Device for improving pre-oxidation in annealing furnace

Abstract

The invention relates to a method for improving the longitudinal uniformity of the temperature of a steel strip (100), in particular a low-carbon alloy steel strip (100) dedicated to a liquid metal coating, which continuously runs at a linear speed in a temperature homogenization section (12) provided with a resistor (2), said temperature homogenization section (12) being located before a post-treatment section (13) and after a furnace section (11), the furnace section (11) is capable of providing radiant heating to the strip (100) in order to bring the strip to a predetermined average temperature, and wherein the temperature homogenization section (12) is further provided with at least one contact roller (3) made of refractory stainless steel having a strip winding angle greater than 90 DEG, preferably greater than 270 DEG or greater, and wherein the temperature homogenization section (12) has a strip winding angle greater than 90 DEG, preferably greater than 270 DEG or greater. And wherein the total contact time and the total contact length of the belt with the at least one contact roller (3) are respectively selected to be greater than or equal to contact time 1 = (A * exp (B * RSTGDP)) * belt thickness, and contact length 1 = contact time 1 * linear velocity, where: RSTGDP = (1-initial roller temperature-actual belt temperature) * 100 initial roller temperature-initial belt temperature, contact length 1 in meters, and the actual belt temperature of the at least one contact roller (3) is greater than or equal to (A * exp (B * RSTGDP). The contact time 1 takes second as a unit, and the linear speed takes meter / second as a unit; and 0.34 lt; alt; 0.92 [mu] m and 0.035 [mu] m; blt; and 0.045% by weight.

Description

Technical Field

[0001] The present invention relates to a heating process in an annealing line for high-strength cold-rolled steel strip, in particular cold-rolled steel strip intended for the automotive industry. The invention also relates to an industrial installation for carrying out the heating process. Background Art

[0002] The heating process commonly used in annealing lines for low carbon tapes uses heat transfer by radiation. Heat transfer by radiation can be obtained by direct flame radiation in the enclosure in which the tape travels, as in the case of the so-called "direct fired furnaces" (DFF). Alternative technologies use electrical heating or radiation by tubes heated internally using flames (radiant tube furnaces or RTFs), the last being the most common due to the high heating power (in Watts per square meter) that can be delivered by the heated surface. If both methods are well known and commonly used in industry, the difference between the two methods lies entirely in the fact that in the case of direct flame heating the combustion gases are in contact with the processed tape, whereas in the alternative method the combustion gases are contained in the heating tubes and are therefore not in direct contact with the tape surface.

[0003] There are other differences, and these can be quickly summarized as follows:

[0004] - the surface facing the belt radiation is at a much higher temperature in the case of a direct fired furnace, typically between 1200° C. and 1450° C. in the case of a direct fired furnace, whereas when the radiant tube is made of refractory steel, this surface is limited to a maximum of 1150° C. or more commonly 950° C. due to creep and thermal expansion phenomena;

[0005] - In the case of radiant tubes, heat transfer is obtained exclusively by radiation, whereas in the case of direct combustion technology, about 10% to a maximum of 20% of the heat transfer is also obtained by convection.

[0006] It is well known that the total heat transfer power results from the sum of the convective heat transfer and the radiative heat transfer and that the radiative heat transfer power is proportional to the fourth power of the temperature difference in Kelvin between the hot and cold bodies. It can be shown that if the temperature difference between the hot and cold bodies is above about 500°C, the temperature of the cold body has no influence and therefore the radiative heat transfer is only proportional to the fourth power of the hot body temperature multiplied by the shape factor and the emissivity of the hot and cold surfaces and multiplied by the Boltzmann constant. Taking into account the classical values ​​of temperature and emissivity used industrially on cold rolled steel annealing cycles, the heating rate from room temperature to 650°C is in the range of 35°C / sec to 45°C / sec for a strip thickness of 1 mm in a direct fired furnace and only 10°C / sec to 15°C / sec in a radiant tube furnace.

[0007] In case of a steel surface with non-uniform emissivity, the heat transfer delivered per surface unit is not the same at each point: a high surface emissivity, a low surface emissivity will cause a higher heat input, a lower heat input respectively into the travelling strip. In case the strip emissivity varies, as is usually the case along and across the strip, non-uniform longitudinal and transverse temperatures can be obtained. The emissivity variations are due to variable cleanliness or oxidation of the surface. For example, it is common to observe variations between the head, tail and middle of a coil, but also between the edge and the centre of a coil. Many reasons in the upstream processes of flat mild steel may explain this fact, even if all of them have not yet been clearly identified. It is also important to note that, even if the strip has thermal conductivity, the thermal conductivity of the strip is significantly lower than that of, for example, copper or aluminium, and is too low to allow a degree of temperature homogenisation, due to the generally large distance between hot and cold parts.

[0008] For example, a 1 mm thick tape with a +10% emissivity variation (ie 0.35 over a 100 mm width of tape and maintained at 0.315 over all the rest of the tape) causes a temperature difference of about 35°C when the average temperature is 650°C.

[0009] It is also known that many measures are being taken today within the framework of industrial CO2 reduction. One of these measures involves increasing the strength of steels used, in particular, in the automotive industry. However, this increase in hardness requires the addition of some alloying elements in addition to carbon, most often Mn, Si, Cr, P and sometimes Nb, V and Ti, the total amount of alloying elements most often being less than 3%.

[0010] It is also known that during the heat treatment before coating by immersion in a liquid metal (like Zn or Al or a mixture thereof), aimed at improving corrosion resistance, the elements added for increasing strength diffuse into the strip surface and oxidize the strip surface. When the surface of the steel is covered with too much oxide, wetting of the surface with the coating liquid metal is strongly disturbed, and the coating causes surface defects and / or poor adhesion to the steel. As a solution to this problem, it is known to propose to perform a surface pre-oxidation of both Fe and alloying elements between 650° C. and 750° C. and preferably between 650° C. and 700° C. However, it is also known that if the oxidation kinetics are quite sensitive to the strip temperature, this is mostly the case in direct-fired furnaces due to the contact between the exhaust gas and the strip. Therefore, when entering the pre-oxidation process step as described in the applicant's previous patent EP 3 286 343 B1 (complementary to this application), the non-uniform strip temperature causes variable oxide thickness and morphology. This may further interfere with the subsequent reduction of the Fe oxides and therefore interfere with the appropriate surface preparation of the strip before entering the coating tank.

[0011] The inventors have demonstrated that the quality of the final product is improved when the oxidation is fairly uniform / homogeneous. Taking into account the above explanations, in practice this means that the strip temperature uniformity must be good enough, which means that the strip temperature is within + / -10°C, but more preferably within + / -5°C when the strip enters the pre-oxidation box.

[0012] A way to compensate for having a constant strip temperature at the exit of the temperature homogenization section when the strip emissivity varies along the strip can be to change the line speed. However, this is not always possible in practice, since temperature variations should be anticipated, e.g. by measuring the strip emissivity at the furnace entrance, which is not common practice.

[0013] Changing the firing rate of the furnace is another possibility, but practice has shown that compensation for local areas that are too cold or too hot causes problems on sections that otherwise have normal emissivity. This is well known to the operators. Without giving too much of an explanation here, this is because the reduction in the firing rate has an effect on the temperature of the furnace and the temperature of the furnace has a rather slow response. The problem is also (and this is well known to the engineers) that it is difficult to measure the real belt temperature with a non-contact system when the emissivity changes unexpectedly.

[0014] Document US2017 / 137906 A1 relates to cold-rolled and hot-dipped high-strength multiphase steels for motor vehicle use, which have high formability characteristics and exhibit high tolerance levels and are intended to be used as structural members and reinforcement materials mainly for motor vehicles. The cold-rolled sheet is heated in an atmosphere with an excess volume percentage of oxygen between 0.2% and 4% in the DFF. The excess volume of oxygen refers to the oxygen present in excess of the amount of oxygen required for the combination of the fuel used to heat the furnace, that is: excess volume percentage of oxygen = (total volume of oxygen-oxygen volume required for combustion) / (total volume of oxygen). Therefore, when excess oxygen is present in the combustion atmosphere in the proportion of the present disclosure, the excess oxygen can be used to react with the steel strip. In the range between 500°C and 750°C, oxidation occurs, i.e., an iron oxide layer is formed on the surface of the steel sheet, and internal oxidation occurs under this iron oxide: thus an internal oxide within a depth of 100 μm is produced, which may contain one or more of Si, Mn, Al, and Ti. If the oxidation depth is greater than 100 μm, the steel surface will be severely oxidized, which will be difficult to reduce, and the coating quality will deteriorate.

[0015] During the heating step, the annealing step and the cooling step, the steel is oxidized and then reduced, i.e. the iron oxide layer at the surface of the steel sheet is completely reduced, while there is an inner oxidation zone with a depth between 200 nm and 100 μm, which contains one or more oxides containing Si, Mn, Al, Ti. This oxidation followed by a reduction step is necessary to make the steel surface suitable for hot dip coating.

[0016] In document EP 3 686 534 B1, the aim is to provide a process for the heat treatment of a high-strength steel strip which makes it possible to obtain on its surface an oxide formation of a more uniform and controlled thickness than in the prior art.

[0017] To this end, the inventors have proposed a process for heat treating a moving high-strength steel strip, the process comprising: a step of performing strip temperature homogenization in a homogenization chamber including at least one radiant heating tube under an oxygen-free atmosphere so as to homogenize the temperature of the strip after the strip has passed through the direct flame heating zone of the previous step; and a step of further performing strip reduction in a reduction zone before the step of oxidizing the strip in an oxidation chamber with an oxidizing atmosphere having an oxygen volume concentration greater than 1%.

[0018] It is known that the use of a direct-fired heating zone allows the temperature of the strip to increase rapidly, thereby compromising the temperature uniformity of the metal product. As in a large number of furnaces, the oxidation chamber is located directly after the direct-fired heating zone, oxidation is performed on a strip where the temperature uniformity is not well controlled.

[0019] The kinetics of the formation of the oxide layer on the surface of the high-strength steel strip depends primarily on the surface temperature of the strip, the composition of the steel and, of course, the time and composition of the oxidizing atmosphere in the oxidation chamber. The oxidation time is defined by the line speed and the length of the section. Good control of the temperature of the strip during its oxidation in the oxidation chamber makes it possible to obtain a surface oxide layer having a more uniform thickness over the entire surface of the strip.

[0020] However, with respect to the desired temperature non-uniformity target (e.g., between 1% and 5%) and belt speed, this patent does not teach the length of the uniformization chamber, nor the time the belt spends in the chamber, making the teaching less practical for technicians to use.

[0021] Purpose of the Invention

[0022] The object of the present invention is to provide a solution aimed at overcoming the disadvantages of the prior art.

[0023] In particular, the invention aims at improving the longitudinal and transverse temperature uniformity of the tape after the step of heating the tape in a DFF or RTF furnace and before the pre-oxidation step, the inhomogeneities being mainly due to local variations in the tape emissivity.

[0024] Furthermore, it is an object of the present invention to provide guidance on designing the required length of the homogenization zone and the residence time of the strip in the homogenization zone in order to obtain a predetermined temperature uniformity target at the outlet of the zone. Summary of the invention

[0025] A first aspect of the present invention relates to a method for improving the longitudinal temperature uniformity of a steel strip, in particular a low-carbon alloy steel strip dedicated to liquid metal coating, the steel strip continuously traveling at a linear speed in a temperature equalization section provided with resistors, the temperature equalization section being located before a key process such as a post-treatment section and after a furnace section, the furnace section being capable of providing radiant heating to the strip to bring the strip to a predetermined average temperature, wherein the temperature equalization section is further provided with at least one contact roll made of refractory stainless steel, the at least one contact roll having a strip winding angle greater than 90°, preferably 270° or greater, and wherein the total contact time and total contact length of the strip with the at least one contact roll are respectively selected to be greater than or equal to

[0026] Contact time 1 = (A * exp(B * RSTGDP)) * strip thickness

[0027] And

[0028] Contact length 1 = Contact time 1 * linear speed

[0029] Where:

[0030]

[0031] - Contact length 1 is in meters, contact time 1 is in seconds, and linear speed is in meters per second; and

[0032] - 0.34 < A < 0.92 and 0.035 < B < 0.045 (note that the A and B coefficients are only for refractory stainless steel rolls),

[0033] The contact time 1 is limited to the value necessary to obtain a predetermined upper limit value of RSTGDP, and the initial roll temperature, the initial strip temperature in the temperature equalization section, and the actual strip temperature are measured by temperature measuring devices.

[0034] According to a preferred embodiment, the method is further limited by at least one of the following characteristics or a suitable combination of these characteristics:

[0035] - Contact time 1 adjusts or limits the total contact time to reduce the longitudinal temperature variation of the strip by 25% and 50% respectively;

[0036] - The temperature of the temperature equalization section provided with rolls is controlled by setting the temperature of these resistors to an average value to obtain the temperature value required for the strip to enter the pre-oxidation section, and the resistors have the function of compensating for heat loss;

[0037] - The temperature equalization section provided with rolls is maintained in an atmosphere with less than 0.5% O2;

[0038] - Maintain the temperature equalization section provided with rollers in an atmosphere having a dew point between -60 °C and 0 °C, preferably between -30 °C and -10 °C;

[0039] - The method also aims to improve the transverse temperature uniformity of the strip, wherein, in the temperature equalization section or in another section, the resistors distributed on the walls of the section heat the section over a length given by the following equation, which depends on the percentage reduction of the transverse strip temperature gradient or STGDP:

[0040] Contact time 2 = A * exp(B * STGDP)

[0041] And

[0042] Contact length 2 = Contact time 2 * Linear velocity

[0043] Where:

[0044]

[0045] - Contact length 2 is in meters, contact time 2 is in seconds, and linear velocity is in meters per second;

[0046] - 2.8 < A < 3.3 and 0.03 < B < 0.04 (note that the A and B coefficients are only for refractory stainless steel rollers);

[0047] - The method is operated with a low-carbon alloy steel strip that is dedicated to being coated with a mixture of liquid Zn and Al, possibly coated with Si, Fe, and inevitable impurities;

[0048] - The method is operated with a pre-oxidation section or an immersion section as a post-treatment section;

[0049] - The method is operated with a direct flame furnace, a radiant tube furnace, or an electric induction furnace as the furnace section;

[0050] - The method is operated with the furnace section maintained in an atmosphere containing less than 1% O2 and one or more components selected from the group consisting of H2, CO, H2O, CO 2、 N2 and mixtures thereof, with the proportion of each of these components depending on the process parameter settings;

[0051] - The strip tension is advantageously between 0.3 kg / mm 2 and 1.5 kg / mm 2 and preferably between 0.5 kg / mm 2 and 1 kg / mm 2 ;

[0052] The temperature difference between the belt entering the temperature homogenization zone and the at least one contact roller is below 100°C, preferably below 50°C.

[0053] Another aspect of the invention relates to an industrial installation for carrying out a method for improving the longitudinal and transverse temperature homogeneity of a steel strip, preferably a low-carbon alloy steel strip dedicated to liquid metal coating, the steel strip running continuously at a linear speed in a temperature homogenization section, the temperature homogenization section comprising a tunnel or a chamber provided with electrical resistances, the strip having a given thickness and tension as described above, wherein the installation comprises in turn:

[0054] - direct flame, radiant tube or induction furnace sections;

[0055] - the temperature homogenization section is further provided with at least one contact roller with a given diameter, depending on the roller diameter / belt thickness ratio and the belt tension, the belt wrapping angle provided by the at least one contact roller is greater than 90°, preferably greater than 270° or more, and the at least one contact roller is capable of providing a total contact time and a total contact length of the belt with the at least one contact roller that meet the above specified conditions, respectively, the contact time depending on the roller-belt temperature gradient reduction percentage;

[0056] - Pre-oxidation section;

[0057] -Deceleration section;

[0058] Therein, these resistors are distributed on the wall of the temperature homogenization section or another section for heating the temperature homogenization section optionally for the time and length given above, the time depending on the percentage of the percentage reduction of the transverse zone temperature gradient.

[0059] According to a preferred embodiment, the facility is further limited by at least one of the following characteristics or a suitable combination of these characteristics:

[0060] - the length of the temperature homogenization section or of another section is selected as a function of the line speed and for a given contact time, the contact time being adjusted or limited so as to reduce the transverse temperature gradient by at least 50%, the transverse temperature gradient being represented by the temperature difference divided by the distance;

[0061] - the length of the temperature homogenization zone or of the further zone is selected according to the target line speed and is suitable to give a residence time of less than 12 seconds in order to reduce the transverse temperature gradient by at least 25%;

[0062] - the temperature homogenization section is lined with refractory material, the resistors being located on the four walls of the section, preferably on the two sides facing the travelling belt in use, or positioned so as to minimize the time and length to reach the target temperature homogenization;

[0063] - the tunnel or chamber located in front of the pre-oxidation section can be arranged horizontally or vertically, but preferably vertically to minimize the influence of the earth's gravity;

[0064] The diameter of the contact roller used as thermal capacity storage is greater than 600 mm, but preferably between 800 mm and 1200 mm, and the thickness of the shell is between 10 mm and 60 mm, but preferably between 20 mm and 40 mm;

[0065] The number of contact rollers at which the belt wrap angle is greater than 90°, preferably greater than 270° or more, is between 1 and 6. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] Figure 1 Represents different heat treatments in an annealing line for preparing high-strength cold rolled steel strip required for metal liquid coating, comprising a temperature homogenization section according to the prior art in which rollers made of refractory stainless steel with a thickness between 20 mm and 30 mm or 40 mm are arranged.

[0067] Figure 2 An example of the evolution of the transverse band temperature gradient over time is shown.

[0068] FIG. 3A to FIG. 3D Different configurations of the temperature homogenization section are shown, the main difference being the number and location of the resistors ( Figure 3A and Figure 3B , horizontal homogenization; Figure 3C and Figure 3D , longitudinal homogenization) and the belt wrap angle of the contact roller and / or roller ( Figure 3C and Figure 3D ).

[0069] Figure 4 The time evolution of the roller / belt temperature gradient is shown for a roller thickness of 25 mm.

[0070] Figure 5 Shown are the corresponding variations in strip temperature and roll surface temperature as a function of time spent in the dedicated homogenization section, considering rolls initially at 650°C and strips entering the section at 600°C (1 mm strip, rolls made of refractory steel and with 25 mm shell thickness).

[0071] Figure 2 , Figure 4 and Figure 5 The results presented in are obtained by thermal simulations based on a simple model of heat transfer that relies on an energy balance with slices and considers radiation, conduction and convection heat transfer modes. The equations of the model are Figure 6 (Source: M. Dubois 1996). DETAILED DESCRIPTION

[0072] When aiming to compensate for emissivity variations in order to obtain a uniform or uniform strip temperature, two situations must be considered.

[0073] First, it is difficult to obtain uniform strip lateral temperature when the surface emissivity varies and when the heating rate is high because heat conduction does not have time to diffuse and build up.

[0074] The inventors have found that in order to improve the lateral temperature uniformity, not only a certain time is required, but also the section where this leveling takes place must also have a uniform temperature equal to the target strip temperature. This is to minimize possible radiant heating differences due to the need to install heating elements to compensate for heat losses. The inventors have also found in this context that the use of a radiant tube furnace (RTF) is not a good solution, due to the small radiant surface compared to the entire surface of the chamber. The situation is even worse with gas flame heated radiant tubes, since it is known that even the temperature of the tube itself is non-uniform and depends on the burning rate due to the flame development in the tube. Non-uniform tube temperatures can be very detrimental to the goal, since some of the radiant heating can compete with the thermal conduction process used to improve the temperature uniformity of the strip.

[0075] The inventors finally found that in order to improve the CO2 efficiency of the overall process, electric heating should be preferred over gas heating.

[0076] Secondly, the inventors have found that the most effective way to improve longitudinal temperature uniformity when it has a limited length (such as the case for the head and tail of the roll) is to use the thermal capacity of the rollers around which the belt is wrapped. This is based on the fact that it is well known that when the belt has a constant temperature, the contact rollers have the same temperature over the contact area and throughout the shell thickness.

[0077] When a belt section with a different temperature reaches the roller, a heat exchange takes place: the heat capacity of the roller is transferred to the belt through contact. Of course, this dynamic effect cannot last too long, because the roller temperature asymptotically tends to the belt temperature.

[0078] It is also clear that the heat transfer exchange depends on the quality of the belt / roller contact and takes a certain time.The inventors have found the best compromise between the reduction of undesired temperature variations and the number of rollers required.

[0079] In order to compensate for the strip heat losses of the segments, electrical resistances are implemented and the temperature is controlled.

[0080] Description of the Preferred Embodiments of the Invention

[0081] Transverse temperature uniformity

[0082] According to a preferred embodiment of the invention, the low carbon strip is subjected to a heat treatment in a heating facility 1 which consists in heating the strip 100 by radiation to a temperature between 650° C. and 750° C. and preferably between 650° C. and 700° C. in a direct fired furnace or a fully radiant tube furnace 11, followed by a dedicated temperature homogenization section 12 ( Figure 1 ). The strip temperature is measured at the end of the temperature heating section 11, typically at its center. The line speed is adjusted in such a way that the target temperature is reached, preferably between 650°C and 700°C.

[0083] In the dedicated homogenization section 12, conduction across the strip width may occur due to the time spent in the chamber, and heating by radiation and / or convection should be minimized. The design of the chamber includes a refractory lining intended to minimize heat losses.

[0084] According to the present disclosure, controlled electric heating is implemented, possibly on the four walls of the section, but preferably on both sides facing the belt (not shown). The electric heating consists only of the electrical resistances 2 (preferably uniformly facing the belt), without the inclusion of the electric radiant tubes. It is therefore intended to provide a target so-called "uniform" wall temperature in the section 12, which means that the temperature variation of the electrical resistances is limited to a temperature lower than about 20° C. The temperature of the electrical resistances is ensured by measuring means contacting these heating elements at different points, but preferably on each side of the section and preferably at those points facing the belt alone. The power of the different electrical resistance panels will advantageously be controlled separately so as to obtain a target temperature preferably between 650° C. and 700° C.

[0085] Since the reduction of the temperature gradient requires a certain time, the inventors have found that this time actually depends on the reduction from the initial temperature gradient to the desired temperature gradient. In the following discussion, we define the belt temperature gradient reduction percentage (STGDP) as follows:

[0086]

[0087] For example, if the initial temperature difference is 20°C across a length of 100mm across the strip, the gradient is 200°C / m. A 50% gradient reduction means the new gradient is 100°C / m, so the temperature difference is only 10°C over 100mm.

[0088] based on Figure 6 The thermal simulation performed for the model includes radiation, convection, and conduction with the following data and / or assumptions:

[0089] - FFD heating of the strip by radiation with a zone temperature of 1250° C. (furnace wall);

[0090] - Over most of the belt surface, the total emissivity of the belt is 0.315. At the center, a 50 mm wide zone has an emissivity 10% higher according to the emissivity variability hypothesis, i.e., 0.35;

[0091] - The goal is to obtain a uniform homogenization zone temperature of 650 °C, i.e., a value slightly higher than the belt temperature when the belt exits the FFD (to take advantage of the radiation effect, considering that the emissivity is now uniform). There is no forced convection in this zone;

[0092] - Thermal conductivity λ 钢 (Tc) = 73.29 - 0.0564(Tc + 273) for a 1 mm thick steel strip, where Tc is the temperature in °C and λ steel is the thermal conductivity in W / m (based on the average estimate of several steels; data from M. Dubois). This thermal conductivity value can even be considered high when compared to alloy steels with usually even lower thermal conductivities.

[0093] Figure 2 Shows the evolution of the temperature gradient over time for the case where the initial temperature difference across the belt width (100 mm) is 32 °C. In this case, the time on the abscissa starts when the belt enters the furnace, but the graph only shows the time when the belt enters the homogenization section of the present invention. As a reference, the dashed line refers to a 50% reduction in the temperature gradient (which also corresponds to 50% of the STDGP). Therefore, it takes 20 seconds to reduce by 50%.

[0094] The inventors have found that the resistance 2 distributed on the walls of the section heats the section over a length given by the following equation, which depends on the percentage reduction in the transverse belt temperature gradient or STGDP:

[0095] Time = A * exp(B * STGDP)

[0096] And

[0097] Length = Time * Linear velocity

[0098] Where:

[0099]

[0100] - The length is in meters, the time is in seconds, and the linear velocity is in meters per second;

[0101] - 2.8 < A < 3.3 and 0.03 < B < 0.04.

[0102] It should be noted that there are no rollers in this configuration here.

[0103] Longitudinal temperature uniformity

[0104] According to another preferred embodiment of the invention, the low carbon belt undergoes a heat treatment consisting in heating the belt 100 by radiation to a temperature between 650° C. and 750° C. and preferably between 650° C. and 700° C. in a direct fired furnace or a fully radiant tube furnace 11, then passing the belt in a dedicated temperature homogenization section 12 comprising at least one contact roller 3 and preferably more contact rollers 3, so as to provide sufficient contact (time) to allow a significant heat exchange between the roller 3 and the belt 100. Different configurations of the section 12 are described in FIG. 3A to FIG. 3D , the main differences are the number and position of contact rollers 3 and / or the belt wrapping angle of rollers 3 .

[0105] At least one contact roller 3 is preferably made of a material having a thickness such as to provide sufficient heat capacity (Cp), such as heat-resistant steel or alternatively a high heat capacity material such as carbon. The shell thickness of each roller 3 is precisely between 10 mm and 60 mm, preferably between 20 mm and 40 mm, to provide sufficient heat capacity.

[0106] The belt tension is controlled at 0.4kg / mm 2 Up to 2kg / mm 2 , preferably between 0.8kg / mm 2 With 1.2kg / mm 2 The range between 0.04 and 0.10 is to ensure good contact without causing excessive plastic deformation of the strip, which is detrimental to the final flatness. The shape of the roller 3 is preferably cylindrical, which means no special crown, but a light crown is acceptable.

[0107] The diameter of the rollers is between 600mm and 1500mm, preferably between 1000mm and 1200mm, to ensure a reasonable contact length of each roller without providing specific layout problems. The rollers are made of refractory stainless steel, with a sleeve thickness between 20mm and 40mm, but preferably between 25mm and 30mm.

[0108] The section 12 is also provided with a plurality of temperature-controlled electrical heating devices 2 , preferably electrical resistors.

[0109] The incoming strip temperature 100 is measured at the end of the temperature heating section 11. The strip temperature at the exit of the series of rollers of the homogenization section 12 is also measured (not shown).

[0110] Since both the reduction and increase of the strip temperature require a certain amount of time, the inventors have found that the required time depends on the desired reduction in temperature change. In the following discussion, we define the percentage reduction in the absolute value of the temperature gradient roll / strip (roller / strip temperature gradient reduction percentage or RSTGDP) in the following way:

[0111]

[0112] Figure 4 Shows the time evolution of the roll / band temperature gradient.

[0113] Figure 5 Shows the corresponding variations of the band temperature and the roll surface temperature with the time spent in the dedicated homogenization section 12, considering an initial roll at 650 °C and a band entering the section at 600 °C (1 mm band, made of refractory steel and having a shell thickness of 25 mm for the roll).

[0114] The number of rolls required and thus the total winding contact according to the invention will depend on the roll size, the non - homogenization reduction target, the line speed and the band thickness.

[0115] The inventors have found that the total contact time and the total contact length of the band with at least the contact roll (3) are respectively selected to be greater than or equal to

[0116] Time = (A * exp(B * RSTGDP)) * band thickness

[0117] And

[0118] Length = Time * line speed

[0119] Where:

[0120]

[0121] - The length is in meters, the time is in seconds, and the line speed is in meters per second; and

[0122] - 0.34 < A < 0.92 and 0.035 < B < 0.045.

[0123] Roughly, the required roll / band contact time (in seconds) can be approximated as

[0124] A’ (seconds / mm) * band thickness (mm)

[0125] Where the value of A’ is as follows:

[0126] - For a 25% temperature gradient reduction percentage (RSTGPD): 1 < A’ < 3;

[0127] - For a 50% temperature gradient reduction percentage (RSTGPD): 3 < A’ < 8;

[0128] - For a 75% temperature gradient reduction percentage (RSTGPD): 8 < A’ < 12.

[0129] Ph. Paulus et al., "Study of the heat buckling and shape problems in continuous heat treating lines and discussion of proposed solutions" (AIME Metallurgical Association, Allegheny County, Pennsylvania, 1985, pp. 419-439) teach that the temperature difference between the roller and the belt should not be too large to avoid belt deformation (buckling). Typically, the initial temperature difference between the roller and the belt should not be greater than 100°C and preferably should not be greater than 50°C.

[0130] In practice, this means that if a 1 mm thick strip is running at 120 mpm (2 mps) and reaches a 650° C. roll at 600° C., the total contact time with the roll required to ensure that the strip ends up at 625° C. (50% gradient reduction) should be at least between 3 and 8 seconds. Taking into account the line speed and the winding angle, the roll diameter, and therefore the number of rolls required, is easily calculated by the skilled person.

[0131] To ensure good contact between the roller and the belt, the belt tension is advantageously between 0.3 kg / mm 2 With 1.5kg / mm 2 and preferably between 0.5 kg / mm 2 With 1kg / mm 2 between.

[0132] List of Reference Numerals

[0133] 1 Heating facilities

[0134] 2 Resistors

[0135] 3 Contact roller

[0136] 11DFF or RTF section

[0137] 12 Homogenization section

[0138] 13 Pre-oxidation section

[0139] 14 Restore Section

[0140] 100 Steel Strip.

Claims

1. A method for improving the longitudinal uniformity of the temperature of a steel strip (100), in particular a low-carbon alloy steel strip (100) specially used for liquid metal coating, the steel strip continuously running at a linear speed in a temperature homogenization section (12) provided with an electric resistor (2), the temperature homogenization section (12) being located before a post-treatment section (13) and after a furnace section (11), the furnace section (11) being capable of providing radiation heating or electric heating to the strip (100) so that the strip reaches a predetermined average temperature, wherein: The temperature equalization section (12) is further provided with at least one contact roller (3) made of refractory stainless steel, and the at least one contact roller has a tape winding angle greater than 90°, preferably greater than 270° or more, and wherein the total contact time and total contact length of the tape with the at least one contact roller (3) are respectively selected to be greater than or equal to Contact time 1 = (A * exp(B * RSTGDP)) * tape thickness and Contact length 1 = Contact time 1 * linear velocity where: - - Contact length 1 is in meters, Contact time 1 is in seconds, and linear velocity is in meters per second; and - 0.34 < A < 0.92 and 0.035 < B < 0.045, The Contact time 1 is limited to the value necessary to reach a predetermined upper limit value of RSTGDP, and the initial roller temperature, the initial tape temperature in the temperature equalization section (12), and the actual tape temperature are measured by a temperature measuring device.

2. The method according to claim 1, wherein: Contact time 1 adjusts or limits the total contact time so that the longitudinal temperature variation of the tape is reduced by 25% and 50% respectively.

3. The method according to claim 1, wherein: The temperature of the temperature equalization section (12) provided with the roller (3) is controlled by setting the temperature of these resistors (2) to an average value to obtain the temperature value required for the tape (100) to enter the pre-oxidation section (13), and the resistors have the function of compensating for heat loss.

4. The method according to claim 1, wherein: The temperature equalization section (12) provided with the roller (3) is maintained in an atmosphere of less than 0.5% O2.

5. The method according to claim 1, wherein: The temperature equalization section (12) provided with the roller (3) is maintained in an atmosphere having a dew point between -60 °C and 0 °C, preferably between -30 °C and -10 °C.

6. The method according to claim 1, further used to improve the lateral uniformity of the temperature of the belt (100), wherein: In the temperature equalization section (12) or in another section, the resistors (2) distributed on the wall of the section heat the section over the length given by the following equation, which depends on the percentage reduction in the transverse tape temperature gradient or STGDP: Contact time 2 = A * exp(B * RSTGDP) and Contact length 2 = Contact time 2 * linear velocity where: - - Contact length 2 is in meters, Contact time 2 is in seconds, and linear velocity is in meters per second; - 2.8 < A < 3.3 and 0.03 < B < 0.

04.

7. The method according to claim 1, wherein: The method operates with a pre-oxidation section or an immersion section as the post-treatment section (13).

8. The method according to claim 1, wherein: The method operates with a direct flame furnace, a radiant tube furnace or an induction furnace as the furnace section (11).

9. The method according to claim 8, wherein: The method operates with the furnace section (11) maintained in an atmosphere containing less than 1% O2 and one or more components selected from the group consisting of H2, CO, H2O, CO2, N2 and mixtures thereof, and the respective proportions of these components depend on the process parameter settings.

10. The method according to claim 1, wherein: The belt tension is advantageously between 0.3 kg / mm 2 With 1.5kg / mm 2 Between and preferably between 0.5kg / mm 2 With 1kg / mm 2 between.

11. The method according to claim 1, wherein: The temperature difference between the tape entering the temperature equalization section (12) and the at least one contact roller (3) is less than 100 °C, preferably less than 50 °C.

12. An industrial installation for carrying out the method for improving the longitudinal and transverse temperature uniformity of a steel strip according to any one of claims 1 to 11, the steel strip preferably being a low-carbon alloy steel strip dedicated for liquid metal coating, the steel strip running continuously at a linear speed in a temperature homogenization section (12), the temperature homogenization section comprising a tunnel or a chamber provided with an electric resistance (2), the strip having a given thickness and tension, wherein: The facility sequentially includes: - A direct flame, radiant tube furnace or induction furnace section (11); - the temperature homogenization section (12), the temperature homogenization section is further provided with at least one contact roller (3) having a given diameter, depending on the roller diameter / belt thickness ratio and the belt tension, the belt winding angle provided by the at least one contact roller is greater than 90°, preferably greater than 270° or more, and the at least one contact roller is capable of providing a total contact time and a total contact length of the belt with the at least one contact roller that meet the conditions specified in claim 1, respectively, the contact time being dependent on the roller-belt temperature gradient reduction percentage RSTGDP; - pre-oxidation section (13); - Reduction section (14); The resistors (2) are distributed on the wall of the temperature homogenization section (12) or another section for heating the temperature homogenization section (12) for a time and length given in claim 6, the time depending on the percentage of the reduction percentage of the transverse zone temperature gradient.

13. The facility of claim 12, wherein: The length of the temperature homogenization section (12) or the further section is selected according to the line speed and for a given contact time which is adjusted or limited to reduce the transverse temperature gradient represented by the temperature difference divided by the distance by at least 50%.

14. The facility of claim 12, wherein: The length of the temperature homogenization zone (12) or the further zone is selected according to the target line speed and is adapted to give a residence time shorter than 12 seconds in order to reduce the transverse temperature gradient by at least 25%.

15. The facility of claim 12, wherein: The temperature homogenization section (12) is lined with refractory material, the resistors being located on the four walls of the section, preferably on the two sides facing the travelling belt in use, or positioned so as to minimize the time and length to reach the target temperature homogenization.

16. The facility of claim 13, wherein: The tunnel or chamber preceding the pre-oxidation section (13) can be arranged horizontally or vertically, but is preferably arranged vertically to minimize the effect of the earth's gravity.

17. The facility of claim 12, wherein: The contact roller (3) used as thermal capacity storage has a diameter greater than 600 mm, but preferably between 800 mm and 1200 mm, and a shell thickness between 10 mm and 60 mm, but preferably between 20 mm and 40 mm.

18. The facility of claim 12, wherein: The number of contact rollers (3) with a belt wrap angle greater than 90°, preferably greater than 270° or more, is between 1 and 6.

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