EP1999287A1

EP1999287A1 - Method for continuously annealing and preparing strip of high-strength steel for the purpose of hot-dip galvanizing it

Info

Publication number: EP1999287A1
Application number: EP07719191A
Authority: EP
Inventors: Michel Bordignon; Xavier Vanden Eynde
Original assignee: Centre de Recherches Metallurgiques CRM ASBL
Current assignee: Centre de Recherches Metallurgiques CRM ASBL
Priority date: 2006-03-29
Filing date: 2007-03-13
Publication date: 2008-12-10
Anticipated expiration: 2027-03-13
Also published as: KR20080111507A; MX2008012494A; ES2331634T3; AU2007231473A1; DE602007002064D1; BRPI0709419A2; RU2426815C2; US20100062163A1; ZA200808424B; US8409667B2; CN101466860A; ATE440156T1; UA92079C2; CA2644459C; AU2007231473B2; JP2009531538A; BE1017086A3; PL1999287T3; WO2007109865A1; JP5140660B2

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

STRESS AND PREPARATION PROCESS FOR CONTINUOUSLY ⁷ A BAM) E HIGH STRENGTH STEEL FOR IN ITS GALVANIZING

QUENCH

Object of the invention

The present invention relates to a new annealing process and continuous preparation of a high-strength steel strip for its coating by hot quenching in a bath of liquid metal, preferably a galvanization or a. so-called "galvannealing" treatment. The technical field considered here is that of galvanization by continuous scrolling in a coating bath composed of zinc or zinc alloy strips of steel. heavily loaded with alloying elements, especially HSS steels (high strength steels). These special steels known to be difficult to galvanize are, for example, steels which may contain levels of alloying elements (aluminum, manganese, silicon, chromium, etc.) up to _. 2% or above, ^" stainless steels," dual phase ", TRIP, TWIP- (up to 25% Mn and 3% Al), etc. These steel strips are generally intended for cutting and placing in subsequent form 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. [0004] Therefore, it has been sought to eliminate the selective oxidation in external mode or to make it migrate inside the steel, at 1 or 2 μm under the outer layer of the surface, to make it possible to present the zinc liquid a layer of substantially pure metal iron, regardless of the alloy composition and promoting the attachment of zinc coating or zinc alloy. This result can be obtained by various processes: - increase of the dew point during the maintenance at high temperature (for example JP-A-2005/068493), so as to switch the selective oxidation of the alloy elements of the external mode 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 -AT-

2005/023348, JP-A-07 034210, etc.) or reduction by the

• free carbon steel mistletoe diffuses, if necessary, through the oxide layer and exchange of oxygen on the surface thereof (see for example BE-A-I 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 maintenance phase, requiring a low dew point and a high hydrogen content (up to 75% of the atmosphere gas). which is an expensive gas. 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. 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 allowing 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 mixture gas in the case of a direct flame furnace or air alone in le case of a radiant furnace;

a second annealing section, separated from the heating section by a conventional airlock, where the strip is kept at the high annealing temperature and which is under an inert atmosphere at overpressure, to prevent the entry of gases from the heating section;

a third reduction section, also separated from the second section by a conventional airlock, under an atmosphere slightly depressed with respect thereto but at a slight overpressure with respect to the ambient; this section is intended to finish the annealing cycle

(end of the holding period), to cool the band and possibly to carry out 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.

Aims 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 which 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. previous cooling and immersion in the zinc bath.

Main Features of the Invention [0013] The present invention relates to a method of continuous annealing and preparation of a high strength steel strip for hot dip coating in a bath of molten metal. according to which said steel strip is treated in at least two sections, comprising successively, considering the direction of progression of the strip: a so-called heating and holding section, in which a heating of the strip is achieved; strip followed by maintaining at a given annealing temperature under an oxidizing atmosphere comprising an air (or oxygen) / non-oxidizing or inert gas mixture, in order to form on the surface of the strip a thin oxide film of which the thickness, preferably between 0.02 and 0. 2 microns, is controlled, said heating of the strip being performed ^• either by direct flame or by radiation; a so-called cooling and transfer section, in which, before it is transferred to the coating bath, at least the annealed strip is cooled and undergoes a complete reduction in iron metal 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 thereof 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 steam.

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 the flame heating means. direct, or by controlled injection of the mixture air (or oxygen) / inert gas in the case of radiation heating or induction. [0019] Preferably, the non-oxidizing or inert gas is nitrogen or argon. [0020] Advantageously, the liquid metal is zinc or one of its alloys.

Still advantageously, the heating zone and maintenance is devoid of 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 ⁰ C, preferably -20 ⁰ C. [0024] According to a preferred operational mode, the strip is heated to a temperature between

65O ⁰ C and 1200 ⁰ C, including the holding temperature.

According to another preferred operational mode, the band is then cooled down to a maximum of δ

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 at carrying out the annealing step preparatory to galvanizing, without the addition of hydrogen, gas which is ten times more expensive than a more common gas such as nitrogen and which is also causing serious fragility defects of resistance steels.

The invention aims to obtain a perfect galvanization for all grades of steel resistance. 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 atmospheric separation throughout the heating / maintenance portion as is the case in other processes (eg JP-A-2003/342645) where reactive zones in depression are included at this part of the. oven.

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 the iron oxide by diffusion of the free carbon towards the steel / iron oxide interface. The alloying elements also contribute to the reduction of iron oxide when they migrate to the steel / iron oxide interface.

However, the air / nitrogen atmosphere of the heating / holding portion will 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 speed of 50 ° C./s in an atmosphere containing 4% of hydrogen and 0.1% of water vapor. which corresponds to a dew point of -2O ⁰ C.

This sheet is then introduced at the temperature of 470 ⁰ C in a zinc bath, containing 0.2% of aluminum, which is maintained at 46O ⁰ C. After immersion of 3 seconds, the coating is wrung out so as to keep a zinc layer of 8 μm. Such a zinc deposit is then perfectly wetting and has adhesion qualities comparable to that obtained for ordinary low-carbon steel.

To cite another example, the same process can be applied to a steel containing inter alia

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 to proceed is to allow the usual flow to establish from the zinc bath to the heating section and to leave the very low hydrogen content (<0.5%), contained in the transfer section. / cooling, react with the oxygen of the heating / holding portion to form water vapor. An additional supply of oxygen, at the exit of the holding section, can be made to neutralize the entry of hydrogen, 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 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 good wettability by the zinc during the immersion of the sheet in the bath.

To be effective, this method should provide for controlling the oxygen content in the oven within the range of between 50 and 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.

The invention has a number of advantages, including the fact that: - that hydrogen addition is much lower than in the state of the art, or even zero, in the zone of maintenance-heating, which constitutes an important economy of exploitation and guarantees the obtaining of high strength steel with fewer frailty defects;

- It no longer separates the heating section of the holding section at the annealing temperature, which saves an airlock and possibly avoid duplication of gaseous atmosphere control equipment;

That this method is much more effective than the processes known in the state of the art, from the point of view of the adhesion of the coating or the wettability of the strip;

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

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, if we consider the direction of progression of the band:

a so-called heating and holding section, in which heating of the strip is carried out followed by maintenance at a given annealing temperature under an oxidizing atmosphere comprising an air (or oxygen) / non-oxidizing or inert gas mixture, to form 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 formed oxide layer; 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 less than 4% and preferably less than 0.5%.

2. Method according to claim 1, characterized in that maintains the controlled oxygen content in the heating section and maintenance between 50 and 400 ppm.

3. Method according to claim 1 or 2, characterized in that the separation of the oxidizing atmosphere from the reducing atmosphere is performed by an overpressure of the oxidizing atmosphere, so that the oxygen entrained by the band through the airlock react completely with the hydrogen contained in the cooling atmosphere by forming water vapor.

4. Method according to claim 1 or 2, characterized in that hydrogen is allowed to react, present in the cooling and transfer section which is overpressure with respect to the heating and holding section, entrained in the gas stream hot directed upstream, with oxygen from the heating and holding section to form water vapor.

5. Method according to any one of the preceding claims, characterized in that 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 air oxidant supplying direct flame heating means, or by controlled injection of the mixture air (or oxygen) / inert gas in the case of radiation heating or induction.

6. Method according to any one of the preceding claims, characterized in that the non-oxidizing or inert gas is nitrogen or argon.

7. Method according to any one of the preceding claims, characterized in that the liquid metal is zinc or one of its alloys.

8. Method according to claim 1, characterized in that the heating and maintenance zone is devoid of reducing atmosphere.

9. The method of claim 1, characterized in that the hot dip coating process is a galvanization or a galvannealing treatment.

10. Method according to any one of the preceding claims, characterized in that the atmosphere in the heating and holding section and in the cooling and transfer section has a dew point less than or equal to -10 ⁰ C, preferably at -20 ^° C.

11. Method according to any one of the preceding claims, characterized in that the strip is heated to a temperature between 650 ⁰ C and 1200 ⁰ C, including the holding temperature.

12. The method of claim 15, characterized in that the band is then cooled to a temperature above 450 ⁰ C, with a cooling rate of between 10 and 100 ° C / s.