CN119604735A

CN119604735A - Device for improving pre-oxidation in annealing furnaces

Info

Publication number: CN119604735A
Application number: CN202380050647.2A
Authority: CN
Inventors: 米歇尔·杜波依斯
Original assignee: John Cockerell Ag
Current assignee: John Cockerell Ag
Priority date: 2022-07-05
Filing date: 2023-06-23
Publication date: 2025-03-11
Also published as: CA3261033A1; EP4303516A1; WO2024008480A1; EP4551887A1

Abstract

Method for improving the temperature longitudinal uniformity of a steel strip (100), in particular a low carbon alloy steel strip (100) dedicated to liquid metal coating, which continuously travels at a linear speed in a temperature homogenizing zone (12) provided with an electric resistance (2), which temperature homogenizing zone (12) is located before a post-treatment zone (13) and after a furnace zone (11), which furnace zone (11) is capable of providing radiant heating to the strip (100) for bringing the strip to a predetermined average temperature, wherein the temperature homogenizing zone (12) is further provided with at least one contact roller (3) made of refractory stainless steel, which at least one contact roller has a strip winding angle of more than 90 °, preferably more than 270 ° or more, and wherein the total contact time and the total contact length of the strip with the at least one contact roller (3) are selected to be greater than or equal to the contact time 1= (a×exp (b× RSTGDP)), and the contact length 1=contact time 1× RSTGDP ×1×1×0.035 initial temperature 1×0.0.035 initial temperature 1×0.0.0 second.

Description

Device for improving pre-oxidation in annealing furnaces

Technical Field

The present invention relates to a heating process for use in the annealing line of high-strength cold-rolled steel strips, in particular cold-rolled steel strips intended for the automotive industry. The invention also relates to an industrial installation for performing the heating process.

Background

The heating process typically used in annealing lines for low carbon strips uses heat transfer by radiation. Heat transfer by radiation can be obtained by direct flame radiation in a housing travelling with a belt, as is the case in a so-called "direct combustion furnace" (DFF). Alternative techniques use electrical heating or the radiation of tubes heated internally by using flames (radiant tube furnace or RTF), the last method being the most common due to the high heating power (in watts per square meter) that the heated surface can deliver. If both methods are well known and commonly used in industry, they differ entirely by the fact that in the case of direct flame heating the combustion gas is in contact with the processed belt, whereas in the alternative the combustion gas is contained in a heating tube and is thus not in direct contact with the belt surface.

Other differences exist and these differences can be summarized rapidly as follows:

The surface facing the radiation is at a much higher temperature in the case of a direct burner, typically between 1200 ℃ and 1450 ℃ in the case of a direct burner, whereas when the radiant tube is made of refractory steel, the surface is limited to a maximum of 1150 ℃ or more commonly 950 ℃ due to creep and thermal expansion phenomena;

in the case of radiant tubes, heat transfer is obtained by radiation only, whereas in the case of direct combustion technology, heat transfer of about 10% to a maximum of 20% is also obtained by convection.

It is well known that the total heat transfer power is produced by the sum of convective heat transfer and radiant heat transfer, and that radiant heat transfer power is proportional to the temperature difference in kelvin between the hot and cold bodies to the power of 4. It can be shown that if the temperature difference between the hot and cold bodies is higher than about 500 ℃, the temperature of the cold body has no effect, and thus the radiant heat transfer is only proportional to the 4 th 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. Considering classical values of temperature and emissivity for industrial use on cold rolled steel annealing cycles, the heating rate from room temperature to 650 ℃ is in the range of 35 ℃ to 45 ℃ per second for 1mm strip thickness in a direct fired furnace and is only 10 ℃ per second to 15 ℃ per second in a radiant tube furnace.

In the case of a steel surface with non-uniform emissivity, the heat transfer delivered per surface unit is different at each point-a high surface emissivity, a low surface emissivity will respectively cause a higher heat input, a lower heat input into the travelling belt. In the case of a variation in the belt emissivity as is typically the case along and across the belt, non-uniform longitudinal and transverse temperatures may be obtained. Emissivity variation is due to variable cleanliness or oxidation of the surface. For example, it is common to observe the variation between the head, tail and middle of a roll, but also between the edges and center of the roll. Many reasons in the upstream process of flat mild steel can explain this fact even though all of these reasons have not been clearly identified. It is also important to note that even if the tape has thermal conductivity, the thermal conductivity of the tape is significantly lower than that of one of copper or aluminum, for example, and because the distance between the hot and cold components is typically large, the thermal conductivity of the tape is too low to allow a degree of temperature homogenization.

For example, a 1mm thick strip with +10% emissivity change (i.e., 0.35 on a 100mm wide strip, while remaining at 0.315 for all the remainder of the strip) causes a temperature difference of about 35 ℃ when the average temperature is 650 ℃.

It is also known that many measures are taken today within the framework of industrial CO ₂ emissions reduction. One of these measures involves an increase in the strength of steel 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 being most often below 3%.

It is also well known that during the heat treatment prior to coating by immersion in a liquid metal like Zn or Al or mixtures thereof, the elements added for increasing strength are intended to diffuse to and oxidize the belt surface in order to improve corrosion resistance. When the surface of the steel is covered with too much oxide, wetting the surface with the coated 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 perform surface pre-oxidation of both Fe and alloying elements between 650 ℃ and 750 ℃ and preferably between 650 ℃ and 700 ℃. However, it is also known that if the oxidation kinetics are quite sensitive to the belt temperature, this is mostly the case in direct fired furnaces due to the contact between the exhaust gas and the belt. Thus, when specifically entering a pre-oxidation process step as described in the inventors' previous patent EP 3 286 343 B1 (to which the present application is complementary), the non-uniform strip temperature causes variable oxide thickness and morphology. This may further interfere with the subsequent reduction of the Fe oxide and thus with the proper surface preparation of the strip before entering the coating tank.

The inventors have demonstrated that the quality of the final product is improved when the oxidation is fairly uniform. In view of the above explanation, it is meant in practice that the belt temperature uniformity must be good enough, meaning that the belt temperature is within +/-10 ℃, but more preferably within +/-5 ℃, as the belt enters the pre-oxidation tank.

The way to compensate for having a constant band temperature at the outlet of the temperature equalization section may be to vary the linear velocity as the band emissivity varies along the band. However, this is not always practical in practice, since temperature variations should be predicted, for example by measuring the belt emissivity at the furnace inlet, which is not a common practice.

Changing the firing rate of the furnace is another possibility, but practice has shown that compensation of localized areas that are too cold or too hot causes problems in sections that would otherwise have normal emissivity. This is well known to the operator. A too detailed explanation is not given here, since the decrease in the combustion rate has an effect on the temperature of the furnace and the temperature of the furnace has a rather slow response. There is also the problem (and this is well known to engineers) that it is difficult to measure the true band temperature with a non-contact system when the emissivity changes unexpectedly.

Document US2017/137906 A1 relates to cold-rolled and hot-dip high-strength multiphase steels for motor vehicle applications, which have high formability characteristics and exhibit high levels of resistance and are intended to be used as structural members and reinforcing materials mainly for motor vehicles. The cold rolled sheet is heated in an atmosphere having an oxygen excess volume percentage of between 0.2% and 4% in the DFF. The excess oxygen volume refers to oxygen present in excess of the amount of oxygen required in combination with the fuel used to heat the furnace, i.e., excess volume oxygen percentage= (total oxygen volume-oxygen volume required for combustion)/(total oxygen volume). Thus, when excess oxygen is present in the combustion atmosphere in the proportions of the present disclosure, the excess oxygen may be available for reaction with the steel strip. In the range between 500 ℃ and 750 ℃, oxidation occurs, i.e. an iron oxide layer is formed on the surface of the steel sheet, while internal oxidation occurs under such iron oxide, thus yielding an internal oxide within a depth of 100 μm, which may comprise one or more of Si, mn, al, ti. If the oxidation depth is more than 100 μm, the steel surface will be severely oxidized, which will be difficult to reduce, and the coating quality will deteriorate.

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 above-mentioned steel sheet is completely reduced, while there is an internal oxidation zone having a depth between 200nm and 100 μm, which internal oxidation zone comprises one or more of the Si, mn, al, ti-containing oxides. This oxidation is followed by a reduction step is necessary to make the steel surface suitable for hot dip coating.

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

To this end, the inventors propose a process for heat treatment of a moving high strength steel strip comprising the steps of carrying out a strip temperature homogenization in a homogenization chamber comprising at least one radiant-heating pipe in 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 further carrying out a step of reducing the strip in a reduction zone before the step of oxidizing the strip in an oxidation chamber with an oxidizing atmosphere having an oxygen concentration greater than 1% by volume.

It is known that the use of direct flame heating zones allows the temperature of the belt to rise rapidly, thereby compromising the temperature uniformity of the metal product. As in a large number of furnaces, the oxidation chamber is positioned directly after the direct flame heating zone, and oxidation is performed on a belt where temperature uniformity is not well controlled.

The kinetics of forming an oxide layer on the surface of a high strength steel strip is mainly dependent on the surface temperature of the strip, the composition of the steel, and of course the time, and the formation of an oxidizing atmosphere in the oxidizing chamber. The oxidation time is defined by the line speed and the section length. The good control of the temperature of the belt during its oxidation in the oxidation chamber makes it possible to obtain a surface oxide layer having a more uniform thickness over the whole surface of the belt.

However, with respect to the intended temperature non-uniformity targets (e.g., between 1% and 5%) and the belt speed, the present patent does not teach the length of the uniformity chamber nor the time the belt spends in the chamber, which makes the teaching a poor practical use for the technician.

Object of the Invention

The object of the present invention is to provide a solution aimed at overcoming the drawbacks of the prior art.

In particular, the object of the present invention is to improve the longitudinal and transverse temperature uniformity of the strip after the step of heating the strip in a DFF or RTF furnace and before the pre-oxidation step, the non-uniformity being mainly due to local variations in the emissivity of the strip.

Furthermore, it is an object of the present invention to provide guidance regarding the length of the homogenization section and the residence time of the belt in the homogenization section required for the design in order to achieve a predetermined temperature uniformity target at the outlet of the section.

Disclosure of Invention

A first aspect of the invention relates to a method for improving the temperature longitudinal uniformity of a steel strip, in particular a low carbon alloy steel strip dedicated to liquid metal coating, which continuously runs at linear speed in a temperature-homogenizing zone provided with an electric resistance, which is located before a critical process like a post-treatment zone and after a furnace zone, which is capable of providing radiant heating to the strip to bring the strip to a predetermined average temperature, wherein the temperature-homogenizing zone is further provided with at least one contact roller made of refractory stainless steel, which has a strip winding angle of more than 90 °, preferably 270 ° or more, and wherein the total contact time and the total contact length of the strip with the at least one contact roller, respectively, are selected to be greater than or equal to

Contact time 1= (a x exp (B x RSTGDP)) @ tape thickness

And

Contact length 1 = contact time 1 x linear velocity

Wherein:

the contact length 1 in meters, the contact time 1 in seconds, the linear velocity in meters/second, and

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

The contact time 1 is limited to the value necessary to obtain the predetermined upper limit value RSTGDP, and the initial roll temperature, the initial belt temperature in the temperature uniformizing section, and the actual belt temperature are measured by the temperature measuring device.

According to a preferred embodiment, the method is further limited by at least one of the following characteristics or an appropriate combination of these characteristics:

Contact time 1 adjusts or limits the total contact time to reduce the longitudinal temperature variation of the belt by 25% and 50%, respectively;

-controlling the temperature of the temperature-homogenizing zone provided with the rollers by setting the temperature of these resistances to an average value, said resistances having the function of compensating for heat losses, so as to obtain the temperature value required for the strip to enter the pre-oxidation zone;

-maintaining the temperature homogenizing zone provided with rollers under an atmosphere having less than 0.5% o ₂;

-maintaining the temperature-homogenizing zone provided with the rollers under an atmosphere having a dew point between-60 ℃ and 0 ℃, preferably between-30 ℃ and-10 ℃;

The method also aims at improving the temperature lateral uniformity of the strip, wherein in said temperature uniformity section or in another section, the resistance distributed on the wall of said section heats the section over a length given by the following equation, which depends on the lateral strip temperature gradient reduction percentage or STGDP:

contact time 2=a×exp (b× STGDP)

And

Contact length 2 = contact time 2 x linear velocity

Wherein:

the contact length 2 is in meters, the contact time 2 is in seconds, and the linear velocity is in meters/second;

-2.8< a <3.3 and 0.03< B <0.04 (note that the a and B coefficients are for refractory stainless steel rolls only);

the method is operated with a low-carbon alloy steel strip dedicated to being coated with a mixture of liquid Zn and Al, possibly with Si, fe and unavoidable impurities;

The process is operated with a pre-oxidation section or a soaking section as a post-treatment section;

The method is operated with a direct flame furnace, radiant tube furnace or electric induction furnace as furnace section;

The process is operated with the furnace section maintained at an atmosphere containing less than 1%O ₂ and one or more components selected from the group consisting of H ₂、CO、H₂O、CO_2、N₂ and mixtures thereof, the proportions of each of these components being dependent on the process parameter settings;

The belt tension is advantageously between 0.3kg/mm ² and 1.5kg/mm ² and preferably between 0.5kg/mm ² and 1kg/mm ²;

the temperature difference between the belt entering the temperature equalization section and the at least one contact roller is lower than 100 ℃, preferably lower than 50 ℃.

Another aspect of the invention relates to an industrial installation for performing a method for improving the longitudinal and transverse temperature uniformity of a steel strip, preferably a low-carbon alloy steel strip dedicated to liquid metal coating, which continuously travels at linear speed in a temperature-homogenizing zone comprising tunnels or chambers provided with electric resistances, said strip having a given thickness and tension as described above, wherein said installation comprises in turn:

-a direct flame, radiant tube or electric induction furnace section;

-said temperature homogenizing zone is further provided with at least one contact roller having a given diameter, which at least one contact roller provides a belt winding angle of more than 90 °, preferably more than 270 ° or more, depending on the ratio roller diameter/belt thickness and said belt tension, and which at least one contact roller is capable of providing a total contact time and a total contact length, respectively, of the belt with the at least one contact roller meeting the conditions specified above, said contact time being dependent on the percentage reduction of the roller-belt temperature gradient;

-a pre-oxidation section;

-a deceleration section;

Wherein these resistances are distributed over the wall of the temperature-homogenizing zone or another zone for heating the temperature-homogenizing zone optionally over the time and length given above, said time being dependent on the percentage of decrease in the transverse band temperature gradient.

According to a preferred embodiment, the facility is further limited by at least one of the following characteristics or an appropriate combination of these characteristics:

The length of the temperature-homogenizing zone or another zone is selected according to the linear velocity and for a given contact time, the contact time being adjusted or limited to reduce the transverse temperature gradient by at least 50%, the transverse temperature gradient being expressed by the temperature difference divided by the distance;

the length of the temperature-homogenizing zone or another zone is selected according to the target line speed and is adapted to give a residence time of less than 12 seconds in order to reduce the transverse temperature gradient by at least 25%;

The temperature-homogenizing zone is lined with refractory material, these resistances being located on four walls of the zone, preferably on the sides facing the travelling belt in use, or positioned so as to minimize the time and length to reach the target temperature homogenization;

the tunnel or chamber located before the pre-oxidation section can be arranged horizontally or vertically, but preferably vertically, to minimize the effect of earth gravity;

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

The number of contact rollers with a belt winding angle of more than 90 °, preferably more than 270 ° or more, is comprised between 1 and 6.

Drawings

Fig. 1 shows different heat treatments in the annealing line for preparing high-strength cold-rolled steel strip required for a metal liquid-coated strip, including a temperature-homogenizing zone according to the prior art in which a roll made of refractory stainless steel having a thickness between 20mm and 30mm or 40mm is provided.

Fig. 2 shows an example of the evolution of the transverse band temperature gradient over time.

Fig. 3A to 3D show different configurations of the temperature equalization sections, mainly in terms of the number and position of the resistances (fig. 3A and 3B, lateral equalization; fig. 3C and 3D, longitudinal equalization) and the contact roller and/or the belt winding angle of the roller (fig. 3C and 3D).

Fig. 4 shows the time evolution of the roller/belt temperature gradient at a roller thickness of 25 mm.

Fig. 5 shows the corresponding variation of the belt temperature and roller surface temperature with time spent in the dedicated homogenization section, taking into account the roller initially at 650 ℃ and the belt entering the section at 600 ℃ (1 mm belt, roller made of refractory steel and having a shell thickness of 25 mm).

The results presented in fig. 2, 4 and 5 are obtained by thermal modeling based on a simple model of heat transfer that relies on energy balance with slices, taking into account radiant heat transfer mode, conductive heat transfer mode and convective heat transfer mode. The equation for this model is shown in FIG. 6 (origin: M.Dubois 1996).

Detailed Description

When aiming to compensate for emissivity variations in order to obtain a uniform or homogeneous band temperature, two cases have to be considered.

First, when the surface emissivity changes and when the heating rate is high, it is difficult to obtain a uniform belt lateral temperature because the heat conduction has no time to diffuse and build up.

The inventors have found that not only does it take a certain time in order to improve the lateral temperature uniformity, but that the section where such leveling takes place must also have a uniform temperature equal to the target belt 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 radiant tube ovens (RTFs) is not a good solution, since radiant tube ovens have a small radiating surface compared to the whole surface of the chamber. The case of gas flame heated radiant tubes is even worse, since it is known that even the temperature of the tube itself is non-uniform and depends on the combustion rate, due to flame development in the tube. Uneven tube temperature can be very detrimental to the target because some radiant heating can compete with the thermally conductive process used to improve the temperature uniformity of the belt.

The inventors have finally found that in order to increase the CO ₂ efficiency of the overall process, electrical heating should be more recommended than gas heating.

Second, the inventors have found that the most effective way to improve longitudinal temperature uniformity when it has a finite length (e.g., for the case of roll heads and tails) is to use the heat capacity of the rolls around which the tape is wound. This is based on the fact that it is well known that when the belt has a constant temperature, the contact roller is on the contact area and has the same temperature throughout the thickness of the shell.

When the belt sections with different temperatures arrive on the rollers, heat exchange takes place in that the heat capacity of the rollers is transferred to the belt by contact. Of course, this dynamic effect cannot last too long because the roll temperature asymptotically tends to the belt temperature.

It is also clear that the heat transfer exchange depends on the quality of the belt/roller contact and requires a certain time. The inventors have found an optimal compromise between the reduction of undesired temperature variations and the required number of rolls.

To compensate for the band heat loss of the section, resistance is implemented and temperature is controlled.

Description of the preferred embodiments of the invention

Lateral temperature uniformity

According to a preferred embodiment of the invention, the low carbon strip is subjected to a heat treatment in a heating installation 1, which heat treatment comprises heating the strip 100 by radiation in a direct-fired furnace or a full radiant tube furnace 11 to a temperature between 650 ℃ and 750 ℃ and preferably between 650 ℃ and 700 ℃, followed by a dedicated temperature equalization section 12 (fig. 1) located before the pre-oxidation chamber 13 and the reduction chamber 14. The belt temperature is measured at the end of the temperature heating section 11 (typically at its center). The linear velocity is adjusted in such a way that the target temperature is reached, preferably between 650 ℃ and 700 ℃.

In the dedicated homogenizing zone 12, conduction across the width of the belt is possible 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 loss.

According to the present disclosure, it is possible to carry out controlled electric heating on four walls of the section, but preferably on both sides facing the belt (not shown). The electrical heating consists only of the resistor 2 (preferably facing the strip uniformly), excluding the electrical radiant tube. Thus, it is intended to provide a so-called "uniform" wall temperature of the target in the section 12, which means that the temperature variation of the electrical resistance is defined as a temperature below about 20 ℃. The temperature of the electrical resistance is ensured by the measuring means contacting the heating elements at different points but preferably on each side of the section and preferably at those points which face the belt individually. The power of the different resistive panels will advantageously be controlled separately in order to obtain a target temperature preferably between 650 ℃ and 700 ℃.

Since a certain time is required for the decrease of the temperature gradient, the present inventors have found that this time is actually dependent on the decrease from the initial temperature gradient to the desired temperature gradient. In the discussion that follows, we define the percent decrease in band temperature gradient (STGDP) as follows:

For example, if the initial temperature difference is 20 ℃ over a length of 100mm transverse to the belt, the gradient is 200 ℃/m. A gradient decrease of 50% means that the new gradient is 100 ℃/m, so the temperature difference is only 10 ℃ over 100 mm.

The thermal simulation performed based on the model of fig. 6 includes radiation, convection, and conduction with the following data and/or assumptions:

FFD heating (furnace walls) of the strip by radiation having a zone temperature of 1250 ℃;

over a large part of the belt surface, the total emissivity of the belt is 0.315. At the center, a 50mm wide region has an emissivity which is assumed to be 10% higher, i.e. 0.35, based on emissivity variability;

The goal is to obtain a uniformity zone temperature of 650 ℃, i.e. a value slightly higher than the band temperature when the band leaves the FFD (to take advantage of the radiation effect, considering that the emissivity is now uniform). This zone is free of forced convection;

1mm thick steel strip with thermal conductivity λ _{Steel and method for producing same} (Tc) =73.29-0.0564 (tc+273), where Tc is the temperature in degrees celsius and λ steel is the thermal conductivity in W/m (based on an average estimate of several steels; data from m.dubois). This thermal conductivity value may even be considered high when compared to alloy steels of generally even lower thermal conductivity.

Fig. 2 shows the evolution of the temperature gradient over time for the case where the initial temperature difference across the width of the band (100 mm) is 32 ℃. 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. For reference, the dashed line refers to a 50% decrease in temperature gradient (this also corresponds to 50% STDGP). Thus requiring 20 seconds to reduce by 50%.

The inventors have found that the resistance 2 distributed over the wall of the section heats the section over a length given by the following equation, which depends on the percentage decrease in the transverse band temperature gradient or STGDP:

time=a×exp (B. STGDP)

And

Length = time x linear velocity

Wherein:

length in meters, time in seconds, linear speed in meters/second;

-2.8< a <3.3 and 0.03< b <0.04.

It should be noted that this configuration is here devoid of rollers.

Longitudinal temperature uniformity

According to another preferred embodiment of the invention, the low carbon strip is subjected to a heat treatment comprising heating the strip 100 by radiation to a temperature between 650 ℃ and 750 ℃ and preferably between 650 ℃ and 700 ℃ in a direct combustion furnace or a full radiant tube furnace 11, and then passing the strip in a dedicated temperature uniformizing section 12 comprising at least one contact roller 3 and preferably more contact rollers 3, so as to provide sufficient contact (time) to allow significant heat exchange between the rollers 3 and the strip 100. The different configurations of the segments 12 are shown in fig. 3A to 3D, the main differences being the number and position of the contact rollers 3 and/or the tape winding angle of the rollers 3.

The at least one contact roller 3 is preferably made of a material having a thickness such that a sufficient heat capacity (Cp) is provided, 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 10mm and 60mm, preferably between 20mm and 40mm, to provide sufficient heat capacity.

The belt tension is controlled in the range of 0.4kg/mm ² to 2kg/mm ², preferably between 0.8kg/mm ² and 1.2kg/mm ² to ensure good contact without causing excessive plastic deformation of the belt, which is detrimental to the final flatness. The shape of the roller 3 is preferably cylindrical, which means that there is no special crown, but a light crown is acceptable.

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

The section 12 is also provided with a plurality of electric heating devices 2, preferably resistances, whose temperature is controlled.

The incoming belt temperature 100 is measured at the end of the temperature heating section 11. The belt temperature (not shown) at the exit of the series of rolls of the homogenizing zone 12 was also measured.

Since a certain time is required for the decrease and increase of the temperature of the steel strip, respectively, the present inventors have found that the required time depends on the decrease of the desired temperature variation. In the following discussion, we define the percent decrease in absolute value of the temperature gradient roller/belt (roller/belt temperature gradient decrease percent or RSTGDP):

fig. 4 shows the temporal evolution of the roller/belt temperature gradient.

Fig. 5 shows the corresponding variation of the belt temperature and roller surface temperature with time spent in the dedicated homogenization section 12, taking into account the roller initially at 650 ℃ and the belt entering the section at 600 ℃ (1 mm belt, roller made of refractory steel and having a shell thickness of 25 mm).

The number of rolls required, and thus the total wrap contact according to the present invention, will depend on the size of the rolls, the non-uniformity reduction target, the line speed and the tape thickness.

The inventors have found that the total contact time and the total contact length of the belt with at least the contact roller (3) are respectively selected to be greater than or equal to

Time= (a x exp (B x RSTGDP)) xband thickness

And

Length = time x linear velocity

Wherein:

-length in meters, time in seconds, linear velocity in meters/second, and

-0.34< A <0.92 and 0.035< b <0.045.

Roughly, the required roller/belt contact time (in seconds) can be approximated as

A' (sec/mm) tape thickness (mm)

Wherein the values of A' are as follows:

-percent decrease (RSTGPD) for 25% temperature gradient 1<A' <3;

-percent decrease (RSTGPD) for 50% temperature gradient 3<A' <8;

-percent decrease (RSTGPD) for 75% temperature gradient 8<A' <12.

Studies of hot buckling and shape problems in continuous heat treatment lines by Paoulus et al, association of the International at the treatment of Leishment at [Study of the heat buckling and shape problems in continuous heat treating lines and discussion of proposed solutions]"(AIME, ariigania, pa., 1985, pages 419-439) teaches that the temperature differential between the roll and the belt should not be too great to avoid belt deformation (buckling). Typically, the initial temperature difference between the roll and the belt should not be greater than 100 ℃ and preferably should not be greater than 50 ℃.

In practice, this means that if a 1mm thick belt travels at 120mpm (2 mps) and reaches a 650 ℃ roller at 600 ℃, the total contact time with the roller required to ensure that the belt is eventually at 625 ℃ (gradient reduced by 50%) should be at least between 3 and 8 seconds. The number of rolls required is therefore easily calculated by the skilled person, taking into account the linear speed and the winding angle, the roll diameter.

In order to ensure good contact between the roller and the belt, the belt tension is advantageously between 0.3kg/mm ² and 1.5kg/mm ² and preferably between 0.5kg/mm ² and 1kg/mm ².

List of reference numerals

1. Heating installation

2. Resistor

3. Contact roller

11DFF or RTF segments

12. Homogenizing zone

13. Pre-oxidation section

14. Reduction section

100. A 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 designed 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 homogenization section (12) is further provided with at least one contact roller (3) made of refractory stainless steel, the at least one contact roller having a strip winding angle greater than 90°, preferably greater than 270° or more, and wherein the total contact time and the total contact length of the strip 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

as well as

Contact length 1 = contact time 1 * linear speed

in:

-

- contact length 1 in meters, contact time 1 in seconds, linear velocity in meters per second; and

-0.34<A<0.92 and 0.035<B<0.045,

The contact time 1 is limited to a value necessary to reach a predetermined upper limit value of RSTGDP, and the initial roll temperature, the initial belt temperature in the temperature homogenization section (12) and the actual belt 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 as to reduce the longitudinal temperature variation of the strip by 25% and 50%, respectively.

3. The method according to claim 1, wherein the temperature of the temperature homogenization section (12) provided with rollers (3) is controlled by setting the temperature of the resistors (2) to an average value to obtain the temperature value required for the strip (100) to enter the pre-oxidation section (13), wherein the resistors have the function of compensating for heat losses.

4. The method according to claim 1, wherein the temperature homogenization section (12) provided with the rollers (3) is maintained under an atmosphere of less than 0.5% _O2 .

5. The method according to claim 1, wherein the temperature homogenization section (12) provided with the rollers (3) is maintained under 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 transverse uniformity of the temperature of the strip (100), wherein, in the temperature homogenization zone (12) or in another zone, resistors (2) distributed on the walls of the zone heat the zone over a length given by the following equation, which depends on the transverse zone temperature gradient reduction percentage or STGDP:

Contact time 2 = A*exp(B*RSTGDP)

as well as

Contact length 2 = contact time 2 * linear speed

in:

-

- contact length2 in meters, contact time2 in seconds, linear velocity in meters/second;

-2.8<A<3.3 and 0.03<B<0.04.

7. The method according to claim 1, wherein the method is operated with a pre-oxidation section or a soaking section as a post-treatment section (13).

8. The method according to claim 1, wherein the method is operated with a direct-fired furnace, a radiant tube furnace or an electric induction furnace as the furnace section (11).

9. The method according to claim 8, wherein the method is operated 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, the respective proportions of these components depending on the process parameter settings.

10. Method according to claim 1, wherein the belt tension is advantageously between 0.3 kg/ ^mm2 and 1.5 kg/ ^mm2 and preferably between 0.5 kg/ ^mm2 and 1 kg/ ^mm2 .

11. Method according to claim 1, wherein the temperature difference between the strip entering the temperature homogenization section (12) and the at least one contact roller (3) is lower than 100°C, preferably lower than 50°C.

12. An industrial installation for carrying out the method according to any one of claims 1 to 11 for improving the longitudinal and transverse temperature homogeneity of a steel strip, preferably 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) comprising a tunnel or a chamber provided with an electric resistance (2), the strip having a given thickness and tension, wherein the installation comprises in sequence:

- direct flame, radiant tube furnace or electric 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. An installation according to claim 12, wherein the length of the temperature homogenization section (12) or the other section is selected according to the line speed and for a given contact time, the contact time being adjusted or limited to reduce the transverse temperature gradient by at least 50%, the transverse temperature gradient being represented by the temperature difference divided by the distance.

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

15. Installation according to 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 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 according to claim 13, wherein the tunnel or chamber preceding the pre-oxidation section (13) can be arranged horizontally or vertically, but preferably vertically, to minimize the influence of the earth's gravity.

17. Plant according to 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. Plant according to 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.