ES2749005T3 - Apparatus for treating a metal strip in a vertical annealing plant - Google Patents

Abstract

An apparatus (1) for treating a metal strip (2) in a vertical annealing plant system comprising a zone where the bright annealing heat treatment of a metal strip (2) is carried out on a controlled atmosphere containing gas, said area comprising in sequence: a first vertical inlet duct (3) of a strip (2), a second horizontal union duct (4), a third vertical outlet duct (5) of the strip (2), and characterized in that it comprises: electromechanical means (8) to block and divide the metal strip (2), suitable for operating downstream of said first vertical inlet duct (3), preferably in said third outlet duct vertical (5), such as to directly block said metal strip (2) and hermetically divide, thus controlling the circulation of gases, delimiting a first zone upstream of said electromechanical means and operating in a controlled atmosphere and a second zone ag downstream of said electromechanical means and operating in a controlled atmosphere or in an ambient atmosphere, where the apparatus further comprises: - means (9) for controlling and regulating a controlled atmosphere, suitable for operating in said second or third conduit in the area upstream and downstream, where said electromechanical locking and partitioning means are positioned; - electromechanical sealing means (10), suitable to control gas circulation, to operate in said second (4) or third vertical duct (5), to act by directly blocking the strip in synchrony with said electromechanical locking and partitioning means (8), wherein said electromechanical locking and partitioning means (8) and said electromechanical sealing means (10) act directly on the strip (2) by means of reversible locking and said electromechanical locking and partitioning means (8) they are suitable for moving in a translational movement parallel to the longitudinal axis of the second or third conduit, preferably by means of guides, rails or slides preferably located within these conduits.

Description

DESCRIPTION

Apparatus for treating a metal strip in a vertical annealing plant

The present invention relates to an apparatus for use in vertical bright annealing furnaces for the treatment of metal products such as stainless steel strips.

The invention consists of a new apparatus, by means of which, in the event of a shutdown of the plant system, the conditions to verify and restore the normal operation of the process are facilitated by a particular control system that controls the engineering of the furnace. and, therefore, the thermal annealing process.

Heat treatments are used to impart specific properties to various types of stainless steel based on their end use.

Thermal treatment consists of an operation or series of operations in the case of a complex treatment process, during which the steel is subjected to one or more thermal cycles, that is, variations within the given temperature limits depending on the weather.

A thermal cycle typically involves heating the steel to a given temperature, possibly keeping it at this temperature for a specified period of time, and then cooling it to room temperature, using different modes depending on the desired effects.

The treatment cycles are selected based on the desired characteristics related to hardness, resistance, microstructure and workability. The operating parameters of the treatment cycle should be established based not only on the type of steel, but also according to the dimensions of the parts and the characteristics of the heating and cooling media.

The annealing process represents a typical heat treatment process, the main objective of which is to eliminate residual stresses and work hardening, microstructure disturbances and segregation, reduce hardness and increase ductility.

For stainless steels, a particular type of controlled atmosphere heat treatment defined as recrystallization or solution annealing depending on whether they are martensitic, ferritic or austenitic steels, is performed to obtain products with a type of bright annealed (BA) surface finish.

Consideration of the problems related to the surface finish of products made of stainless steel, self-passivating materials, is important to guarantee adequate aesthetic characteristics and to optimize the behavior of such products in certain corrosive environments, or to improve inherent characteristics such as hygienic properties. thereof.

Bright annealing (BA) is carried out by applying a particular thermal cycle to metal products such as stainless steel sheets and strips, in which the surface pass lamination is carried out after cold rolling previously carried out with polished rollers.

The BA controlled heat treatment is applied to practically all types of stainless steels, including martensitic, ferritic and austenitic steels, and allows obtaining a product with a particularly shiny surface, since during the heating stage, oxide films do not They are formed precisely due to the absence of an oxidizing atmosphere during treatment.

In general, the controlled atmospheres for these types of annealing furnaces are based on inert gases (nitrogen or argon) with additions of hydrogen. Nitrogen is the main component of these mixtures and prevents oxidation; hydrogen serves to reduce oxides and to obtain a clean and shiny surface.

Depending on the type of metal and technical specifications, the hydrogen content can vary between 5 and 100% expressed as a percentage by volume.

In addition to temperature and pressure, the dew point, typically between -62 ° C and -73 ° C, is also an important operating parameter for this type of treatment.

Type BA furnaces for stainless steels can be horizontal or vertical, and the latter is preferred since they allow to avoid damage to the shiny finish of the material surface that is processed during the annealing process.

The typical structure of a BA vertical furnace, which includes heights ranging from 20 to 65 meters, is carried out using a schematic inverted U-shaped structure that exhibits in a sequence: a first vertical entry section, a second horizontal section and a third vertical outlet section.

With reference to the engineering design of a vertical furnace for the heat treatment of a product being processed, such as a stainless steel strip, in the first vertical section it comprises a first entrance zone, a second heating zone and a third heating zone. cooling; in the second horizontal section it comprises a fourth retention area and in the third vertical outlet section, a fifth retention area and product outlet.

The heating zone comprises direct heating systems, such as electric radiant elements, or indirect heating systems, such as gas muffle furnaces, or a combination of these two heating systems.

With direct heating systems, the product that is processed is heated by radiant elements made with electrical resistances of the molybdenum and / or tungsten alloy type, which are therefore expensive and sensitive to changes in temperature with the risk of sublimation in the case of rapid temperature changes.

The main advantage of using systems with direct heating by means of electrical resistances lies in the highest production rates and the highest achievable temperatures, with the resistances of the electrical elements capable of reaching temperatures of 1400-1500 ° C.

The cooling system, which is placed downstream of the heating system, can be carried out in one or more stages according to the desired cooling rate.

All the following treatment zones, up to the exit zone, contribute to the positive realization of the process and, therefore, it is important that throughout the oven, from the entrance zone to the exit zone, the operating parameters are respected accurate, in terms of temperatures, gaseous atmospheres, pressures and dew point, to obtain products with high surface quality.

Considering the dimensions of the furnace, particularly the heights, which can also be of the order of 65 meters, there must be a particularly careful regulation and management of the volumes of circulating gases and of the operating parameters under normal operating conditions, and the operational management is extremely troublesome in the case of a non-steady state operation, for example because the metal strip being processed breaks or rips into two pieces, a situation that generally occurs in the vertical down section.

The non-steady state conditions related to the breakage of the metal strip lead to the shutdown of the plant system and, in general, it takes approximately 170 hours to restore normal operating conditions, resulting in considerable production losses.

From non-steady state conditions, this situation can easily degenerate under critical conditions, since in the presence of a controlled atmosphere containing nitrogen / hydrogen, problems such as fires and explosions can arise.

Document US 6015526 describes a plant system with a vertical annealing furnace, where an attempt is made to find a solution to the problems related to high productivity by proposing a complex structure of a muffle furnace that is capable of expansion. However, the document does not provide a solution in cases where the plant system is stopped to verify and restore normal process operation. Document US 2009/158975 A1 discloses a device that limits the risk of the formation of an explosive atmosphere in the furnace of a continuous line for heat treatment of metal strips whose sections are under an atmosphere consisting of a mixture of inert gas and hydrogen, whose hydrogen content by volume is between 5 and 100.

JP H 158750 discloses a sealing gate, respectively, on the upstream side and on the downstream side of a metal strip leading to a heat treatment furnace, forming a purge chamber, supplying an inert gas to the chamber purge and maintaining positive pressure.

Therefore, a solution to the technical problems mentioned and still existing in the state of the art is highly desirable.

An object of the present invention is to offer an apparatus that is capable of facilitating procedures to restore production operations between two stable states in the event of accidental breakage of the metal strip being processed and a stoppage of the resulting production, making possible to minimize procedures and time required to restore steady state operation, while facilitating restoration of procedures and conditions to return to steady state status, minimizing risk of fire and explosion, and reducing Minimize the need to replace costly electrical resistors in the direct heating furnace due to damage.

An object of the present invention is an apparatus for treating a metal strip in a vertical annealing plant system, in which the metal strip passes through a central area of the system, where the bright annealing heat treatment it is carried out in a controlled atmosphere containing gas, and comprising in sequence a first vertical strip inlet conduit, a second horizontal joint conduit, a third vertical strip outlet conduit, the latter comprising locking means and electromechanical partitions, suitable to operate downstream of said first vertical inlet duct, preferably in said third vertical outlet duct, to act directly blocking the metal strip and to hermetically divide, thus controlling the circulation of gases, delimiting said second or third conduit a first zone upstream of this electromechanical medium and operating in a controlled atmosphere and a s second zone downstream of this electromechanical medium and operating in a controlled atmosphere or in an ambient atmosphere.

The apparatus disclosed by the present invention further comprises:

- means for controlling and regulating a controlled atmosphere, suitable for operating in said second or third conduit in the upstream zone and the downstream zone in which said electromechanical blocking and partitioning means are placed

- electromechanical hermetic closing means, suitable to control gas circulation, to operate in said second or third vertical duct, to act by directly blocking the strip in synchrony with said electromechanical blocking and partitioning means, wherein the blocking and partitioning means electromechanical and electromechanical sealing means are suitable to operate directly on the strip by means of a reversible lock,

- and the electromechanical locking and partitioning means are suitable for moving in translational movement parallel to the longitudinal axis of the second or third conduit, preferably by means of guides, rails or slides preferably placed within these conduits.

The means for controlling and regulating a controlled atmosphere are suitable for entering or eliminating gases or gas mixtures and for regulating temperature, dew point, and pressure and such means comprise detection and analysis sensors, gas storage systems, systems intake and extraction systems and connection and union systems.

The electromechanical sealing means for controlling gas circulation are suitable for opening and closing in synchrony with said electromechanical locking and partitioning means in such a way that they divide and limit the passage of gas in zones of controlled atmosphere.

The electromechanical partitioning and locking means and said electromechanical sealing means comprise moving elements, preferably metal, suitable for acting directly on the metal strip and suitable for covering with elastic material, including natural or synthetic rubbers, preferably silicone rubbers, resistant to temperatures above ambient temperature, preferably to temperatures in the range of 20 ° C to 90 ° C.

The movable locking and partitioning elements are suitable to act simultaneously on both larger sides of the metal strip, achieving a tight seal towards both larger surfaces of the strip and are performed in a manner selected from the group comprising: at least one metal plate that slides into the strip, at least one pair of metal plates that can slide into the strip, various metal plates that are slidable, telescopically operated, and as to extend from the outside inward toward the strip, and at least one pair of elements that are movable towards the strip and suitable to rotate on at least one pair of hinges towards the strip.

The moving elements of the electromechanical sealing means comprise elements, preferably metal elements suitable for reversibly moving in translational motion and rotating until contacting the metal strip, and are made with a cover made of suitable material to prevent marks from being left behind. on the strip and to ensure a hermetic seal, preferably made with material selected from the set that includes: felt, synthetic or natural fabric, or cotton.

The means for controlling and restoring a controlled atmosphere are suitable for entering at least one gaseous form selected from the group comprising: hydrogen, nitrogen, a mixture of hydrogen and nitrogen, air, and said metal strip is made of steel, preferably stainless steel .

Heretofore, a general description of the present invention has been provided.

With the help of the attached figures and examples, a more detailed description of particular embodiments will now be provided in order to offer a better understanding of the objectives, characteristics, advantages and modes of application of the invention.

Fig. 1 - Figure 1 is a diagram of a vertical annealing line.

Fig. 2 - Figure 2 is a diagram of the central section of the vertical annealing line, where a general diagram of an embodiment of the object of the present invention is represented.

Fig. 3 - Fig. 3 is a detailed diagram of an embodiment of the present invention.

Fig. 1 is a diagram of a vertical annealing line, in which a metal strip 2 is initially wound on an uncoiler, sent, passing through an inlet accumulation tower, in a central strip treatment section 1 for treatment.

The strip then travels through the outlet accumulation tower, the surface pass rolling line, and, in conclusion, is wound on the winder.

Figure 2 represents a diagram of a central treatment section of an annealing line incorporating the object of the present invention. In Figure 2, the central treatment section is indicated entirely by number 1 and the strip being treated in the plant system is indicated by number 2. During the annealing process, strip 2 first passes through vertical duct 3, then horizontal duct 4 and finally exits vertical duct 5.

The furnace is in controlled atmosphere conditions and operates under overpressure conditions with respect to atmospheric pressure. Once strip 2 has entered conduit 3, it undergoes a thermal cycle and travels first through the indirect heating furnace and then through the direct heating furnace, both of which are indicated entirely by the number 6 Strip 2 then travels through the slow and rapid cooling zone, which is indicated entirely by the number 7. Strip 2 then travels through duct 4 and begins to travel through duct 5, but In non-steady state operation, as an abnormal situation involving the break of the strip, it is blocked inside the duct 5.

In the absence of what constitutes the object of the present invention, to resolve the abnormal situation that arose with the breakage of the strip, it would be necessary to try to extract the strip upstream, by the action of the unwinder with a high risk of damaging not only the rollers support, but also thermally active zones, particularly heating furnaces and cooling areas, and to extract the strip downstream, by action of the reel, thus endangering not only the product being processed but also future processing .

In order to resolve the abnormal situation related to the breakage of the strip, normally in two pieces, the apparatus 8 constituting the object of the present invention intervenes by moving in translational movement in relation to the conduit 5 or 4 depending on the position occupied by the broken strip section, blocks the same strip upstream of the break point achieving a hermetic seal with respect to the gases present and divides the duct into an area upstream of the break point, where the operating conditions and parameters (is That is, the pressure, temperature, dew point, H2 / N2 mixture) are kept under control, and an area downstream from the break point, where you can work under maintenance temperature and atmosphere conditions and conditions to restore the steady state operation.

During the transition procedures involved in strip repair, which take place in the area downstream of the break point in an air-containing environment, following the blocking of the strip, the broken strip and a backing or terminal strip are After repairs, they join and weld in the area downstream of the break point, and the section of the duct involved is adequately flushed with nitrogen.

Subsequently, to restore normal operating conditions in the area between the break point of the strip and the outlet, after having restored the appropriate operating parameters only in the area downstream of the break point and not in the entire central section of heat treatment, by means of control systems of temperature, pressure, dew point and gas supply 9 and by means of hermetic sealing systems 10, the production line becomes operational.

The direct intervention in the zone of rupture of the strip allows to avoid having to restore all the conditions related to the operating parameters in the entire central section of heat treatment.

In this way, considerable savings can be achieved in terms of the gas to be used and replaced, considering the high volumes of the treatment areas, as well as considerable energy savings by preserving thermal operating conditions, considering the high temperatures for treatment , to minimize the risk of fire and explosion, and to minimize the need to replace costly tungsten and / or molybdenum based electrical resistors in the direct heating furnace.

Finally, considering the time involved in operation between the beginning and end of the problem, between transitory and normal operation, there is a marked reduction in downtime, and therefore production resumes over a considerable period of time. short.

Example 1

An AISI 304 austenitic steel strip is being processed on a BA bright annealing line and during a production stage, typically lasting 60 minutes per 25t spool, a strip break situation has been simulated in the vertical outlet in the central section of heat treatment.

The strip in the treatment process is 1000 mm wide and 1 mm thick, at a speed of 28 m / min. Once the strip has entered the vertical inlet duct in the center section of the heat treatment system, it travels first through the indirect heating muffle furnace, where it is heated to a temperature of approximately 1000 ° C, and then into the furnace direct heating, where it is heated to temperatures of approximately 1100 ° C by means of electrical resistances.

In the operating environment there is an atmospheric overpressure of approximately 10 390 mm H ² O and a dew point of -40 ° C. Subsequently, the strip travels through the slow cooling zone and then the rapid cooling zone and cools to approximately 50 ° C.

The strip then travels through the horizontal duct and begins to travel through the vertical outlet duct, but due to a simulated break in the strip in approximately 1/3 of the passage through the vertical outlet duct 5, the line of production stops.

In this case, the apparatus that constitutes the object of the present invention intervenes and moving in translational movement parallel to the conduit 5 according to the position occupied by the edge of the broken strip, it blocks the strip by means of a system of coated elements of elastomer that rotate around a hinge (Fig. 3).

The closure elements lock the strip in such a way that they also ensure the hermetic seal with respect to the gases present. Through the hermetic seal, the outlet duct is divided into a zone downstream from the break point and into a zone upstream from the strip break point, where the operating conditions and parameters (pressure, dew point and gaseous atmosphere) are verified.

At the point of breakage of the strip, downstream of the blocking and partitioning elements, a welding procedure is carried out between the broken strip upstream and a backing strip or terminal downstream, by means of which the production is will restart again.

After repair of the broken strip, in the area downstream of the break point, the area previously occupied by air is suitably washed with gas (nitrogen) and then the gaseous hydrogen / nitrogen atmosphere is restored to normal operation.

Subsequently, all correct operating parameters are reset by operating parameter control and management systems, and then the production line is restarted.

By means of the apparatus that constitutes the object of the present invention and inserted in the production line, the time dedicated to the supplementary maintenance work has decreased ten times and due to the hermetic sealing achieved, the oxygen in the air has not come into direct contact With the heating furnace electrical resistors, therefore, it is possible to avoid having to proceed with the replacement of these resistors due to contact with oxygen and the resulting sublimation and avoid having to restore the operating parameters throughout the system, which enables a marked overall increase in productivity.

Example 2

An AISI 430 ferritic steel strip is being processed on a BA bright annealing line and during the production stage, typically lasting 60 minutes per 25t reel, a strip break situation has been simulated in the outlet conduit vertical in the central section of heat treatment.

The strip in the treatment process is 1500 mm wide and 1.2 mm thick, at a speed of 22 m / min. Once the strip has entered the vertical inlet duct in the center section of the heat treatment system, it travels first through the indirect heating muffle furnace, where it is heated to a temperature of approximately 1000 ° C, and then into the furnace direct heating, where it is heated to temperatures of approximately 1000 ° C by means of electrical resistances.

In the operating environment there is an atmospheric overpressure of 10 390 mm H ² O and a dew point of -40 ° C. Subsequently, the strip travels through the slow cooling zone and then the rapid cooling zone and cools to approximately 50 ° C.

The strip then travels through the horizontal duct and begins to travel through the vertical outlet duct, but Due to a simulated tear of the strip in approximately 2/3 of the passage through the vertical outlet duct 5, the production line stops.

In this case, the apparatus of the present invention intervenes by moving in translational movement parallel to the conduit 5 according to the position occupied by the edge of the broken strip, and blocks the strip by means of a system of sliding panels covered with elastomer.

The closure elements lock the strip in such a way that they also ensure the hermetic seal with respect to the gases present. Through the hermetic seal, the outlet duct is divided into a zone downstream from the break point and into a zone upstream from the strip break point, where the operating conditions and parameters (pressure, dew point and gaseous atmosphere) are verified.

At the point of breakage of the strip, downstream of the blocking and partitioning elements, a welding procedure is carried out between the broken strip upstream and a backing strip or terminal downstream, by means of which production will be restarted .

After repair of the broken strip, in the area downstream of the break point, the area previously occupied by air is adequately washed with gas (nitrogen) and then the hydrogen / nitrogen atmosphere is restored for normal operation.

Subsequently, the correct operating parameters are restored by operating parameter control and management systems, and then the production line is restarted.

By means of the apparatus that constitutes the object of the present invention and inserted in the production line, the time dedicated to the supplementary maintenance work has decreased ten times and due to the hermetic sealing achieved, the oxygen in the air has not come into direct contact With the heating furnace electrical resistors, therefore, it is possible to avoid having to proceed with the replacement of these resistors due to contact with oxygen and the resulting sublimation and avoid having to restore the operating parameters throughout the system, which enables a marked overall increase in productivity.

Claims (7)

1. An apparatus (1) for treating a metal strip (2) in a vertical annealing plant system comprising an area where the bright annealing heat treatment of a metal strip (2) is carried out in a controlled atmosphere containing gas, said area comprising in sequence: a first vertical inlet duct (3) of a strip (2), a second horizontal union duct (4), a third vertical outlet duct (5) of the strip (2), and characterized in that it comprises: electromechanical means (8) to block and divide the metal strip (2), suitable for operating downstream of said first vertical inlet conduit (3), preferably in said third conduit vertical outlet (5), such as to directly block said metal strip (2) and hermetically divide, thus controlling the circulation of gases, delimiting a first zone upstream of said electromechanical means and operating in a controlled atmosphere and a second zone downstream of said electromechanical means and operating in a controlled atmosphere or in an ambient atmosphere, where the apparatus further comprises: - means (9) for controlling and regulating a controlled atmosphere, suitable for operating in said second or third conduit in the upstream zone and in the downstream zone, where said electromechanical partitioning and blocking means are positioned; - electromechanical sealing means (10), suitable to control gas circulation, to operate in said second (4) or third vertical duct (5), to act by directly blocking the strip in synchrony with said electromechanical blocking and partitioning means (8), wherein said electromechanical locking and partitioning means (8) and said electromechanical sealing means (10) act directly on the strip (2) by means of reversible locking and said electromechanical locking and partitioning means (8) they are suitable for moving in a translational movement parallel to the longitudinal axis of the second or third conduit, preferably by means of guides, rails or slides preferably located within these conduits. The apparatus (1) of claim 1, wherein said means (9) for controlling and regulating a controlled atmosphere are suitable for introducing or eliminating gases or gas mixtures and for regulating temperature, dew point, and Pressure and such means comprise detection and analysis sensors, gas storage systems, intake and extraction systems and connection and union systems. The apparatus (1) of claim 1, wherein said electromechanical sealing means (10) for controlling gas circulation are suitable for opening and closing in sync with said electromechanical locking and partitioning means (8) in such a way so that they divide and limit the passage of gas in areas of controlled atmosphere. The apparatus (1) of any one of claims 1 to 3, wherein said electromechanical locking and partitioning means (8) and said electromechanical sealing means (10) comprise moving elements, preferably metal, suitable for operating directly on the metal strip and covered with elastic material such as natural or synthetic rubbers, preferably silicone rubbers, resistant to temperatures above room temperature, preferably to temperatures in the range of 20 ° C to 90 ° C. 5. The apparatus (1) of claim 4, wherein said movable partition and lock elements are suitable to act simultaneously on both larger sides of the metal strip (2), achieving a seal towards both larger surfaces of the strip and are made in a shape selected from the group comprising: at least one metal plate that can slide towards the strip (2), at least one pair of metal plates that can slide towards the strip, a series of sliding metal plates, operating telescopically, and such as to extend from the outside to the inside towards the strip (2), and at least one pair of elements that are movable towards the strip (2) and suitable to rotate at minus a pair of hinges to the strip (2). The apparatus (1) of claim 5, wherein said movable elements of said electromechanical sealing means (10) preferably comprise metal elements suitable for reversibly moving in translational motion and rotating until contacting the metal strip and they are made with a cover made of suitable material to prevent marks from being left on the strip (2) and to ensure an airtight seal preferably selected from the group comprising: felt, synthetic or natural fabric, or cotton. The apparatus (1) of any one of the preceding claims, wherein said means (9) for controlling and restoring a controlled atmosphere are suitable for entering at least one gaseous form selected from the group comprising: hydrogen, nitrogen, a mixture hydrogen and nitrogen and air, and said metal strip (2) is made of steel, preferably stainless steel.