Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27B—FURNACES, KILNS, OVENS OR RETORTS IN GENERAL; OPEN SINTERING OR LIKE APPARATUS
  • F27B9/00—Furnaces through which the charge is moved mechanically, e.g. of tunnel type; Similar furnaces in which the charge moves by gravity
  • F27B9/30—Details, accessories or equipment specially adapted for furnaces of these types
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
  • C21D1/76—Adjusting the composition of the atmosphere
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
  • C21D9/54—Furnaces for treating strips or wire
  • C21D9/56—Continuous furnaces for strip or wire
  • C21D9/561—Continuous furnaces for strip or wire with a controlled atmosphere or vacuum
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
  • C21D9/54—Furnaces for treating strips or wire
  • C21D9/56—Continuous furnaces for strip or wire
  • C21D9/62—Continuous furnaces for strip or wire with direct resistance heating
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27B—FURNACES, KILNS, OVENS OR RETORTS IN GENERAL; OPEN SINTERING OR LIKE APPARATUS
  • F27B9/00—Furnaces through which the charge is moved mechanically, e.g. of tunnel type; Similar furnaces in which the charge moves by gravity
  • F27B9/14—Furnaces through which the charge is moved mechanically, e.g. of tunnel type; Similar furnaces in which the charge moves by gravity characterised by the path of the charge during treatment; characterised by the means by which the charge is moved during treatment
  • F27B9/20—Furnaces through which the charge is moved mechanically, e.g. of tunnel type; Similar furnaces in which the charge moves by gravity characterised by the path of the charge during treatment; characterised by the means by which the charge is moved during treatment the charge moving in a substantially straight path
  • F27B9/24—Furnaces through which the charge is moved mechanically, e.g. of tunnel type; Similar furnaces in which the charge moves by gravity characterised by the path of the charge during treatment; characterised by the means by which the charge is moved during treatment the charge moving in a substantially straight path being carried by a conveyor
  • F27B9/2407—Furnaces through which the charge is moved mechanically, e.g. of tunnel type; Similar furnaces in which the charge moves by gravity characterised by the path of the charge during treatment; characterised by the means by which the charge is moved during treatment the charge moving in a substantially straight path being carried by a conveyor the conveyor being constituted by rollers (roller hearth furnace)
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27B—FURNACES, KILNS, OVENS OR RETORTS IN GENERAL; OPEN SINTERING OR LIKE APPARATUS
  • F27B9/00—Furnaces through which the charge is moved mechanically, e.g. of tunnel type; Similar furnaces in which the charge moves by gravity
  • F27B9/28—Furnaces through which the charge is moved mechanically, e.g. of tunnel type; Similar furnaces in which the charge moves by gravity for treating continuous lengths of work
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
  • F27D19/00—Arrangements of controlling devices
  • F27D2019/0028—Regulation
  • F27D2019/0059—Regulation involving the control of the conveyor movement, e.g. speed or sequences
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F27—FURNACES; KILNS; OVENS; RETORTS
  • F27D—DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
  • F27D19/00—Arrangements of controlling devices
  • F27D2019/0028—Regulation
  • F27D2019/0059—Regulation involving the control of the conveyor movement, e.g. speed or sequences
  • F27D2019/0062—Regulation involving the control of the conveyor movement, e.g. speed or sequences control of the workpiece stay in different zones

Definitions

• the present invention relates to the heating process used in annealing lines of cold rolled steel strips of high strength, especially those intended for the automotive industry.
• the invention also concerns the industrial installation for carrying out the heating process.
• the heating process that is commonly used in annealing lines for low carbon strips uses heat transfer by radiation.
• the latter may be obtained by direct flame radiation in the enclosure where the strip is travelling as it is the case in a so-called “direct fired furnace” (DFF).
• DFF direct fired furnace
• An alternative technology uses the radiation of a tube internally heated either electrically or by using a flame (radiant tube furnace or RTF), that last method being the most common due to the high heating power (in watt per m 2 ) that can be delivered by the heating surface. If both methods are well-known and commonly used industrially, they completely differ by the fact that in case of direct flame heating the combustion gas are in contact with the processed strip whereas, in the alternative method, they are contained in the heating tube and so have no direct contact with the strip surface. [0003] Other differences exist and they can be quickly summarized as follows :
• the surface radiating toward the strip is at much higher temperature in case of a direct fired furnace, typically between 1200 and 1450°C in case of direct fired furnace while it is limited to maximum 1150°C or more commonly 950°C when the radiant tubes are made of refractory steels due to creeping and heat expansion phenomena ;
• the total heat transfer power results from the sum of convective and radiative heat transfers and that the radiative one is proportional to the difference of temperature in kelvin at the 4 th power between the hot and the cold body. It can be shown that if the temperature difference between the hot and the cold body is higher that about 500°C the temperature of the cold body has no effect and the radiative heat transfer is then only proportional to the 4 th power of hot body temperature multiplied by the shape factor and the emissivity of the hot and cold surfaces as well as by the Boltzmann constant.
• the heating rate from room temperature to 650°C is in the range of 35 to 45°C/sec for a 1 mm strip thickness in a direct fired furnace and only of 10 to 15°C/sec in a radiant tube furnace.
• the heat transfer delivered per surface unit is not identical in each point : a high surface emissivity, low surface emissivity respectively, will induce a higher heat input, lower heat input respectively, into the running strip.
• the strip emissivity varies as it is usually the case along and across the strip, non-uniform longitudinal and transversal temperatures may be obtained.
• the emissivity variations are due to variable cleanliness or oxidation of the surface. It is for example common to observe variations between head, tail and middle of a coil but also between edge and center thereof. Many causes in the upstream process of flat low carbon steel can explain that fact even if all of them are not yet well identified. It is also important to note that, even if the strip has a thermal conductivity, this one is significantly lower than the one of copper or aluminium for example, and is too low to allow some uniform ization of the temperature due to the generally large distance between hot and cold parts.
• the inventors confirm that the quality of the final product is improved when the oxidation is reasonably uniform/homogeneous. Considering the explanations above, it practically means that the strip temperature uniformity must be good enough, that means a strip temperature within +/-10°C but more preferably within +/-5°C, at the time the strip enters in the preoxidation box.
• Document US 2017/137906 A1 relates to cold rolled and hot dip high-strength multiphase steels, for motor vehicles use, which have high formability properties and exhibit high resistance levels, and are intended to be used as structural members and reinforcing materials primarily for motor vehicles.
• the cold rolled sheet is heated in a DFF within an atmosphere having an excess volume oxygen percentage between 0.2% and 4%.
• the iron oxide layer at the surface of the steel sheet, mentioned above, is fully reduced, while an internal oxidized zone, with a depth between 200nm and 100pm, comprising one or more of Si, Mn, Al, Ti containing oxides, is present.
• This oxidation followed by a reduction step is necessary so that the steel surface is suitable for hot dip coating.
• the inventors propose a process for the heat treatment of a moving high-strength steel strip, comprising a step of strip temperature homogenization in a homogenization chamber comprising at least one radiant heating tube under a no-oxygen atmosphere, so as to homogenize the temperature of the strip after it has passed through the direct flame heating zone of the previous step, and before a step of oxidizing the strip in an oxidation chamber with an oxidizing atmosphere having an oxygen concentration by volume greater than 1 %, and further a step of strip reduction in a reduction zone.
• the kinetics of formation of an oxide layer on the surface of a strip of high strength steel depends mainly on the surface temperature of the strip, the steel composition and of course time, as well as the composition of the oxidizing atmosphere in the chamber of oxidation.
• the oxidation time is defined by the line speed and the section length. So 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 homogeneous thickness over the entire surface of the strip.
• this patent does not teach the length of the homogenization chamber, nor the time to be spent by the strip in that chamber, which renders the teaching of poor practical use for the skilled person.
• the present invention aims to provide a solution intended to overcome the drawbacks of prior art.
• the invention aims at improving longitudinal and transversal temperature homogeneity of a strip after the step of heating the same in a DFF or a RTF furnace and before the step of preoxidation, inhomogeneity being mostly attributed to local variation of strip emissivity.
• the invention aims at providing guidelines in terms of designing the length of a homogenization section needed and residence time of the strip therein in order to attain a predetermined temperature homogeneity target at the exit of the section.
• the method is further limited by at least one of the following characteristics or a suitable combination of these characteristics :
• the total contact time ContactTimel is adjusted or limited to obtain a reduction by 25%, respectively by 50%, of the longitudinal temperature variation of the strip ;
• the temperature of the temperature homogenization section provided with rolls is controlled by setting the temperature of the electric resistances at the average value to obtain the temperature value required for the strip entering the preoxidation section, said electric resistances having the function of compensating thermal loss ;
• the temperature homogenization section provided with rolls is maintained under an atmosphere with less than 0.5%O2 ;
• the temperature homogenization section provided with rolls is maintained under an atmosphere having a dew point between -60 and 0°C, preferably between -30 and -10°C ;
• the method is also intended for improving temperature transversal homogeneity of the strip, wherein, in said temperature homogenization section or in another section, electric resistances distributed on the walls of the said section are heating the section on a length that is given in the following equation that depends on the percentage of the transversal strip temperature gradient decrease percentage or STGDP:
• the method is operated with low carbon alloyed steel strip dedicated to be coated with in a mixture of liquid Zn and Al, possibly with Si, Fe, and with inevitable impurities ;
• the method is operated with a preoxidation section or a soaking section as post-treatment section ;
• the method is operated with a direct flame furnace, a radiant tube furnace or an electric induction furnace as furnace section ;
• the method is operated with a furnace section being maintained under an atmosphere containing less than 1 %O2 and one or more components selected from the group consisting of H2, CO, H2O, CO2, N2 and a mixture thereof, in respective proportions depending on processing parameter settings ;
• the strip tension is advantageously comprised between 0.3 and 1.5kg/mm 2 , and preferably between 0.5 and 1 kg/mm 2 ; the temperature difference between the strip entering the temperature homogenization section and the at least one contact roll is lower than 100°C, preferably lower than 50°C.
• Another aspect of the invention relates to an industrial installation for carrying out the method for improving longitudinal and transversal temperature homogeneity of a steel strip, preferably a low carbon alloyed steel strip dedicated to liquid metal coating, continuously running at a line speed in a temperature homogenization section comprising a tunnel or a chamber provided with electric resistances, as described above, said strip having given thickness and tension, wherein said installation successively comprises :
• said temperature homogenization section which is further provided with at least one contact roll with a given diameter, providing a strip wrapping angle, depending on the ratio roll diameter/strip thickness and on said strip tension, higher than 90°, preferably higher than 270° or more, and able to provide a total contact time, a total contact length respectively, of the strip with the at least contact roll fulfilling the conditions specified above, said contact time depending on the roll-strip temperature gradient decrease percentage ;
• the installation is further limited by at least one of the following characteristics or a suitable combination of these characteristics :
• the temperature homogenization section or the other section has a length selected according to the line speed and for giving a contact time adjusted or limited to reduce the transversal temperature gradient, expressed by temperature difference divided by distance, at least by 50% ;
• the temperature homogenization section or the other section has a length selected according to the target line speed and adapted for giving a residence time lower than 12 seconds in order to reduce the transversal temperature gradient at least by 25% ;
• the temperature homogenization section is lined with refractory material, the electrical resistances being located on the four walls of the section, and preferably on the two sides facing the running strip in use or located so that to minimize the time and length to attain the targeted temperature homogenization ;
• the tunnel or chamber located ahead of the preoxidation section can be disposed horizontally or vertically, but is preferably disposed vertically to minimize the effects of earth gravity ;
• the contact roll(s) used as heat capacity reservoir have a diameter higher than 600mm, but preferably between 800 and 1200mm, and a thickness of the shell between 10 and 60mm, but preferably between 20 and 40mm ;
• the number of contact roll(s) having a strip wrapping angle higher than 90°, preferably higher than 270°, is comprised between 1 and 6.
• Fig. 1 is representing the different thermal treatments in an annealing line of cold rolled steel strips of high strength needed to prepare the strip for metal liquid coating, including a temperature homogenization section according to prior art, in which rolls are provided, made of refractory stainless steel with a thickness between 20 and 30 or 40 mm
• Fig. 2 shows an example for the evolution of the transversal strip temperature gradient in function of time.
• Fig. 3A to 3D show different configurations for a temperature homogenization section, mainly differing by the number and the location of the resistances (Fig. 3A and 3B, transversal homogenization ; Fig. 3C and 3D, longitudinal homogenization) and contact rolls and/or the strip wrapping angle of the rolls (Fig. 3C and 3D).
• Fig. 4 shows the time evolution of the roll/strip temperature gradient for a roll thickness of 25mm
• Fig. 5 shows the respective variations of strip temperature and of roll surface temperature with the time spent in the dedicated homogenization section, considering a roll initially at 650°C and a strip entering in the section at 600°C (1 mm strip, rolls made of refractory steel and with a shell thickness of 25mm).
• the inventors have found that, in order to improve the transversal temperature uniformity, not only a certain time is required but also that the section where this levelling will take place must also have a uniform temperature equal to the target strip temperature. This is to minimize the possible radiative heating difference due to the heating element that needs to be installed to compensate for the heat losses.
• the inventors have also found in that context that the use of a radiant tubes furnace (RTF) is not a good solution due to the small radiative surface thereof compared to the whole surface of the chamber.
• RTF radiant tubes furnace
• gas-flame heated radiant tubes is even worse as it is known that, due to the flame development in the tube, even the temperature of the tube itself is not uniform and depends on the firing rate.
• a non-uniform tube temperature may be very detrimental to the target as some radiative heating may be in competition with the thermal conductive process that is used to improve the temperature uniformity of the strip.
• the inventors have found that the most efficient way to improve the longitudinal temperature uniformity when it is of limited length, as it is for example the case with coil heads and tails, is to use the heat capacity of rolls the strip wraps around. This is based on the fact that it is well-known that, when the strip has a constant temperature, the contact roll has the same temperature on the contact area and all through the shell thickness.
• a low carbon strip undergoes a heat treatment in a heating installation 1 that consists in heating the strip 100 by radiation either in a direct fired furnace or a full radiant tube furnace 11 to a temperature between 650 and 750°C, and preferably between 650 and 700°C, followed by a dedicated temperature homogenization section 12 located before a preoxidation chamber 13 and a reduction chamber 14 ( Figure 1 ).
• the strip temperature is measured at the end of the temperature heating section 11 , usually in its center.
• the line speed is adjusted in such a way that the target temperature, preferentially between 650 and 700°C, is reached.
• a controlled electric heating is implemented possibly on the four walls of the section but preferably on the two sides facing the strip (not shown).
• the electric heating is exclusively made of resistances 2 (preferably facing the strip uniformly), electric radiant tubes being excluded. It is therefore intended to provide a targeted so-called “uniform” wall temperature in section 12, that means a temperature with temperature variations of the resistances defined to be lower than about 20°C.
• the temperature of the resistances is ensured with a measuring device contacting the heating elements on different points but preferentially on each side of the section and preferably on those facing the strip individually.
• the power of the different resistance panels will be advantageously controlled separately in order to obtain a target temperature that is preferentially between 650 and 700°C.
• the gradient is 200°C/m.
• a gradient reduction of 50% means that the new gradient is 100°C/m so the temperature difference is only 10°C on 100mm.
• the overall emissivity of the strip is 0.315 over most of its surface.
• a 50mm-wide zone has an emissivity 10% higher, i.e. 0.35, according to an emissivity variability assumption ;
• the target is to obtain a uniform homogenization zone temperature of 650°C, i.e. a value slightly higher than the strip temperature when it exits the FFD (to take advantage of a radiation effect, considering that emissivity is now uniform).
• This zone is free of forced convection ;
• Figure 2 shows the evolution of the temperature gradient with time for a case where the initial temperature difference across the strip width (100mm) is 32°C. Time in abscissa is in this case started when the strip enters the furnace but the graph only shows time when the strip enters the homogeneous section of the invention.
• the dotted line refers to 50% of temperature gradient decrease (which also corresponds to a STDGP of 50%). It comes then that 20sec are required for 50% reduction.
• the low carbon strip undergoes a heat treatment that consists in heating the strip 100 by radiation either in a direct fired furnace or a full radiant tube furnace 11 to a temperature between 650 and 750°C, and preferably between 650 and 700°C followed by passing the strip in a dedicated temperature homogenization section 12 containing at least one contact roll 3, and preferably more contact rolls 3, in order to provide sufficient contact (time) to allow a significant heat exchange between the roll(s) 3 and the strip 100.
• a dedicated temperature homogenization section 12 containing at least one contact roll 3, and preferably more contact rolls 3, in order to provide sufficient contact (time) to allow a significant heat exchange between the roll(s) 3 and the strip 100.
• Different configurations of section 12 are shown on figures 3A to 3D, mainly differing by the number and the location of contact rolls 3 and/or the strip wrapping angle of the rolls 3.
• the at least one contact roll 3 is preferably made of a material having of thickness such that to provide sufficient heat capacity (Cp), for example a refractory steel or alternatively a high heat capacity material like carbon.
• Each roll 3 has a shell thickness between 10 and 60mm, and preferably between 20 and 40mm precisely to provide sufficient heat capacity.
• the strip tension is controlled in the range of 0.4 to 2kg/mm 2 , and preferably between 0.8 and 1.2kg/mm 2 to ensure a good contact without inducing excessive plastic deformation of the strip that could be detrimental for final flatness.
• the shape of the rolls 3 is preferably cylindrical that means without special crown but a light crown would be acceptable.
• the rolls have a diameter between 600 and 1500mm, and preferably between 1000 and 1200mm to ensure reasonable contact length per roll without providing specific layout problems.
• the rolls are made of refractory stainless steel with the sleeve thickness between 20 and 40mm, but preferably between 25 and 30mm
• Section 12 is also provided with a number of electric heating devices 2, preferably electric resistances, that are controlled in temperature.
• 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 rolls of the homogenization section 12 is also measured (not shown).
• Figure 4 shows the time evolution of the rol l/strip temperature gradient.
• Figure 5 shows the respective variations of strip temperature and of roll surface temperature with the time spent in the dedicated homogenization section 12, considering a roll initially at 650°C and a strip entering in the section at 600°C (1 mm strip, rolls made of refractory steel and with a shell thickness of 25mm).
• the number of rolls required and so the total wrapping contact according to the invention will depend on the size thereof, the inhomogenization reduction target, the line speed and the strip thickness.
• the inventors have discovered that the total contact time, the total contact length respectively, of the strip with the at least contact roll (3) are selected greater or equal to
• the strip tension is advantageously comprised between 0.3 and 1.5kg/mm 2 , and preferably between 0.5 and 1 kg/mm 2 .

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• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
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• Physics & Mathematics (AREA)
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• Crystallography & Structural Chemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• General Engineering & Computer Science (AREA)
• Heat Treatment Of Strip Materials And Filament Materials (AREA)

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• 2022
  • 2022-07-05 EP EP22183162.1A patent/EP4303516A1/de not_active Withdrawn
• 2023
  • 2023-06-23 CA CA3261033A patent/CA3261033A1/en active Pending
  • 2023-06-23 WO PCT/EP2023/067151 patent/WO2024008480A1/en not_active Ceased
  • 2023-06-23 CN CN202380050647.2A patent/CN119604735A/zh active Pending
  • 2023-06-23 EP EP23735008.7A patent/EP4551887A1/de active Pending

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