the first aspect of the invention can further comprise a step of pre-oxidation, or of internal selective oxidation, of the surface of the strip produced in a preheating, heating or holding section. at the temperature of the metal strip, placed before the cooling section.

The residue removal method according to the first aspect of the invention can be carried out on a continuous line comprising a dip coating section of the metal strip in a molten bath, and can further comprise, after step residue reduction, a step of heating or cooling the metal strip to a temperature close to the temperature of the bath. The hydrogenated atmosphere is for example composed of a mixture of nitrogen and hydrogen.

According to a third aspect of the invention, there is proposed a use of a continuous treatment line for a metal strip comprising a first step of rapid cooling of said metal strip with water, or a mixture of water. and a gas, and a second step of rapid cooling of the strip with a non-oxidizing liquid solution for the metal strip and pickling for the oxides present on the surface of the strip, or by a mixture of this liquid solution and a gas, the use further comprising a step of reducing residues with hydrogen, said residues being formed during rapid cooling of the strip.

According to a fourth aspect of the invention, there is provided a line for the continuous treatment of a metal strip comprising a section for rapidly cooling the metal strip by spraying thereon with a liquid solution, or a mixture. of a liquid solution and of 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 not oxidizing for the metal strip and stripping for the oxides present on 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 reducing residues formed during the cooling and present on the strip, said reducing section being designed to implement a suppression method according to one aspect of the invention, or one or more of its improvements.

The reduction section may include means for reducing the residues by hydrogen comprising a means for blowing hydrogen, or a hydrogenated atmosphere, on the metal strip to expose the metal strip to an atmosphere with a hydrogen content. is between 5% and 100% by volume, and at a temperature between 500 ° C and 600 ° C. The residue reduction section may include, at the input in the direction of travel of the strip, a rapid heating device for bringing the strip to a temperature close to or equal to a predetermined temperature at which chemical reactions are initiated to reduce the residues. residues.

This rapid heating device is necessary when the power of the strip heating means present in this section does not allow this temperature to be quickly reached. Indeed, if the power density of the heating means is low, the time necessary to reach the temperature necessary for the reduction step will have to be added to the residence time of the strip in the section. On a new line, this would lead to lengthening the length of the section, and on an existing line, this would reduce the speed of the belt. 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 to obtain the targeted metallurgy, thus limiting unwanted metallurgical transformations.

In one possibility, the residue reduction section is part of an overaging section.

The residue reduction section may include a means for blowing hydrogen, or a hydrogen 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 internal selective oxidation, of the surface of the strip arranged in a preheating section, a section of heating or a section for maintaining the temperature of the metal strip, said section being placed upstream of the rapid cooling section in the direction of travel of the strip.

According to another aspect of the invention, there is provided a computer program product comprising instructions which lead a line according to the fourth aspect of the invention, or one or more of its improvements, to execute the steps of a process according to first aspect of the invention, or one or more of its improvements.

Advantageously, the residue reduction step is carried out while the strip is in an atmosphere in 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 exemplary embodiment of the invention, the thermal and metallurgical cycle of the strip may include one or more of the following steps:

• Preheating and pre-oxidation of the strip carried out in a direct flame DFF section, the pre-oxidation being intended to form a layer of iron oxide on the surface of the strip and thus limit the quantity of oxides d. addition elements present on the surface of the strip upstream of the rapid cooling.

• Heating and maintaining a temperature in two sections with RTF radiant tubes, under a reducing atmosphere of nitrogen and hydrogen, to obtain the desired metallurgy before rapid cooling, in particular the target proportion of austenite. During this heating and maintenance, the iron oxides present on the strip surface are gradually reduced by the hydrogen. As long as the iron oxide layer is not completely reduced, it prevents the migration of additive elements to the surface of the strip. It is therefore advantageous that the layer of iron oxides is not completely removed until the end of the maintenance, before the start of rapid cooling. If iron oxides remain at the end of the holding, they will be pickled during cooling. Their presence at the end of maintenance is however not desired because the pickling products would pollute the sprayed solution for cooling. It would thus be necessary to renew it more often, leading to an increased consumption of acid and water. demineralized. The oxides formed from the addition elements are themselves not reduced in the RTF.

• Quenching with a non-oxidizing and pickling fluid in a cooling section in order to obtain the desired metallurgy, in particular the transformation of part of the austenite into martensite. This non-oxidizing quenching removes oxides on the surface which could be harmful to the quality of the galvanization but leaves residues on the strip.

• Reheating of the strip to a temperature close to the starting temperature of the residue reduction reactions present on the strip in an induction heating section.

• At the inlet of the overaging section, a hydrogen blast, or a hydrogenated atmosphere, on the belt to bring its temperature to that necessary to start the residue reduction reactions.

• Cooling of the tape to the overaging temperature.

• Maintaining the overaging temperature to freeze the metallurgical structure during which the surface of the strip is cleaned (residues formed during quenching and present on the surface of the strip after quenching are eliminated).

For an annealing line, the thermal cycle then includes cooling the strip to room temperature.

For a hot dip galvanizing line, the thermal cycle then includes:

• Reheating or cooling the strip to a coating temperature in an induction heating section or a gas cooling section.

• Coating the strip by hot dipping in a zinc bath.

• Final cooling of the strip in a cooling tower. Examples of reactions between hydrogen and the residues implemented according to the process of the invention are given below, for the case of manganese:

• (HCOO) ₂ Mn + H ₂ - CO + C0 ₂ + H ₂ 0 + Mn

• (HCOO) ₂ Mn + H ₂ ^ 2HCOOH + Mn

• HCOOH + H ₂ ^ H ₂ CO + H ₂ 0 (formaldehyde)

• H ₂ CO + H ₂ ^ CHsOH (methanol)

Similar reactions occur for iron and other addition elements.

The efficiency of the reaction is conditioned 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 reactants, and the flow rate of hydrogen, or hydrogenated atmosphere, at the surface of the strip.

To simplify the description of the invention, we will consider hereafter that the removal of residues is carried out in an overaging section. It can also be done in a section dedicated to this function, especially if the line does not include an overaging section.

According to one embodiment of the invention on a line comprising a large overaging section, and / or when the overaging is carried out at a high temperature, the method according to the invention is implemented in this section d 'overaging by placing it under an atmosphere enriched in hydrogen. This atmosphere has a volume content of hydrogen of between 5% and 100%, depending on the residence time and the temperature of the strip in the overaging section. Preferably, the hydrogen content is greater than or equal to 10%. The entire overaging section can be maintained at this higher hydrogen concentration where only a portion of it can be, depending on the band residence time required at this higher concentration for removal. residue. In this configuration, it is not necessary to provide any particular device for injecting hydrogen other than those usually present on this device. section. This solution is possible when the overaging temperature is sufficient to initiate the chemical reactions for reducing the residues and when the residence time in the section is sufficient to have the time necessary for the elimination of the residues. This embodiment of the invention is thus limited to lines comprising a large overaging section and / or to steel grades allowing a high overaging temperature. It is equally applicable when the end of cooling temperature is equal to the overaging temperature, as shown in FIG. 4, or when the latter is lower than the overaging temperature, as shown in FIG. 5. When it is 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 to initiate the chemical reactions of reduction of the residues, the strip, or the film on the surface of the strip, is brought at a temperature sufficient to initiate the chemical reactions before bringing it, or bringing it back, to the overaging temperature. Advantageously, the strip is brought to a temperature sufficient to trigger the chemical reactions at the inlet of the overaging section. This higher temperature and the possible residence time at this temperature are limited to those necessary to initiate the chemical reactions so as not to influence the metallurgy and the mechanical properties of the strip. As with the previous embodiment, the hydrogen content is increased in the overaging section to promote residue reduction.

As illustrated in FIGS. 4 and 6, when the metallurgy of the steel does not require cooling the strip below the temperature necessary to initiate the chemical reactions for reducing the residues, the cooling of the strip is advantageously stopped at this temperature. As illustrated in Figures 5 and 7 to 9, when the metallurgy of steel requires cooling the strip below the temperature necessary to engage the chemical reactions to reduce residues, it is first necessary to bring the strip, or the film on the surface of the strip, to this temperature. As we will see below, this rise to the temperature necessary to initiate the chemical reactions can be carried out with equipment according to the state of the art or by dedicated equipment according to the invention. It can be done gradually or in stages. It can be more or less rapid depending on the type of heating applied.

In the remainder of the description of the invention, we will describe two examples of heating means for bringing the strip, or the film on the surface of the strip, to the temperature required to start the chemical reactions for reducing the residues. Other means not described may be used.

A first heating means for bringing the strip to the temperature required to start the chemical reactions is induction heating. It has the advantage of allowing a high power density for a rapid rise in temperature.

A second heating means for bringing the strip to the temperature required to initiate the chemical reactions is convection heating. It consists of blowing on the strip hydrogen, or a hydrogenated atmosphere with a hydrogen content of between 5% and 100% and preferably greater than 10%, and at high temperature, for example 800 ° C. This solution makes it possible to quickly reach a film temperature at the surface of the strip sufficient to initiate the chemical reactions, without the need to bring the entire thickness of the strip to this temperature. Stirring the atmosphere near the surface of the strip also speeds up chemical reactions.

As a variant, the rise to the temperature required to start the chemical reactions is carried out in two stages, the first for example with induction heating, the second with blowing heating as described above.

The blowing device can, for example, comprise nozzles, slots, tubes or perforated plates. To simplify the description of the invention, we will deal subsequently only with the case of nozzles without this being in any way restrictive.

The blowing device is advantageously placed at the inlet of the overaging section. It can nevertheless be placed at any point of the section for which the remaining length downstream in the overaging section allows sufficient residence time to eliminate residues. The blowing device can comprise several gas jets over the width of the strip and on each of the two large faces of the strip. The jets can be placed opposite, or offset over the width and / or the length of the strip.

The pitch between two nozzles in the same row can be chosen according to the jet opening angle and the distance between the nozzles and the strip so as to cover the entire width of the strip while limiting the overlap between the jets. It can typically be between 50 mm and 200 mm.

Depending on the maximum web running speed, several rows of nozzles can be placed on each side of the web. The pitch between two rows of nozzles is defined according to the maximum web running speed. It can 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 direction of flow of the strip (downstream) or in the direction opposite to the movement of the strip (upstream). Preferably, the distance from the blowing orifices to the strip is typically between 40 mm and 200 mm.

The gas supply flow rate to the nozzles can be controlled individually by row of nozzles, by pair of rows of nozzles (the pair comprising two rows placed substantially facing each other on either side of the strip), or according to any other configuration.

Preferably, the gas exhaust speed from the nozzles is between 10 m / s and 160 m / s and preferably between 80 m / s and 130 m / s. The quantity of gas blown onto the strip can be controlled according in particular to its temperature, its hydrogen content, the running speed of the strip and the film temperature of the strip. It can typically be between 0.5 and 15 kg / m ² of strip, for example 1 and 3 kg / m ² of strip for a gas containing 10% hydrogen, a mixing temperature of 800 ° C, a strip 1 m wide moving at 100 m / min and a residence time in the overaging section of 30 s.

The nozzles can be supplied with a gas the temperature of which is typically between 500 ° C and 800 ° C. The hydrogen concentration in the gas blown onto the strip can be related in particular to its temperature and to its blowing speed, as well as to the film temperature targeted on the strip. The lower the gas temperature and blowing speed levels are.

The temperature of the gas blown onto the strip causes heat exchange with it, mainly by convection. The blowing parameters, for example the gas temperature, can be controlled so that the heat input to the web by the blown gas is limited to the amount of heat necessary to wear and / or hold the web, or the film. on the surface of the strip, at the desired temperature. The heating power of the heating devices at the inlet of the overaging section, or upstream thereof, is controlled so that they provide the amount of additional heat that may be required to bring, or maintain, the strip to the desired temperature. The gas blown onto the strip can be recirculated with a recirculation fan whose suction is connected to the overaging section and whose discharge is connected to the supply to the nozzles. The recirculation circuit may include means for controlling the temperature of the gas (heating and or cooling) in order to bring the gas to the desired temperature at the exhaust from the nozzles. A partial renewal of the atmosphere of the overaging section can be carried out continuously to maintain the desired hydrogen concentration at the exhaust from the nozzles. The device can also include at least one injection point for new gas in cross-section, this gas possibly having the desired hydrogen concentration at the nozzle exhaust or a higher concentration of up to 100% hydrogen.

The hydrogen content of the overaging section, and / or that of the gas possibly blown onto the strip, is also chosen according to the type of steel to be treated and the desired coating quality. The hydrogen content can be reduced depending on the amount of residual residue tolerated.

Depending on the nature and the content of addition elements present in the steel to be treated, it may be necessary to add a pre-oxidation, or an internal selective oxidation, in one of the heating sections upstream of the cooling, as described above.

Brief description of the figures

Other characteristics and advantages of the invention will become apparent on reading the detailed description which follows, for the understanding of which reference is made to the accompanying drawings in which:

[Fig. 1] is a schematically and partially shown view, in longitudinal view, of a vertical furnace galvanizing line according to an exemplary embodiment of the invention;

[Fig.2] is an enlargement of the rapid cooling section of Fig. 1;

[Fig.3] is an enlargement of the rapid cooling section according to an alternative embodiment of the invention;

[Fig. 4] is a graph schematically illustrating the variation of the exchange coefficient at the surface of the strip as a function of the strip temperature, during its rapid cooling;

[Fig.5] is an enlargement of the induction heating and overaging 106 sections 105 of Fig. 1; [Fig. 6] is a diagram illustrating the temperature of the strip as a function of time according to a first example of implementation of the method according to the invention;

[Fig. 7] is a diagram illustrating the temperature of the strip as a function of time according to a second example of implementation of the method according to the invention;

[Fig. 8] is a diagram illustrating the temperature of the strip as a function of time according to a third example of implementation of the method according to the invention; [Fig. 9] is a diagram illustrating the temperature of the strip as a function of time according to a fourth example of implementation of the method according to the invention;

[Fig. 10] is a diagram illustrating the temperature of the strip as a function of time according to a fifth example of implementation of the method according to the invention;

[Fig. 11] is a diagram illustrating the temperature of the strip as a function of time according to a sixth example of implementation of the method according to the invention;

[Fig. 12] is a diagram illustrating, for a galvanizing line, the temperature of the strip as a function of time for 3 different overaging temperatures according to the first example of implementation of the method according to the invention illustrated in FIG. 8 .

Detailed description of the invention

Since the embodiments described below are in no way limiting, it is in particular possible to consider variants of the invention comprising only a selection of characteristics described, subsequently isolated from the other characteristics described, if this selection of characteristics is sufficient. to confer a technical advantage or to differentiate the invention from the state of the prior art. This selection includes at least one feature, preferably functional without structural details, or with only part of the structural details if only this part is sufficient to confer a technical advantage or to differentiate the invention from the state of the prior art.

In the remainder of the description, elements having an identical structure or similar functions will be designated by the same references.

Referring to the diagram of Figure 1 of the accompanying drawings, one can see schematically and partially shown, in longitudinal view, a vertical furnace galvanizing line according to an exemplary embodiment of the invention, in which a metal strip 1 circulates. It comprises, successively and in the direction of travel of the strip, a direct flame preheating section 100 in which a pre-oxidation of the strip is carried out, a heating section 101, a holding section 102, a section 103 of slow gas cooling, a section 104 for rapid cooling by an aqueous liquid solution, an induction heating section 105, an overaging section 106, a furnace outlet section 107, and a dip galvanizing section 108.

Referring to the diagram of Figure 4 of the accompanying drawings, a graph can be seen illustrating schematically the variation of the exchange coefficient at the surface of the strip as a function of the strip temperature, during its rapid cooling in section 104 On the abscissa, we have the temperature of the strip and on the ordinate, the exchange coefficient. On this graph, the change in the exchange coefficient during the cooling of the strip can be read from right to left. Until the temperature indicated by the letter L on the abscissa axis is reached, the strip is cooled in stable mode due to the presence of a film of vapor on the surface of the strip. This temperature L is the so-called Leidenfrost temperature. From this temperature L and until the strip reaches the temperature marked by the letter N on the abscissa axis, the strip is cooled in a transition mode with an unstable vapor film. The exchange coefficient thus increases sharply due to the rupture of the insulating vapor layer. Then, from temperature N until the end of cooling, it is carried out in a nucleated boiling regime.

The Leidenfrost temperature is a critical point which depends on many parameters, in particular the spray characteristics such as the surface density of the projected water, the speed and the diameter of the drops, the mesh of the nozzles, the distance of the nozzles to the belt. , the temperature and the nature of the fluid. These parameters can be determined experimentally for different types of spray nozzles in order to constitute tables applicable to industrial production cases. Typical Leidenfrost temperature values are between 200 ° C and 700 ° C depending on the cooling efficiency. An experimental database makes it possible to know the determination of the Leidenfrost temperature associated with each case of production of the line. Referring to the diagram of Figure 2 of the accompanying drawings, an enlarged view of the section 104 for rapid cooling by an aqueous liquid solution of Figure 1 can be seen. It comprises at the inlet a means 5 for separating atmospheres allowing the separation of atmospheres. 'prevent the reducing atmosphere of the gaseous slow cooling section 103 upstream from being polluted with water vapor from section 104.

In this section 104, the strip is cooled by spraying a liquid onto it, or a mixture of a liquid and a gas, by means of nozzles 3 arranged on either side. Of the band. This cooling section comprises two zones 109, 110, located on two different strip strands. In the example shown, the first strand is descending and the second is ascending. As a variant, the first strand could be upward and the second downward.

On the descending strand of zone 109, the strip is cooled with water, or with a mixture of water and a gas. This cooling in zone 109 makes it possible to bring the strip to a temperature substantially equal to the Leidenfrost temperature. On the rising strand of zone 110, cooling is carried out by a non-oxidizing liquid solution for the strip and a stripper for the oxides present at the surface of the strip, or by a mixture of this liquid solution and a gas.

On the horizontal strand placed between the descending strand of the zone 109 and the rising strand of the zone 110 is arranged an atmosphere separation lock 5. This airlock prevents vapors of the non-oxidizing liquid from zone 110 from entering zone 109 and polluting the water vapor present in this zone. The vapors drawn off in this lock can be condensed and the liquid obtained can be returned to the cooling liquid recirculation circuit of zone 110. Upstream of the lock 5, gas knives 15 make it possible to limit the quantity of water carried away. by the strip from zone 109 to zone 110. These gas knives 15 blow gas on the strip at high speed in order to expel the water present thereon. By limiting the entry of water from zone 109 into zone 110, we limit the dilution of the liquid solution used in section 110.

Each zone 109, 110 has a collection tank 16 making it possible to collect the water runoff from the zone before returning it to the nozzles in the zone with means not shown, in particular a pump. At the outlet of the liquid cooling section 104 is placed a means 5 for separating atmospheres preceded by gas knives 15. They make it possible to prevent the reducing atmosphere of the downstream induction heating section 105 from being polluted by gas. steam from section 104 or water carried by the belt. Referring to the diagram of Figure 3 of the accompanying drawings, one can see a representation of the rapid cooling section 104 according to an alternative embodiment of the invention. In these, the two zones 109, 110 are arranged on the same strip strand. This strand is descending in the example illustrated but it could also be ascending.

The induction heating section 105 includes an inductor 2 for heating the strip. The overaging section 106 comprises other means for separating atmospheres 5, arranged at the inlet and at the outlet of said section. The atmospheric separation means 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 only 4%. The atmospheric separation means may be of the roller type, with a single pair of rollers placed facing each other on either side of the strip or more. Advantageously, they comprise two pairs of rollers and a withdrawal is carried out between the two pairs of rollers to increase the efficiency 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 immersion temperature of the strip in the coating bath 7, the strip is either cooled in the cooling chamber or heated by the inductor 6.

Referring to the diagram of FIG. 5 of the accompanying drawings, an enlargement of FIG. 1 can be seen schematically showing in more detail the sections 105 of induction heating and 106 of overaging. These two sections include injection points 10, 12 and exhaust points 11, 13 for the gas mixture forming the atmosphere of these sections. The overaging section comprises means 8, 14 for heating the strip intended to bring the strip, or the film to the surface of the strip, to a temperature sufficient to initiate the chemical reactions for reducing the residues, in particular when the overaging temperature is not sufficient for this. The heating means 8 is for example radiative or by induction. It is chosen from those allowing a large heat transfer to the strip over a short length. It must in fact make it possible to rapidly bring the strip to the temperature necessary to initiate the chemical reactions so as to limit the residence time of the strip at a temperature above the overaging temperature. The heating means 14 is convective. It consists in blowing a gas at high temperature on the strip, for example at 800 ° C. The overaging section may comprise only one of the means 8, 14 for heating the strip. If it comprises both, the heating means 8 can be placed 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 includes a strip cooling means 9 for quickly bringing the strip back to the overaging temperature.

Referring to the diagrams of Figures 6 to 12 of the accompanying drawings, one can see schematically represented examples of thermal cycles of the strip as a function of time, according to examples of applications of the method according to the invention. On these diagrams, the temperature of the strip is on the ordinate and the time is on the abscissa. For all these examples, we consider the same tape format and the same tape running speed. The curves in these diagrams start with a plateau illustrating the end of the hold M, at a temperature TM, in section 102, followed by slow gas cooling RL, to temperature TRL, in section 103, then cooling. rapid liquid RR in section 104 to temperature TRR, overaging 0, to temperature TO, in section 105.

In the example of FIG. 6, the steel grade and the targeted metallurgical structure do not require cooling the strip below the overaging temperature. Likewise, they lead to an overaging temperature TO sufficient to trigger the chemical reactions of reduction of the residues and the length of the overaging section is such that the residence time of the strip at the overaging temperature is sufficient for remove residues. The strip is cooled in section 104 to the overaging temperature TO, and it is maintained at this temperature in the overaging section 106 by the heating means of the section, for example radiant tubes. The induction heating section 105 is not used. The atmosphere of the overaging section includes a hydrogen content suitable for this steel and 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 targeted metallurgical structure require cooling the strip to a temperature TRR lower than the overaging temperature. The overaging temperature TO is always sufficient to initiate the chemical reactions for reducing the residues and the length of the overaging section is such that the residence time of the strip at the overaging temperature makes it possible to eliminate the residues. The web is cooled in section 104 to temperature TRR. The inductor of the heating section 105 makes it possible to raise the temperature of the strip to the overaging temperature TO and it is maintained at this temperature in the overaging section 106 by the heating means of the section. In the example of FIG. 8, the steel grade and the targeted metallurgical structure lead to an overaging temperature insufficient to initiate the chemical reactions for reducing the residues. On the other hand, they do not require the strip to be cooled below the temperature TE necessary to start the chemical reactions. The strip is cooled in section 104 to temperature TE, for example 400 ° C. The induction heating section 105 is not used. The level E at the temperature TE is limited to the time necessary to start the chemical reactions, for example one minute. Depending on the running speed of the strip, this level can be obtained when the strip passes through the induction heating section 105, the thermal insulation of which prevents the strip from cooling. If the residence time of the strip in the induction heating section is not sufficient, stage E ends at the start of the overaging section. RE cooling is then carried out to bring the strip to the overaging temperature TO. Depending in particular on the format of the strip, its speed of movement and the temperature difference between TE and TO, cooling can be obtained simply by controlling, in particular stopping, the heating equipment placed at the entry of the section. overaging. If this is not sufficient, a cooling means 9 makes it possible to cool the strip to the temperature TO. This means consists for example in blowing a gas on the strip at an appropriate temperature. The strip is then maintained at the overaging temperature by the section heating means.

In the example of Figure 9, the steel grade and the metallurgical structure targeted lead to an overaging temperature that is still insufficient to trigger the chemical reduction reactions. In addition, they require cooling the strip below the temperature TE necessary to start the chemical reactions. The web is thus cooled in section 104 to temperature TRR. The induction heating section 105 is used to heat the strip to the temperature TE. Again, the level E at temperature TE is limited to the time necessary to start the chemical reactions.

In the example of FIG. 10, the steel grade and the targeted metallurgical structure require cooling the strip below the overaging temperature TO. In addition, they lead to an overaging temperature insufficient to initiate the chemical reduction reactions when this 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 to a temperature T1, lower than the overaging temperature TO. After this first heating C1, a second heating CC makes it possible to bring the strip to the overaging temperature. This DC heater is produced by blowing a hot gas on the strip, for example at 800 ° C., by means 14 visible in FIG. 2. It leads to a film temperature at the surface of the strip at least equal to the temperature. TE necessary to initiate chemical reactions to reduce residues, while the core of the strip can remain at a lower temperature. It is thus not necessary to bring the strip to a temperature above the overaging temperature in order to initiate these reactions. The example of FIG. 11 is close to that of FIG. 10. It differs from the latter in that the overaging temperature TO is appreciably lower. The second DC heating is also carried out by blowing a hot gas on the strip by means 14. It leads to a film temperature at the surface of the strip at least equal to the temperature TE necessary to start the chemical reactions for reducing the residues. , while the core of the band only reaches a temperature TS lower than TE, but this is here higher than the overaging temperature TO. As in the example of FIG. 8, RE cooling is then carried out to bring the strip to the overaging temperature.

Figure 12 illustrates three examples of thermal cycles depending on the overaging temperature for a galvanizing line. The first example shown in solid lines corresponds to the case of Figure 6, namely that the overaging temperature T01 is lower than the temperature Tl at which the strip must be immersed in the coating bath. The strip is heated from T01 to T1 in the furnace outlet section 107 by means of the inductor 6. For these 3 examples, after exiting the bath, FC cooling brings the strip to room temperature. The second example shown in broken lines corresponds to the case where the overaging temperature T02 is equal to the temperature T1 at which the strip must be immersed in the coating bath. The strip simply passes through the furnace outlet section 107 without being heated or cooled. The third example represented by a succession of crosses corresponds to the case where the overaging temperature T03 is greater than the temperature T1 at which the strip must be immersed in the coating bath. The strip is cooled from T03 to T1 in the furnace outlet section 107. This cooling is achieved by blowing a gas on the strip, for example a mixture of nitrogen and hydrogen.

Of course, the invention is not limited to the examples which have just been described and numerous modifications can be made to these examples without departing from the scope of the invention. In addition, the different characteristics, shapes, variants and embodiments of the invention can be associated with each other in various combinations as long as they are not incompatible or mutually exclusive.

Claims

1. A method of rapidly cooling a metal strip (1) traveling in a continuous line carried out in a section (104) of said line and removing residues formed during said rapid cooling in a section (106) of said line, characterized in that it comprises a first step of cooling with water, or by a mixture of water and a gas, followed by a second step of cooling with a non-oxidizing liquid solution for the strip and a stripper for the oxides present at the surface of the strip, or by a mixture of this liquid solution and a gas, said second step leading to the presence of residues on the surface of the strip, followed by a step of removing said residues obtained by a reduction of said residues with hydrogen.

2. The method of claim 1, wherein the first cooling step cools the strip to a temperature greater than or equal to the Leidenfrost temperature.

3. The method of claim 1 or 2, wherein the second cooling step cools the strip from a temperature less than or equal to the Leidenfrost temperature.

4. Method according to any one of the preceding claims, wherein the residue removal step is carried out while the metal strip (1) has a temperature between 50 ° C and 600 ° C and for a period of between 15 seconds and 300 seconds.

5. Method according to any one of the preceding claims, wherein the step of removing residues is carried out while the metal strip (1) is in an atmosphere in which the hydrogen content is between 5% and 100% in volume, and preferably greater than or equal to 10%.

6. The method of claim 1, further comprising a step of pre-oxidation, or internal selective oxidation, of the surface of the metal strip (1), carried out in a section (100) for preheating the strip or a section (101) for heating the strip, or a section (102) for maintaining the temperature of the strip, said section (100, 101, 102) being disposed upstream of the section (104) for rapidly cooling the strip, according to the direction of travel of the tape.

7. The method of claim 1, carried out on a continuous line comprising a section (108) of dip coating a metal strip (1) in a molten bath (7), further comprising, after step removal of residues, a strip heating step, or a strip cooling step, to bring the strip to a temperature close to the temperature of the bath.

8. Continuous treatment line for a metal strip (1) comprising a section (104) for rapid cooling of the strip and a section (106) for removing residues formed during the cooling of the strip with a non-liquid solution. oxidizing for the strip and stripper for the oxides present at the surface of the strip, or by a mixture of this liquid solution and a gas, said rapid cooling and residue removal sections being able to implement a cooling process and removing residues according to any one of claims 1 to 7.

9. Line according to the preceding claim, wherein the section (106) for removing residues comprises, at the input in the direction of travel of the strip, a rapid heating device (14) for bringing the strip to a temperature close to or equal. at a predetermined temperature at which chemical residue reduction reactions start.

10. The line of claim 8 or 9, wherein the residue removal section (106) is part of an overaging section.

11. A line according to any one of claims 8 to 10, wherein the residue removal section comprises means for blowing hydrogen (14), or a hydrogen atmosphere, onto the metal strip (1).

12. Line according to claim 11, further comprising a chamber for pre-oxidation, or internal selective oxidation, of the surface of the strip disposed in a preheating section (100), a heating section (101) or a heating section (101). section (102) for maintaining the temperature of the metal strip (1), said section being placed upstream of the rapid cooling section (104) in the direction of travel of the strip.

13. A computer program product comprising instructions which lead a line according to any one of claims 8 to 12 to perform the steps of a method according to any one of claims 1 to 7.