Title of Invention

AN AUTOMATED SYSTEM OF DETERMINING COOLING RATE OF HOT-ROLLED COIL OVER ITS LENGTH AND ACROSS ITS THIKNESS .

Abstract

An automated on-line system of determining cooling rate of hot rolled coil over its length and across its thickness, comprising the steps of: a) determining the off-line temperature value of the coil through the adaptation of a one-dimensional heat conduction relationship being solved by explicit finite difference procedure; b) developing of a correlation for heat-transfer coefficient at strip- water interface for different grades of steel at different thickness; c) receiving input data in respect of temperature, speed of the coil and signal of opening of water-cooling valve at Run out Table of a Hot Strip Mill (HSM) from level-1 automation; d) analyzing the input data from level-1 automation and the signals of top and bottom header openings of water cooling valves through out the length of the coil for cooling of a coil, thereby converting the off- line system to an on-line system; e) estimating coiling temperature over the length of the coil by using captured data and analyzing the same using the on-line system, the output from the on-line system being the temperature over the through thickness and length of the coil; f) estimating cooling rate based on the estimated coiling temperature.

Full Text

FIELD OF INVENTION; The present invention relates to a method for on-line estimation of the through-thickness coiling temperature over the length of a coil in the Hot Strip Mill of Steel Plant. The invention further relates to, as a precursor, development of an off-line model and modification of the same to an on-line model with the input flow of data from the field devices of the HSM. The on-line simulator thus developed provides an on-line estimation of through thickness coiling temperature over the length of a coil in a Run out table (ROT) of a HSM. The present invention is intended to be practiced preferably with the application of a programmable computer. BACKGROUND OF THE INVENTION. Good mathematical models for simulating the cooling behavior of a hot strip in a ROT of HSM are not readily available. The available simplified mathematical models for simulating coil temperature are generally restricted to Off-line application. Thus, monitoring the cooling rate of a hot strip over its entire length during the cooling stage in ROT assumes significant importance as an On-line estimation could only provide the grain size or the micro-structure of the strip which in turn determines the metallurgical and mechanical properties of the product. Background material is available in the following books/publications. REFERENCES: 1. Colas, R., and Sellars, CM., 1987, "Computed Temperature Profiles of Hot Rolled Plate and Strip during accelerated Cooling", Proceedings of the international Symposium on Accelerated cooling of Rolled Steel. Winnipeg, Canada, Eds. G.E. Ruddle and A. F. Crawley, Pergamon Press, London, Vol. 3, pp. 121-130. 2. Kumar. A., McCulloch, C, Hawbolt, E.B. and Samarasekara, I.V., April 1991, "Modelling Thermal and Microstructural Evolution on Runout Table of Hot Strip Mill", Material Science and Technology, Vol.7, pp. 369-368. 3. Evans, J.F., Roebuck, I.D., and Watkins, H.R., 1993, "Numerical Modelling of Hot Strip Mill Run out Table Cooling", Iron and Steel Engineer, Vol.70, No. 1, pp.50-55. 4. Guo, R.M., August 1993," Heat Transfer of Laminar Flow Cooling During Strip Acceleration on Hot Strip Mill Run out Tables", Iron and Steel Maker, pp. 49-59. 5. Hatta, N., and Osakabe, H., 1989, " Numerical Modeling for Cooling Process of a Moving Hot Plate by a Laminar Water Curtain", ISIJ International, Vol. 29, No. 11, pp. 919-925. 6. Hernandez, V.H., Samarasekera, I.V., and Brimacombe J.K., 1994, " Heat Transfer Model of Run-out Table Cooling: A Fundamental Approach", 36th Mechanical Working and Steel Processing Conference, Vol. XXXII, pp. 345-356. 7. Filipovic, J., Viskanta, R., and Incropera, F.P., 1994, " Cooling of a Moving Steel Strip by an Array of Round Jets", 35th Mechanical Working and Steel Processing Conference, ISS - AIME, Vol. XXXI, pp. 317-327. 8. W.K. Son and W.Y.D Yuen, "Flow Visualization of the Boiling Heat Transfer at the Run- Out Table", 41st Mechanical Working and Steel Processing Conference, ISS, 1999, Vol. XXXVII, pp. 707-715. 9. Liu, Z.D., Fraser, D., and Samarsekera, I. V., 2002, "Experimental Study and Calculation of Boiling Heat Transfer on Steel Plates during Run-out Table Operation", Canadian Metallurgical Quarterly, Vol.41, No. 1, pp. 63-74. 10. Sun, C.G., Han, H.N., Jin, Y.S., and Hwang, S.M., 2002, "A Finite Element Model for the Prediction of Thermal and Metallurgical Behavior of Strip on the Run-out-Table in Hot Rolling", ISIJ International, Vol. 42, No. 4, pp. 392-400. 11. Kato, T., Hayasi, Y., Kuraishi, T., Ayano, S., and Kashiwazaki, T., 1994, "New temperature Control System of Hot Strip Mill Run Out Table", 35th Mechanical Working and Steel Processing Conference, ISS -AIME, Vol. XXXI, pp. 311-316. 12. Auzinger, D., Pfaffermayr, M., Pichler, R., and Schlegl, B., 1994, "Advanced process Model for Today's Hot Strip Mill", Proceedings of National Conference on Flat Products, The Indian institute of Metals, Jamshedpur, India, pp. 101-115. BACKGROUND OF THE INVENTION, (contd..) Laminar flow cooling is an efficient method of cooling a hot strip for the reason of high value of local heat transfer coefficient between cooling water and hot strip. A Run-out Table (ROT) in a hot strip rolling has a laminar cooling system between the last finishing stand and the down coiler and cooling of the strip over the length of coil is performed in the ROT. The microstructure of the grain size of the strip rolled in Hot Strip Mill is decided by the cooling rate over the length of the coil. The metallurgical transformation from austenite phase to ferrite phase in the grain brings a dramatic change in the mechanical properties of the strip during the ROT cooling of the strip. The quality of the final product or the hot strip coil can change for a variation of cooling rate or the coiling temperature. Hence, the cooling rate is important as it in turn determines the Metallurgical properties and Mechanical properties. Hence, the precise determination of the coiling temperature or the cooling rate is a useful tool. If the Mechanical property is uniform or almost same over the length of the coil, the tensile strength, Ultimate tensile strength and % Elongation of the coil is same at every point in the coil, which can guarantee better performance of the coil for use. Uniform mechanical properties over the length can be achieved by uniform cooling rate. To have the uniform mechanical property over the length of the coil, the steel manufacturers must ensure the uniform cooling rate over the length of the coil in the ROT of a Hot Strip Mill. Due to the complex nature of heat transfer mechanism in ROT, effective numerical models for simulating coil temperature are restricted in Off-line by their nature. However, the available on-line models are more concentrated on the specific plant. No general consensus has been established so far about the particular method to determine the temperature profile. Previous numerical studies have evaluated the heat transfer co-efficient in a round about way or from experimental method by measuring the water jet diameter and coolant flow. It is very difficult to predict the amount of the heat flux or the heat transfer coefficient associated with the transition boiling. So far, not a single expression for heat transfer coefficient could be determined by measurement. Early methods on estimation of coiling temperature can be found in the work of Colas et al. [1]. Colas et al. [4] have used a constant heat transfer coefficient for the zone where the water flows parallel to the surface, whereas in the jet impingement zone, another value was applied. By using the above-mentioned values of heat transfer co-efficient, a reasonable good agreement with observations has been made in the study. Kumar et al. [2] have suggested a single value of heat transfer coefficient, which is constant, so as to fit experimental results for each bank of jets along the length of ROT. The use of constant value heat transfer coefficient provides better agreement in their study with the experimental results. In the numerical model of Evans et al.[3], the cooling for top header and bottom cooling has been treated independently. The parabolic distribution of the average heat transfer coefficient has been plotted from a correlation involving Prandtl Number (Pr), Reynolds Number (Re), thermal conductivity of the coolant and the width of impingement zone. The developed correlation predicts strip temperature for a wide range of coiling temperatures. However the single-phase convection mechanism for heat transfer contradicts the established fact that boiling exists in the jet impingement area at a wall superheat smaller than approximately 100°C. The model developed by Guo [4], although is not involved with any adaptation, correlates heat transfer coefficient with strip thickness, velocity, strip surface temperature and water flow rate by a power law. The operating data a Hot Strip Mill have been used to determine the powers, which are in the range of 0.8-1.4. A method of cooling phenomenon on a moving steel plate by Hatta and Osakabe [5] has observed, in the vicinity of the impact zone, an observable dark line shows that this zone might have the highest heat transfer. A correlation of heat transfer coefficient, which is a function of water temperature, saturation temperature of coolant and the steel plate temperature, has evolved out in their model with its validity in film boiling zone. Hernandez et al. [6] provided an insight into the fundamental evaporation mechanism through a model for parallel flow boiling curves. For the modelling purpose, most of the cooling in the run out table has been assumed to be in the transition-boiling regime. A detailed model on boiling has been developed based on macro layer evaporation The cooling process outside the zone of transition boiling has been assumed to be affected by Film Boiling. Radiation heat transfer has been considered as the main mechanism of heat loss between the water banks. An analytical model for turbulent Film Boiling in parallel flow on a moving strip has been established in the research of Filipovic et al. [7], where a correlation for heat flux in the stagnation line of a planar jet in a heat flux control experiment was used. For the film-boiling region between two water headers, a correlation has been adopted from earlier literature. In a flow visualization experiment, Soh and Yuen [8] studied a stationary strip with an initial temperature of 240°C. Although the study has captured the images of the striking jet on the plate, only splashing of the free surface of the water followed by vapour bubbles from the boiling was observed. The computational model has assumed nucleate boiling and employed the correlations to evaluate the heat flux without any description of the determination of heat transfer coefficient and the model have been constructed to gain further information on the heat transfer and an estimate has been formed of the duration for the water being in direct contact with the heated strip. The experimental study of Liu et al. [9] indicates film boiling on a stationary plate at an initial temperature of 900°C and a two-dimensional heat conduction has conduction equation has been solved by finite element method to calculate heat fluxes ana heat transfer coefficient along the surface of the plate. In another recent study, Sun et al. [10] have used different correlations at top and bottom surface of ROT in the form of a power law for heat transfer coefficients of water-cooling, whereby powers of strip surface temperature and strip velocity have been considered. The above-mentioned numerical models for simulation of temperature of coil strips in the HSM are basically off-line model in nature and the method has been carried out at Laboratory scale and also at reduced temperature compared to the actual temperature of the strip at the plant. Most of the prior art are restricted so far to experimental stage on the cooling model of run out table of a Hot Strip Mill, having performed in the laboratory scale. Even experimental works at laboratory scale are confined to a stationary plate and at a temperature of 250-350°C, whereas in reality a temperature drop from 890°C to 550°C (near down coiler) for a moving strip is experienced. Kato et al. [11] have developed new cooling nozzles having ability to maintain uniform cooling even with the changes in heat transfer coefficient and thus avoiding the possibility of nucleate boiling. A heat transfer coefficient correlation, which is a power law of water flow density, strip velocity, and strip temperature, has been developed in the same method. The model has been made on-line with feed forward and feedback control technique and the mathematical equation has been modified by an adaptation technique after the correction of heat transfer coefficient. The study of Auzinger et al. [12] has mentioned only the nature of heat transfer coefficient from the strip to the coolant as a strong non-linear function of strip temperature and water flow density in the cooling model. Their study shows that the profile of the heat transfer coefficient follows the exact nature of the boiling curve. Thus, the above-mentioned prior art suggest that the power correlations for heat transfer coefficient are functions of many variables at the same time. In most of the cases, heat transfer co-efficient value at strip-water interface is based on the determined values at laboratory or a complex correlation involving powers of details of plant data. In the laboratory scale, the actual temperature of the strip of a hot strip is never reproduced. It is therefore the object of the invention to provide a method for estimation of the through thickness cooling temperature over the length of a coil from an on-line model to achieve the estimated cooling temperature of the coil. Another object of the invention is to develop an off-line model for modification to an on-line model with the input flow data captured from the field devices of the ROT of HSM. SUMMARY OF THE INVENTION Accordingly there is provided a method for estimation of the through-thickness coiling temperature over the length of a coil in Hot Strip Mills. The method comprises the steps of - development of an off-line model through adaptation of a one-dimensional heat conduction equation being solved by explicit finite difference procedure; development of correlation for heat transfer coefficient for different grades of steel; receipt of input data in respect of temperature, speed of the coil from Level-0 and in Level-1 of HSM; analyzing signals of top and bottom header of the ROT in respect of water cooling of the coil, thereby converting the off-line model to an on-line numerical model; estimation of coiling temperature over the coil using the captured data and analyzing the same using the on-line model, the output from the on-line model being the coiling temperature over the through thickness and length of the coil; estimating of cooling rate based on the estimated coiling temperature. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS FIG. 1 - A general layout of strip cooling system in ROT of HSM FIG. 2 - A computational domain of the strip used for development of the off-line model. FIG. 3 - Automation concept for ROT cooling. FIG. 4 - Displays the representative result of the temperature estimation of the strip over the length of a coil for rolled strip 2.2 mm. DETAILED DESCRIPTION OF THE INVENTION A general layout of strip cooling system in ROT is depicted in Figure 1. Strip is cooled by eleven number of banks installed between the last finishing mill and the down coiler. Out of eleven water banks, there are ten banks of macro cooling and one bank of micro cooling. Each water bank has 2*34 numbers of cooling nozzles. Two pyrometers are installed, one near exit of last finishing stand and another one near the down coiler to measure the temperature eof the strip, which are passed on through level-2 automation. To estimate the temperature profile of strip, a numerical model has been developed to evaluate the through-thickness temperature of the strip. The method was initiated with the goal of an on-line temperature prediction model over the length of the coil and more importance was placed on achieving the prediction of the coiling temperature in the ROT. Hence, the on-line simulator has been developed with totally based on the level-2 automation to receive information on FRT, strip speed, strip thickness and the record of the opening /closing of the valves. The development of an off-line model becomes necessary before putting a model as on-line in a process industry. Pursuant to requirement, the same equation has been incorporated in the development of an Off-line model for estimation of through-thickness temperature of the strip in ROT of HSM. However, the heat conduction phenomenon that determines the dissipation of heat from the hot strip during the cooling at ROT. Different modes of heat transfer, viz. Conduction, Convection and Radiation contribute to the analysis of ROT cooling. Radiation and convection to the ambient air affect the zone of cooling from the last finishing stand to the water-cooling and from the end of water-cooling to the down coiler. The area of the strip under the water-cooling is a combination of complex phenomenon of conduction to the strip and the convection mechanism by the water. Heat conduction to the rolls has been considered to be negligible compared to the heat dissipation by water-cooling and radiation in the ROT bed. Compared to the thickness of the strip, the strip is wide enough and hence the temperature gradient transverse to the direction of movement is practically zero when the velocity of the strip is high. As a result, heat transfer in the direction of strip movement and along the direction of the strip width is substantially smaller than the heat transfer along the strip thickness. Hence, the model based on a one-dimensional heat conduction equation has been adopted for the present numerical study. The numerical model solves the following equation by Explicit Finite Difference method: where p, c and K denote density, specific heat and thermal conductivity respectively. A heat generation term (the last term in RHS of the equation) in the conduction equation has been considered to account for the heat generated due to the phase transformation (y→) that occurs during cooling of the strip. Transformation heat, q, is an important heat source in the numerical study of Run-out Table cooling zone where transformation occurs. BOUNDARY CONDITIONS AND TIME STEP To cover all of the above mentioned heat transfer mechanisms, boundary conditions or heat transfer coefficient values at different zones on the ROT are considered. Suitable boundary conditions have been applied according to the mode of heat transfer in a particular zone. Hence, only the boundary conditions get modified during the solution of the Eq. (1) for that zone. The following initial and boundary conditions have been used for solving the above equation: where, Tf =Finish Rolling Temperature (FRT). Measured temperature at the end of the finishing mill is taken as the input temperature for the start of the simulation of the off-line model for ROT cooling. Here, L: the strip thickness, H : Heat transfer coefficient, Ts: Strip temperature Ta: Ambient temperature To obtain a stable solution by the finite difference method, a time step has been selected by the stability criterion, which is given by: Stability Criterion: where, Biot Number, , Bi = h * Ax / k , Fourier Number, Fo =a At/ (Ax)2 a = Thermal Diffusivity At= Time Step used for Numerical Simulation. Ax = Step Size of computational domain The computational domain used for the analysis of the numerical study is shown in Figure 2. DETEREMINATION OF HEAT TRANSFER COEFFICIENT In the zone of ROT exposed to convective-radiative heat transfer, a psuedo heat transfer coefficient, h airhas been calculated. The pseudo heat transfer coefficient (hair-inst) value for radiation to air has been calculated using Stefan-Boltzmann equation: e = Emissivity, and o = Stefan-Boltzmann constant (5.669e-8 W/m-K4) The heat transfer coefficient (h) at water-strip surface has been determined by the method described as follows. As supported by the flow visualization work of Soh et al. [8], the heat transfer coefficient for boiling phenomenon should be similar to the nature of the boiling curve and it is a function of strip surface temperature. The prior arts have established the dominance of Film Boiling phenomenon around the point of impingement of jet from cooling water from water bank and the possible correlations of heat transfer coefficient during Film Boiling phenomenon have been explored. A cubic correlation of strip surface temperature has been suggested for use of heat transfer coefficient value at water-strip interface. For different grades of steel, the equation suggested for heat transfer coefficient: which gives better result. The determination of values of the constants a, b, c and d in the above equation of the heat transfer coefficient has been performed by a curve fitting method so as to obtain desired CT from the numerical simulation by the use of the constants. ON-LINE DATA (INPUT AND OUTPUT! The invented numerical model receives the input data regarding temperature, speed and other parameters from LeveI-0 field devices of HSM. The process related data are captured through field devices such as Pyrometer for recording FRT, CT, tachometer for the speed and solenoid valves for the opening of valves of water-cooling at ROT. The data thus captured are moved upwards to level-1 PLC (Programmable Logic Controller). Data segmentation of the data file received has been performed to account for the speed of the strip. The data capturing by field devices and receiving through computer have been depicted in automation concept. Based on data from automation, the signals of top and bottom header for water-cooling have been analysed using the numerical code and thus the same off-line model has been converted to work as an on-line model. The output of the on-line model is the temperature over the length and through the thickness of the coil. This is shown on the CRT display. The on-line system described above has been applied for use in the prediction of coiling temperature over the length of coil in ROT bed in HSM. ft has been verified for its versatility by checking it for a wide range of grades and thickness, ranging from 1.6 to 6 mm. Figure 4 displays the representative result of the temperature prediction of the strip over the length of the coil for a rolled strip of 2.2 mm. One line represents the predicted value by adaptation technique from the model, whereas the other displays the actual (measured) temperature from the pyrometer. WE CLAIM 1. An automated on-line system of determining cooling rate of hot rolled coil over its length and across its thickness, comprising the steps of; a) determining the off-line temperature value of the coil through the adaptation of a one-dimensional heat conduction relationship being solved by explicit finite difference procedure; b) developing of a correlation for heat-transfer coefficient at strip-water interface for different grades of steel at different thickness; c) receiving input data in respect of temperature, speed of the coil and signal of opening of water-cooling valve at Run out Table of a Hot Strip Mill (HSM) from level-1 automation; d) analyzing the input data from level-1 automation and the signals of top and bottom header openings of water cooling valves through out the length of the coil for cooling of a coil, thereby converting the off-line system to an on-line system; e) estimating coiling temperature over the length of tehcoil by using captured data and analyzing the same using the on-line system, the output from the on-line system being the temperature over the through thickness and length of the coil; f) estimating cooling rate based on the estimated coiling temperature. Characterized in that the said system receive the input data regarding temperature, speed and other parameters from level-field devices of HSM, and the process related data are captured through field devices such as pyrometer for recording FRT (Finish Rolling Temperature), CT (Coiling Temperature), tachometer for the speed and solenoid valves for the opening of top and bottom cooling at Run-out Table (ROT), the data thus captured are moved to PLC (Programmable logic controller) or Level-1 automation, and segmentation of the data file received has been performed to account for the speed of the strip, the signals of top and bottom header for water cooling have been analysed and thus the same off-line system has been converted to work as an on-line system and the output of the on-line system is the temperature over the length and through the thickness of the coil and the cooling rate of the coil. An automated on-line system of determining cooling rate of hot rolled coil over its length and across its thickness, comprising the steps of: a) determining the off-line temperature value of the coil through the adaptation of a one-dimensional heat conduction relationship being solved by explicit finite difference procedure; b) developing of a correlation for heat-transfer coefficient at strip- water interface for different grades of steel at different thickness; c) receiving input data in respect of temperature, speed of the coil and signal of opening of water-cooling valve at Run out Table of a Hot Strip Mill (HSM) from level-1 automation; d) analyzing the input data from level-1 automation and the signals of top and bottom header openings of water cooling valves through out the length of the coil for cooling of a coil, thereby converting the off- line system to an on-line system; e) estimating coiling temperature over the length of the coil by using captured data and analyzing the same using the on-line system, the output from the on-line system being the temperature over the through thickness and length of the coil; f) estimating cooling rate based on the estimated coiling temperature.

Documents:

345-kol-2004-granted-abstract.pdf

345-kol-2004-granted-claims.pdf

345-kol-2004-granted-correspondence.pdf

345-kol-2004-granted-description (complete).pdf

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345-kol-2004-granted-examination report.pdf

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345-kol-2004-granted-form 2.pdf

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