EP1999287B1 - Method for continuously annealing and preparing strip of high-strength steel for the purpose of hot-dip galvanizing it - Google Patents

Abstract

The present application relates to a method for continuously annealing and preparing a strip of high-strength steel for the purpose of hot-dip coating it in a bath of liquid metal, in which said steel strip is treated in at least two sections, comprising in succession, when considering the direction of advance of the strip: a section called the heating and holding section, in which the strip is heated and then held at a given annealing temperature in an oxidizing atmosphere; and a section called the cooling and transfer section, in which the annealed strip at least is cooled and undergoes complete reduction, in a reducing atmosphere, of the iron oxide present in the oxide layer formed in the previous section, in such a way that the oxidizing atmosphere is separated from the reducing atmosphere, a controlled oxygen content is maintained in the heating and holding section between 50 and 1000 ppm, and a controlled hydrogen content is maintained in the cooling and transfer section at a value of less than 4% and preferably less than 0.5%.

Description

Object of the invention

The present invention relates to a novel process for the continuous annealing and preparation of a high strength steel strip for hot dip coating in a bath of liquid metal, preferably a galvanization or a so-called galvannealing ".

The technical field considered here is that of galvanization by continuous scrolling, in a coating bath composed of zinc or zinc alloy, of steel strips heavily loaded with alloying elements, more particularly HSS steels ( high strength steels ). These special steels, which are known to be difficult to galvanize, are, for example, steels which may contain contents of alloying elements (aluminum, manganese, silicon, chromium, etc.) up to 2% or more, stainless steels, "dual phase ", TRIP, TWIP (up to 25% Mn and 3% Al), etc. These steel strips are generally intended for cutting and subsequent shaping by stamping, folding, etc., for applications for example in the automotive or construction sector.

State of the art

It is well known that some steels do not respond well to galvanizing or galvannealing treatment, given their specific surface reactivity. The galvanizing power essentially depends on the proper removal of the rolling oil residues and the prevention of excessive surface oxidation prior to immersion in the liquid metal bath. Thus, a lack of wettability of liquid zinc on steel grades heavily loaded with alloying elements can be encountered during the continuous galvanizing process. This decrease in zinc wetting is explained by the presence of a layer of selective oxides in the outer layer of the surface of the strip ("extreme surface"). These selective oxides are created by the segregation of the alloying elements and their oxidation by water vapor, during the continuous annealing prior to immersion in the zinc bath. The water vapor is generated here by the reduction of the iron oxide, always present on the cold-rolled sheet, by the hydrogen contained in the atmosphere of the annealing furnaces.

• augmentation du point de rosée pendant le maintien à haute température (par exemple JP-A-2005/068493 ), de manière à faire basculer l'oxydation sélective des éléments d'alliage du mode externe au mode interne ;
• oxydation totale du fer pendant l'étape de chauffe, en augmentant par exemple le rapport air/gaz combustible dans les brûleurs du four à flammes directes, puis réduction en fer métallique pendant le maintien à haute température par l'hydrogène (par exemple JP-A-2005/023348 , JP-A-07 034210 , etc.) ou réduction par le carbone libre de l'acier qui diffuse, le cas échéant, au travers de la couche d'oxyde et échange de l'oxygène à la surface de celle-ci (voir par exemple BE-A-1 014 997 ) ;
• pré-dépôt de fer ou de nickel (par exemple JP-A-04 280925 , JP-A-2005/105399 ).

Therefore, we tried to eliminate the oxidation in external mode or to migrate inside the steel, 1 or 2 microns under the outer layer of the surface, to allow to present liquid zinc a virtually pure metal iron layer, regardless of the alloy composition and promoting the attachment of zinc coating or zinc alloy. This result can be obtained by different methods:

• increased dew point during high temperature maintenance (eg JP-A-2005/068 493 ), so as to switch the selective oxidation of the alloy elements from the external mode to the internal mode;
• total oxidation of the iron during the heating step, for example by increasing the air / fuel gas ratio in the burners of the direct flame furnace, then reduction in metallic iron during the maintenance at high temperature by hydrogen (for example JP-A-2005/023348 , JP-A-07 034210 , etc.) or reduction by the free carbon of the steel diffusing, if necessary, through the oxide layer and exchange of oxygen on the surface thereof (see for example BE-A-1,014,997 );
• pre-deposition of iron or nickel (for example JP-A-04 280925 , JP-A-2005/105399 ).

These processes generally require working in a reducing atmosphere for the steel during the high temperature holding phase, requiring a low dew point and a high hydrogen content (up to 75% of the atmospheric gas) which is a gas. expensive. They all make it possible to improve the "galvanizability" of high strength steels with a significant but insufficient efficiency, especially in the case of certain steels containing, for example, significant silicon contents (approximately 1.5% by weight). Moreover, the processes requiring a pre-deposit have very high costs.

• une première section de (pré)chauffage assurant le chauffage de la bande jusqu'à une température permettant la formation d'un film d'oxyde d'épaisseur adéquate (environ 50 nanomètres) pour sa réduction ultérieure ; cette section se trouve sous une atmosphère rendue oxydante par adjonction d'air ou d'oxygène, par exemple sous la forme d'un mélange air/gaz combustible dans le cas d'un four à flamme directe ou d'air seul dans le cas d'un four radiant ;
• une deuxième section de recuit, séparée de la section de chauffage par un sas conventionnel, où la bande est maintenue à la haute température de recuit et qui se trouve sous une atmosphère inerte en surpression, pour y empêcher l'entrée des gaz de la section de chauffe ;
• une troisième section de réduction, également séparée de la deuxième section par un sas conventionnel, sous une atmosphère en légère dépression par rapport à celle-ci mais en légère surpression par rapport à l'ambiante ; cette section est destinée à terminer le cycle de recuit (fin de la période de maintien), à refroidir la bande et éventuellement à effectuer un survieillissement avant de la transférer dans le bain de métal liquide via une trompe d'immersion ; dans cette zone, la couche d'oxyde créée dans la première section est idéalement réduite complètement par une atmosphère hydrogène/gaz inerte à très bas point de rosée.

According to an example of a method already known in the state of the art, an annealing installation and preparation of a steel strip for galvanizing typically comprises, in the direction of progression of the strip:

• a first (pre) heating section heating the strip to a temperature enabling the formation of an oxide film of adequate thickness (about 50 nanometers) for its subsequent reduction; this section is under an oxidizing atmosphere by adding air or oxygen, for example in the form of an air / fuel gas mixture in the case of a direct flame furnace or of air alone in the case of a radiant furnace;
• a second annealing section, separated from the heating section by a conventional airlock, where the strip is held at the high annealing temperature and which is under an inert overpressure atmosphere, to prevent entry of gases from the section heating;
• a third reduction section, also separated from the second section by a conventional airlock, in an atmosphere slightly depressed relative thereto but slightly overpressure relative to the ambient; this section is intended to complete the annealing cycle (end of the holding period), to cool the band and possibly to perform a survival before transferring it into the bath of liquid metal via an immersion tube; in this zone, the oxide layer created in the first section is ideally reduced completely by a hydrogen / inert gas atmosphere with a very low dew point.

Of course, there are also known simpler or more complex annealing furnaces, typically comprising between one and four distinct sections, to perform the respective functions of (pre-) heating, maintenance, cooling, over-aging, etc.

Goals of the invention

The present invention aims to provide a solution that makes it possible to overcome the disadvantages of the state of the art.

In particular, the invention aims to provide a method of annealing and preparation for galvanizing high strength steels that is more economical, the latter being carried out with or without accompanying heat treatment galvannealing type.

The invention also aims to allow a preparation of high strength steels for galvanizing, which are free of brittleness defects.

In particular, the invention aims to provide a confined atmosphere annealing process free of added hydrogen.

An additional object of the invention is to prevent the selective oxidation of alloying elements in the outermost layer of the surface of the strip during the total oxidation step during continuous annealing prior to cooling. and immersion in the zinc bath.

Main characteristic elements of the invention

• une section dite de chauffe et de maintien, dans laquelle est réalisé un chauffage de la bande suivi d'un maintien à une température donnée de recuit sous une atmosphère oxydante comprenant un mélange air (ou oxygène)/gaz non oxydant ou inerte, en vue de former sur la surface de la bande un fin film d'oxyde dont l'épaisseur, comprise de préférence entre 0,02 et 0,2 µm, est contrôlée, ledit chauffage de la bande étant effectué soit par flamme directe, soit par rayonnement ;
• une section dite de refroidissement et de transfert, dans laquelle, avant son transfert au bain de revêtement, la bande recuite au moins est refroidie et subit une réduction complète en fer métallique de l'oxyde de fer présent dans la couche d'oxyde formée dans la section de chauffe et de maintien, sous une atmosphère réductrice comprenant un mélange à basse teneur en hydrogène et gaz inerte, les deux dites sections étant séparées l'une de l'autre par un sas conventionnel ;

caractérisé en ce qu'on sépare au moins partiellement l'atmosphère oxydante de l'atmosphère réductrice, en ce qu'on maintient une teneur en oxygène contrôlée dans la section de chauffe et de maintien entre 50 et 1000 ppm et en ce qu'on maintient une teneur en hydrogène contrôlée dans la section de refroidissement et transfert à une valeur inférieure à 4 % et de préférence inférieure à 0,5 %.The present invention relates to a method of continuously annealing and preparing a high-strength steel strip for hot-dip coating in a bath of molten metal, wherein said steel strip is treated in at least two sections, comprising successively, considering the direction of progression of the band:

• a so-called heating and holding section, in which a heating of the strip is carried out followed by a maintenance at a given annealing temperature under an oxidizing atmosphere comprising an air (or oxygen) / non-oxidizing or inert gas mixture, with a view to forming on the surface of the strip a thin oxide film whose thickness, preferably between 0.02 and 0.2 μm, is controlled, said heating of the strip being carried out either by direct flame or by radiation ;
• a so-called cooling and transfer section, in which, before it is transferred to the coating bath, the at least one annealed strip is cooled and undergoes a complete reduction in metallic iron of the iron oxide present in the oxide layer formed in the heating and holding section, under a reducing atmosphere comprising a mixture of low hydrogen content and inert gas, the said two sections being separated from each other by a conventional airlock;

characterized in that the oxidizing atmosphere is at least partially separated from the reducing atmosphere by maintaining a controlled oxygen content in the heating and holding section between 50 and 1000 ppm and in that maintains a controlled hydrogen content in the cooling section and transfer to a value of less than 4% and preferably less than 0.5%.

It is meant by complete reduction of iron oxide, a reduction of it to at least 98%.

Advantageously, the controlled oxygen content is maintained in the heating and holding section between 50 and 400 ppm.

According to a first preferred embodiment of the invention, the separation of the oxidizing atmosphere from the reducing atmosphere is carried out by an overpressure of the oxidizing atmosphere, so that the oxygen entrained by the strip in the cooling zone and transfer through the airlock, following this overpressure, react completely with the hydrogen contained in the cooling atmosphere by forming water vapor.

According to a second preferred embodiment of the invention, the hydrogen is allowed to react, present in the cooling and transfer section, entrained in the hot gas stream directed upstream, with oxygen from the heating and holding section to form water vapor. In this case, the cooling and transfer section is maintained in overpressure with respect to the heating and holding section. As the gas under pressure can not escape to the bath of liquid metal, it goes back to the heating and maintenance zone.

According to the invention, the control of the oxygen content of the oxide layer formed in the heating and holding section is obtained either by modifying the gaseous mixture containing combustion air supplying direct flame heating means, or by controlled injection of the air (or oxygen) / inert gas mixture in the case of radiation or induction heating.

Preferably, the non-oxidizing or inert gas is nitrogen or argon.

Advantageously, the liquid metal is zinc or one of its alloys.

Still advantageously, the heating and maintenance zone is devoid of a reducing atmosphere.

Preferably, the hot dip coating process is a galvanization or a galvannealing treatment.

Still according to the invention, the atmosphere both in the heating and holding section and in the cooling and transfer section has a dew point less than or equal to -10 ° C, preferably -20 ° C.

According to a preferred operational mode, the strip is heated to a temperature of between 650 ° C. and 1200 ° C., including the holding temperature.

According to another preferred operational mode, the band is then cooled down to a temperature above 450 ° C, with a cooling rate between 10 and 100 ° C / s.

Description of a preferred embodiment of the invention

An economical method, proposed according to the invention, aims to perform the annealing step preparatory to galvanization, without the addition of hydrogen gas which is ten times more expensive than a more common gas such as nitrogen and which is In addition, it causes serious fragility defects in resistance steels.

The aim of the invention is to obtain perfect galvanization for all grades of resistance steel. To avoid oxidation of the alloy elements at the extreme surface, it is proposed to inject an air / nitrogen mixture into the oven during the entire cycle of (pre-) heating and holding the sheet at high temperature.

This method therefore does not require any atmospheric separation throughout the heating / holding part as is the case in other processes (for example JP-A-2003/342645 where negative reactive zones are included at this part of the furnace.

• l'oxydation du fer par l'oxygène en extrême surface avec croissance de l'oxyde de fer par diffusion de fer en surface. Ainsi, tant qu'une fine couche d'oxyde de fer subsiste en surface de la tôle, les éléments d'alliage, à l'exception du manganèse, sont bloqués à l'interface acier/oxyde de fer ;
• la réduction subséquente de l'oxyde de fer par diffusion du carbone libre vers l'interface acier/oxyde de fer.

The oxygen contained in the air / nitrogen mixture will have the effect of creating in the annealing section two simultaneous and competitive reactions:

• the oxidation of iron by oxygen at the extreme surface with growth of iron oxide by diffusion of iron at the surface. Thus, as long as a thin layer of iron oxide remains on the surface of the sheet, the alloying elements, with the exception of manganese, are blocked at the steel / iron oxide interface;
• the subsequent reduction of iron oxide by diffusion of free carbon towards the steel / iron oxide interface.

The alloying elements also contribute to the reduction of iron oxide when migrating at the steel / iron oxide interface.

The air / nitrogen atmosphere of the heating / holding portion will, however, have to be separated and partially isolated from the non-oxidizing atmosphere of the cooling and transfer stages of the strip into the zinc bath. For this purpose, the oxidizing atmosphere will preferably be maintained at an excess pressure relative to the non-oxidizing atmosphere such that the oxygen entrained by the sheet reacts completely with the hydrogen contained in the atmosphere of the cooling.

In such a configuration, a steel containing, for example, 1.2% of aluminum will for example be heated and annealed to a temperature of 800 ° C. in an atmosphere containing 100 ppm of oxygen in nitrogen. At the end of the hold, which lasts one minute, the sheet is cooled to 500 ° C. at a rate of 50 ° C./s in an atmosphere containing 4% hydrogen and 0.1% water vapor. which corresponds to a dew point of -20 ° C. This sheet is then introduced at the temperature of 470 ° C. in a zinc bath, containing 0.2% of aluminum, which is maintained at 460 ° C. After immersion for 3 seconds, the coating is wrung out so as to keep an 8 μm zinc layer. Such a zinc deposit is then perfectly wetting and has adhesion qualities comparable to that obtained for ordinary low-carbon steel.

To give another example, the same process can be applied to a steel containing, for example, 1.5% silicon. In this case, however, it will be necessary to increase the oxygen content during the heating / holding step to 300 ppm to obtain a comparable result. This increase in oxygen content is necessary because silicon slows the diffusion of iron by providing a silicon oxide barrier at the steel / iron oxide interface.

Another way of proceeding is to let the usual flux settle from the zinc bath to the heating section and to leave the very low hydrogen content (<0.5%), contained in the transfer / cooling section, react with the oxygen of the heating / holding part to form water vapor. An additional supply of oxygen, at the outlet of the holding section, may be made to neutralize the hydrogen inlet, the contents used being always located very far from the dangerous, that is to say, explosive ( 4% H ₂ in air).

A high hydrogen content is indeed not necessary in the cooling section because the carbon of the steel will be sufficient to reduce the thin layer of iron oxide created in the heating / holding part and the metal iron thus prepared will ensure a good wettability by the zinc during the immersion of the sheet in the bath.

To be effective, this method will have to provide for controlling the oxygen content in the furnace within the range of 50 to 1000 ppm. In fact, a content that is too low will not make it possible to produce a layer of iron oxide sufficiently impervious to the diffusion of the alloying elements towards the extreme surface, and a content that is too high in oxygen will produce a layer of iron oxide that is too thick. , which can not be reduced during the cooling and transfer steps to the zinc bath. This oxygen content will preferably be in the range of 50 to 400 ppm.

• qu'on procède à un ajout d'hydrogène beaucoup plus faible que dans l'état de la technique, voire nul, dans la zone de chauffe-maintien, ce qui constitue une importante économie d'exploitation et garantit l'obtention d'acier de haute résistance présentant moins de défauts de fragilité ;
• qu'on ne sépare plus la section de chauffe de la section de maintien à la température de recuit, ce qui permet d'économiser un sas ainsi que d'éviter éventuellement un dédoublement des équipements de contrôle de l'atmosphère gazeuse ;
• que ce procédé est beaucoup plus efficace que les procédés connus dans l'état dé la technique, du point de vue de l'adhérence du revêtement ou de la mouillabilité de la bande ;
• que l'atmosphère gazeuse utilisée est moins fragilisante pour l'équipement (par exemple les tubes radiants), notamment suite à la réduction de la teneur de celle-ci en hydrogène.

The invention has a number of advantages, including:

• Hydrogen addition is much lower than in the state of the art, or even zero, in the maintenance zone, which represents a significant operating saving and guarantees the obtaining of steel. high strength with fewer frailty defects;
• that the heating section of the holding section is no longer separated at the annealing temperature, which makes it possible to save an airlock and possibly to avoid duplication of the gaseous atmosphere control equipment;
• that this method is much more effective than the methods known in the state of the art, from the point of view of the adhesion of the coating or the wettability of the strip;
• that the gaseous atmosphere used is less fragile for the equipment (for example the radiant tubes), in particular following the reduction of the content thereof in hydrogen.

Claims (12)

1. Method for continuously annealing and preparing a strip of high-strength steel in order to coat it by hot dipping in a bath of molten metal, according to which said steel strip is treated in at least two sections, comprising successively, if considered in the flow direction of the strip:

   - a "heating and temperature-maintenance" section, in which the strip is heated, then maintained at a given annealing temperature under oxidising atmosphere with an air (or oxygen)/non-oxidising or inert gas mixture, in order to form a thin oxide film on the surface of the strip, whose thickness, preferably between 0.02 and 0.2 µm, is controlled, said heating of the strip being achieved either by direct flame or by radiation;

   - a "cooling and transfer" section in which, before it is transferred to the coating bath, the annealed strip is at least cooled and undergoes complete reduction to metallic iron from the iron oxide present in the oxide layer formed in the heating and temperature-maintenance section, under reducing atmosphere with a mixture of low level of hydrogen and inert gas, both said sections being separated from each other by a conventional airlock;

   wherein the oxidising atmosphere is at least partially separated from the reducing atmosphere, wherein a controlled oxygen level is maintained between 50 and 1,000 ppm in the heating and temperature-maintenance section and wherein a controlled hydrogen level is maintained in the cooling and transfer section at a value lower than 4% and preferably lower than 0.5%.
2. Method as in Claim 1, wherein the controlled oxygen level in the heating and temperature-maintenance section is maintained between 50 and 400 ppm.
3. Method as in Claim 1 or 2, wherein the oxidising atmosphere is separated from the reducing atmosphere by over-pressurising the oxidising atmosphere so that the oxygen introduced by the strip through the airlock completely reacts with the hydrogen of the cooling atmosphere by forming water steam.
4. Method as in Claim 1 or 2, wherein the hydrogen present in the cooling and transfer section, which is at a pressure higher than the heating and temperature-maintenance section and which is introduced into the gaseous flow directed upstream, is allowed to react with the oxygen coming from the heating and temperature-maintenance section so as to form water steam.
5. Method as in any of the above claims, wherein the control of the oxygen content of the oxide layer formed in the heating and temperature-maintenance section is achieved either by modifying the gaseous mixture with combustion air feeding the direct-flame heating means, or by the controlled injection of the air (or oxygen/inert gas mixture in the case of radiation or induction heating.
6. Method as in any of the above claims, wherein the non-oxidising or inert gas is nitrogen or argon.
7. Method as in any of the above claims, wherein the molten metal is zinc or one of its alloys.
8. Method as in Claim 1, wherein the heating and temperature-maintenance zone is free of any reducing atmosphere.
9. Method as in Claim 1, wherein the hot-dip coating method is galvanisation or a galvannealing treatment.
10. Method as in any of the above claims, wherein the atmosphere in the heating and temperature-maintenance section and in the cooling and transfer section has a dewpoint lower than or equal to -10°C and preferably to - 20°C.
11. Method as in any of the above clams, wherein the strip is heated up to a temperature between 650°C and 1,200°C, which includes the maintenance temperature.
12. Method as in Claim 1, wherein the strip is then cooled to a temperature higher than 450°C at a cooling speed between 10 and 100°C/s.