Method for heating a rolled steel strip and installation thereof

Abstract

The invention relates to a method for heating a rolled steel strip that comprises the following steps: - continuously driving a rolled steel strip inside a furnace, - introducing an entry gas flow into a device for heating gas that comprises: • a pipe, allowing gas to flow inside of it, • an insert located inside said pipe, being electrically conductive, • an inductor located close to said pipe, - feeding energy into said insert using said inductor, so that said energy fed into the insert is transferred to the gas inside said device for heating gas, to heat said gas to a temperature from 500°C to 1500°C, - circulating said heated gas into a radiant tube located inside the furnace, so that said heated gas transfers heat to said radiant tube, making said radiant tube release the transferred heat inside the furnace to heat said rolled steel strip.

Description

[0001] Method for heating a rolled steel strip and installation thereof

[0002]

[0001] The present invention relates to a method for heating a rolled steel strip and the associated installation. More precisely, the present invention relates to a method for heating a rolled steel strip with a gas heated using induction heating.

[0003]

[0002] In the process of making steel products, thermal treatments are done to give the steel its desired properties. It is done by heating homogeneously the steel product to be treated at one or several target temperatures that corresponds to the desired properties. There are various furnaces that are designed for the heat treatment of steel products.

[0004]

[0003] For a steel strip, radiant tubes furnaces are a type of furnace to perform heat treatments after rolling. It is a preferred method to heat steel grades that can be sensitive to a reaction with the heating means, as it uses an indirect heating method. For this, a burner is disposed at the entry of the radiant tube that produces a combustion gas that circulates inside the tube and heats the tube. The steel strip inside the furnace is heated by the radiation that is emitted by the radiant tubes.

[0005]

[0004] However, this process causes CO2 emissions because of the combustion gas and CO2 emissions need to be reduced in the steel industry. There is also a problem that the temperature distribution in the radiant tube is not even as the burner heats the tube more at the entrance. This temperature unevenness causes creep and thus reduces the life of the radiant tube. There is thus a need for an alternative method that solve these problems.

[0006]

[0005] The aim of the present invention is therefore to remedy the aforementioned drawbacks by providing a method that provides a homogeneous heating of a rolled steel strip that is more energy efficient and that increases the life of the radiant tubes while reducing CO2 emissions.

[0006] For this purpose, a first object of the present invention consists in a method for heating a rolled steel strip wherein said method comprises the following steps:

[0007] - continuously driving a rolled steel strip 6 to be heated inside a furnace 5, - introducing an entry gas flow into at least one device for heating gas 1 , said device for heating gas 1 comprising:

[0008] o a pipe 2 allowing gas to flow inside of it,

[0009] o an insert 3 located inside said pipe 2, said insert being electrically conductive,

[0010] o an inductor 4 located in the vicinity of said pipe 2,

[0011] - feeding energy into said insert 3 using said inductor 4 of said device for heating gas 1 so that said energy fed into the insert 3 is transferred to the gas introduced into said device for heating gas 1 , to heat said gas to a temperature from 500°C to 1500°C,

[0012] - circulating said heated gas into at least one radiant tube 10 located inside the furnace 5, so that said heated gas transfers heat to said at least one radiant tube 10, making said at least one radiant tube 10 release the transferred heat inside the furnace 5 to heat said rolled steel strip 6.

[0013]

[0007] The method for heating a rolled steel strip according to the invention may also have the optional features listed below, considered individually or in combination:

[0014] - the rolled steel strip 6 is heated to a temperature from 300°C to 1 100°C, - the method comprises an additional step of retrieving said heated gas exiting said at least one radiant tube 10 to be used again to heat said rolled steel strip 6,

[0015] - the retrieved gas is mixed with the entry gas flow before being heated again with the device for heating gas 1 ,

[0016] - the driving speed of the rolled steel strip 6 inside of the furnace 5 is from 10 m / min to 600 m / min,

[0017] - the heated gas is circulating through several radiant tubes 10 arranged to heat the inner volume of the furnace 5, - at least one device for heating gas 1 is connected to each of said radiant tubes 10,

[0018] - the entry gas flow is composed of one or more of the following gases:

[0019] syngas, CO2, CO, H2, H2O, hydrocarbons, N2.

[0020]

[0008] A second object of the invention consists in an installation for heating a rolled steel strip, wherein said installation comprises:

[0021] - a furnace 5 allowing a rolled steel strip 6 to be driven inside of it,

[0022] - at least one device for heating gas 1 , said device for heating gas 1 comprising:

[0023] o a pipe 2 allowing gas to flow inside of it,

[0024] o an insert 3 located inside said pipe 1 , said insert 3 being electrically conductive,

[0025] o an inductor 4 located in the vicinity of said pipe 2 and able to feed energy into said insert 3, so that said energy fed into said insert 3 is transferred to the gas flowing into said pipe 2, to heat said gas is to a temperature from 500°C to 1500°C.

[0026] - a means for introducing an entry gas flow into the device for heating gas 1 ,

[0027] - at least one radiant tube 10, located inside the furnace 5 and connected to the device for heating gas 1 , allowing the heated gas to circulate inside of it and for releasing heat into the furnace 5 to heat said rolled steel strip 6.

[0028]

[0009] The installation according to the invention may also have the optional features listed below, considered individually or in combination:

[0029] - the installation further comprises a means for retrieving the gas that exits the radiant tubes 10 to be used again to heat the rolled steel strip 6, - the installation further comprises a mixing station 8 able to mix said retrieved gas with the entry gas flow before being introduced into said at least one device for heating gas 1 ,

[0030] - the furnace 5 comprises two or more sections inside of it, - the insert 3 is designed to have an exchange surface with the gas to be heated that ranges from 0.5m2to 100m2by meter of pipe 2,

[0031] - the insert 3 shape is designed to provide a turbulent flow to the gas flowing into the pipe 2.

[0032]

[0010] Other characteristics and advantages of the invention will be described in greater detail in the following description.

[0033]

[0011] The invention will be better understood by reading the following description, which is provided purely for purposes of explanation and is in no way intended to be restrictive, with reference to:

[0034]

[0012] Figure 1 , which is a general view of the device for heating gas according to the invention,

[0035]

[0013] Figure 2, which is a cross-sectional view of the interior of the pipe of the device for heating gas according to one embodiment,

[0036]

[0014] Figures 3a and 3b, which are representations of embodiments of the insert of the device for heating gas according to the invention,

[0037]

[0015] Figure 4, which is a representation of an embodiment of the installation for heating a rolled steel strip according to the invention.

[0038]

[0016] With reference to Figure 1 , 2 and 4, the method for heating a rolled steel strip and the installation for heating a rolled steel strip according to the invention will be described in detail.

[0039]

[0017] The method for heating a rolled steel strip according to the invention comprises four steps that can be done simultaneously. The first step consists in continuously driving the rolled steel strip 6 to be heated inside a furnace 5. For the second step, an entry gas flow is introduced into at least one device for heating gas 1 . The device for heating gas 1 comprises a pipe 2 that allows gas to flow inside of it, an insert 3 located inside the pipe 2 and that is electrically conductive as well as an inductor 4 located in the vicinity of the pipe 2. The third step consists in feeding energy to the insert 3. To achieve this, the inductor 4 creates a magnetic field that induces a current in the insert 3. This energy fed to the insert 3 results in the heating of the insert 3. This energy is then transferred from the hot insert 3 to the gas flowing into the pipe 2 that is thus heated to a temperature from 500°C to 1500°C. Preferably to a temperature from 500°C to 1300°C. More preferably to a temperature from 600°C to 1300°C. The heated gas is then circulated into at least one radiant tube 10 that is located inside the furnace 5 and is connected to the device for heating gas 1 . The heated gas transfers its energy to the at least one radiant tube 10 and said at least one radiant tube 10 releases the transferred energy inside the furnace 5 to increase the temperature inside the furnace 5 and thus, heating the rolled steel strip 6.

[0040]

[0018] This method is a method of indirect heating as the heating medium, the gas, is not directly in contact with the rolled steel strip 6. This allows to avoid the rolled steel strip 6 to be damaged by reactions with the hot gas.

[0041]

[0019] Another advantage of the method is that since the gas circulating in the radiant tubes 10 is heated beforehand using induction heating, the distribution of temperature inside the radiant tube 10 is homogeneous, contrary to a classical use of radiant tubes 10 with a burner at the entrance of the tube where the temperature is very high at the entrance compared to the rest of the radiant tube 10. This allows to avoid localized creep and thus increases the life of the radiant tube 10.

[0042]

[0020] Preferably, the rolled steel strip 6 is heated to a temperature from 300°C to 1 100°C. Preferably to a temperature from 400°C to 1000°C. To heat the rolled steel strip 6 to a desired temperature, it is preferable to heat the gas to a temperature higher than that of the desired temperature to take into account the heat losses as not all energy from the gas is transferred to the rolled steel strip 6.

[0043]

[0021] Preferably, the method for heating a rolled steel strip according to the invention comprises an additional step of retrieving the heated gas exiting the at least one radiant tube 10 and using it again to heat the rolled steel strip 6.

[0044]

[0022] Preferably, the retrieved gas is introduced into a mixing station 8 that mixes it with the entry gas flow. The mix of entry gas flow and retrieved gas can then be introduced into the at least one device for heating gas 1 so that the retrieved gas is heated again together with the entry gas flow.

[0045]

[0023] This step is a recycling step that allows to reuse the gas after it has been used to heat the rolled steel strip 6. It allows to save gas and energy as the retrieved gas is hotter than the entry gas flow and thus the mix of retrieved gas and entry gas flow requires less energy to heat than a gas composed only of entry gas flow that is preferably at ambient temperature. This step thus allows to reduce even more the CO2 emissions.

[0046]

[0024] Alternatively, the mixing station 8 can be positioned after the device for heating gas 1 to mix the retrieved gas with the heated gas just before being injected into the radiant tube 10.

[0047]

[0025] Preferably, the driving speed of the rolled steel strip 6 inside of the furnace is from 10m / min to 600m / min. This range of speed allows to perform a various number of thermal treatments for most of steel grades.

[0048]

[0026] Preferably, the gas is circulating through several radiant tubes 10 arranged to heat the inner volume of the furnace 5. This allows the rolled steel strip 6 to be heated homogeneously.

[0049]

[0027] Preferably, at least one device for heating gas 1 is connected to each of the radiant tubes 10.

[0050]

[0028] Preferably, the furnace 5 comprises two or more sections inside of it, each section comprising at least one radiant tube 10. This allows the rolled steel strip 6 to be heated at different temperatures successively. It allows to perform some thermal treatments that needs successive heating at different temperatures.

[0051]

[0029] Preferably, the rolled steel strip 6 is used in a subsequent process after being heated. Preferably, said subsequent process is a step of galvanization.

[0052]

[0030] An embodiment of the invention is represented in Fig 4. In this embodiment, the rolled steel strip 6 is continuously driven inside a vertical furnace 5 thanks to rolls 9 that support the rolled steel strip 6. The entry gas flow goes to the mixing station 8 where it is mixed with the gas retrieved from the radiant tubes 10. The mix of gas is then distributed to several devices for heating gas 1 and after being heated and exiting the devices for heating gas 1 , the heated gas is released into the radiant tubes 10 through feeding pipes 7. The heated gas flows inside the radiant tubes 10 and transfers its energy to the tube 10. The radiant tubes 10 being located inside the furnace 5, their energy is released inside the furnace 5 by radiation. This results in an increase in temperature inside the furnace 5 that heats the rolled steel strip 6. The radiant tubes 10 are each connected to a single device for heating gas 1 . The radiant tubes 10 are arranged to heat the inner volume of the furnace 5. The gas is retrieved at the exit of each radiant tube 10 and is sent to the mixing station 8 to be mixed with the entry gas flow and heated again.

[0053]

[0031] In Fig 4, only a section of the furnace 5 is represented but it can have multiple sections, each one having several radiant tubes 10.

[0054]

[0032] The invention is not limited to the embodiment represented in Fig 4 and other configurations can be considered.

[0055]

[0033] The insert 3 is a piece that is located inside the pipe 2. Its purpose is to transfer energy to the gas flowing into the pipe 2. This transfer occurs as the gas flows into the pipe 2 and enters in contact with the exchange surface of the insert 3. The gas is thus heated by forced convection and / or radiation.

[0056]

[0034] For the structure of the insert 3, increasing the exchange surface will improve the efficiency of the heating as the insert 3 can transfer more energy to the gas over the length of the pipe 2. The exchange surface corresponds to the surface of the insert 3 that the gas can enter in contact with. The higher the surface exchange of the insert 3 is, the shorter the pipe 2 length required to reach a given target temperature is as well.

[0057]

[0035] For each meter of length of pipe 2, the insert 3 preferably has an exchange surface from 0.5 m2to 100 m2. For an exchange surface below 0.5 m2for each meter of length of pipe 2, the efficiency of the heating can be insufficient to reach the target temperature. For an exchange surface above 100 m2for each meter of pipe 2, the gas could reach the target temperature way before the end of the pipe 2 thus reducing the efficiency of the heating compared to the material used. Preferably, for each meter of length of pipe 2, the insert 3 has an exchange surface from 0.5 m2to 80 m2or from 0.5 m2 to 60 m2or from 0.5 m2to 50 m2or from 0.5 m2to 40 m2or from 0.5 m2to 30 m2or from 0.5 to 25 m2or from 0.5 m2to 20 m2or from 0.5 m2to 15 m2or from 0.5 m2to 10 m2or from 0.5 m2to 7.5 m2or from 0.5 m2to 5 m2or from 0.5 to 2.5 m2or from 1 m2to 100 m2or from 1 m2to 80 m2or from 1 m2to 60 m2or from 1 m2to 50 m2or from 1 m2to 40 m2or from 1 m2to 30 m2or from 1 to 25 m2or from 1 m2to 20 m2or from 1 m2to 15 m2or from 1 m2to 10 m2or from 1 m2to 7.5 m2or from 1 m2to 5 m2or from 1 m2to 2.5 m2.

[0058]

[0036] The efficiency of the heating can also be improved by imposing an agitation to the gas flow inside the pipe 2. This is provided by creating a turbulent flow inside the pipe 2.

[0059]

[0037] The flow patterns are usually characterized by the Reynolds number (Re). Usually, the Reynolds number is defined as:

[0060] puL

[0061] Re = - —

[0062] A

[0063] p: density of the fluid (kg / m3)

[0064] u: flow speed (m / s)

[0065] L: characteristic length (m)

[0066] p: dynamic viscosity (kg / (m.s))

[0067] A turbulent flow is characterized by a Reynolds number Re > 2800.

[0068]

[0038] The shape of the insert 3 has an influence on the Reynolds number as it modifies the characteristic length L of the system. A shape designed to provide turbulent flow to the gas flowing into the pipe 2 can thus improve the efficiency of the heating of the gas.

[0069]

[0039] To further improve the efficiency, the insert 3 preferably extends from the entrance of the pipe 2 to the exit of the pipe 2. The insert 3 also preferably has a section that represents from 50% to 100% of the free section inside the pipe 2, that is to say, the section where the gas is flowing. Having an insert 3 with a section that is almost the same as the free section of the pipe 2 allows to avoid that a large fraction of the gas flows around the insert 3 without being heated by the insert 3. However, having a smaller section allows the insert 3 to expand more freely with thermal dilatation without damaging the device for heating gas 1 . An insert 3 with a section of 100% of the free section of the pipe 2 can be considered with a material such as Invar™ that has a thermal expansion rate sufficiently low so as not to damage the pipe 2. With a section lower than 50% of the free section of the pipe 2, the efficiency of the heating could be too low. Preferably, the section of the insert 3 comparatively to the free section of the pipe 2 is from 50% to 97.5% or from 50% to 95% or from 50% to 92.5% or from 50 to 90% or from 50% to 85% or from 50% to 80% or from 50% to 75% or from 60% to 97.5% or from 60% to 95% or from 60% to 92.5% or from 60 to 90% or from 60% to 85% or from 60% to 80% or from 60% to 75% or from 65% to 97.5% or from 65% to 95% or from 65% to 92.5% or from 65 to 90% or from 65% to 85% or from 65% to 80% or from 65% to 75% or from 70% to 97.5% or from 70% to 95% or from 70% to 92.5% or from 70 to 90% or from 70% to 85% or from 70% to 80% or from 70% to 75%.

[0070]

[0040] Examples for insert designs are represented in Fig 3a and Fig 3b. The insert represented in Fig 3a has a screw shape and the insert represented in Fig 3b has a perforated twisted tape shape. These designs provide a high exchange surface comparatively to the length of the pipe 2 and can also provide a turbulent flow for a large range of gas speeds.

[0071]

[0041] The designs of the insert 3 are not limited to the ones represented in Fig 3a and Fig 3b and other designs can be considered.

[0072]

[0042] In order to be heated, the insert 3 has to be made of a material that is electrically conductive. The insert 3 is preferably made of magnetic steel or graphite but can also be made of any electrically conductive material. An electrically conductive material is any material that has an electrical conductivity value o > 10 000 (Q.m)-1.

[0073]

[0043] Magnetic steel can be used as a material for the insert 3 for heating non oxidizing gas to temperatures up to 1000°C, preferably up to 1 100°C without material degradation. For higher temperatures, it is preferable to use graphite as a material for the insert 2 as graphite can reach temperatures higher than 3000°C. Tungsten and molybdenum can also be considered as material for the insert 3 for high temperatures as they can resist to temperatures higher than 3000°C. For heating oxidizing gases, stainless steel can be used as the induced material.

[0044] The insert 3 can also be covered by a coating to make it more resistant to oxidation or creep and thus increasing its life.

[0074]

[0045] The inductor 4 is preferably a coil and is located in the vicinity of the pipe 2. Preferably, the inductor 4 is located inside the pipe 2 close to its wall as represented in Fig 2. This configuration allows to avoid heating the pipe 2 alongside the insert 3 if the pipe 2 is made of a material that is also electrically conductive. In this configuration, a refractory material is placed between the insert 3 and the inductor 4 to serve as a thermal insulator. A magnetic yoke is also preferably installed inside the pipe 2, between the inductor 4 and the wall of the pipe 2. It can be a plate of electric steel. Its purpose is to confine the magnetic field inside the pipe 2 so as not to heat the pipe 2, to increase the energy efficiency. Alternatively, the inductor 4 can be placed outside the pipe 2, as in Fig 1 , if the pipe 2 is made of a material that is not electrically conductive.

[0075]

[0046] A various number of gases can be heated by the device for heating gas 1 used in the method according to the invention. The following Table 1 gives a list of gases that can be heated using two specific materials for the insert 3.

[0076]

[0077] Table 1

[0047] The gases that are listed in Table 1 are only listed as examples and the gases that can be heated in the invention are not limited to these gases. In the same way, the materials that are listed in Table 1 are only listed as examples and the materials that are used for the insert in the invention are not limited to these materials.

[0078]

[0048] The pipe 2 preferably has a cylindrical shape. To reach a certain power, a minimal surface and / or mass of induced material is needed in the form of the insert 3. The shape of the insert 3 and the dimensions of the pipe 2 are arranged to provide optimal efficiency and compacity of the device for heating gas 1 depending on the power needed for the application it is used for. As the pipe 2 is not heated in the device for heating gas 1 according to the invention, the thickness of the pipe 2 does not have an influence on the efficiency of the heating.

[0079]

[0049] The global flow rates for the gas that are considered for the invention depend on the temperature that need to be attained for the rolled steel strip 6 and the gas used. The higher the temperature needed, the higher the flow rate needed. Moreover, the more thermal radiation a gas provides, the less flow rate needed to heat the rolled steel strip 6.

[0080]

[0050] In the applications of the method according to the invention, a given global gas flow rate of a given composition is heated to a target temperature. Depending on the value of this global gas flow rate and the target temperature, the required global heating power can range from 1 kW to 50MW. For low power values, for example in the order of 1 kW to 10kW, a single device for heating gas 1 would be sufficient to provide this level of power. For higher power range, for example in the order of 10MW to 50MW, depending on the technology available to the person skilled in the art, a single device for heating gas 1 can be insufficient to obtain this level of power. In this case, the global gas flow rate to be heated can be divided in several streams of lower flow rate that will each be heated by a single device for heating gas and require much lower power to reach the same target temperature.

[0081]

[0051] The method according to the invention allows to successfully provide a homogeneous heating of a rolled steel strip that is more energy efficient and that increases the life of the radiant tubes while reducing CO2 emissions.

Claims

CLAIMS1 ) A method for heating a rolled steel strip wherein said method comprises the following steps:- continuously driving a rolled steel strip (6) to be heated inside a furnace (5),- introducing an entry gas flow into at least one device for heating gas (1 ), said device for heating gas (1 ) comprising:o a pipe (2) allowing gas to flow inside of it,o an insert (3) located inside said pipe (2), said insert being electrically conductive,o an inductor (4) located in the vicinity of said pipe (2),- feeding energy into said insert (3) using said inductor (4) of said device for heating gas (1 ) so that said energy fed into the insert (3) is transferred to the gas introduced into said device for heating gas (1 ), to heat said gas to a temperature from 500°C to 1500°C,- circulating said heated gas into at least one radiant tube (10) located inside the furnace (5), so that said heated gas transfers heat to said at least one radiant tube (10), making said at least one radiant tube (10) release the transferred heat inside the furnace (5) to heat said rolled steel strip (6).2) A method for heating a rolled steel strip according to claim 1 , wherein said rolled steel strip (6) is heated to a temperature from 300°C to 1100°C.3) A method for heating a rolled steel strip according to claim 1 or 2, wherein said method comprises an additional step of retrieving said hot gas exiting said at least one radiant tube (10) to be used again to heat said rolled steel strip (6).4) A method for heating a rolled steel strip according to claim 3, wherein said retrieved gas is mixed with the entry gas flow before being heated again with the device for heating gas (1 ).5) A method for heating a rolled steel strip according to any of the preceding claims wherein the driving speed of the rolled steel strip (6) inside of the furnace (5) is from 10 m / min to 600 m / min.6) A method for heating a rolled steel strip according to any of the preceding claims wherein said heated gas is circulating through several radiant tubes (10) arranged to heat the inner volume of the furnace (5).7) A method for heating a rolled steel strip according to claim 6, wherein at least one device for heating gas (1 ) is connected to each of said radiant tubes (10).8) A method for heating a rolled steel strip according to any of the preceding claims, wherein said entry gas flow is composed of one or more of the following gases: syngas, CO2, CO, H2, H2O, hydrocarbons, N2.9) An installation for heating a rolled steel strip, wherein said installation comprises:- a furnace (5) allowing a rolled steel strip (6) to be driven inside of it, - at least one device for heating gas (1 ), said device for heating gas (1 ) comprising:o a pipe (2) allowing gas to flow inside of it,o an insert (3) located inside said pipe (1 ), said insert (3) being electrically conductive,o an inductor (4) located in the vicinity of said pipe (2) and able to feed energy into said insert (3), so that said energy fed into said insert (3) is transferred to the gas flowing into said pipe (2), to heat said gas is to a temperature from 500°C to 1500°C.- a means for introducing an entry gas flow into the device for heating gas (1 ),- at least one radiant tube (10), located inside the furnace (5) and connected to the device for heating gas (1 ), allowing the heated gas tocirculate inside of it and for releasing heat into the furnace (5) to heat said rolled steel strip (6).10) An installation for heating a rolled steel strip according to claim 9, wherein said installation further comprises a means for retrieving gas that exits the radiant tubes (10) to be used again to heat the rolled steel strip (6).1 1 ) An installation for heating a rolled steel strip according to claim 10, wherein said installation further comprises a mixing station (8) able to mix said retrieved gas with the entry gas flow before being introduced into said at least one device for heating gas (1 ).12) An installation for heating a rolled steel strip according to any of claim 9 to 1 1 , wherein said furnace (5) comprises two or more sections inside of it.13) An installation for heating a rolled steel strip according to any of claims 9 to 12, wherein said insert (3) is designed to have an exchange surface with the gas to be heated that ranges from 0.5m2to 100m2by meter of pipe (2).14) An installation for heating a rolled steel strip according to any of claims 9 to 13, wherein said insert (3) shape is designed to provide a turbulent flow to the gas flowing into the pipe (2).

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