WO2008000963A1

WO2008000963A1 - Continuous thermal treatment installation intended for bright annealing a strip of stainless steel

Info

Publication number: WO2008000963A1
Application number: PCT/FR2007/001095
Authority: WO
Inventors: Fabien Illy; Jean-Luc Dodergny
Original assignee: Dms
Priority date: 2006-06-30
Filing date: 2007-06-28
Publication date: 2008-01-03
Also published as: EP2041325A1; FR2903121A1; FR2903121B1

Abstract

The invention concerns an installation for continuous thermal treatment intended for bright annealing a strip of stainless steel, consisting of at least one chamber under a controlled atmosphere in which the means of heating and cooling the strip have been integrated, these means of heating and cooling being made up respectively of a vertical rapid heating section (3) using induction and at least one vertical rapid cooling section (4) positioned on the vertical strand that raises or lowers the metallic strip, through which sections (3, 4) the strip is passed freely, without touching, as the temperature of the strip exceeds 250°C. According to the invention, said space consists of: the chamber (2) with the non-oxidising controlled atmosphere comprising a mixture of an inert gas and hydrogen or pure hydrogen, plus a chamber (5) kept under an inert gas atmosphere by a safety device thereby creating a buffer zone between the external atmospheric air and the hydrogenated atmosphere in order to restrict the ingress of external atmospheric air into the hydrogen containing chamber (2); a device (10) for moving the strip, integrated directly into said space in order to minimise the weight of the strip at the heating point, allow the strip to be inserted without sliding and smooth out the variations in the traction forces on the free vertical strand that moves the metallic strip, when the line is operating normally, consisting of at least one powered roller.

Description

Title: Continuous heat treatment plant for the bright annealing of a stainless steel strip.

The present invention relates to a continuous heat treatment plant for the bright annealing of a stainless steel strip.

Stainless steel refers more particularly the 200, 300 and 400 series of the American standard AISI.

Glossy annealing provides a high quality, low grain, BA grade surface finish. This surface state is obtained during the rolling preceding the annealing step, this last step making it possible only to reveal it without damaging it.

To do this, the bright annealing is carried out in a closed chamber furnace and under a non-oxidizing controlled atmosphere, in order to avoid the formation of oxide on the strip. The atmosphere is composed of a mixture of hydrogen and nitrogen with a maximum proportion of 25% nitrogen, or even pure hydrogen.

In addition, in order not to damage and mark the band, the band must not be touched when the local temperature of the band exceeds 250 ⁰ C. Generally, the band is affected only from 150 ⁰ C. Bright annealing is traditionally done using an electric resistance furnace, a muffle gas furnace, or a hybrid furnace combining both technologies. Currently, the lines in operation are small, low capacity units, whose annual production is generally less than 100,000 tons. In comparison, the alternative is a chemical pickling annealing plant with an annual production capacity of between 180,000 tons and 230,000 tons.

The surface state of the steels resulting from annealing and pickling is nevertheless of inferior quality, because of the use of chemical agents, in particular of acids, used to remove the layer of scale resulting from the oxidation of the strip in the oven. Bright-annealed steels, however, are considered over-quality for everyday applications and represent only a niche market. These steels find, for example, a use in electronics for the realization of precision welds. Today, controlled atmosphere furnaces are also known, including a transverse flow induction heating section and a gas fast cooling section. These recent technologies can be used for bright annealing.

The transverse induction heating technique makes it possible to raise the temperature of the stainless steel strip, especially up to 1200 ^° C. and according to a temperature gradient greater than 100 ° C./sec.

The fast cooling technique allows blowing a cooled mixture of nitrogen and hydrogen, to lower the temperature of the strip with a temperature gradient equal to or greater than 100 ° C / sec. These new technologies theoretically allow to increase the running speed of the strip of brilliant annealing installations well beyond 100 m / min and thus to increase their capacity.

However, there is still no brilliant annealing installation as powerful because such bandwidth speeds are at the origin of new obstacles and in particular:

- safety problems with the risk of formation of an explosive mixture by infiltration of air, in particular oxygen in the hydrogen-containing chamber,

- Tape driving problems with tensile forces on the upper band at the origin of the rupture of the metal strand, especially at the hot spot in the oven.

The security problems are related to the thermal inertia of the rapid cooling system compared to the thermal inertia of the rapid heating system. During normal operation of the line, and for a high bandwidth, especially greater than 100 m / min, the fan motors of the rapid cooling system blow, at very high speed, a gaseous mixture on the band composed of nitrogen and hydrogen, cooled by water heat exchanger.

During a sudden stop of the line, for example, in case of breakage of the belt, the rapid heating system by transverse induction can be stopped almost instantaneously while the rapid cooling system, because of the mechanical inertia of the organs turners that constitute it, continues to blow the gaseous mixture cooled in the room.

The internal temperature of the chamber is thus greatly lowered and this results in a contraction of the internal atmosphere. The controlled atmosphere chamber is provided with input and output devices for bands intended to seal between the two hydrogen and air atmospheres for limited relative pressure differences.

However, in case of sudden stoppage of the band, the pressure differences between the two atmospheres far exceed the operating limits of these devices which can not prevent an infiltration of atmospheric air outside, including oxygen composed to the atmosphere with hydrogen content.

In addition, the intensity of the internal depression is accentuated in bright annealing furnaces in which the controlled atmosphere chamber is of a smaller volume than that of the chamber of an oven, for example, used for galvanizing, unable to provide the role of buffer volume.

The problems of tape driving are related to the requirements of bright annealing furnaces which must make it possible to obtain a high quality of surface finish, in particular of BA quality according to the American AISI standard. As previously explained, this surface quality is obtained during the rolling preceding the bright annealing step, the latter only revealing it.

It is therefore imperative not to deteriorate the surface condition after this rolling step and in particular: to avoid any relative sliding between the strip and the pipe rolls so as not to scratch said strip, - To avoid touching the band when it is hot, so as not to mark said band.

Today, it is thus common to touch the metal strip for temperatures of less than or equal to 150 ⁰ C, and it is strictly forbidden to touch it for temperatures greater than 250 ^° C.

In a vertical bright annealing furnace, the strip is thus not touched over the entire heating section and at least a portion of the cooling section. Throughout this region, the strand of tape is subjected to the stresses due not only to its own weight, but also to the tensile stresses required for tape driving.

The band height can not exceed a certain value, at risk of rupture of the band, especially at the hot spot of the band, the hot spot of the band corresponding to the hottest point in the oven. Furthermore, it should be noted that the control of the tape drive requires tensile forces on the tape, tensile forces which then become more critical with the speed of scrolling.

The cumulative height of the heating section and the cooling section is thus limited. The production capacity of the oven is thus affected, the running speed of the strip in the oven is determined according to the quality of the rapid heating to reach the minimum temperature required for the annealing step, as well as to minimize the forces on the vertical strand.

The object of the present invention is to overcome the aforementioned drawbacks by proposing a continuous thermal treatment plant, intended for the bright annealing of a strip of stainless steel, making it possible to increase the production capacities of the current lines of bright annealing, well over 100,000 tons per year.

Another object of the present invention is to propose a heat treatment installation allowing tape running speeds well above 100 m / min, and in complete safety, particularly in case abrupt stoppage of the line.

Other objects and advantages of the present invention will become apparent from the description which follows, which is given for information only and which is not intended to limit it. The invention relates to a continuous thermal treatment plant for the bright annealing of a stainless steel strip, comprising at least one controlled atmosphere enclosure incorporating means for heating the strip and means for cooling the strip, heating means and the cooling means of the strip being constituted respectively by a vertical section of rapid heating induction and by at least one vertical section of rapid cooling, positioned on the vertical strand, rising or falling of the metal strip, on which sections the strip is free from driving, unaffected, when the strip temperature exceeds 250 ° C, characterized in that: - said enclosure is constituted by a controlled atmosphere non-oxidizing chamber, the content of which consists of a mixture of inert gas and hydrogen, or even pure hydrogen, and by a chamber maintained under inert gas a device d e security for creating a buffer zone between the outside atmospheric air and the hydrogen-containing atmosphere to limit the insertion of outside atmospheric air into the hydrogen-containing chamber,

a band-driving device is integrated directly into said enclosure in order to minimize the weight of the hot-spot band part, making it possible to drive the strip without sliding and to smooth the variations of the tensile forces on the free vertical strand of conduct of the metal strip, during normal operation of the line.

The invention will be better understood on reading the description, accompanied by the appended drawings, among which:

FIG. 1 is a schematic illustration of a heat treatment installation intended for the bright annealing of a strip of stainless steel and according to the invention according to a first embodiment, FIG. 2 is a schematic illustration of a continuous thermal treatment installation intended for the bright annealing of a strip of stainless steel and according to the invention according to a second embodiment. The invention relates to a continuous heat treatment plant 1 for the bright annealing of a strip of stainless steel comprising at least one controlled atmosphere enclosure.

The enclosure incorporates means for heating the strip and cooling means of the strip, consisting respectively of a vertical section 3 of rapid induction heating and at least one vertical section 4, of rapid cooling, both positioned on the vertical strand, rising or falling, of the metal strip.

According to two examples illustrated in the figures, the rapid heating section and the rapid cooling section are on the ascending strand of the metal strip.

It should be noted that the rapid heating section and the rapid cooling section can be positioned on the descending run of the metal strip according to another embodiment.

The vertical section 3 rapid heating may be transverse induction or longitudinal induction and allow to raise directly and quickly the temperature of the strip to the desired temperature to obtain the annealing of the strip, for example 1.200 ⁰ C at the point hot.

The vertical section 4 rapid cooling allows the temperature of the strip to drop to a temperature at least less than 250 ⁰ C to touch the band again, including a driving roller and without the risk of marking. The temperature of the strip at the outlet of the cooling section may be, for example, less than 150 ^° C.

The rapid heating section 3 and the rapid cooling section 4 allow heating or cooling with a temperature gradient equal to or greater than 100 ° C / sec. As illustrated in the figures on said sections 3, 4 for heating and cooling, the strip is free of conduct, unaffected, when the strip temperature exceeds 250 ⁰ C, to avoid marking the strip. According to the invention, the enclosure 2, 5 is constituted by a controlled atmosphere chamber 2, non-oxidizing, the content of which consists of a mixture of inert gas and hydrogen and a chamber 5 maintained under inert gas. a security device.

The mixture of the chamber 2 comprises at least 75% of hydrogen, for example 95%. The atmosphere can still be pure hydrogen.

The safety device makes it possible to create a buffer zone between the outside atmospheric air and the hydrogen-containing atmosphere. This safety device makes it possible to limit the insertion of outside atmospheric air into the chamber with a hydrogen content and thus limits the formation of an explosive atmosphere mixture.

The installation further has a tape driving device 10. This device makes it possible to drive the strip without slipping in order to avoid scratching the strip. This device also smooths the variations of tensile forces on the vertical strand, upward or downward, free conduct of the metal strip, during normal operation of the line.

The tape driving device 10 is integrated directly into said enclosure to minimize the weight of the hot spot band portion. It can be constituted by at least one motorized roller 11. When the rapid heating and rapid cooling sections are on the rising strand of the strip, as illustrated, the pipe device 10 is positioned upstream of the heating section. fast. If the processing sections are on the downstroke, the conduit 10 is positioned downstream of the quench section. In this way, and according to the illustrated examples, most of the hot spot strip can be subjected to heat input. of the rapid heating system, the device 10 further allowing to smooth the variations on the upstream strand, thus allowing tape running speeds well above 100 m / sec, without risk of rupture.

Moreover, in the event of a sudden stop of the line, the chamber 5 with a neutral gas content, in particular nitrogen, serves to act as a buffer zone in the event of a depression of the hydrogen-content chamber 2.

According to an embodiment illustrated in Figure 1, the tape driving device 10 is integrated in the chamber 5 with a neutral gas content of the safety device. According to another embodiment illustrated in Figure 2, the tape driving device 10 is integrated in said controlled atmosphere chamber 2 to hydrogen content.

According to another embodiment not illustrated, a portion of the mechanical devices of the device 10 can be distributed in the chamber 2 with a hydrogen content and in the chamber 5 with a neutral gas content.

The tape driving device 10 may comprise an S-shaped block 12 for driving the metal strip without slipping. This block comprises at least two rollers, one of which is said to be at least one motorized roller 11. This block makes it possible to apply the strip on the surface of the motorized roller 11 with a sufficient pressure to prevent any sliding phenomenon.

The tape driving device 10 may furthermore have a dancer roll 13, in particular able to move in a limited path to smooth the variations of the tensile forces on the upward or downward vertical strand of the free metal pipe of the pipe. especially during normal operation of the line.

It should be noted that the motorized roller 11 also smooths the variations of tensile forces on the vertical strand. However, the sensitivity of the motorized roll 11 is lower than that of the dancer roll 13. The dancer roll 13 is located between the S-block 12 and the sections 3, 4 of rapid heating and rapid cooling, in a vertical manner, especially below said sections.

The dancer roll 13 may be actuated by a jack, in particular an electric jack, a pneumatic cylinder or a counterweight. The tape driving device 10 may furthermore comprise a tensiometer 14, in particular integrated in the controlled atmosphere chamber 2, in particular downstream of the rapid cooling section 4 in the figures, in line with the above-mentioned above. section 4 cooling. This tensiometer 14 allows the measurement of the tension of the strip 20 on a high position of the vertical strand, upward or downward, free of driving.

Advantageously, the installation furthermore has means for driving the tape drive device 10 in pairs, and means for controlling said tape driving device 10 as a function of the value of the measuring signal of the tension meter 14.

In particular, the motorized roller 11 can be slaved into torque as a function of the value of the signal of the tension meter 14. In the same way, the dancer roll 13 can be slaved into torque as a function of the value of said signal of the tensiometer 14. may further have a motorized drive roller 16, in particular integrated in the controlled atmosphere chamber 2, in particular positioned downstream of the tensiometer 14 in the figures. This roller 16 can drive the band and is controlled in speed. The installation thus has means for driving in speed the motorized drive roller 16.

The installation may furthermore have a guide roller 15, in particular integrated in the controlled atmosphere chamber, downstream of the fast cooling section 4, directly above said section. This guide roller 15 makes it possible to reduce the lateral positioning effects of the strip, in particular on a high position of the upstrand of the metal strip, and as illustrated. The effects of Lateral positioning are still called saber effects.

Saber effect means a deflection of the strip in a plane with respect to an imaginary longitudinal axis of the strip.

As illustrated in FIG. 1 or 2, the guide roller 15 and / or the motorized drive roller 16 also constitute deviating rollers.

As illustrated, the safety device further comprises a strip inlet device 6 in the buffer zone from outside air and a strip inlet device 7 in the hydrogen chamber since the buffer zone, as well as a strip discharge device 8 from the hydrogen content chamber to the buffer zone and a strip discharge device 9 from the buffer zone to the outside atmospheric air.

These band input and output devices provide a relative seal between the atmospheres. Of course, other modes of implementation, within the reach of those skilled in the art, could still be envisaged without departing from the scope of the invention.

Claims

1. Continuous heat treatment plant, intended for the bright annealing of a stainless steel strip, comprising at least one controlled atmosphere enclosure incorporating means for heating the strip and means for cooling the strip, the means for heating and cooling means of the strip being constituted respectively by a vertical section (3) of rapid induction heating and at least by a vertical section (4) of rapid cooling, positioned on the vertical strand, upward or downward, of the metal strip, on which sections (3, 4) the strip is free from driving, unaffected, when the temperature of the strip exceeds 250 ⁰ C, characterized in that:

said enclosure (2, 5) consists of a non-oxidizing controlled atmosphere chamber (2), the content of which consists of a mixture of inert gas and hydrogen or even pure hydrogen, and of a chamber ( 5) maintained under an inert gas belonging to a safety device for creating a buffer zone between the outside atmospheric air and the hydrogen-containing atmosphere in order to limit the insertion of outside atmospheric air into the chamber (2) to hydrogen content,

said security device comprises a device (6) for entering the buffer zone into the buffer zone from the outside air, and a device (7) for entering the chamber into the hydrogen content chamber from the buffer zone. , as well as a band outlet device (8) from the hydrogen-containing chamber to the buffer zone, and a band-exit device (9) from the buffer zone to the outside air - a device (10) is integrated directly into said enclosure (2, 5) to minimize the weight of the hot spot web portion, thereby driving the web without slipping and smoothing the variations of the tensile forces on the web. the vertical strand free from driving the metal strip, during normal operation of the line.

2. Installation according to claim 1 wherein the device (10) for driving tape is integrated in said chamber. controlled atmosphere (2) with hydrogen content.

3. Installation according to claim 1 wherein the device (10) for driving tape is integrated in said chamber (5) with a neutral gas content of the safety device.

4. Installation according to one of claims 1 to 3 wherein the device (10) for driving web is constituted by at least one motorized roller (11).

5. Installation according to claim 4, wherein the band driving device (10) for minimizing the weight of the hot spot band height comprises an S-block (12) for driving the metal band without slipping, the block comprising at least two rollers, one of which is said at least one motorized roller (11).

6. Installation according to claim 5, wherein the device (10) for driving to minimize the weight of the hot spot band height further has a dancer roll (13) located between the S-block (12). and the sections (3, 4) for rapid heating and rapid cooling, in line with said sections, the dancer roll (13) for smoothing the variations of the tensile forces on the vertical strand of the free metal strip driving during normal operation of the line.

7. Installation according to one of claims 1 to 6, wherein the device (10) for driving tape comprises a tensiometer (14) integrated in the controlled atmosphere chamber (2), for measuring the voltage of the strip on a high position of the free vertical conduct strand.

8. Installation according to claim 7, having means for driving in torque the device (10) for driving tape, and means for controlling said tape driving device (10) as a function of the value of the measuring signal of the tensiometer. (14).

9. Installation according to one of claims 7 or 8, having a motorized drive roller (16) integrated in the controlled atmosphere chamber (2) and means for controlling said roller speed motorized drive (16).

10. Installation according to one of claims 1 to 8, having a guide roller (15) integrated in the controlled atmosphere chamber, for reducing the lateral positioning effects of the band on a high position of the vertical strand of the metal band.