Annealing apparatus and related process

Abstract

An annealing apparatus for a metal strip (2) advancing along a longitudinal direction, said apparatus comprising in sequence - at least one pre-heating section (17); - at least one heating section (3); - at least one temperature maintenance section (4); - at least one cooling section (23); wherein said at least one pre-heating section (17) comprises a plurality of heating rollers (16) for advancing the metal strip (2); wherein said at least one cooling section (23) comprises a plurality of cooling rollers (22) for advancing the metal strip (2); wherein there is provided a circuit of heat-transfer fluid, said circuit being configured to cross both the cooling rollers (22) of said at least one cooling section (23), in order to remove heat energy from the strip, thus obtaining a heat-transfer fluid at high temperature, and the heating rollers (16) of said at least one pre- heating section (17) in order to transfer heat energy to the strip, thus obtaining a heat-transfer fluid at low temperature.

Description

[0001] ANNEALING APPARATUS AND RELATED PROCESS

[0002] ************

[0003] Field of the invention

[0004] The present invention relates to the field of annealing systems for metal sheets or strips, in particular made of steel, such as for example an annealing and galvanizing line, known as a “hot-dip galvanizing line” (HDGL), or a “continuous annealing line” (CAL). The invention also concerns a related annealing process for heating and annealing a strip of ferrous or non-ferrous metallic material.

[0005] Background art

[0006] As is known, steel strips after cold rolling must be subjected to an annealing cycle in order to achieve the recrystallization of the grains constituting the crystalline structure of the product. In fact, such grains have undergone a crushing effect due to the plastic deformation applied during cold rolling.

[0007] The annealing furnace usually consists of the following zones:

[0008] - pre-heating section,

[0009] - open flame heating section,

[0010] - radiant tube heating section,

[0011] - temperature maintenance section,

[0012] - controlled cooling section.

[0013] A possible alternative involves using radiant tubes only. In this case the furnace is reduced to the following zones:

[0014] - radiant tube heating section,

[0015] - temperature maintenance section,

[0016] - controlled cooling section.

[0017] The controlled cooling section usually involves the use of a mixture of technical gases, e.g., nitrogen and hydrogen, or the use of hydrogen alone when oxidation of the surface of the strip in the furnace is to be avoided. Said controlled cooling section is provided with jet coolers.

[0018] Instead, if oxidation of the strip is allowed, ambient air is preferably used.

[0019] The purpose of the gaseous flow is to remove thermal energy from the strip, thus lowering the temperature thereof to make it suitable for subsequent processing. In the case of an annealing and galvanizing line, known as a “hot-dip galvanizing line” (HDGL), the strip is cooled from the target temperature of the maintenance section (650°C - 950°C) to the temperature of the zinc bath (about 460°C).

[0020] In the case of a continuous annealing line (CAL), the strip is cooled from said target temperature of the maintenance section to a final temperature, at the outlet from the annealing furnace, lower than 100° C.

[0021] The thermal energy removed through the mixture of technical gases or the ambient air is transferred through an appropriate gas / water heat exchanger to the cooling water circuit of the plant.

[0022] The cooled mixture of technical gases is recirculated through a closed circuit to the jet coolers of the controlled cooling section to continuously remove thermal energy from the strip.

[0023] Instead, the thermal energy transferred to the cooling water is dissipated / dispersed into the environment through the evaporative towers of the cooling water closed circuit.

[0024] This manner of operating the annealing plants implies critical issues, namely:

[0025] - loss of thermal energy that was transferred to the metal strip to achieve recrystallization; in fact, then being removed in the controlled cooling section, this unrecovered thermal energy leads to consumption of chemical and electrical energy resulting in the emission of associated carbon dioxide;

[0026] - this thermal energy removed from the strip in the controlled cooling section is transferred to the cooling water and, finally, transferred to the environment through the evaporative towers, resulting in water consumption;

[0027] - the energy used for the recirculation of the cooling water involves a further emission of carbon dioxide, linked to the production of the electricity consumed.

[0028] Considering the above, the need for an innovative review of annealing systems is evident, in order to eliminate the above-mentioned drawbacks while at the same time maximizing the amount of thermal energy recovered and reused in the heat treatment furnace. Summary of the invention

[0029] It is the object of the present invention to make an annealing apparatus for a metal strip, which is capable of completely or partially recovering the thermal energy that is removed from the annealed metal strip in a controlled cooling step and of reusing it by transferring this recovered energy to the metal strip at the inlet of the apparatus (usually at room temperature), thus obtaining a pre-heating of the product to be annealed with clear advantages for the environment and the reduction of the operating costs of the furnace.

[0030] It is another object of the invention to make an annealing apparatus that allows for high production flexibility, while creating a thermal energy storage system useful for all cases in which the amount of thermal energy removed from the product during the cooling step does not coincide (either shortfall or excess) with the amount of energy required by the product in the pre-heating step.

[0031] It is another object of the invention to make a related highly efficient annealing process with low environmental impact.

[0032] The innovative solution can be applied in all continuous annealing plants for ferrous and non-ferrous strips.

[0033] Therefore, the present invention aims to achieve the purposes discussed above by making an annealing apparatus for a metal strip advancing along a longitudinal direction, said apparatus comprising in sequence:

[0034] - at least one pre-heating section;

[0035] - at least one heating section;

[0036] - at least one temperature maintenance section;

[0037] - at least one cooling section; where said at least one pre-heating section comprises a plurality of heating rollers for advancing the metal strip; where said at least one cooling section comprises a plurality of cooling rollers for advancing the metal strip; where there is provided a closed circuit of heat-transfer fluid, said circuit being configured to cross both the cooling rollers of said at least one cooling section, in order to remove heat energy from the strip by conduction, thus obtaining a heattransfer fluid at high temperature, and the heating rollers of said at least one pre- heating section in order to transfer heat energy to the metal strip by conduction, thus obtaining a heat-transfer fluid at low temperature.

[0038] A further aspect of the invention relates to an annealing process for annealing a cold-rolled metal strip, advancing along a longitudinal direction, the process being carried out by the aforesaid apparatus and comprising the following steps:

[0039] - pre-heating the metal strip in said at least one pre-heating section;

[0040] - heating the metal strip in said at least one heating section;

[0041] - maintaining the temperature of the metal strip in said at least one temperature maintenance section;

[0042] - cooling the metal strip in said at least one cooling section; wherein the heat-transfer fluid of the closed circuit crosses both the cooling rollers of said at least one cooling section, removing heat energy from the metal strip by conduction and obtaining a heat-transfer fluid at high temperature, and the heating rollers of said at least one pre-heating section, transferring heat energy to the metal strip by conduction and obtaining a heat-transfer fluid at low temperature.

[0043] Other advantages of some embodiments of the invention include:

[0044] - High production flexibility regardless of the horizontal or vertical configuration of the annealing apparatus;

[0045] - Marked reduction in production costs achieved through significant energy savings for heating the metal strip;

[0046] - Significant reduction in direct and indirect carbon dioxide emissions;

[0047] - Marked simplification of the closed circuit for cooling water while achieving a reduction in water consumption due to evaporative losses thereof;

[0048] - High flexibility of the cooling process, being able to obtain cooling speeds between 10 and 100 C7s in relation to the metallurgical quality to be obtained.

[0049] The dependent claims describe preferred embodiments of the invention.

[0050] Brief description of the drawings

[0051] Further features and advantages of the invention will become more apparent in light of the detailed description of preferred, but not exclusive, embodiments of an apparatus shown by way of non-limiting example, with the aid of the accompanying drawings, in which: Figure 1 depicts a diagram of a first embodiment of an annealing apparatus according to the invention;

[0052] Figure 2 depicts a diagram of a pre-heating section with supply of heat-transfer fluid in parallel to the heating rollers;

[0053] Figure 3 depicts an alternative diagram of said pre-heating section with supply of heat-transfer fluid in series to the heating rollers;

[0054] Figure 4 depicts a diagram of a cooling section with supply of heat-transfer fluid in parallel to the cooling rollers;

[0055] Figure 5 depicts an alternative diagram of said cooling section with supply of heattransfer fluid in series to the cooling rollers;

[0056] Figure 6 depicts a diagram of the components of the apparatus according to the invention;

[0057] Figure 7 depicts a diagram of further components of the apparatus according to the invention;

[0058] Figures 8a, 8b and 8c depict three examples of the thermal cycle of an annealing apparatus according to the invention;

[0059] Figure 9 depicts a diagram of a second embodiment of an annealing apparatus according to the invention;

[0060] Figure 10 depicts a diagram relating to the management of a water / heat -transfer fluid heat exchanger for the production of hot water;

[0061] Figure 11 depicts some examples of internal channels in a heating roller or cooling roller of the apparatus according to the invention.

[0062] The same reference numerals in the figures identify the same elements or components.

[0063] Detailed description of preferred embodiments of the invention

[0064] With reference to the Figures, some exemplary embodiments of an annealing apparatus for metal strips according to the invention are shown. As known, the metal strip is a product having a dimension, i.e., thickness, considerably smaller than the other two dimensions, i.e., length and width.

[0065] In all embodiments of the invention, the annealing apparatus for a cold-rolled metal strip 2, preferably advancing along a longitudinal direction, comprises in sequence - at least one pre-heating section 17;

[0066] - at least one heating section 3;

[0067] - at least one temperature maintenance section 4;

[0068] - at least one cooling section 23.

[0069] Advantageously, the at least one pre-heating section 17 comprises a plurality of heating rollers 16 for advancing the metal strip 2; the at least one cooling section 23 comprises a plurality of cooling rollers 22 for advancing the metal strip 2; and there is provided a closed circuit of heat-transfer fluid, said circuit being configured to cross both the cooling rollers 22, in order to remove heat energy from the metal strip by conduction, thus obtaining a heat-transfer fluid at high temperature, and the heating rollers 16 in order to transfer heat energy to the metal strip by conduction, thus obtaining a heat-transfer fluid at low temperature.

[0070] It is preferable to provide only one pre-heating section 17, only one heating section 3, only one temperature maintenance section 4, and only one cooling section 23. Therefore, in steady state operation, the heat-transfer fluid allows the thermal energy that is removed from the annealed metal strip in the cooling section 23 to be completely or partially recovered and reused by transferring it to the metal strip in the pre-heating section 17, at the inlet of the apparatus, thus obtaining a preheating of the product to be annealed with clear advantages for the environment and the reduction of the operating costs of the heating section 3.

[0071] In a preferred variant both the cooling rollers 22 and the heating rollers 16 are provided with at least one respective internal channel 39, formed so that the cooling rollers 22 can be crossed, preferably along their longitudinal extension, by said heat-transfer fluid at low temperature for cooling the metal strip by conduction, and so that the heating rollers 16 can be crossed, preferably along their longitudinal extension, by said heat-transfer fluid at high temperature for preheating the metal strip by conduction, said cooling rollers 22 and said heating rollers 16 being adapted to be in direct contact with the strip during the advancement thereof.

[0072] Preferably, along the circuit, considering an advancement direction of the heattransfer fluid (see arrows in Figures 1 and 10), the following are provided - a first storage and movement system 25, 27 for storing and moving the heattransfer fluid at high temperature, arranged along a first stretch 70 of the circuit from said cooling section 23 to said pre-heating section 17;

[0073] - and a second storage and movement system 19, 20 for storing and moving the heat-transfer fluid at low temperature, arranged along a second stretch 71 of the circuit from said pre-heating section 17 to said cooling section 23.

[0074] In a preferred variant, the first storage and movement system 25, 27 comprises a first tank 25 for storing the heat-transfer fluid at high temperature and a first pumping unit 27 configured to adjust the flow rate of the heat-transfer fluid towards the pre-heating section 17.

[0075] Said first tank 25 preferably has insulated walls 40 for minimizing the energy lost into the environment (Figure 7).

[0076] In fact, this tank 25 forms a thermal energy storage where the heat-transfer fluid at high temperature is stored, which will be used to pre-heat the metal strip 2 at the inlet of the annealing furnace, i.e. , at the inlet of the heating section 3.

[0077] In essence, by decoupling the cooling section 23 from the pre-heating section 17, whenever the thermal energy removed from the strip 2 in the cooling step is different from that required by the strip to be pre-heated at the inlet of the annealing apparatus, the excess or shortfall is stored in, or removed from, the tank 25, respectively.

[0078] The first tank 25 has no heating means. In particular, no heating means are required therein to perform the thermal energy storage function.

[0079] Preferably, the flow of the heat-transfer fluid at high temperature from the tank 25 is controlled by the pumping unit 27, in which the pump drive motor is a variable speed motor in order to achieve a control or modification mode of the flow rate towards the pre-heating section 17 through a supply duct 15. The heat-transfer fluid at high temperature thus reaches the pre-heating section 17 through the supply duct 15.

[0080] Likewise, the second storage and movement system 19, 20 comprises a second tank 19 for storing the heat-transfer fluid at low temperature and a second pumping unit 20 configured to adjust the flow rate of said heat-transfer fluid towards the cooling section 23. Said second tank 19 preferably has walls 61 which are not insulated (Figure 6). The second tank 19 has no cooling means, in particular cooling means arranged inside it, as they are not necessary.

[0081] Preferably, the flow of the heat-transfer fluid at low temperature from the tank 19 is controlled by the pumping unit 20, in which the pump drive motor is a variable speed motor in order to achieve a control or modification mode of the flow rate towards the cooling section 23 through a supply duct 21 . The heat-transfer fluid at low temperature thus reaches the cooling section 23 through the supply duct 21.

[0082] Optionally, a heating device 26 is provided downstream of the first tank 25, preferably between the first tank 25 and the first pumping unit 27, for heating the heat-transfer fluid in case of cold starting of the annealing apparatus, for example after a long maintenance break.

[0083] By way of example, the heating device 26 is provided with at least one heating element 47, e.g., an armored electrical resistor immersed in an internal volume of the device 26 crossed by the heat-transfer fluid.

[0084] In a variant of the annealing apparatus of the invention, a further gas jet cooling section 5, for preliminarily cooling the strip, is provided between the temperature maintenance section 4 and the cooling section 23.

[0085] After completing the recrystallization cycle in the temperature maintenance section 4, the metal strip 2 thus enters this gas jet cooling section 5 where it undergoes a first slow cooling step due to a cold gas, preferably distributed by upper and lower cooling plenums 10, 11 (Figures 1 and 10). This cooling section 5 is used whenever slow cooling of the metallurgical recipe is required as a function of the quality of the product to be obtained.

[0086] Optionally, the cooling section 5 can be equipped with further heat recovery systems, as can be seen in Figures 1 and 10, in which the gas flows ejected from the upper and lower cooling plenums 10, 11 , once heated due to having taken heat from the product, are taken by means of a pumping or compression system 7 and sent to a further heat exchanger 6, which cools the hot gas coming from an inlet pipe 8, sending it back to the cooling plenums 10, 11 , once cooled, through an outlet pipe 9. Preferably, the cooling speed in this cooling section 5 is in a range from 5 to 15 °C / s.

[0087] After the cooling section 5, the metal strip 2 reaches a temperature from 650 to 900°C and enters the cooling section 23.

[0088] In a further variant of the annealing apparatus of the invention, a post-heating section 12, for heating the metal strip to a lower temperature than the solubilization temperature, is provided downstream of the cooling section 23. The function of this post-heating section 12 is purely metallurgical. In fact, since the post-heating takes place at a temperature lower than the solubilization temperature, it allows activating diffusion phenomena which induce a stable, coherent phase formation.

[0089] Preferably, a further gas jet cooling section 13, for taking the metal strip 2 to a target temperature for exiting from the apparatus, is provided downstream of said post-heating section 12.

[0090] In the example in Figures 1 and 10, both the gas jet cooling section 5 and the post-heating section 12 followed by the further gas jet cooling section 13 are provided in the annealing apparatus of the invention.

[0091] In some variants of the apparatus, in relation to the quality of the incoming strip and to the desired metallurgical quality, the gas jet cooling section 5 and the post heating section 12, as well as possibly the gas jet cooling section 13, can be removed.

[0092] Figures 8a, 8b and 8c show three examples of reference cycles. In particular, Figures 8a, 8b show a thermal cycle for an annealing and galvanizing line, known as “hot-dip galvanizing line” (HDGL), and a continuous annealing line (CAL), respectively.

[0093] Figure 8a shows a thermal cycle for an annealing apparatus provided in sequence with a pre-heating section 17 with heating rollers 16, heating section 3, temperature maintenance section 4, gas jet cooling section 5, cooling section 23 with cooling rollers 22, and further gas jet cooling section 13.

[0094] Figure 8b shows a thermal cycle for an annealing apparatus provided in sequence with pre-heating section 17 with heating rollers 16, heating section 3, temperature maintenance section 4, gas jet cooling section 5, cooling section 23 with cooling rollers 22, post-heating section 12, and further gas jet cooling section 13. Figure 8c shows a thermal cycle for an annealing apparatus provided in sequence with pre-heating section 17 with heating rollers 16, heating section 3, temperature maintenance section 4, and cooling section 23 with cooling rollers 22.

[0095] Preferred, but not exclusive, variants of the cooling section 23 and the pre-heating section 17 are described below.

[0096] In a preferred variant, each of the cooling rollers 22 of the cooling section 23 is provided with at least one internal channel 39 communicating with the aforementioned supply duct 21 so as to be crossed, preferably along the longitudinal extension thereof, by the heat-transfer fluid at low temperature and to cool the metal strip by conduction.

[0097] For example, the internal channels of the different cooling rollers 22 are connected in the circuit mutually in parallel; or they are connected in series by means of intermediate pipes so that the heat-transfer fluid crosses the cooling rollers 22 in counter-current with respect to the advancement of the metal strip on said cooling rollers 22.

[0098] In particular, as illustrated in Figure 4 in a parallel connection configuration, a number of pipes 35 can depart from the supply duct 21 , equal to the number of cooling rollers 22, which supply, at a first end of the respective roller 22, the at least one internal channel with heat-transfer fluid at low temperature. At a second end of the respective roller 22, the at least one internal channel communicates with a respective pipe 36. The number of pipes 36 is also preferably equal to the number of cooling rollers 22. The pipes 36 flow into an outlet duct 24 to convey the heat-transfer fluid at high temperature, which has removed heat from the strip, from the cooling section 23 towards the first tank 25.

[0099] The outlet duct 24 is suitably coated with an insulating material 49 (Figure 7) in order to minimize energy losses to the environment.

[0100] Instead, as illustrated for example in Figure 5 in a series connection configuration, the supply duct 21 supplies with heat-transfer fluid at low temperature the at least one internal channel of the last cooling roller 22 of the cooling section 23, at which the metal strip 2 exits after being cooled by the cooling section 23. The supply of the heat-transfer fluid to the at least one internal channel of the subsequent rollers 22, upstream of said last roller, occurs by means of respective intermediate pipes 35’ interposed between the cooling rollers 22. A flow of the heat-transfer fluid is thus obtained in countercurrent with respect to the advancement direction of the strip, so that while the metal strip 2 cools down passing from the first to the last roller 22, the heat-transfer fluid instead heats up passing from the last to the first roller 22, beyond which an outlet duct 24 is provided to convey the heat-transfer fluid at high temperature, which has removed heat from the strip, from the cooling section 23 towards the first tank 25.

[0101] Likewise, each of said heating rollers 16 of the pre-heating section 17 is provided with at least one internal channel 39 communicating with the aforementioned supply duct 15 so as to be crossed, preferably along the longitudinal extension thereof, by the heat-transfer fluid at high temperature and to heat the metal strip by conduction.

[0102] For example, the internal channels of the different heating rollers 16 are connected in the circuit mutually in parallel; or they are connected in series by means of intermediate pipes so that the heat-transfer fluid crosses in sequence the heating rollers 16 in counter-current with respect to the advancement of the metal strip on said heating rollers 16.

[0103] In particular, as illustrated in Figure 2 in a parallel connection configuration, a number of pipes 28 can depart from the supply duct 15, equal to the number of heating rollers 16, which supply, at a first end of the respective roller 16, the at least one internal channel with heat-transfer fluid at high temperature. At a second end of the respective roller 16, the at least one internal channel communicates with a respective pipe 29. The number of pipes 29 is also preferably equal to the number of heating rollers 16. The pipes 29 flow into an outlet duct 18 to convey the heat-transfer fluid at low temperature, which transferred heat to the strip, from the pre-heating section 17 towards the second tank 19. The outlet duct 18 is preferably not insulated.

[0104] Instead, as illustrated for example in Figure 3 in a series connection configuration, the supply duct 15 supplies with heat-transfer fluid at high temperature the at least one internal channel of the last heating roller 16 of the pre-heating section 17, at which the metal strip 2 exits after being pre-heated by the pre-heating section 17. The supply of the heat-transfer fluid to the at least one internal channel of the subsequent rollers 16, upstream of said last roller, occurs by means of respective intermediate pipes 28’ interposed between the heating rollers 16. A flow of the heat-transfer fluid is thus obtained in countercurrent with respect to the advancement direction of the strip, so that while the metal strip 2 is heated passing from the first to the last roller 16, the heat -transfer fluid is instead cooled passing from the last to the first roller 16, beyond which an outlet duct 18 is provided to convey the heat-transfer fluid at low temperature, which transferred heat to the strip, from the pre-heating section 17 towards the second tank 19.

[0105] In a further preferred variant, for an optimal control of the pre-heating process in the pre-heating section 17, the following can be provided:

[0106] - at least one first temperature sensor 30 at the inlet of the pre-heating section 17 for measuring the strip temperature;

[0107] - at least one second temperature sensor 31 at the outlet of the pre-heating section 17 for measuring the strip temperature;

[0108] - at least one first flow rate transducer 32 for measuring a first flow rate of the heat-transfer fluid at high temperature entering the pre-heating section 17.

[0109] It is possible to provide a single first temperature sensor 30, a single second temperature sensor 31 , and a single first flow rate transducer 32.

[0110] Advantageously, at least one automatic control unit 33 can be provided, preferably a single automatic control unit, capable of receiving data from said first temperature sensor 30, said second temperature sensor 31 and said first flow rate transducer 32 and finely adjusting the flow rate of the heat transfer fluid entering the pre-heating section 17 accordingly, preferably by acting on a first proportional valve 34.

[0111] The proportional valve 34 can act in combination with the pumping unit 27. Preferably, the pump drive motor of the pumping unit 27 is a variable speed motor in order to achieve rough control of the flow rate of the heat-transfer fluid towards the pre-heating section 17, while the proportional valve 34 achieves fine control of said flow rate.

[0112] Similarly, for an optimal control of the cooling process in the cooling section 23, the following can be provided: - at least one third temperature sensor 62 at the inlet of the cooling section 23 for measuring the strip temperature;

[0113] - at least one fourth temperature sensor 63 at the outlet of the cooling section 23 for measuring the strip temperature;

[0114] - at least one second flow rate transducer 64 for measuring a second flow rate of the heat-transfer fluid entering the cooling section 23.

[0115] It is possible to provide a single third temperature sensor 62, a single fourth temperature sensor 63, and a single second flow rate transducer 64.

[0116] Said automatic control unit 33 is also adapted to receive data from said third temperature sensor 62, said fourth temperature sensor 63 and said second flow rate transducer 64 and therefore finely adjust the flow rate of the heat transfer fluid entering the cooling section 23, preferably by acting on a second proportional valve 34’.

[0117] The proportional valve 34’ can act in combination with the pumping unit 20. Preferably, the pump drive motor of the pumping unit 20 is a variable speed motor in order to achieve rough control of the flow rate of the heat transfer fluid towards the cooling section 23, while the proportional valve 34’ achieves fine control of said flow rate.

[0118] To further improve the control of the pre-heating process in the pre-heating section 17, the first tank 25 is provided (Figure 7) with a first temperature transducer 45 to measure the temperature of the heat transfer fluid inside said first tank, and with a first automatic level meter 46 to detect the level of the heat transfer fluid inside said first tank. Said automatic control unit 33 is also adapted to receive data from said first temperature transducer 45 and said first automatic level meter 46 to better adjust the pre-heating in the pre-heating section 17.

[0119] Injection means 41 can be provided to inject inert gas so as to pressurize the first tank 25 so as to avoid degradation of the heat-transfer fluid due to oxidation phenomena.

[0120] Likewise, to further improve the control of the cooling process in the cooling section 23, the second tank 19 is provided (Figure 6) with a second temperature transducer 42 to measure the temperature of the heat transfer fluid inside said second tank, and with a second automatic level meter 43 to detect the level of the heat transfer fluid inside said second tank. Said automatic control unit 33 is also adapted to receive data from said second temperature transducer 42 and said second automatic level meter 43 to better adjust the cooling in the cooling section 23.

[0121] Also in this case, injection means 41 can be provided to inject inert gas so as to pressurize the second tank 19 so as to avoid degradation of the heat -transfer fluid due to oxidation phenomena.

[0122] In a further variant of the apparatus of the invention, shown in Figures 9 and 10, along the first stretch 70 of the circuit, at the outlet of the cooling section 23 and upstream of the first tank 25, there can be provided a first water / heat-transfer fluid heat exchanger 50 for producing hot water so as to remove heat energy, previously removed from the metal strip, which is in excess with respect to the heat energy required by the strip in the pre-heating section 17.

[0123] Likewise, along the stretch 71 of the circuit, at the outlet of the pre-heating section 17 and upstream of the second tank 19, there can be provided a second water / heat-transfer fluid heat exchanger 50’ for producing hot water so as to minimize the temperature of the heat-transfer fluid and promote cooling in the cooling section 23.

[0124] Preferably, both for the first heat exchanger 50 and the second heat exchanger 50’ there are provided (Figure 11 ):

[0125] - temperature transducers 55, 57 for measuring the temperature of the incoming and outgoing water from the heat exchanger;

[0126] - a flow rate transducer 56 for measuring the flow rate of the water at the inlet of the heat exchanger;

[0127] - further temperature transducers 52, 53 for measuring the temperature of the incoming and outgoing heat-transfer fluid from the heat exchanger;

[0128] - a further flow rate transducer 54 for measuring the flow rate of the heat-transfer fluid at the inlet of the heat exchanger.

[0129] The aforesaid automatic control unit 33 is adapted to also receive data from said temperature transducers 55, 57, said flow rate transducer 56, said further temperature transducers 52, 53, and said further flow rate transducer 54, and thus to adjust the flow rate of the water entering the respective heat exchanger 50, 50’, in relation to the thermal energy to be removed in real time from the heat-transfer fluid in order to produce hot water up to a maximum temperature of preferably 80- 90°C.

[0130] In particular, for example, a proportional valve 60 is provided along a water supply pipe 58, arranged upstream of the temperature transducer 55 and the flow rate transducer 56, and controlled by said automatic control unit 33; while the temperature transducer 57 is installed along the pipe 59 of the water exiting from the exchanger 50’, 50.

[0131] Instead, for example, along the outlet duct 18 of the heat-transfer fluid from the pre-heating section 17, or along the outlet duct 24 of the heat-transfer fluid from the cooling section 23, which flows into the respective heat exchanger 50’, 50, there are in succession the flow rate transducer 54 and the temperature transducer 52; while the temperature transducer 53 is installed along the pipe 51 of the heat-transfer fluid exiting from the exchanger 50’, 50.

[0132] In all embodiments of the apparatus of the invention (Figures 1 and 10), it is preferable that the pre-heating section 17 is suitably contained within a metal casing 37, insulated so as not to disperse heat into the environment, and which can be pressurized with appropriate inert gas or kept in air depending on whether oxidative phenomena are to be avoided or encouraged.

[0133] Similarly, the cooling section 23 can be suitably contained inside a metal casing 38, insulated so as not to disperse heat into the environment, and which can be pressurized with appropriate inert gas or kept in the air depending on whether oxidative phenomena are to be avoided or encouraged.

[0134] Preferably, the heat-transfer fluid that can be used in the annealing apparatus of the invention is diathermic oil or another fluid with an equivalent function.

[0135] The advantages of using diathermic oil include the following:

[0136] - outstanding performance at high temperature, including excellent thermal stability and low vapor pressure;

[0137] - constant and reliable performance in transferring heat for long periods of time;

[0138] - it provides an even, reliable, efficient process heat, without requiring high pressures; - the high boiling point contributes to reducing volatility and problems of fluid loss associated with other fluids;

[0139] - it is not corrosive for the metals commonly used in the construction of heattransfer systems.

[0140] Examples of diathermic oils particularly suitable for being used in the system of the invention are the products commercially known as Therminol, preferably Therminol products from 55 to 75, e.g., Therminol 66.

[0141] In relation to the heating rollers 16 and the cooling rollers 22, these can be made with an external steel or copper jacket. The latter material is preferred to maximize the heat exchange by conduction between the strip and the rollers.

[0142] The number of rollers 16, 22 can be defined as a function of the speed of the strip and the size thereof. Preferably, the number of rollers is within a range of 3 to 6. In the example of the Figures the number of rollers 16, 23 is four.

[0143] The rollers 16, 22 can have the inlet of the heat-transfer fluid into the internal channels of the roller and the outlet of the heat-transfer fluid from said internal channels through the hubs of the roller itself.

[0144] The heat exchange capacity is a function of the contact surface between the strip 2 and the rollers 16, 22: the best results are obtained in configurations in which the roller is wound by the strip by at least 135°. However, if it is necessary to vary the heat exchange, e.g., as a function of the thickness of the product, it is possible to vary the angle of embrace of the roller by the strip, increasing it to exchange more heat or reducing it to exchange less heat.

[0145] Preferably, the internal channels of the rollers 16, 22, preferably peripheral internal channels, can define a rectilinear axis, substantially parallel to the longitudinal axis of the respective roller, or a helical or spiral axis, which is wound inside the longitudinal axis of the respective roller.

[0146] Only one internal channel, or internal jacket, can be present inside each roller, which can be annular or non-annular in shape, for example; or several internal channels or internal cavities can be present, possibly communicating with one another.

[0147] Some non-limiting examples of peripheral internal channels 39 can be seen in the cross-sections of a roller 16, 22, shown in Figure 11 . The internal channels 39 can have any cross-section shape, not only a round section. For example, the crosssection can be polygonal.

[0148] An annealing process performed using an apparatus according to any one of the variants described above is described below.

[0149] In all embodiments of the invention, the process comprises the following stages:

[0150] - pre-heating the metal strip in the pre-heating section 17;

[0151] - heating the metal strip in the at least one heating section 3;

[0152] - maintaining the temperature of the metal strip in the temperature maintenance section 4;

[0153] - cooling the metal strip in the cooling section 23.

[0154] Advantageously, when the apparatus of the invention is fully operational, the heattransfer fluid of the closed circuit crosses both the cooling rollers 22, removing heat energy from the metal strip by conduction and obtaining a heat-transfer fluid at high temperature, and the heating rollers 16, transferring heat energy to the metal strip by conduction and obtaining a heat-transfer fluid at low temperature.

[0155] In a preferred variant of the process, the heat-transfer fluid of the closed circuit crosses both at least one respective internal channel 39 of the cooling rollers 22 and at least one respective internal channel 39 of the heating rollers 16; said cooling rollers 22 and said heating rollers 16 being in direct contact with the strip during the advancement thereof.

[0156] Preferably, considering an advancement direction of the heat-transfer fluid in the circuit, the heat-transfer fluid at high temperature is stored and moved, by means of the first storage and movement system 25, 27, along the first stretch 70 of the circuit from the cooling section 23 to the pre-heating section 17; whereas the heattransfer fluid at low temperature is stored and moved, by means of the second storage and movement system 19, 20, along a second stretch 71 of the circuit from said pre-heating section 17 to said cooling section 23.

[0157] In particular, for example, the heat-transfer fluid at high temperature is stored in the first tank 25 and the flow rate thereof towards the pre-heating section 17 is adjusted by the first pumping unit 27; whereas the heat -transfer fluid at low temperature is stored in the second tank 19 and the flow rate thereof towards the cooling section 23 is adjusted by the second pumping unit 20. The process of recovery and reuse of thermal energy through the cooling section 23 and the pre-heating section 17, respectively, can exhibit the following energy balance or imbalance conditions. By indicating the power subtracted from the strip in the cooling section 23 with Pc, and Ph for the power supplied to the strip in the pre-heating section 17 we can find ourselves in the following conditions: Pc > Ph, Pc = Ph or Pc < Ph.

[0158] The case Pc > Ph can occur in two different operating conditions of the plant, namely:

[0159] 1 ) in fully operational conditions, the thermal energy removed from the strip is greater than that supplied to the strip because the temperature difference during cooling is greater than that during pre-heating. This effect is mainly due to the fact that, while during heating there is the constraint of the maximum temperature reachable by the diathermic oil, which is about 350- 400 °C depending on the type of oil used whereby considering the incoming strip at an average temperature of 50°C at the outlet of the pre-heating system it is calculated not to exceed 300°C therefore with a AT=250 °C, during cooling there could be a AT> 250 °C, up to a maximum value of 490°C for an annealing and galvanizing line (HDGL) furnace and up to 700 °C for a continuous annealing (CAL) furnace,

[0160] 2) or, in a process condition due to a transient, for example due to a change in the section of the strip to be processed or a change in the thermal recipe of the furnace when the strip roll is changed or due to the combined effect of both conditions, the power removed from the strip is higher than that supplied to the strip. This is a temporary effect that lasts for the welding passage time, between the portions of strip with different sections, from the inlet of the annealing apparatus to the cooling section 23. The transient time t can vary, depending on the furnace geometry and process parameters, from 3 minutes to 20 minutes. This type of phenomenon is appropriately managed thanks to the volumes of the tanks 25 and 19.

[0161] In the case Pc = Ph, the energy removed from the strip is exactly the same as that supplied to the strip in the pre-heating step. The flow rate of the cold branch thus coincides with the flow rate of the hot branch of the circuit, and there is no storage of thermal energy in the tanks as the process is perfectly balanced.

[0162] Finally, the case Pc < Ph corresponds to a process condition due to a transient, due to a change in the section of the strip to be processed or a change in the thermal recipe of the furnace when the strip roll is changed or due to the combined effect of both conditions, where the power removed from the strip is lower than that supplied to the strip. This is a temporary effect that lasts for the welding passage time, between the portions of strip with different sections, from the inlet of the annealing apparatus to the cooling section 23. The transient time t can vary, depending on the furnace geometry and process parameters, from 3 minutes to 20 minutes. This type of phenomenon is appropriately managed thanks to the volumes of the tanks 25 and 19.

[0163] For case 1 ) with Pc > Ph, having a continuous difference in power removed with respect to that supplied to the strip, it could be considered to remove this surplus thermal energy by producing hot water with the aforementioned heat exchanger 50 located on the stretch 70 of the circuit where high temperature heat-transfer fluid flows.

[0164] Preferably, the water / heat-transfer fluid heat exchanger 50 can be appropriately sized using the following formula:

[0165] AP= Pc max - Ph min where:

[0166] AP = Exchange power of the water / heat-transfer fluid heat exchanger 50;

[0167] Pc max = maximum power removed from the strip in the cooling section 23; Ph min = minimum power supplied to the strip in the pre-heating section 17.

[0168] For case 2) with Pc > Ph, to appropriately size the tanks 25 and 29 it is preferable to use a relationship that links the mass M of heat-transfer fluid in the tank 25 to some process parameters through the following formula:

[0169] M = - r*(1-r)*t / (ln(T / T1 )) where r = mass flow rate of the heat-transfer fluid in kg / s; r = loss factor %, which is a dimensionless factor calculated as the ratio between the section of the strip during pre-heating and the section of the strip during cooling, both expressed in mm2; t = transient duration in seconds (s); T = initial temperature in the tank 25 at t=0;

[0170] T 1 = final temperature in the tank 25 at t=transient end time;

[0171] By dividing the mass M by the density of the heat transfer fluid, the volume in m3of the tanks 25 and 19 is obtained, for the heat-transfer fluid at high temperature and for the heat-transfer fluid at low temperature, respectively.

Claims

CLAIMS1 . An annealing apparatus for a metal strip (2) advancing along a direction, said apparatus comprising in sequence- at least one pre-heating section (17);- at least one heating section (3);- at least one temperature maintenance section (4);- at least one cooling section (23); wherein said at least one pre-heating section (17) comprises a plurality of heating rollers (16) for advancing the metal strip (2); wherein said at least one cooling section (23) comprises a plurality of cooling rollers (22) for advancing the metal strip (2); wherein there is provided a closed circuit of heat-transfer fluid, said circuit being configured to cross both the cooling rollers (22) of said at least one cooling section (23), in order to remove heat energy from the metal strip by conduction, thus obtaining a heat-transfer fluid at high temperature, and the heating rollers (16) of said at least one pre-heating section (17) in order to transfer heat energy to the metal strip by conduction, thus obtaining a heat-transfer fluid at low temperature.

2. An apparatus according to claim 1 , wherein both the cooling rollers (22) and the heating rollers (16) are provided with at least one respective internal channel (39), formed so that the cooling rollers (22) can be crossed by said heat-transfer fluid at low temperature for cooling the metal strip by conduction, and so that the heating rollers (16) can be crossed by said heat-transfer fluid at high temperature for preheating the metal strip by conduction, said cooling rollers (22) and said heating rollers (16) being adapted to be in direct contact with the strip during the advancement thereof.

3. An apparatus according to claim 1 or 2, wherein along said circuit, considering an advancement direction of the heat-transfer fluid, there are provided- a first storage and movement system (25, 27) for storing and moving the heattransfer fluid at high temperature, acting as a thermal energy store (TES) and arranged along a first stretch (70) of the circuit from said at least one cooling section (23) to said at least one pre-heating section (17);- and preferably a second storage and movement system (19, 20) for storing and moving the heat-transfer fluid at low temperature, arranged along a second stretch (71 ) of the circuit from said at least one pre-heating section (17) to said at least one cooling section (23).

4. An apparatus according to claim 3, wherein the first storage and movement system (25, 27) comprises a first tank (25) for storing the heat-transfer fluid at high temperature, configured as a thermal energy store (TES), and a first pumping unit (27) configured to adjust the flow rate of said heat-transfer fluid at high temperature towards the at least one pre-heating section (17); and preferably wherein the second storage and movement system (19, 20) comprises a second tank (19) for storing the heat-transfer fluid at low temperature and a second pumping unit (20) configured to adjust the flow rate of said heattransfer fluid at low temperature towards the at least one cooling section (23); preferably wherein the first tank (25) has no heating means and optionally has insulated walls (40) to minimize the energy lost into the environment, and preferably wherein the second tank (19) has no heating means.

5. An apparatus according to claim 4, wherein a heating device (26) is provided downstream of the first tank (25), preferably between said first tank (25) and said first pumping unit (27), for heating the heat-transfer fluid in case of cold starting of the apparatus.

6. An apparatus according to any one of claims 2 to 5, wherein a supply duct (21 ) is provided along the circuit for supplying the heat-transfer fluid at low temperature to said at least one cooling section (23); wherein said at least one internal channel (39) of each of said cooling rollers (22) communicates with said supply duct (21 ); preferably wherein said at least one internal channels of the cooling rollers (22) are connected in the circuit mutually in parallel; or wherein said at least one internal channels of the cooling rollers (22) are connected in series by means of intermediate pipes (35’) so that the heat-transfer fluid crosses the cooling rollers (22) in counter-current with respect to the advancement of the metal strip on said cooling rollers (22).

7. An apparatus according to any one of claims 2 to 6, wherein a supply duct (15) is provided along the circuit for supplying the heat-transfer fluid at high temperature to said at least one pre-heating section (17); wherein said at least one internal channel (39) of each of said heating rollers (16) communicates with said supply duct (15); preferably wherein said at least one internal channels of the heating rollers (16) are connected in the circuit mutually in parallel; or wherein said at least one internal channels of the heating rollers (16) are connected in series by means of intermediate pipes (28’) so that the heat-transfer fluid crosses in sequence the heating rollers (16) in counter-current with respect to the advancement of the metal strip on said heating rollers (16).

8. An apparatus according to any one of the preceding claims, wherein a further gas jet cooling section (5), for preliminarily cooling the strip, is provided between the at least one temperature maintenance section (4) and said at least one cooling section (23).

9. An apparatus according to any one of the preceding claims, wherein a postheating section (12) for heating the metal strip to a lower temperature than the solubilization temperature is provided downstream of said at least one cooling section (23); and preferably wherein a further gas jet cooling section (13) for taking the metal strip to a target temperature for exiting from the apparatus is provided downstream of said post-heating section (12).

10. An apparatus according to any one of the preceding claims, wherein there are provided- at least one first temperature sensor (30) at the inlet of the pre-heating section (17) for measuring the strip temperature;- at least one second temperature sensor (31 ) at the outlet of the pre-heating section (17) for measuring the strip temperature;- at least one first flow rate transducer (32) for measuring a first flow rate of the heat-transfer fluid entering the pre-heating section (17);- at least one third temperature sensor (62) at the inlet of the cooling section (23) for measuring the strip temperature;- at least one fourth temperature sensor (63) at the outlet of the cooling section (23) for measuring the strip temperature;- at least one second flow rate transducer (64) for measuring a second flow rate of the heat-transfer fluid entering the cooling section (23); and wherein there is provided at least one automatic control unit (33) adapted to receive data from said at least one first temperature sensor (30), said at least one second temperature sensor (31 ) and said at least one first flow rate transducer (32), and therefore adjust said first flow rate, and adapted to receive data from said at least one third temperature sensor (62), said at least one fourth temperature sensor (63), and said at least one second flow rate transducer (64), and therefore adjust said second flow rate.

11. An apparatus according to claim 10, wherein said at least one automatic control unit (33) is adapted to adjust said first flow rate by means of a first proportional valve (34) and to adjust said second flow rate by means of a second proportional valve (34’).

12. An apparatus according to claim 11 , wherein said first proportional valve (34) is configured to act in combination with a first pumping unit (27) configured to adjust the first flow rate of said heat-transfer fluid at high temperature from a first tank (25), acting as a thermal energy store (TES) and arranged along a first section (70) of the circuit from the at the least one cooling section (23) to the at least one pre-heating section (17), towards said at least one pre-heating section (17); preferably wherein said second proportional valve (34’) is configured to act in combination with a second pumping unit (20) configured to adjust the second flow rate of said heat-transfer fluid at low temperature from a second tank (19), arranged along a second section (71 ) of the circuit from the at least one preheating section (17) to the at least one cooling section (23), towards said at least one cooling section (23).

13. An apparatus according to claim 12, wherein both the drive motor of said first pumping unit (27) and the drive motor of said second pumping unit (20) are a variable speed motor.

14. An apparatus according to any one of claims 4 to 13, wherein the first tank (25) is provided with a first temperature transducer (45) for measuring the temperature of the heat-transfer fluid therein and with a first automatic level meter (46) for detecting the level of the heat-transfer fluid therein, and wherein there is provided at least one automatic control unit (33) adapted to receive data from said first temperature transducer (45) and said first automatic level meter (46) for better adjusting the pre-heating in the pre-heating section (17); and preferably wherein the second tank (19) is provided with a second temperature transducer (42) for measuring the temperature of the heat-transfer fluid therein and with a second automatic level meter (43) for detecting the level of the heat-transfer fluid therein, and wherein said at least one automatic control unit (33) is adapted to receive data from said second temperature transducer (42) and said second automatic level meter (43) for better adjusting the cooling in the cooling section (23); preferably wherein there are provided injection means (41 ) for injecting inert gas so as to pressurize said first tank (25) and / or said second tank (19).

15. An apparatus according to any one of claims 4 to 14, wherein along said first stretch (70) of the circuit, at the outlet of said cooling section (23) and upstream of the first tank (25), there is provided a first water / heat-transfer fluid heat exchanger (50) for producing hot water so as to remove heat energy from the metal strip which is in excess with respect to the heat energy required by the strip in the preheating section (17); and preferably wherein along said second stretch (71 ) of the circuit, at the outlet of said pre-heating section (17) and upstream of the second tank (19), there is provided a second water / heat-transfer fluid heat exchanger (50’) for producing hot water so as to minimize the temperature of the heat-transfer fluid and promote cooling in the cooling section (23).

16. An apparatus according to claim 15, wherein for the first heat exchanger (50) and preferably for the second heat exchanger (50’) there are provided- temperature transducers (55, 57) for measuring the temperature of the incoming and outgoing water;- a flow rate transducer (56) for measuring the flow rate of the incoming water;- further temperature transducers (52, 53) for measuring the temperature of the incoming and outgoing heat-transfer fluid;- a further flow rate transducer (54) for measuring the flow rate of the incoming heat-transfer fluid; wherein there is provided at least one automatic control unit (33) adapted to receive data from said temperature transducers (55, 57), said flow rate transducer (56), said further temperature transducers (52, 53), and said further flow rate transducer (54), and therefore adjust the flow rate of the water entering the respective heat exchanger (50,50’).

17. An annealing process carried out by an apparatus according to any one of the preceding claims, the process comprising the following stages:- pre-heating the metal strip in said at least one pre-heating section (17);- heating the metal strip in said at least one heating section (3);- maintaining the temperature of the metal strip in said at least one temperature maintenance section (4);- cooling the metal strip in said at least one cooling section (23); wherein the heat-transfer fluid of the closed circuit crosses both the cooling rollers (22) of said at least one cooling section (23), removing heat energy from the metal strip by conduction and obtaining a heat-transfer fluid at high temperature, and the heating rollers (16) of said at least one pre-heating section (17), transferring heat energy to the metal strip by conduction and obtaining a heat-transfer fluid at low temperature.

18. A process according to claim 17, wherein the heat-transfer fluid of the closed circuit crosses both at least one respective internal channel (39) of the cooling rollers (22) and at least one respective internal channel (39) of the heating rollers (16); said cooling rollers (22) and said heating rollers (16) being in direct contact with the strip during the advancement thereof.

19. A process according to claim 17 or 18, wherein, considering an advancement direction of the heat-transfer fluid in the circuit, said heat-transfer fluid at high temperature is stored and moved, by means of a first storage and movement system (25, 27), acting as a thermal energy store (TES), along a first stretch (70)of the circuit from said at least one cooling section (23) to said at least one preheating section (17); and preferably wherein said heat-transfer fluid at low temperature is stored and moved, by means of a second storage and movement system (19, 20), along a second stretch (71 ) of the circuit from said at least one pre-heating section (17) to said at least one cooling section (23).

20. A process according to claim 19, wherein said heat-transfer fluid at high temperature is stored in a first tank (25), acting as a thermal energy store (TES), and the flow rate thereof towards the at least one pre-heating section (17) is adjusted by a first pumping unit (27); whereas preferably said heat-transfer fluid at low temperature is stored in a second tank (19) and the flow rate thereof towards the at least one cooling section (23) is adjusted by a second pumping unit (20).21 . A process according to any one of claims 17 to 20, wherein there are provided- at least one first temperature sensor (30) at the inlet of the pre-heating section (17) for measuring the strip temperature;- at least one second temperature sensor (31 ) at the outlet of the pre-heating section (17) for measuring the strip temperature;- at least one first flow rate transducer (32) for measuring a first flow rate of the heat-transfer fluid entering the pre-heating section (17);- at least one third temperature sensor (62) at the inlet of the cooling section (23) for measuring the strip temperature;- at least one fourth temperature sensor (63) at the outlet of the cooling section (23) for measuring the strip temperature;- at least one second flow rate transducer (64) for measuring a second flow rate of the heat-transfer fluid entering the cooling section (23); and wherein at least one automatic control unit (33) receives data from said at least one first temperature sensor (30), said at least one second temperature sensor (31) and said at least one first flow rate transducer (32), and therefore adjusts the first flow rate of the heat-transfer fluid entering the pre-heating section (17), and also receives data from said at least one third temperature sensor (62), said at least one fourth temperature sensor (63), and said at least one second flowrate transducer (64), and therefore adjusts the second flow rate of the heattransfer fluid entering the cooling section (23).

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