Method for controlling rapid cooling through contact with a liquid of a metal strip moving in a continuous line

Abstract

The invention relates to a method for controlling the rapid cooling through contact with a liquid of a section (S) of a strip (1), comprising means for measuring the temperature and the shape of the strip, and a thermal predictive model coupled to a metallurgical predictive model, both of which are coupled to a thermomechanical predictive model, and the real-time adjustment of the control parameters of the line in order to achieve an overall thermomechanical stress amplitude in the strip that is lower than a predefined limit value (Lσ).

Description

METHOD FOR CONTROLLING RAPID COOLING BY CONTACT WITH A LIQUID OF A METAL STRIP MOVING IN A CONTINUOUS LINE Designation of the technical field concerned

[0001] The invention relates to a rapid cooling method during continuous heat treatment of metal strips. In particular, it is applicable to the production of advanced high strength steel AHSS requiring a high cooling rate above 200 °C / s, carried out by spraying liquid or liquid and gas mixture to obtain a complex distribution of metallurgical phases, including austenite, ferrite, pearlite, bainite and martensite.

[0002] The device and method according to the invention make it possible to control the temperature distribution of the strip throughout the cooling process in order to achieve the expected final quality of the product, including flatness and homogeneity of mechanical properties, within the framework of reliable and stable production by using the relationship between the temperature distribution in the width of the strip and in the running direction during the cooling process and the final characteristics of the product. Technical problems addressed by the invention

[0003] The heat treatment to be applied to the strip in the continuous line depends on the chemical composition of the steel and the mechanical properties expected at the end of the treatment. It includes a heating step up to a temperature at the end of heating between 750 and 950 °C, a holding step at the temperature reached at the end of heating and a cooling step down to room temperature or an intermediate temperature with a specific cooling rate for each metallurgical quality.

[0004] For example, obtaining a given steel may require an annealing temperature higher than its austenitizing temperature, then a holding time at this temperature, followed by slow cooling for a partial transformation of the austenite into ferrite, and finally rapid cooling for the transformation of the austenite into martensite.

[0005] The mechanical properties obtained at the end of the heat treatment depend on the chemical composition of the steel and the control of the thermal process to obtain a particular microstructure.

[0006] The quality of the products at the end of the heat treatment, i.e. the uniformity of mechanical properties, including yield strength, tensile strength, elongation and flatness of the strip, depends on the evolution of the strip temperature throughout the stages of the thermal process.

[0007] The conventional method of producing high-strength cold-rolled steel sheets using continuous annealing equipment, including rapid cooling means, is based on the relationship between the tensile strength of the steel sheet and production parameters such as steel grade, sheet thickness, line speed, rapid cooling start and stop temperature, and tempering temperature after rapid cooling. According to this method, it is possible to reduce the strength variation for steel sheets with an expected tensile strength below 700 MPa, with an allowable deviation of about 3%.

[0008] For advanced ultra-high strength steels, characterized by tensile strength up to 1 GPa or more, the high sensitivity to chemical composition and thermal process parameters, especially high cooling rate, implies a large variation in the final mechanical properties, which may not correspond to the required range of properties.

[0009] Different technologies are used to rapidly cool steel strips in a continuous line, in particular liquid contact cooling allows to achieve the cooling rate required for the production of high strength steel, above 200 °C / s.

[0010] The three common types of technology are: spray cooling of a gas and liquid mixture using bi-fluid nozzles, spray cooling of a liquid using mono-fluid nozzles, quenching by immersion in a liquid, possibly combined with liquid spraying.

[0011] All these technologies involve heterogeneous cooling resulting from the typical thermal phenomena encountered when the hot steel plate comes into contact with a liquid. In fact, cooling by contact with a liquid is characterized by three different boiling regimes, depending on the surface temperature of the strip.

[0012] At high temperatures, typically above about 600 °C, cooling occurs in the film boiling regime, where the strip surface is insulated from the liquid by a vapor film, resulting in uniform, low, and stable heat exchange.

[0013] The Leidenfrost temperature, typically observed between 600 °C and 250 °C depending on the characteristics of the cooling technology, defines the upper limit of the transition boiling regime where the vapor layer breaks due to uneven surface contact with the liquid, resulting in increased and unstable heat exchange.

[0014] At lower temperatures, typically below 250 °C, the temperature where maximum heat flux is observed defines the upper limit of the nucleate boiling regime where heat exchange decreases rapidly while the entire surface is in contact with the liquid.

[0015] The temperature range of metallurgical transformations for advanced high-strength steels, typically between about 550 °C for bainitic transformation and about 350 °C for martensitic transformation, covers the heterogeneous cooling range of the boiling transition regime, resulting in non-uniform thermal and microstructural behaviors impacting the final product quality.

[0016] The increasing quality requirement for steel products, in particular the reduction of the tolerance of deviation of mechanical properties in the longitudinal direction and in the width direction in a coil, as well as the reduction of the tolerance of flatness defects, implies the development of a method dedicated to the control of the cooling process, in particular for very high strength steel grades, more critical than conventional high strength steels.

[0017] The final quality control of the product is based on the evaluation of the thermomechanical behavior of the strip during the heat treatment process carried out in the line.

[0018] State-of-the-art thermal predictive models exist to predict the evolution of the strip temperature during rapid cooling by contact with a liquid, taking into account the heat exchanges between the strip and the cooling fluid as a function of many parameters. These parameters include the initial thermal profile of the strip, its geometric profile, the temperature and flow rate of the cooling fluid, as well as the size, distribution and velocity of the liquid droplets.

[0019] Metallurgical predictive models exist, according to the state of the art, to predict the metallurgical transformations and the evolution of the resulting physical properties during the cooling of the strip by considering its initial microstructure at the entrance to the cooling device, according to the operations carried out before the strip enters the line, such as rolling, and according to the thermal cycle carried out upstream of the cooling device on the line.

[0020] These coupled models can consider a distribution of cooling characteristics and strip characteristics that vary at each point across the strip width and along the cooling device. When these models are complete and detailed, they require significant computing power and time, making them impossible to run in real time using the line control system. Therefore, it is not possible to adjust the parameters of the rapid cooling device in real time based on the results of these models. Technical background

[0021] Insufficient product quality after heat treatments in continuous lines, in particular flatness defects and heterogeneity of mechanical properties, is related to the thermal history of all points of the strip passing through the cooling device.

[0022] The origin of the flatness defects observed after the rapid cooling process is the distribution of thermomechanical stresses exceeding critical values. It follows that the flatness defects arise from the temperature distribution and nonlinear properties of the material.

[0023] The origin of the deviation of material properties in length and width of a coil is the distribution of cooling rate exceeding critical values. It follows that the heterogeneity of material properties also arises from the temperature distribution in the strip.

[0024] Therefore, the final quality of the products is based on the control of the thermal history of all points of the strip throughout the cooling device by means of the control of the cooling efficiency, depending on the properties of the product, including the metallurgical properties, thermo-physical properties and the evolution of the mechanical properties throughout the cooling process, related to the chemical composition of the product and the applied thermal cycle, the design of the cooling device, including the nozzle arrangement and nozzle characteristics, the parameters of the cooling process, including the liquid flow rate, the liquid temperature, the heat transfer coefficient related to the spray characteristics, the initial shape of the strip and its evolution in the cooling device, the transverse thermal profile of the strip upstream and downstream of the cooling stage.

[0025] The location of the sensors required for temperature and flatness measurement must take into account the constraints of an installation in an industrial context. In particular, infrared thermal imaging methods or optical sensors have limited use in an unfavorable environment with water and steam. Therefore, temperature is commonly measured using a linear scanner or pyrometers upstream and downstream of the liquid contact cooling unit.

[0026] In current processes, cooling control is performed by adjusting the cooling length and water flow rate along the length and width of the strip to achieve the target final temperature and a uniform temperature profile across the entire width of the strip, measured using a pyrometer or a linear scanner. This overall cooling control, based solely on the initial and final temperature as a function of temperature measurement upstream and downstream of the cooling device, does not allow for the assessment of the strong temperature gradients occurring locally across the width of the strip and in the direction of travel of the strip linked to the physical phenomenon, namely the Leidenfrost effect, induced by spraying a liquid onto a hot surface.

[0027] Furthermore, the coupled effects of thermal and metallurgical phenomena are not considered in the current cooling strategy based on the minimum overall cooling rate to obtain the required microstructure. The combined effect of local thermal gradients and physical property gradients on the thermomechanical stress distribution involving flatness defects is generally not considered in current cooling strategies.

[0028] Final shape is a key characteristic for product quality assessment. Flatness tolerance is defined based on downstream process requirements. Poor flatness can lead to reduced productivity, resulting in belt vibration, belt breakage, and product rejection.

[0029] In continuous heat treatment lines, flatness defects result from thermomechanical stresses along the entire width and length of the product, generated by non-uniform temperature distribution during cooling.

[0030] A flatness monitoring system, usually optical flatness sensors, is sometimes used to assess final flatness downstream of the cooling process. No changes in flatness are usually assessed during the cooling process.

[0031] The evolution of flatness defects during the cooling process can also be effectively evaluated using finite element thermomechanical simulation models, taking into account for the model input data the evolution of material properties resulting from the metallurgical model and the two-dimensional temperature distribution resulting from the thermal model.

[0032] These models are not commonly suitable for real-time industrial use for dynamic cooling control.

[0033] In the usual cooling control method, the temperature measuring device and the flatness measuring device are installed downstream of the cooling process to evaluate the product quality obtained with the applied cooling parameters.

[0034] Another key feature for product quality assessment is the evaluation of mechanical properties, usually performed offline, only for samples taken from the steel strip at the beginning and end of the coil. These measurements are performed at a limited number of points across the strip width (usually 3 to 5). Other non-destructive measuring systems exist, usually installed at the exit of the processing line, without the possibility of real-time adjustment of the upstream process.

[0035] In the current process, product quality is assessed after complete heat treatment, not allowing a closed loop with cooling adjustment for product quality improvement.

[0036] Other methods have been proposed to control flatness or mechanical properties during the cooling stage of the continuous annealing process.

[0037] WO2010 / 079452 of the applicant describes a method for controlling the homogeneity of spray cooling of a liquid or a mixture of gas and liquid along the width and / or length of a metal strip, by determining the area where the vapor film disappears and adapting the cooling parameters such as liquid temperature, velocity, flow rate or drop size and gas flow rate for spray cooling of a mixture of gas and liquid to maintain the vapor phase at all points in order to reduce the risks of flatness defects and heterogeneity of material properties.

[0038] This method is based solely on the thermal critical temperature related to boiling regimes and does not consider the coupled effects of microstructure change and temperature gradients during cooling.

[0039] The invention comprises a temperature measuring device installed in the cooling device where the strip is supposed to reach the rewetting point, between 250 and 450 °C.

[0040] The invention does not describe the installation of a measuring device allowing a reliable assessment of the thermal critical point inside the liquid contact cooling device.

[0041] WO2021 / 074500 of the applicant describes a method for reducing flatness defects in a strip subjected to cooling by spraying liquid or a mixture of gas and liquid, along a cooling zone of a continuous heat treatment line, the cooling intensity being adjusted in the running direction in order to obtain a relative position between the Leidenfrost temperature and the temperatures where the metallurgical structure changes to minimize the internal stresses of the strip thanks to a compensation effect of the thermal stresses resulting from the non-linear thermal gradient in the running direction of the product linked to the successive boiling regimes, and of the microstructural stresses resulting from the inversion of the curve of the coefficient of thermal expansion during the metallurgical transformations.

[0042] The method is limited to flatness control by means of thermomechanical stress control considering only nonlinearities in the strip length. Thermal gradients and associated microstructural gradients in the strip width are not considered.

[0043] JP5428292 describes a method for producing a high-strength cold-rolled steel sheet based on expressing the nonlinear relationship between the material quality of the steel product (the tensile strength) and the factors that affect the material quality by means of a neural network linking the quenching temperature, the tempering temperature and the material composition.

[0044] The control method according to these solutions is based only on the final state of the product, it does not take into account the combined influence of thermal efficiency, metallurgical transformation and the evolution of flatness defects during the entire cooling process on the final quality of the product.

[0045] CN113637837 describes a method for improving the flatness of a product in a continuous annealing line equipped with a water mist cooling device, characterized by dividing the strip into 20 sections in the strip width direction, allowing analytical calculation of the strip temperature as a function of the cooling parameters applied and allowing analytical calculation of the resulting strip deformation.

[0046] Section temperature control is achieved by controlling the number of nozzles, nozzle opening degree, pressure and spray flow rate to achieve target flatness.

[0047] This cooling control is based solely on the relationship between thermal gradients across the strip width and thermal stresses. It does not take into account the superposition of thermal stresses resulting from thermal gradients and metallurgical stresses resulting from the nonlinearity of physical properties in the event of microstructure change.

[0048] None of these patents describe a solution for controlling the cooling device based on real-time calculations of strip cooling, metallurgical transformations and overall thermomechanical stresses during the cooling process.

[0049] Improving the production of AHSS steels in processing lines, combined with reducing flatness defects and reducing the deviation of mechanical properties, requires a method to evaluate in real time the temperature distribution of the strip during the passage through the cooling device, in order to automatically adjust the cooling process parameters to meet the flatness tolerance and the deviation tolerance of mechanical properties at the end of heat treatment.

[0050] However, all the cooling control methods according to the state of the art are limited to the knowledge of the transverse thermal profile upstream and downstream of the rapid cooling or the flatness defect after cooling. They do not allow to evaluate the coupled influence of the evolution of the material properties and the shape on the temperature distribution during the crossing of the cooling device. They do not allow to evaluate the local thermal gradients involved in the flatness defects and the heterogeneity of the mechanical properties after cooling. None of them allows to quantify the flatness defects and the deviation of the material properties to allow the control of the cooling parameters according to the predefined quality tolerance.

[0051] The aim of the invention is to make possible the automatic control of rapid cooling by contact with a liquid of a strip moving in a continuous line for an improvement of the flatness and homogeneity of the mechanical properties of the final product, by allowing an implementation of thermomechanical predictive models in real time in an industrial context.

[0052] The invention is in the form of a method for controlling the cooling of a metal strip in a cooling device of a continuous annealing or galvanizing line by spraying a liquid or a mixture of a liquid and a gas onto the strip, the device comprising means for measuring the temperature of the strip at the inlet and / or outlet of the cooling device, means for measuring the shape of the strip at the inlet and / or outlet of the cooling device, a database of parameters of the heat treatment process of the strip in the line and means for adjusting the cooling efficiency of the strip over its width and / or in its running direction,

[0053] characterized in that it comprises a thermal predictive model which predicts the evolution of the temperature of a section S of the strip at the inlet of the cooling device along the cooling device coupled with a metallurgical predictive model which predicts the metallurgical properties of the section S of the strip as a function of the evolution of the temperature, coupled with a thermomechanical predictive model which predicts the thermomechanical stresses which result from the combination of thermal stresses due to temperature gradients, metallurgical stresses due to transformations of metallurgical phases and mechanical stresses due to the application of external forces to the section S, the method comprising the real-time adjustment of line control parameters to achieve an overall thermomechanical stress amplitude in the section S of the strip lower than a predefined limit value Lσ.

[0054] External forces at section S that act on the belt are, for example, the tensile forces exerted by the rotating rollers to ensure the belt is driven along the line.

[0055] Advantageously according to the invention, the overall thermomechanical constraints are calculated by taking into account the derivation of the temperature distribution, and the distribution of the physical properties of the strip, according to a grid constituted by the segmentation of the strip width and the segmentation of the cooling length.

[0056] Advantageously according to the invention, the segmentation of the width of the strip is defined by the combination of singular points of a function representing the thermal transverse profile and singular points of a function representing the geometric profile of a section S of the strip at the entrance to the cooling device.

[0057] Advantageously according to the invention, the segmentation of the strip length is defined by the points corresponding to the occurrence of the cooling rate change temperatures and to the occurrence of the metallurgical phase transformation temperatures for all the characteristic points of the segmentation of the strip width, during the movement of the section S of the strip along the cooling device.

[0058] Advantageously according to the invention, the adjustment of the line comprises the adjustment of the speed and the belt tension and the adjustment of the parameters of the cooling device such as the flow rate of cooling fluid, the spraying distance, the spraying angle or any other parameter dependent on the particular geometry of the device.

[0059] Advantageously according to the invention, the adjustment of the line is applied to minimize the excess of the internal constraints relative to the predefined limit value Lσ, in a homogeneous manner at all points of the grid.

[0060] According to an exemplary embodiment of the invention, the calculations are carried out offline for a preliminary adjustment of the cooling device, aimed at minimizing the overall thermomechanical constraints from the recording of the thermal process data.

[0061] According to the invention, the method successively comprises: a step E1 of determining the transverse thermal profile of a section S of the strip at the inlet of the cooling device, a step E2 of determining the transverse geometric profile of the section S of the strip at the inlet of the cooling device, a step E3 of segmenting the width of the strip, defined by the combination of the singular points of a function representing the transverse thermal profile determined in step 1 and the singular points of a function representing the geometric profile determined in step 2.

[0062] Advantageously, the singular points comprise the edges of the strip and the extrema of the mathematical function representing the geometric profile and of the mathematical function representing the thermal profile of the strip.a step E4 of calculating the evolution along the cooling device, of the temperature at the singular points of the segmentation of the strip width defined in step 3,a step E5 of segmentation of the length of the strip defined by the points corresponding to the occurrence of the cooling rate change temperatures and to the occurrence of the metallurgical phase transformation temperatures for all the characteristic points of the segmentation of the strip width, during the movement of the section S of the strip along the cooling device,a step E6 of determining a grid corresponding to a longitudinal representation of the displacement of the section of the strip along the cooling device, by means of the combination of longitudinal lines xj passing through the segmentation of the width of the strip determined in step 3, and, on the other hand, of transverse lines yi passing through the segmentation of the length of the strip determined in step 5, a step E7 of calculating the overall thermomechanical stresses σ by the combination of the thermal stresses due to the temperature gradients and the metallurgical stresses due to the transformations of metallurgical phases and the mechanical stresses due to the application of external forces, for all the points of the grid determined in step 6,a step E8 of comparing the value of the overall thermomechanical stress σ calculated for all the points of the grid with a predefined limit value Lσ for each point of the grid as a function of the temperature and the mechanical properties of the strip at this point to avoid the formation of a permanent shape defect.,

[0063] If a thermomechanical stress amplitude value σ at a point on the grid is greater than the predefined limit value Lσ, the method further comprises a step E9 of determining an adjustment of the line control parameters making it possible to bring the amplitude of the thermomechanical stress σ at this point below the predefined limit value Lσ.

[0064] Line adjustment includes adjusting the running speed and web tension and adjusting cooling device parameters such as coolant flow rate, spray distance, spray angle or any other parameter dependent on the particular geometry of the device.

[0065] If the adjustment capability of the line parameters does not allow the limit value Lσ to be reached, the line adjustment is applied to achieve a thermomechanical stress value of minimum amplitude.

[0066] If the limit Lσ cannot be reached at all points of the grid, the setting of the cooling device must be adjusted to minimize homogeneously at all points of the grid the exceeding of the amplitude of the thermomechanical stresses relative to the limit value Lσ.

[0067] This optimization is carried out by applying standard optimization methods according to the state of the art, for example the least squares method.

[0068] The characterization of singular points along the width and length of the strip, representative of the thermomechanical stresses of the strip along the cooling device, makes it possible to limit the stress calculations to these points.

[0069] The method according to the invention makes it possible to avoid the usual use of finite element modeling for solving thermoelasticity problems, which is time-consuming and incompatible with real-time calculation in industrial conditions.

[0070] The method according to the invention makes it possible to predict the maximum localized thermomechanical stresses, according to the state of the art, at the occurrence of variations in the cooling rate and at the occurrence of transformations of metallurgical phases.

[0071] Since these points are few in number, the computing resources are limited, which allows for online implementation, in real time, by the line control system, or by a dedicated computing means connected to the line control system.

[0072] Thus, the method according to the invention is advantageously implemented in real time, while the strip section is moving along the line.

[0073] Thus, by applying state-of-the-art knowledge on the buckling behavior of a thin plate, the flatness of the strip is deduced from the amplitude and location of the maximum thermomechanical stresses.

[0074] The method according to the invention further comprises a step E10 of applying the line adjustment determined in step E9 before the section of the strip enters the cooling device.

[0075] In one embodiment of the invention, the adjustment of the line, making it possible to bring the amplitude of the global thermomechanical stress σ at a point below the predefined limit value Lσ, is carried out on the cooling device.

[0076] In another embodiment of the invention, the adjustment of the line aimed at reducing the amplitude of the thermomechanical stress σ below the predefined limit value Lσ is carried out by adjusting process parameters not specific to the cooling device, for example the running speed or the traction of the strip.

[0077] According to one aspect of the invention, the transverse geometric profile and the transverse thermal profile of the section S of the strip at the inlet of the cooling device are represented by mathematical functions.

[0078] The width segmentation is first defined by the singular points of the functions, namely the minima, maxima and inflection points.

[0079] The initial width segmentation can be advantageously refined based on the shape difference between the boundaries of each initial segment in order to define an admissible linear approximation for optimal definition on each segment of spray characteristics such as spray distance, spray angle and liquid flow distribution.

[0080] The strip length segmentation is defined by the occurrence of temperatures characteristic of cooling rate variations or metallurgical phase transformations along the strip cooling history between the inlet to the cooling device and the outlet from the cooling device, for each point of the previously defined strip width segmentation.

[0081] The characteristic temperatures related to metallurgical phenomena are evaluated using a standard model for the evaluation of the microstructure and physical and mechanical properties throughout the cooling process depending on the chemical composition and the thermal cycle applied to the product.

[0082] The temperature evolution of the characteristic points of the bandwidth segmentation is evaluated by means of a usual thermal model based on convection heat transfer, including an experimental heat transfer coefficient evaluated for a specific spraying equipment and a range of operating parameters.

[0083] The thermal model takes into account the influence of the band shape and edge effects due to liquid runoff depending on the location of each grid point.

[0084] The thermal model takes into account the geometrical peculiarities of the cooling device such as spray distance, spray angle, discontinuity between the cooling units constituting the entire cooling device.

[0085] The thermal model takes into account the particularities of the heat transfer evolution curve observed in the case of cooling by a liquid linked to the succession of different boiling regimes.

[0086] The segmentation of the strip length determined by the combination of the occurrence of thermal and metallurgical phenomena can be represented by a piecewise multilinear function.

[0087] The grid is constructed in the plane of the strip by means of the intersection of the segmentation of the strip width determined by the combination of the singular points of the functions representing the transverse geometric profile and the transverse thermal profile of the section S of the strip at the inlet of the cooling device and, on the other hand, the segmentation of the strip length determined by the combination of the characteristic temperatures related to the occurrence of thermal and metallurgical phenomena in the direction of travel.

[0088] The cooling rate distribution is deduced by derivation of the strip temperature distribution according to the grid.

[0089] Thus, by applying state-of-the-art knowledge on the production of AHSS steels and the correlation between the cooling rate and the mechanical properties, the homogeneity of distribution of the mechanical properties of the product is deduced by derivation of the temperature distribution according to the grid by comparison with a predefined experimental admissible cooling rate.

[0090] The distribution of thermomechanical stresses along the grid is deduced from the temperature distribution and the physical properties of the strip along the grid by applying the usual differential equations which make it possible to describe the behavior of the strip under the combined effect of thermal, metallurgical and mechanical stresses.

[0091] The usual equations of thermoelasticity can be solved analytically in the simple case of representing the transverse distribution and the longitudinal evolution of the temperature by piecewise multilinear functions.

[0092] Thus, by applying state-of-the-art knowledge on the buckling behavior of a thin plate exposed to thermal and microstructural gradients, the flatness of the strip is deduced from the predefined experimental admissible thermomechanical stresses.

[0093] The method according to the invention makes it possible to evaluate in real time the relationship between the cooling parameters applied and the final quality of the strip, including the flatness characteristics and the mechanical properties.

[0094] The process allows for real-time adjustment of cooling parameters to modify the required cooling efficiency to improve quality.

[0095] Advantageously, the method according to the invention makes it possible to anticipate changes in parameters of the heat treatment process, such as the change in chemical composition of the steel, the change in strip dimension or the change in target temperature of the strip, in order to quickly and efficiently adjust the cooling parameters to ensure consistent quality.

[0096] The cooling control according to the invention is based on adjusting the cooling intensity in the direction of travel of the strip and in the width of the strip to reach the target temperature and balance the thermomechanical stresses resulting from the temperature distribution and the change in microstructure according to the grid.

[0097] The cooling control strategy depends on the temperature associated with each grid cell and the corresponding boiling regime.

[0098] The cooling control required in cells where the film boiling regime occurs consists of adjusting the cooling efficiency across the strip width and in the strip travel direction to achieve the first critical temperature related to the first occurrence of the microstructural change and balance the thermal stresses resulting from the thermal gradients in the travel direction.

[0099] Depending on the adjustment capabilities of the cooling equipment, cooling control is applied in the film boiling regime stage, taking into account the initial strip shape influencing the spray impact area, for example, by decreasing the spray distance or decreasing the water pressure to reduce the cooling efficiency in selected areas.

[0100] The cooling control required after entering the transition boiling regime consists of adjusting the cooling efficiency in the strip running direction to reach the last critical temperature related to the last occurrence of the microstructure change and controlling the cooling efficiency across the strip width to reduce the thermomechanical stresses resulting from the combination of thermal stresses due to temperature gradients and microstructural stresses due to the heterogeneous phase distribution.

[0101] Depending on the adjustment capabilities of the cooling equipment, the cooling control in the transition boiling regime stage is applied taking into account the evolution of the band shape that influences the liquid discharge, for example, by increasing the water flow rate or by directing the water spray towards the area to be supercooled.

[0102] In this area, control of local heterogeneity requires operation of the nozzles in the highest water pressure range.

[0103] The cooling control required after the last occurrence of microstructure change is to adjust the cooling efficiency in the strip running direction to achieve the final target temperature by adjusting the water flow rate according to the capabilities of the cooling equipment.

[0104] The necessary cooling control in the strip width and in the cooling length is evaluated using the thermal model taking into account the initial microstructure, the initial temperature and the shape of the product, as well as the evolution of the microstructure, the temperature and the shape during the passage of the cooling device, the effective spraying distance, the effective spraying angle and the water discharge depending on the current shape of the strip.

[0105] Adjustment of cooling parameters can be performed in real time by applying predefined adjustment tables.

[0106] Steps E1 to E9 of the process can be carried out offline, using theoretical data and / or data from steel coils whose passage through the line is programmed. The results obtained can be used to feed predefined adjustment tables.

[0107] According to one embodiment of the invention, an Artificial Intelligence model is proposed for automatic learning of the optimal adjustment of the cooling parameters by means of the correlation of the data measured upstream and downstream of the heat treatment and the evaluation of the quality of the product by the method of deriving the temperature distribution and the distribution of the physical properties to evaluate the thermomechanical constraints according to the grid and the comparison of these values ​​with experimental threshold values.

[0108] Advantageously according to the invention, the properties of the materials are calculated in real time using conventional metallurgical models, as a function of the chemical composition and the thermal cycles applied, allowing the evaluation of the evolution of the microstructure and the physical and mechanical properties of the strip as a function of the evolution of its temperature during passage through the cooling device.

[0109] The implementation of the process is based on the combination of equipment, databases, predictive models and an Artificial Intelligence system linking the measurement results, the results of the predictive models and the automatic control of the cooling device.

[0110] According to a particular embodiment of the invention, the method comprises one or more of the following features:an adjustable cooling device for precisely adjusting the cooling efficiency in the width and length of the strip as required, the device possibly comprising movable parts of the spray bars for adjusting the spraying inclination and distance or masking of the edges of the products or subdivision of the water supply in the length of the spray bars for precise transverse adjustment or any similar equipment according to the state of the art,a device for measuring the temperature of the product before entering the cooling device and after leaving the cooling device, by means of an infrared linear scanner or pyrometers,a device for measuring the flatness of the product before entering the cooling device and after leaving the cooling device by means of an optical flatness sensor, a computer-aided predictive model for calculating the thermal distribution according to the grid, a computer-aided predictive model for calculating the metallurgical properties of the products as a function of the temperature evolution, a computer-aided predictive model for calculating the thermomechanical stresses according to the grid and comparing these values ​​with experimental threshold values. an automatic control system for the cooling device.,

[0111] In order to be able to apply the method according to the invention, taking into account the environmental constraints in industrial conditions, a linear scanner is necessarily installed before the inlet of the rapid cooling device in order to evaluate the possibly asymmetrical transverse thermal profile resulting from the upstream thermal process.

[0112] A linear scanner is advantageously installed after the outlet of the rapid cooling device in order to verify that the admissible target temperature is reached at all points and to evaluate the final uniformity of the thermal profile of the strip.

[0113] Flatness should be assessed upstream of the rapid cooling device in order to evaluate the initial flatness defect which will influence the cooling efficiency and therefore the cooling control for quality improvement. Flatness is advantageously assessed downstream of the rapid cooling device for the validation of the predictive model.

[0114] The method according to the invention requires the creation of databases.

[0115] A database of process parameters is created, including the parameters required for the production of advanced high-strength steels, such as the thermal cycles applied according to the steel grades and the expected final properties, in particular as a function of the thermal gradient and the cooling rate.

[0116] A database of material properties is created using a conventional metallurgical model, based on chemical composition and applied thermal cycles, evaluating the evolution of the microstructure and physical and mechanical properties as a function of the temperature evolution in the cooling device.

[0117] A cooling parameter preset table is created, including water flow rate, water temperature, spray angle, spray distance related to the relationship between cooling efficiency control and the initial thermal and geometric state of the strip.

[0118] Additionally, the process includes predefined rules to automatically apply the optimal cooling setting to the cooling equipment to achieve reliable and stable production with expected final product quality, including consistency of flatness and mechanical properties, regardless of production (steel grades, line speed, thermal cycles).

[0119] In addition, the advanced automatic control is based on an Artificial Intelligence model that collects and analyzes all the data acquired during production and the simulation results according to the method of the invention in order to optimize the cooling parameters to be applied for the improvement of the quality of the strip.

[0120] According to another aspect of the invention, there is provided a computer program comprising code instructions which, when executed by a computing unit, lead to implementing a method according to the first aspect of the invention, or one or more of these improvements. Brief description of the figures

[0121] Other characteristics and advantages of the invention will emerge from the detailed description which follows, for the understanding of which reference is made to the attached drawings, in which:is a schematic and partial longitudinal view of a vertical processing line according to an embodiment of the invention;is a schematic, perspective and partially cutaway view of the entry of the strip into the cooling zoneis a schematic view showing a flowchart of the steps of the method according to the invention;is a view of a flow diagram illustrating the sequence of all the operations carried out according to the invention;is a graph schematically illustrating the determination of the segmentation of the width of the strip based on the combination of the singular points of a convex function representing the geometric profile of the section S, and of a concave function representing the thermal profile of the section S, the abscissa axis indicating the position of the points in the width of the strip represented by the letter w and the main ordinate axis indicating the height of the flatness defect represented by the letter f and the secondary ordinate axis indicating the temperature represented by the letter T;is a graph schematically illustrating the determination of the segmentation of the width of the strip based on the combination of the singular points of an asymmetric polynomial function for the geometry and of a convex function for the thermal;is a graph schematically illustrating the variation, during rapid cooling, of the heat exchange coefficient at the surface of the strip represented by the letters HTC on the ordinate axis as a function of the temperature of the strip represented by the letter T on the abscissa axis;is a graph schematically illustrating the variation of the coefficient of thermal expansion represented by the letter d during cooling, in the event of a change in microstructure;is a graph illustrating the determination of the segmentation of the strip length based on the occurrence of characteristic temperatures in the cooling length;is a schematic view of the grid in the plane of the strip;is a schematic view of the distribution of thermomechanical stresses according to the grid in the plane of the strip;is a graph illustrating the thermomechanical stresses at the intersection of the points xj of the segmentation of the strip width and the points yi of the segmentation of the strip length, relative to the threshold value Lσ according to the invention.is a graph illustrating the thermomechanical stresses after optimal adjustment of the cooling device; Detailed description of the invention

[0122] According to a first embodiment of the invention, the rapid cooling device of a vertical processing line 2 as shown in the is arranged to cool the strip 1 by spraying it with a liquid, or with a mixture of a gas and a liquid, by means of a cooling device 3 allowing the adjustment of the water flow rate, the adjustment of the spray angle and the spray distance, the in-line cooling control method comprising temperature 4 and flatness 5 sensors installed upstream of the cooling device 3 and downstream of the cooling device, as well as a system for analyzing the properties of the materials 6 downstream of the thermal process.

[0123] It schematically represents, in perspective and partially cut away, the entry of the strip into the cooling zone in a vertical heat treatment line. The section S of the strip at the entrance to the cooling device is represented by a transverse line in the strip width.

[0124] Graphically shows the sequence of steps of the online cooling control method according to the invention.

[0125] Illustrates the sequence of all operations performed according to the invention. In this figure, the parallelogram shape represents input / output data, the cylinder represents a stored database, the hexagonal shape represents adjustment data, the rectangular shape represents a process, the diamond shape represents a decision. The arrow represents the direction of flow.

[0126] Key for: D1: Temperature profile measured in the width of the strip upstream of the cooling device D2: Flatness measured upstream of the cooling device D3: Database of thermal process parameters D4: Database of material properties D5: Cooling parameter preset table D6: Heat transfer coefficient table D7: Temperature profile measured in the width of the strip after cooling has stopped D8: Material properties measured at the outlet of the thermal process D9: Flatness measured after cooling has stopped M1: Cooling setting M2: Thermal model M3: Grid construction M4: Stress calculation model M5: Artificial Intelligence model R1: Shape function R2: Temperature distribution in the cooling device R3: Cooling rate distribution R4: Grid R5: Stress distribution according to the grid C1: Decision test forthe final target temperatureC2: Decision test for the tolerance of mechanical properties related to the cooling rate thresholdC3: Decision test for the tolerance of flatness related to the stress threshold

[0127] Step 1 (E1) consists of measuring the transverse thermal profile of a section S of the strip (data D1) upstream of the cooling device 3 using a temperature measuring sensor 4.

[0128] Step 2 (E2) consists of measuring the flatness characteristics of a section S of the strip (data D2) upstream of the cooling device 3 using a flatness measuring sensor 5.

[0129] Step 3 (E3) consists of determining the singular points of a function representing the initial thermal transverse profile of the section S of the strip at the inlet of the cooling device (data D1) and the singular points of a function representing the initial transverse geometric profile (data D2), then segmenting the width of the strip according to the combination of these singular points P 1j (R1 Results).

[0130] Illustrates a first example of the determination of singular points P 1j for a strip with a symmetrical V-shaped geometric profile in the width represented by a continuous line and an inverse symmetrical thermal profile in the width represented by a discontinuous line. The two edges form the singular points P 11 and P 15 , and the center of the band forms the singular point P 13. To have a sufficient number of points in the width of the strip, intermediate singular points P 12 and P 14 are added.

[0131] Illustrates a second example of the determination of singular points P 1j for a strip with an asymmetrical W-shaped geometric profile and a symmetrical V-shaped thermal profile across the width of the strip. The two edges form the singular points P 11 and P 17 , the extrema form the singular points P 12 , P 14 and P 16 . Inflection points characterized by a change in convexity add two additional singular points P 13 and P 15 .

[0132] Step 4 (E4) consists of determining the evolution of temperatures T 1j at singular points P 1jin the width of the strip along the cooling device from the temperature measurement (data D1) at the inlet of the cooling device.

[0133] Step 4 includes the preparation of the cooling adjustment parameters (Process M1) linked to the process database (Data D3) and the material properties database (Data D4) included in the cooling control system and to the strip thermal characteristics (Data D1) and strip flatness characteristics (Data D2) measured upstream of the cooling device.

[0134] The process database (Data D3) includes all required parameters, including thermal cycles applied according to steel grades and expected final properties, in particular final product temperature and cooling rate.

[0135] The material properties database (Data D4) includes strip properties, including chemical composition, microstructure distribution, physical and mechanical properties as a function of temperature evolution in the cooling device, resulting from the conventional metallurgical model.

[0136] Cooling parameters are derived from the preset tables (Data D5) including water flow rate, water temperature, spray angle, spray distance or other control parameters depending on the specific design of the cooling device in relation to the control of cooling efficiency and the final control of flatness and mechanical properties.

[0137] Step 4 also consists of calculating the temperature distribution (Results R2) in the strip during the crossing of the cooling device using a thermal model (Process M2) based on experimental tables of heat transfer coefficients (Data D6) for the singular points P 1j in the width of the band related to the initial shape of the band and to the temperature T 1j associated (Results R1).

[0138] The continuation of the process is conditioned by the correlation of the final temperature deduced from the calculated temperature distribution (Results R2) with the measured temperature (Data D7) at the outlet of the cooling device.

[0139] The measured temperature must reach the target temperature defined in the process data (Decision C1) and the cooling rate target (Decision C2) so that the mechanical properties of the strip are as expected.

[0140] Step 5 (E5) consists of determining the occurrence of cooling rate change temperatures and the occurrence of metallurgical phase transformation temperatures for all characteristic points of the strip width segmentation, during the movement of the strip section S along the cooling device. Step 5 thus consists of locating the abscissas Li along the cooling device where the points P 1j of the transverse segmentation reach the characteristic thermal and metallurgical temperatures.

[0141] Step 6 (E6) consists of determining a grid G ​​corresponding to a longitudinal representation of the movement of the section S along the cooling device 3. This is achieved by combining longitudinal lines xj passing through the singular points P 1jin the width of the strip determined in step 3 (E3), and, on the other hand, transverse lines yi passing through the locations L i determined in step 5 (E5).

[0142] Illustrates the grid obtained for the strip with a V-shaped geometric profile combined with an inverse thermal profile shown in the, with characteristic temperatures evaluated along the cooling device shown in the.

[0143] Step 7 (E7) consists of determining a total thermomechanical stress σ at each point of intersection Pij of the longitudinal and transverse lines of the grid G ​​by solving the usual thermoelasticity equations for which analytical solutions exist, according to the state of the art, for the representation by simple piecewise multilinear functions of the variation of the temperature in the width of the strip and in the length of the cooling device.

[0144] Step 8 (E8) consists of comparing the calculated value of the thermomechanical stress σ at each intersection point Pij of the longitudinal and transverse lines of the grid G, with a predefined limit value Lσ. If a stress value σ at a point Pij, is greater than the predefined limit value Lσ, the method further comprises a step E9 consisting of determining an adjustment of the lines L i to bring the thermomechanical stress σ at this point Pij below the predefined limit value Lσ.

[0145] The method according to the invention is applied to the rapid cooling of a carbon steel composed of 0.1% carbon, 1% manganese and 1% silicon, annealed at a temperature between 850 and 960 °C for complete austenitization according to the chemical composition and the expected microstructure, as well as the properties expected after cooling.

[0146] Rapid cooling is defined to start in the temperature range required for the production of AHSS steels, typically at 650°C after a preliminary slow cooling step, and is defined to stop at 150°C.

[0147] The minimum cooling rate for a complete martensitic transformation, i.e. without transformation of austenite into another phase such as bainite or pearlite, can be determined from the transformation curves established for the composition of the steel, i.e. 200 °C / s for the example considered.

[0148] Therefore, only one characteristic temperature is observed on the thermal expansion coefficient evolution curve schematically illustrated by the curve in the joint, corresponding to the total martensitic transformation of the austenitic phase, at the characteristic temperature identified by the point Ms on the graph.

[0149] The cooling device includes spray bars equipped with conical jet nozzles operating with a water flow rate of 20 l / min / nozzle and a water pressure of 6 bar, uniform for the entire length of the cooling device.

[0150] According to the spray characteristics of the cooling device, the thermal characteristic point separating the film boiling regime and the boiling transition regime is estimated to be 450 °C, as shown in Figure 1 by point L.

[0151] The second thermal characteristic point separating the boiling transition regime and the nucleate boiling regime is identified by point M.

[0152] Cooling control is applied to a strip entering the rapid cooling device with an initial convex-shaped flatness defect combined with an initial concave-shaped thermal profile across the width of the strip, as shown in.

[0153] The segmentation of the band width for the definition of the grid is based on the representation of the initial shape of the band by a symmetric convex function characterized by 2 singular points, the minimum and the maximum combined with the representation of the initial symmetric concave thermal profile characterized by 2 singular points, the minimum and the maximum.

[0154] The initial segmentation of the band width is subdivided into 2 segments in order to obtain an admissible linear approximation on each segment of the modeling of thermal phenomena.

[0155] Bandwidth segmentation by P points 1jand temperatures T 1j associated, is illustrated in the.

[0156] The segmentation in the length is determined by the intersection between the temperature evolution calculated in the cooling length from the temperature T 1j associated with each characteristic point P 1j of the segmentation of the bandwidth and the occurrence of thermal or metallurgical characteristic temperatures.

[0157] For example, only one microstructural change is observed during cooling from the austenitic phase before cooling to the martensitic phase at 330 °C during cooling (Ms point).

[0158] Other characteristic points include the Leidenfrost temperature marked by point L at 450°C on the graph of the and the maximum heat flux temperature, marked by point M at 260°C on the graph of the.

[0159] Therefore, for each singular point of the bandwidth previously defined, three characteristic points cross the thermal path in the cooling length.

[0160] For each segment in the strip width, the first segment in the cooling length is limited by reaching the Leidenfrost temperature. In this segment, cooling operates in a film boiling regime characterized by a homogeneous and constant heat flux leading to stable and easily controllable operation. The microstructure remains uniform.

[0161] The second segment in the cooling length is limited by the Leidenfrost temperature and the onset of the martensitic transformation. In this segment, cooling operates in a boiling transition regime characterized by a high heat flux that increases the pre-existing temperature differences in the band width.

[0162] Therefore, heterogeneous cooling efficiency leads to heterogeneous temperature distribution and heterogeneous material properties and strip deformation in case of insufficient cooling control, in the length and width of the product.

[0163] The third segment in the cooling length is limited by the martensitic transformation onset temperature and the nucleate boiling regime temperature.

[0164] This sequence of defining segments related to thermal and metallurgical characteristic points in the cooling length of the strip is repeated for all segments of the strip width.

[0165] The definition of the segments related to the characteristic points encountered in the cooling length is illustrated in.

[0166] Lamontre shows the grid constructed in the plane of the strip according to the method of the invention, comprising 4 segments linked to the singular points in the width of the strip delimited by the abscissas x1 to x5 and 10 segments linked to the occurrences of characteristic temperatures in the cooling length for each segment of the width and delimited by the ordinates y1 to y11.

[0167] According to the method of the invention, the grid makes it possible to evaluate a temperature distribution limited to the characteristic points in the plane of the strip during the cooling crossing.

[0168] Following this grid, the temperature distribution can be represented by a piecewise multilinear function in the strip width and by a piecewise multilinear function in the strip length.

[0169] From this temperature distribution, the thermomechanical stresses are calculated by solving the usual thermoelasticity equations, based on the derivation of the temperature distribution and the physical properties in the width and length of the strip.

[0170] Highlights the thermomechanical stresses calculated at the intersection of the characteristic points of the grid.

[0171] Highlights the evolution in the cooling length of the stresses calculated at the intersection of the characteristic points of the grid and the Lσ value defining the limit of the admissible compressive stresses, according to a predefined tolerance.

[0172] The amplitude of the compressive thermomechanical stresses is greater than the amplitude of the limit value Lσ at the edge of the strip represented by the ordinate x1, for the characteristic points P 51 and P 61corresponding to the abscissas y5 and y6 of the segmentation of the strip length.

[0173] Highlights the evolution in the cooling length of the calculated stresses after optimal adjustment of the cooling device to bring the amplitude of the global thermomechanical stress σ below the amplitude of the Lσ value for all the characteristic points of the grid, for example by means of the adjustment of the sprayed water flow rate in the length and width of the cooling device.

[0174] After adjustment, occurrences of thermal and metallurgical characteristic points are merged, decreasing the number of characteristic points in the cooling length to 9.

[0175] According to another exemplary embodiment, the initial shape of the strip is represented by an asymmetric polynomial function and the transverse thermal profile by a convex function.

[0176] The segmentation of the band width is based on the 7 singular points of the polynomial function including 3 extrema, the 2 edges of the band and 2 inflection points corresponding to the inversion of the convexity.

[0177] The singular points of the convex function representing the transverse thermal profile are the 2 edges and the minimum of the function; they are confused with the singular points of the polynomial function.

[0178] The construction of the segments defined by the 7 singular points is illustrated in.

[0179] The present invention further provides a computer program comprising code instructions which, when executed by a computing unit, lead to implementing a method according to the first aspect of the invention, or one or more of these improvements.

[0180] An electronic module is also provided which can be implemented in the form of an electronic module comprising a memory in which the computer program product comprising instructions intended to be executed by the computing unit is stored.

Claims

Method for controlling the cooling of a metal strip (1) in a cooling device (3) of a continuous annealing or galvanizing line (2) by spraying a liquid or a mixture of a liquid and a gas onto the strip, the device comprising means for measuring the temperature of the strip at the inlet and / or outlet of the cooling device, means for measuring the shape of the strip at the inlet and / or outlet of the cooling device, a database of parameters of the heat treatment process of the strip in the line and means for adjusting the cooling efficiency of the strip over its width and / or in its running direction,characterized in that it comprises a thermal predictive model which predicts the evolution of the temperature of a section (S) of the strip at the inlet of the cooling device along the cooling device coupled with a metallurgical predictive model which predicts the metallurgical properties of the section (S) of the strip as a function of the evolution of the temperature, coupled with a thermomechanical predictive model which predicts the overall thermomechanical stresses which result from the combination of thermal stresses due to temperature gradients, metallurgical stresses due to transformations of metallurgical phases and mechanical stresses due to the application of external forces to the section (S), the method comprising the real-time adjustment of line control parameters to achieve an overall thermomechanical stress amplitude in the section (S) of the strip lower than a predefined limit value (Lσ)., Method according to claim 1, characterized in that the overall thermomechanical stresses are calculated by taking into account the derivation of the temperature distribution, and the distribution of the physical properties of the strip, according to a grid constituted by the segmentation of the strip width and the segmentation of the cooling length. Method according to claim 2, characterized in that the segmentation of the width of the strip is defined by the combination of singular points of a function representing the thermal transverse profile and singular points of a function representing the geometric profile of a section (S) of the strip at the entrance to the cooling device. Method according to claim 2, characterized in that the segmentation of the strip length is defined by the points corresponding to the occurrence of the cooling rate change temperatures and to the occurrence of the metallurgical phase transformation temperatures for all characteristic points of the segmentation of the strip width, during the movement of the section (S) of the strip along the cooling device. A method according to claim 1, characterized in that the line adjustment comprises adjusting the web speed and tension and adjusting the parameters of the cooling device such as the coolant flow rate, the spray distance, the spray angle or any other parameter dependent on the particular geometry of the device. Method according to one of claims 1 or 5, characterized in that the adjustment of the line is applied to minimize the excess of the internal constraints relative to the predefined limit value (Lσ), in a homogeneous manner at all points of the grid. Method according to one of the preceding claims, characterized in that the calculations are carried out offline for a preliminary adjustment of the cooling device, aimed at minimizing the overall thermomechanical constraints from the recording of the thermal process data. [Rectified according to rule 91, 17.01.2025]Computer program product comprising code instructions which, when executed by a calculation unit, lead to implementing a method according to any one of the preceding claims.

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