EP3686534B1 - Method and furnace for thermal treatment of a high-resistance steel strip including a temperature homogenisation chamber - Google Patents

Description

Technical area

According to a first aspect, the invention relates to a process for the heat treatment of a high strength steel strip. According to a second aspect, the invention relates to a furnace for the heat treatment of a high resistance steel strip.

State of the art

Commonly used high strength steels include alloying elements, for example manganese, silicon, chromium and / or aluminum. During an annealing step, the alloying elements present in high strength steel can diffuse towards the surface of the steel and oxidize rapidly because of their high affinity for oxygen, even in areas with radiant tubes where the atmosphere is nevertheless reducing for iron oxides. This selective oxidation creates surface defects which make it difficult to adhere the zinc coating (or other metal or alloy) applied when galvanizing the surface. This problem of wettability is a limiting aspect of galvanizing which cannot be carried out correctly.

Studies have been carried out in order to understand the kinetics of these oxidation phenomena and to provide solutions to the problems posed during galvanization. A particularly studied method consists in subjecting, in the annealing furnace, the surface of the strips to temperature and atmospheric conditions suitable for rapidly and deeply oxidizing the alloying elements and thus preventing their surface migration. During this operation, an iron oxide layer is formed which will subsequently be removed in subsequent zones of the annealing furnace under a reducing atmosphere.

It is known from the documents of the state of the art, and in particular from EP 2 732 062 B1 , that oxidation of metal products can be achieved during direct flame heating. According to this document, the oxidation potential of the atmosphere around the metallic product during direct flame heating can be adjusted by modifying the excess oxygen. US 9,279,175 B2 emphasizes the importance of forming an oxide layer as well homogeneous as possible in order to constitute an effective diffusion barrier. EP 2 732 062 B1 clarifies, however, that the specific adjustment of an oxide thickness, i.e. obtaining a uniform distribution over the surface of the steel, can only be controlled with great difficulty. What also describes EP 2 010 690 B1 . The documents US 2017/137906 A1 and KR 20160085830 A describe an oxidation of a metal strip as it passes through a direct flame heating section with an excess of oxygen in the combustion atmosphere. The documents US 2010/173072 A1 and KR 20160085830 A describe the oxidation of a metal strip with injection of an oxidant during its passage through a radiant tube heating section.

Thus, a problem generally encountered during the heat treatment of metal products with oxidation and reduction of the surface is to obtain a non-homogeneous surface state before the galvanizing step.

Summary of the invention

According to a first aspect, one of the aims of the present invention is to provide a method of heat treatment of a high resistance steel strip making it possible to obtain on its surface an oxide formation with a more homogeneous and more controlled thickness. .

1. a) chauffage de la bande dans une zone de chauffage à flamme directe;
2. b) homogénéisation en température de la bande dans une chambre d'homogénéisation comprenant au moins un tube de chauffage radiant, de sorte à homogénéiser une température de la bande après son passage dans la zone de chauffage à flamme directe de l'étape précédente;
3. c) oxydation de la bande dans une chambre d'oxydation avec une atmosphère oxydante ayant une concentration volumique en oxygène supérieure 1 %;
4. d) réduction de la bande dans une zone de réduction.

To this end, the inventors propose a process for the heat treatment of a steel strip of high running resistance, said process comprising the following steps:

1. a) heating the strip in a direct flame heating zone;
2. b) temperature homogenization of the strip in a homogenization chamber comprising at least one radiant heating tube, so as to homogenize a temperature of the strip after it has passed through the direct flame heating zone of the previous step;
3. c) oxidation of the strip in an oxidation chamber with an oxidizing atmosphere having an oxygen concentration by volume greater than 1%;
4. d) reduction of the band in a reduction zone.

The method of the invention allows during the heat treatment, thanks to the temperature homogenization step, the oxidation of the strip presenting a more homogeneous surface in temperature. This allows growth of an oxide layer having a more homogeneous thickness over the entire strip surface. A more homogeneous oxide thickness at the surface of the strip makes it possible to have a subsequent reduction of said oxide layer better controlled. Indeed, variations in the thickness of the oxide layer formed during the oxidation step require an adaptation of the reduction time during the reduction step in order to reduce the oxide over the entire surface. strip surface. Such an adaptation of the reduction time is based on the greatest oxide thicknesses. The method of the invention allows better control of the time of the reduction step because it guarantees a more homogeneous oxide thickness on the strip surface.

The method of the invention is particularly advantageous because it makes it possible to compensate for the temperature inhomogeneity of the strip, in particular of the surface of the strip during step a) of heating the strip by direct flame. Indeed the use of a direct flame heating zone allows a rapid rise in temperature of the strip to the detriment of the temperature homogeneity of the metal product. However, in a large number of furnaces, the oxidation chamber is positioned directly after the direct flame heating zone, so that the oxidation is carried out on a strip whose temperature homogeneity is not well controlled.

As indicated above, good control of the temperature of the strip during its oxidation in the oxidation chamber makes it possible to obtain a surface oxide layer having a more homogeneous thickness over the entire surface of the strip. It appears that the kinetics of formation of an oxide layer on the surface of a high strength steel strip depends mainly on the surface temperature of the strip as well as on the composition of the oxidizing atmosphere in the chamber. oxidation. Temperature inhomogeneities at the surface of the strip can therefore cause large variations in the thickness of the oxide layer at the surface of the strip.

When reducing the oxide layer in the reduction zone, it is necessary to reduce the entire thickness of oxide formed in the oxidation chamber. Now, when an oxide layer has a variable thickness, it is necessary to ensure sufficient reduction to reduce the oxide layer at the places where it is thickest. This can result in a slowing down of the travel speed in the reduction zone, or in a reducing atmosphere in the reduction zone, which is richer in hydrogen, or even by lengthening the reduction zone in order to maintain an acceptable production rate. Thus the inhomogeneity of the surface temperature of a high resistance steel strip during the oxidation of the latter in an oxidation chamber can have consequences on the efficiency of the heat treatment process in terms of the rate of production and / or costs.

An oxidation carried out during heating by direct flame (step a)) makes an adjustment of the thickness of the FeO layer formed very difficult to control. Indeed, in EP 2 010 690 A1 , it was found that for the same oxidizing conditions in the atmosphere during heating by direct flame, a high scroll speed shows a thinner FeO layer compared to lower scroll speeds, demonstrating the high sensitivity from the iron oxide formation process to the various parameters involved.

An advantage of the process of the invention over processes where the oxidation is carried out at the same time as the heating of the strip in a direct flame heating zone is that the process of the invention makes it possible to dissociate the heating of the strip. the strip, temperature homogenization and its oxidation with separate stages and furnace chambers. This allows better control of the iron oxide formation parameters at the surface of the strip while allowing strip heating by direct flame. Thus, the invention makes it possible to overcome the drawbacks of heating by direct flame by introducing a temperature homogenization chamber. Thanks to the invention, it is therefore possible to have an oven exhibiting a very good quality of heat treatment, as well as a better surface condition of the strip before it is galvanized, and this for reasonable operating costs.

Throughout the document, oxygen concentration (volume) should be understood to mean an O ₂ (volume) concentration. The steps of the process of the invention are to be carried out in the following order: step a), step b), step c), and step d).

Preferably, the reduction zone has a reducing atmosphere having a hydrogen volume concentration greater than 3% and of preferably greater than 5%, and even more preferably greater than 8%. An advantage associated with such volume hydrogen concentrations in the reduction zone for these preferred embodiments is to increase the assurance that reduction will take place. Preferably, the remainder of the composition of the atmosphere of the reduction zone comprises nitrogen.

Throughout the document, it is necessary to understand by concentration (volume) of hydrogen, a concentration (volume) of H ₂ .

It has also been found that the process of the invention is particularly effective for high strength steel strips, for example having a Cr composition by weight of less than 5%, preferably less than 3% and even more preferably less. at 1%. For the purposes of the present invention, the term “high strength steel” is understood to mean a steel comprising alloying elements such as manganese, silicon, chromium and / or aluminum. Preferably, the strip has a thickness between 0.3 mm and 3.2 mm.

The homogenization chamber comprising at least one radiant heating tube is intended to allow standardization / homogenization of the temperature of the strip when the latter is present in the homogenization chamber. The temperature uniformization of the strip takes place gradually during its passage through the homogenization chamber in order to obtain a temperature that is as uniform as possible at the outlet of the homogenization chamber. The homogenization chamber is not primarily intended to vary the average temperature of the strip but rather is intended to make the temperature of the strip uniform.

In the homogenization chamber may be present radiant and / or heating elements which have a power that can be changed quickly, which makes it possible to adjust the temperature quickly so as to maintain an optimum temperature at the entrance to the chamber. oxidation and ensure regular oxidation of the surface of the steel strip.

Preferably, the temperature homogenization chamber comprises two, three, or four radiant heating tubes.

The temperature of the strip is included in the present application as being a temperature measured at the surface of the strip and representing the temperature over the entire thickness of the strip. In fact, for strips having a thickness between 0.6 mm and 2.5 mm, the diffusion of heat throughout the thickness is very rapid and it can therefore be estimated that a temperature of the strip at a point of the strip at its surface is representative of the temperature throughout the thickness of the strip. This is particularly true when the strip is in a chamber that is substantially homogeneous in temperature. Thus a homogeneity or an inhomogeneity of temperature can be characterized by measurements of the surface temperature of the strip at very distinct places. For example, a temperature inhomogeneity is observed on a strip section when there is a temperature difference greater than 5%, preferably greater than 2% and even more preferably greater than 1% between a point located at the center of the strip. and a point on the edge of the strip. The strip temperature is for example an average strip temperature taken on a section of strip at several different points, for example the strip temperature is the average of the temperatures measured at the level of the two edges as well as at its center. A target band temperature is reached when the average band temperature and the target band temperature are equal or in any case have a deviation of less than 2%, preferably less than 1%. In the temperature homogenization chamber, the strip temperature remains essentially the same but it is surface homogenized.

Preferably, the oxidizing atmosphere in the oxidation chamber has a volume oxygen concentration of between 1.5% and 5% and even more preferably between 2% and 5%.

Preferably, the oxidation chamber of the invention does not include a radiant heating tube inside thereof. For example, the oxidation chamber is confined, for example insulated within a radiant heating furnace section so that it is indirectly heated by the radiant heating tubes of the radiant heating furnace section.

• une aspiration d'au moins une partie du gaz oxydant hors de la chambre d'oxydation,
• un refroidissement de ladite au moins une partie du gaz oxydant,
• un entraînement par un ventilateur de ladite au moins une partie dudit gaz oxydant,
• un enrichissement en oxygène de ladite au moins une partie dudit gaz oxydant par injection d'air,
• une réinjection de ladite au moins une partie dudit gaz oxydant dans la chambre d'oxydation.

Advantageously, the method according to the invention further comprises a step of homogenization of the oxidizing gas from the oxidation chamber, comprising:

• suction of at least part of the oxidizing gas out of the oxidation chamber,
• cooling said at least part of the oxidizing gas,
• a drive by a fan of said at least part of said oxidizing gas,
• an oxygen enrichment of said at least part of said oxidizing gas by injection of air,
• reinjection of said at least part of said oxidizing gas into the oxidation chamber.

It has been observed that the homogenization of the oxidizing gas makes it possible to improve the control over the oxidation step and to obtain the formation of an oxide layer on the surface of the steel strip, the thickness of which is more homogeneous and / or reproducible.

Direct flame heating is used to clean high strength steel strip (eg degreasing). The cleaning makes it possible in particular to remove organic residues present on the surface of the steel strip.

Preferably, the oxidation step is carried out at a strip temperature of between 650 ° C and 750 ° C.

It has been observed that when said oxidation step is carried out in the temperature range between 650 ° C and 750 ° C, this provides good control over the thickness of the iron oxide layer formed during step d. oxidation and provides stability to the entire annealing process.

A strip temperature of between 650 ° C and 750 ° C allows good control of the oxidation kinetics of the strip surface during its passage in an oxidation chamber in which the volume oxygen concentration is greater than 1% . Preferably, the volume concentration of oxygen in the oxidation chamber is between 1.5% and 5% and even more preferably between 2% and 5%. Control of the homogeneity of the oxidation kinetics is ensured by passing the strip through the homogenization chamber.

It has been observed that an increased oxygen content in the oxidation chamber reduces the harmful impact of gas leaks. However, too high an oxygen content has the consequence of oxidizing the steel strip too deeply, which requires a subsequent step of removing said iron oxide layer of a longer duration, which represents a disadvantage in terms of time and therefore cost. The inventors have determined that volume oxygen concentration values in the oxidation chamber of between 1.5% and 5% and even more preferably between 2% and 5% allow an oxidation step which is not. or little influenced by any gas leaks, without generating an excessively thick layer of iron oxide.

Preferably, the duration of exposure of said steel strip in the oxidation chamber is between 2 and 8 seconds, preferably ranging from 2 to 4 seconds.

Advantageously, the oxidation step is carried out in a confined or relatively confined manner in the oxidation chamber.

For the purposes of the invention, it is understood by the terms “relatively isolated” or “confined” that a relative seal is guaranteed in the element considered. Appropriate technical means can be implemented to control this relative tightness in order to reduce as much as possible the gas exchanges between the oxidizing atmosphere of said oxidation chamber and the atmosphere prevailing outside said oxidation chamber, by example in the rest of the RTF. An RTF is a section of a furnace essentially comprising radiant heating tubes and is an acronym known to a person skilled in the art (RTF for Radiant Tubes Furnaces).

Preferably, the oxidation step is homogeneous in that it allows homogeneous oxidation at the surface of said steel strip.

Preferably, the oxidation step is carried out by propelling an oxidizing gas by means of a carrier gas, preferably nitrogen.

It has been observed that this propulsion by means of a carrier gas makes it possible to bring the oxidizing gas to the surface of said steel strip and to cross the boundary layer entrained by the steel strip. As an advantageous effect, then, at least one of the iron layers located below said surface of said steel strip can also be oxidized. Better control and better reproducibility and / or homogeneity can thus be guaranteed during the formation of the iron oxide layer formed during the oxidation step.

Advantageously, the method according to the invention comprises the application of a pressure inside said oxidation chamber and in the rest of the furnace, said pressures being substantially equal.

It has been observed that the risk of gas transfers between the oxidation chamber and the rest of the furnace used in the process according to the invention are greatly reduced when the pressure in the oxidation chamber and in the device are substantially equal.

In addition, the process according to the invention makes it possible to maintain an easily controllable oxidation which avoids the disturbances caused by the atmosphere which surrounds the oxidation chamber.

Preferably, the heating step a), the temperature homogenization step b) as well as the reduction step d) are carried out with a reducing atmosphere having a volume concentration of hydrogen greater than 3%.

Preferably, the reducing atmosphere in the reduction zone has an atmosphere having a hydrogen concentration of between 3% and 5%. Preferably, the reduction zone has a composition comprising a hydrogen concentration of between 3% and 5%, the remainder of the composition comprising nitrogen.

Preferably, the temperature homogenization step is carried out at a strip temperature of between 650 ° C and 750 ° C.

As mentioned above, such a temperature range allows good control of the kinetics of oxide formation in the oxidation chamber, that is to say in the presence of a volume concentration of oxygen generally between 1% and 5. %. In addition, it is particularly advantageous to homogenize the strip temperature at a target temperature. Homogenization at a target temperature means that there is a supply of heat at the strip strictly equal to the heat lost by the strip. Thus, the homogenization is carried out with an essentially zero heat input / loss balance so as to prevent the introduction of other temperature inhomogeneities to the strip.

Preferably, step a) of heating is carried out so as to obtain a strip temperature of between 650 ° C and 750 ° C.

Such a strip temperature range is easily attainable by direct flame heating, which makes step a) relatively easy to implement. Preferably, step a) is carried out under reducing condition in the presence of carbon monoxide and hydrogen. Such conditions are generated by using a non-stoichiometric fuel / oxidizer mixture which is particularly poor in oxygen.

The temperature homogenization step is carried out with an atmosphere having an oxygen volume concentration of less than 0.01% by volume, preferably with an oxygen-free atmosphere.

Although the temperature homogenization step is carried out in a chamber adjacent to the oxidation chamber, the atmosphere in the homogenization chamber can be kept poor, or even very poor, in oxygen. This can be made possible according to a preferred embodiment, by the presence of confinement means positioned between the oxidation chamber and the homogenization chamber, for example by using an airlock.

Such containment means may be particularly desired because large or poorly controlled gas passages between the oxidation chamber and the reduction zone and / or the temperature homogenization chamber, can cause harmful gas exchanges between the different oven chambers. When oxygen escapes from the oxidation chamber to a chamber under a reducing atmosphere, the water vapor content increases in this zone. Then, the increase in the water vapor content influences the dew point and can give rise to unwanted oxidation phenomena, such as for example the oxidation of alloy compounds on the surface of the steel. As already explained, these alloying compounds have a high affinity for oxygen, and their selective oxidation has a deleterious influence on the adhesion of the coating obtained after galvanization.

Furthermore, the volume concentration of oxygen in the oxidation chamber (greater than 1%, or even between 1.5% and 5% according to a preferred embodiment), can represent a volume concentration which is particularly sensitive to unwanted gas exchanges. with adjacent rooms. Containment means positioned between the oxidation chamber and the homogenization chamber, for example an airlock, make it possible to further control the oxygen concentration in the oxidation chamber. This is important because when hydrogen escapes from a chamber under a reducing atmosphere to the interior of the oxidation chamber, the oxidation is no longer as efficient as part of the oxygen is consumed. by reaction with hydrogen. These phenomena negatively influence the properties of the iron oxide layer formed during the oxidation step. This problem is amplified when the oxygen content is relatively low in the oxidation chamber, since the oxygen will then be consumed even more quickly by reaction with the hydrogen.

Generally speaking, these leaks therefore greatly reduce the control of the conditions of the annealing process, which consequently causes a lack of control over the quality of a galvanized high strength steel obtained after galvanizing the strip treated according to the first aspect. of the invention, in particular in terms of adhesion of the coating layer to the surface of the steel strip.

Step a) of heating is carried out with an atmosphere having an oxygen volume concentration of less than 0.01% by volume, preferably with an oxygen-free atmosphere.

It is particularly important to preheat the steel strip with an oxygen-poor and preferably oxygen-free atmosphere so that the steel strip does not start to oxidize on the surface before it enters the chamber. oxidation. Thus, this results in better control of the oxidation thickness when the latter is carried out only in the oxidation chamber. In addition, since the temperature of the strip is not homogeneous during heating step a), it is important that no oxidation is made under such conditions of strip temperature inhomogeneity.

Preferably, the temperature homogenization step is carried out by the movement of the strip near said at least one radiant heating tube.

The advantage of running the strip near a radiant heating tube is that it allows a well-controlled amount of heat to be supplied to the strip over the entire width of the strip. Thus, the movement of the strip near the radiant heating tube allows heat exchange between the strip and the radiant heating tube. This makes it possible to maintain the strip at a temperature, for example at the target temperature, while allowing homogenization of the temperature of the strip. The invention thus makes it possible to benefit from the advantages of heating by direct flame while compensating for the drawbacks associated with heating by direct flame (inhomogeneity of strip temperature). For example, the strip runs at a distance from a radiant heating tube of between 0.1 m and 0.2 m.

Preferably, said homogenization section comprises at least two radiant heating tubes. Preferably, the metal product passes between said two radiant heating tubes.

The movement of the steel strip in front of two radiant heating tubes allows an improvement in the temperature uniformity of the steel strip by allowing more time for the steel strip to equilibrate in temperature while receiving a amount of heat from the radiant heating tubes to maintain a target band temperature. The target band temperature is generally between 650 ° C and 750 ° C and corresponds to a temperature at which the oxidation of the band in the oxidation chamber is well controlled. The same reasoning can be applied for three, four, six radiant heating tubes.

Preferably, the heating of the strip in step a) is carried out until a target strip temperature of between 650 ° C and 750 ° C is reached, and the temperature homogenization of the strip in step b) is reached. is produced so as to homogenize the temperature of the strip according to said temperature target. Preferably, the homogenization step of step b) makes it possible to maintain the strip at the target temperature.

During the temperature homogenization step, the heat communicated by the radiant heating tube (s) to the strip has the sole purpose of maintaining the temperature of the strip according to the target temperature as well as of homogenizing the temperature of the strip. this. Preferably, the radiant heating tubes during the temperature homogenization step radiate uniformly towards the strip, allowing good temperature homogenization of the strip, at the surface as well as according to the thickness of the strip.

• une section de four à chauffage direct comprenant :
  • une zone de chauffage à flamme directe;
• une section de four à chauffage radiant comprenant :
  • une chambre d'oxydation ;
  • une zone de réduction ;
  • une chambre d'homogénéisation en température positionnée après la zone de chauffage à flamme directe et avant la chambre d'oxydation, la chambre d'homogénéisation comprenant au moins un tube de chauffage radiant.

According to a second aspect, one of the aims of the present invention is to provide a furnace for the heat treatment of a high resistance steel strip by scrolling allowing oxide formation on the surface of the strip with a more homogeneous thickness. and more controlled. To this end, the inventors propose a furnace for the heat treatment of a high resistance metal strip by scrolling comprising:

• a direct heating furnace section comprising:
  • a direct flame heating zone;
• a radiant heating furnace section comprising:
  • an oxidation chamber;
  • a reduction zone;
  • a temperature homogenization chamber positioned after the direct flame heating zone and before the oxidation chamber, the homogenization chamber comprising at least one radiant heating tube.

Thus, the temperature homogenization chamber is positioned between the direct flame heating zone and the oxidation chamber. A radiant heated furnace section is an RTF. The homogenization chamber is located in the radiant heating furnace section, as is the oxidation chamber.

Preferably, said homogenization chamber comprises at least two radiant heating tubes and even more preferably at least three radiant heating tubes.

The number of radiant heating tubes in the homogenization chamber makes it possible to define its length, along which the strip can equilibrate in temperature while remaining at the target strip temperature. The number of radiant heating tubes and the length of the temperature homogenization chamber depend on the direct flame heating zone and the temperature inhomogeneity of the strip which emerges from it as well as the desired temperature homogeneity of the strip in the oxidation chamber. The number of radiant tubes and the length of the homogenization chamber can also depend on the target temperature at the outlet of the homogenization chamber.

Preferably, in the temperature homogenization chamber, the metallic product is positioned scrolling between at least two radiant heating tubes. Such an embodiment allows better homogenization of the temperature of the strip as described for the process according to the first aspect of the invention.

For example, the furnace further comprises a first and a second rollers for guiding the moving web, the first roll being positioned downstream of the direct flame heating zone and the second roll being positioned downstream of the oxidation chamber. . The strip is preferably kept under tension in the homogenization chamber so that when moving, said strip describes an essentially rectilinear path as it passes through the homogenization chamber and into the reduction zone.

For example, the first and second rollers are positioned so that said metal strip is stretched in a substantially vertical orientation between said rollers. An essentially vertical strip orientation corresponds to a strip orientation with respect to a flat ground describing an angle with the normal to the flat ground between 0 ° and 15 °. The strip is under tension in the oven so that it is stretched as it passes through the homogenization chamber and then into the oxidation chamber.

In another possible embodiment, the furnace is configured such that the metal strip is stretched in a substantially horizontal orientation.

In a preferred embodiment, the oxidation chamber is further delimited by two locks which are each formed by at least two lock rollers. In such a case, although there is no permanent contact between the strip and such airlock rollers, it is possible for the strip to come into contact with them, for example as a result of movement of the strip.

The oxidation chamber is confined from the homogenization chamber and from the reduction zone by two confinement means allowing the strip to travel through said oxidation chamber, for example the two confinement means are two locks. The associated advantages described for the process of the invention apply to the oven, mutatis mutandis.

Preferably, the oxidation chamber is provided with vents in order to balance the incoming and outgoing volumes to balance the pressure inside the chamber and also to reduce the possible transfers of gas by leaks.

Brief description of the figures

• la Fig.1 montre un mode de réalisation selon l'invention;
• la Fig.2 montre un mode de réalisation selon l'invention;
• la Fig.3 montre une vue schématique de l'amenée d'une bande vers une chambre d'homogénéisation en température, puis vers une chambre d'oxydation et le cheminement de la bande vers une zone de réduction.

Les dessins des figures ne sont pas à l'échelle et ne sont pas limitatifs. Généralement, des éléments semblables sont dénotés par des références semblables dans les figures. La présence de numéros de référence aux dessins ne peut être considérée comme limitative, y compris lorsque ces numéros sont indiqués dans les revendications.These and other aspects of the invention will be clarified in the detailed description of particular embodiments of the invention, with reference being made to the drawings of the figures, in which:

• the Fig. 1 shows an embodiment according to the invention;
• the Fig. 2 shows an embodiment according to the invention;
• the Fig. 3 shows a schematic view of the supply of a strip to a temperature homogenization chamber, then to an oxidation chamber and the path of the strip to a reduction zone.

The drawings of the figures are not to scale and are not limiting. Generally, like elements are denoted by like references in the figures. The presence of reference numbers in the drawings can not be considered as limiting, including when these numbers are indicated in the claims.

Detailed description of certain embodiments of the invention

The figure 1 shows a schematic illustration of the oven 1 according to the second aspect of the invention making it possible to implement the method according to the first aspect of the invention. The furnace 1 comprises in the direction of travel of the strip 5, a direct flame heating zone 10, a temperature homogenization chamber 20, an oxidation chamber 30 and a reduction zone 40 for the reduction of the temperature. oxide and heat treatment of the tape. The furnace 1 comprises a direct heating furnace section 2 comprising the direct flame heating zone 10 and a radiant heating furnace section 3 comprising the temperature homogenization chamber 20, the oxidation chamber 30 and the heating zone. reduction 40.

The method according to the invention comprises the implementation of step a) of heating the strip 5 by direct flame in the direct flame heating zone 10. The method then comprises the implementation of step b) , that is to say the movement of the strip 5 near at least one radiant heating tube 25 so, for example, to allow time for the strip 5 preheated to a target temperature, to homogenize in temperature while maintaining said target temperature. According to another possible scenario, the strip 5 can be heated in the homogenization chamber 20 so as to have an outlet (homogenized) temperature higher than the inlet temperature. The method then comprises the implementation of the oxidation step c), that is to say the movement of the strip 5 in the oxidation chamber 30 comprising a volume concentration of oxygen greater than 1% and preferably between 1.5% and 5%. During step c), an oxide layer forms on the surface of the strip 5. The oxide formed is essentially iron II, II-III or III oxide in general. The method of heat treatment of a steel strip 5 comprises after step c), step d) during which, the steel strip 5 oxidized in step c) undergoes a heat treatment at a temperature of band up to 800 ° C and preferably up to 850 ° C. During this step d), the strip 5 is subjected to a reducing atmosphere preferably comprising a volume concentration of hydrogen greater than 3%, and more preferably between 3% and 5%. The remaining volume fraction being nitrogen in general. The temperature of the heat treatment in the reduction zone during step d) can be changed relatively easily without, however, steps a), b) and c) being changed significantly.

The figure 2 shows an overall view of a furnace 1 according to the second aspect of the invention, with a schematic representation of the path of the strip 5 through the direct flame heating zone 10, the homogenization chamber 20, the chamber of oxidation 30 and the reduction zone 40 included in the furnace 1. The strip 5 describes a succession of vertical passes during which it passes through the direct heating furnace section 2 then the radiant heating furnace section 3 After having passed through the direct flame heating zone 10, the strip 5 enters the radiant heating furnace section 3 through the homogenization chamber 20. In the non-limiting example shown in FIG. figure 2 , the direct flame heating zone 10 comprises two pass lines. Then the strip 5 is directed towards the temperature homogenization chamber 20.

The pass line comprising the temperature homogenization chamber 20 and the oxidation chamber 30 is located in the RTF section (radiant heating furnace section) of furnace 1. Thus, the oxidation chamber 30 is at a similar temperature. of the RTF section which surrounds it while being preferably isolated in terms of the oxygen and hydrogen content.

After leaving the oxidation chamber 30, the strip 5 enters the reduction zone 40 for its heat treatment. The reduction zone 40 comprises a series of vertical passes surrounded by radiant heating tubes 25 allowing adjustment of the temperature of the strip 5 in order to achieve the desired heat treatment of the high strength steel strip 5.

The figure 3 shows a schematic view of the feed of the strip 5 to the temperature homogenization chamber 20, then to the oxidation chamber 30 and the path of the strip 5 to the reduction zone 40. The figure 3 shows a particular embodiment of the chamber temperature homogenization 20 which exemplarily illustrates three radiant heating tubes 25 arranged so that the strip 5 passes close by as it travels through the temperature homogenization chamber 20. The temperature homogenization chamber 20 illustrated allows good homogenization of the temperature of the strip 5 at a target temperature, the target temperature being defined as a function of the composition of the steel. Thus a precisely defined and homogeneous oxide thickness over the entire surface of the strip 5 can be obtained.

For example, in operation, a strip of steel is fed into a direct flame heating zone 10 and is heated under a reducing condition in the presence of carbon monoxide and hydrogen, preferably so as to reach a strip temperature. between 650 to 750 ° C. The steel strip is then fed to the oxidation chamber 30 which is confined in the section of the radiant heating furnace (RTF), where the oxidation takes place with an oxygen content greater than 1%. This oxidation step allows the formation at the surface of a layer of iron oxide, for example. Then, the oxide layer is removed during the heat treatment step in a reducing atmosphere in order to proceed with the galvanizing step according to a method well known to those skilled in the art.

The present invention has been described in relation to specific embodiments, which have a purely illustrative value and should not be considered as limiting. In general, the present invention is not limited to the examples illustrated and / or described above. The use of the verbs "to understand", "to include", "to include", or any other variant, as well as their conjugations, can in no way exclude the presence of elements other than those mentioned. The use of the indefinite article "a", "a", or of the definite article "the", "the" or "the", to introduce an element does not exclude the presence of a plurality of these elements. Reference numbers in the claims do not limit their scope.

Claims (12)

1. Method of heat treatment of a scrolling high-strength steel strip (5), said method comprising the following steps:

   a) heating the strip (5) in a direct flame heating zone (10), the heating step being performed with an atmosphere having an oxygen volume concentration of less than 0.01% by volume, preferably with an oxygen-free atmosphere,

   b) temperature homogenisation of the strip (5) in a homogenisation chamber (20) comprising at least one radiant heating tube (25), so as to homogenise the strip (5) in temperature after its passage in the direct flame heating zone (10) in the previous step, the temperature homogenisation step being performed with an atmosphere having an oxygen volume concentration of less than 0.01% by volume, preferably with an oxygen-free atmosphere,

   c) oxidation of the strip (5) in an oxidation chamber (30) with an oxidising atmosphere having an oxygen volume concentration of greater than 1%,

   d) reduction of the strip (5) in a reduction zone (40).
2. Method according to the preceding claim characterised in that the reduction zone (40) has a reducing atmosphere having a volume concentration of hydrogen greater than 3%.
3. Method according to any one of the preceding claims characterised in that the oxidation step is performed at a strip (5) temperature comprised between 650 °C and 750 °C.
4. Method according to any one of the preceding claims characterised in that said oxygen volume concentration is comprised between 1.5% and 5%, preferably comprised between 2% and 5%.
5. Method according to any one of the preceding claims characterised in that the temperature homogenisation step is performed at a strip (5) temperature comprised between 650 °C and 750 °C.
6. Method according to any one of the preceding claims characterised in that the heating step a) is performed so as to obtain a strip (5) temperature comprised between 650 °C and 750 °C.
7. Method according to any one of the preceding claims characterised in that the temperature homogenisation step is performed by scrolling the strip (5) close to said at least one radiant heating tube (25).
8. Method according to the preceding claim, characterised in that said homogenisation section comprises two radiant heating tubes (25) and in that the strip (5) scrolls between said two radiant heating tubes (25).
9. Furnace (1) for heat treatment of a high-strength steel strip (5) by scrolling comprising:

   - a direct heating furnace section (2) comprising:

   • a direct flame heating zone (10),

   - a radiant heating furnace section (3) comprising:

   • an oxidation chamber (30),

   • a reduction zone (40),

   characterised in that the radiant heating furnace section (3) further comprises a temperature homogenisation chamber (20) positioned between said direct flame heating zone (10) and said oxidation chamber (30), said homogenisation chamber (20) comprising at least one radiant heating tube (25),
   and in that the oxidation chamber (30) is isolated from the homogenisation chamber (20) and from the reduction zone (40) by two means of containment (35) allowing the scrolling of the strip (5) through said oxidation chamber (30).
10. Furnace (1) according to the preceding claim characterised in that said homogenisation chamber (20) comprises at least two radiant heating tubes (25) and more preferably at least three radiant heating tubes (25).
11. Furnace (1) according to the preceding claim characterised in that it is configured so that in the homogenisation chamber (20), the high-strength steel strip (5) is able to be positioned in scrolling between at least two radiant heating tubes (25).
12. Furnace (1) according to claim 9 characterised in that the two means of containment (35) are two airlocks.

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