Title of Invention

A PROCESS AND APPARATUS FOR BATCH ANNEALING OF COILS IN A COLD ROLLING MILL

Abstract

A process of batch annealing of coils in a cold rolling mill which comprises stacking a plurality of coils in a furnace, and heating the coils in the furnace in a cycle determined by data including coil dimensions, furnace dimensions operating parameters and thermal cycle to obtain coils having predetermined grain size and tensile strength, said cycle defining the temperature and time to be set for heating, soaking and cooling of the coils in the furnace feeding and processing the data into a simulator comprising three modules[i] thermal module; [ii] microstructural module and [iii] mechanical property module as herein describep and obtaining the optimum temperatures and time to be set for the annealing cycle for the predetermined grain size and tensile strength and setting the furnace to heat soak and cool the coils stacked therein in accordance with the set temperatures and time to obtain annealed coils.

Full Text

FORM-2 THE PATENTS ACT, 1970 (39 of 1970) COMPLETE Specification SECTION-10 A PROCESS AND APPARATUS FOR BATCH ANNEALING OF COILS IN A COLD ROLLING MILL TATA CONSULTANCY SERVICES, (a Division of TATA SONS LIMITED), of Bombay House, Sir Homi Mody Street, Mumbai 400 023, Maharashtra, India, an Indian Company 24 JUL 2003 GRANTED 24-7-2008 THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE NATURE OF THIS INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED:- This invention relates to a process and apparatus for batch annealing in a cold rolling mill and particularly to a process for optimizing the operation. In a cold rolling mill, hot rolled coil strips are further rolled at room temperature to improve the quality and mechanical properties of the strips. The room temperature deformation in a cold rolling operation increases the hardness and yield strength of the sheet. However, it is also accompanied by a loss in ductility and formability of the sheet. As a result, for the subsequent forming operations, e.g. deep drawing of auto body parts, annealing of the cold rolled sheets becomes essential. During the annealing operation, the cold rolled sheet is stress-relieved by recovery, recrystallization and grain growth mechanisms. The two available technologies for the annealing of cold rolled sheets are batch and continuous annealing processes. Batch annealing operation is commonly used in cold rolling mills. In the conventional batch annealing process, the steel coils separated by convector plate are stacked on a furnace base. A protective cover encloses the coil where an inert or reducing gas (H2 or N2+H2) is circulated. A gas-fired furnace externally heats this enclosure. The outer and inner surfaces of the coil get heated by convection from the circulating inert gas and by radiation between cover and coil. The inner portion of the coil gets heated through the conduction process, which is retarded due to the gap between the sheets. As a result, during the annealing operation, different locations in the coil undergo different thermal cycles. Since recovery and recrystallization are thermally activated processes, this thermal lag (AT between hot and cold spots) leads to spatial variation in microstructure with an associated variation in the properties of the coil. In order to reduce the property variation, the annealing time is often increased with an associated decrease in the productivity. Traditionally, the batch annealing operations are optimized through plant scale trial and error method. However, due to high cost involved with plant scale trials - in terms of production loss, production waste, and long time - the processes are often run at the sub optimal levels. It has been observed that generally the specified product quality are achieved through higher safety margins resulting in lower throughput, inconsistent product quality, higher rejection, higher energy consumption and lower process efficiency. The annealing operation comprises three segments, i.e. heating, soaking and cooling segments. In the heating segment, the control temperature is at first raised from room temperature (30°C) to 650°C in 6 hours, subsequently, and subsequently from 650°C to 725°C in 5 hours. In the soaking segment, the control temperature is maintained at 725°C. However, in the conventional batch annealing operation, the operator is unable to see the microstructure and mechanical properties during the progress of annealing, and therefore the end of annealing is based on temperature criterion, which is monitored by thermocouples. In this case, when the temperature difference between the base thermocouple and control thermocouple is less than 30°C, the soaking phase of the annealing is deemed to be complete and cooling cycle is initiated. The high temperature requirements (600-700 °C) and long annealing cycles (40-60 hrs) makes this operation energy intensive. In addition, being one of the final operations, it has important bearing on the product quality, which is crucial for application of cold rolled sheets in auto body and white goods. Also, it must be noted that while other unit operations in cold rolling mill - e.g. pickling, cold rolling, galvanizing, and skin pass mill - are continuous process, batch annealing is the only batch process requiring 40-70 hrs of processing time. Therefore, it can be a major bottleneck to the overall material flow and productivity of the Cold Rolling Mill. Because of its relevance on such key plant performance parameters, it is of paramount importance to optimize the batch annealing operation for maximization of the furnace productivity and minimization of the energy cost while achieving the specified product quality. The major variables in a batch annealing operation are: steel grade, coil size, hot/cold-rolling conditions, annealing cycle (temperature/time). During the annealing cycle, the cold spot must be completely recrystallized while the hot spot must not have excessive grain growth. There is an optimum annealing cycle for a given sheet type and coil dimension, which maximizes the product quality and furnace productivity. It is not only difficult but also expensive (in terms of time, cost and production interruption) to estimate this optimum annealing cycle for new grades of steel and coil dimensions through plant trials. There is therefore a need for a process, which optimizes the process for different coils. An object of this invention is to provide a method of batch annealing in a furnace with the ability to optimize the process by fine tuning the parameters to predict variation - within a coil and coil to coil - in temperature, microstructure and final mechanical properties of a coil undergoing batch annealing. Another object of this invention is not only predict the thermal history of the coil, also predict the microstructure and mechanical properties of the product and use this history to carry out the batch annealing process in accordance with this invention. It is a further object of this invention to provide a tool for optimization of batch annealing cycles with maximum furnace productivity and reduced energy consumption while meeting the specified product quality standards. According to this invention there is provided a process of batch annealing in a cold rolling mill which comprises stacking a plurality of coils in a furnace, and heating the coils in the furnace in a cycle determined by data including coil dimensions, furnace dimensions operating parameters and thermal cycle to obtain coils having predetermined grain size and tensile strength, said cycle defining the temperature and time to be set for heating, soaking and cooling of the coils in the furnace feeding and processing the data into a simulator comprising three modules[i] thermal module; [ii] microstructural module and [iii] mechanical property module as herein described and obtaining the optimum temperatures and time to be set for the annealing cycle for the predetermined grain size and tensile strength and setting the furnace to heat soak and cool the coils stacked therein in accordance with the set temperatures and time to obtain annealed coils. In accordance with a preferred embodiment of the invention processing data in the thermal module comprises calculating the transient temperatures cycles at different locations in the coils. In accordance with another embodiment of the invention processing data in the micro structure module comprises inputting the processed results from the thermal module and calculating grain size on the basis of recrystallization and grain growth kinetics as herein described. In accordance with yet another embodiment of the invention processing data in the mechanical property module comprises estimating the mechanical properties of the annealed coils based on the microstructure property correlation as herein described. The invention will now be described with reference to the accompanying drawings in which: Figure 1 of the drawings shows a labeled schematic diagram of a typical batch-annealing furnace. Figure 2a shows the presence of hot spots and cold spots in a coil and Figure 2b shows the influence of these spots on the thermal cycle and recrystallization kinetics. Figure 3 of the accompanying drawings shows a typical batch annealing process in accordance with this present invention. Figure 4 of the drawings which is a block diagram for the creation of the BAFSIM simulator. Figure 5 is a block diagram of the method of this invention showing the variables, which are inputted, and the simulation results received using the BAFSIM simulator of Figure 4. Figure 6 of the accompanying drawings illustrates a typical method in accordance with this invention showing how the BAFSIM simulator is practically applied in the process of this invention. Figure 7 is a graph showing the end of soaking time determined conventionally . Figures 8 and 9 show the determination of soaking time according to the present invention for the two desired parameters. Referring to the drawings, a batch annealing furnace in which annealing operations are carried out is shown schematically in Figure 1. The furnace generally indicated by the reference numeral 10 consists of furnace wall F enclosing therewithin a cavity for loading coils C within a cover CO. The said coils C placed within the furnace f are separated by convector plates CP. Burners B generate flue gases FG which flow throughout the furnace 10, whereas an inert gas IG which is typically Hydrogen is made to circulate around the coils C within the cover CO. Two thermocouples or thermocouples sets are provided to indicate the temperature of the coils C placed within the cover CO. These thermocouples are the base thermocouple BT and the control thermocouple CT. The typical process steps can be enumerated as follows as seen in Figure 3 of the drawings: A. Coil Stacking: First the coils (3-5), are stacked on the furnace base. Convector plates separate the coils. B. Cover Placement: After stacking the coils, the inner cover shell is placed. The cover is sealed airtight to the base. C. Nitrogen Purging: Before firing the burners, the cover is purged with nitrogen to remove the oxygen gas from the base. D. Furnace Placement: Subsequent to the nitrogen purging, the furnace (heating hood) is placed on the cover. E. Hydrogen Flow: After the nitrogen purging is complete, hydrogen is purged into the base. The hydrogen is recirculated inside the cover through base fans. F. Heating/Soaking Cycle: The burners are fired on the furnace. Typically two rows of burners are there on the bottom of the heating hood, each containing 4-6 burners. The flue gas heats the cover through convection and radiation. The cover in turn heats the coil through hydrogen gas convection and radiation. The core of the coils is heated by conduction. Temperature monitoring inside the furnace is done by thermocouples located at flue gas, inert gas and on the bottom of the coils. G. Furnace Removal: At the end of the heating cycle, the furnace is removed from the base. H. Cooling Hood Placement: The cooling hood is placed over the cover. I. Cooling Cycle: During the cooling cycle, air is passed between the cover and cooling hood with blowers. In some cases water-cooling is also done on the cooling hood. J. Cooling Hood Removal: At the end ofthe cooling cycle, cooling hood is removed. K. Cover Removal: When the coil core reaches the specified temperature (-150-175 °C), the cover is removed. L. Coil Removal: At the end of the cycle, the coils are removed from the base and placed under dehumidified air for cooling to room temperature. In the hitherto known process of annealing, the annealing operation comprises three segments, i.e. heating, soaking and cooling segments. In the heating segment, the control temperature is at first raised from room temperature (30°C) to 650°C in 6 hours, subsequently, and subsequently from 650°C to 725°C in 5 hours. In the soaking segment, the control temperature is maintained at 725°C. However, in the conventional batch annealing operation, the operator is unable to see the microstructure and mechanical properties during the progress of annealing, and therefore the end of annealing is based on temperature criterion, which is monitored by thermocouples BT and CT. In this case, when the temperature difference between the base thermocouple BT and control thermocouple CT is less than 30°C, the soaking phase of the annealing is deemed to be complete and cooling cycle is initiated. The high temperature requirements (600-700 °C) and long annealing cycles (40-60 hrs) makes this operation energy intensive. In the process of annealing the coils are not heated uniformly throughout. Cold and hot spots are prevalent in the coils as seen in Figure 2a and Figure 2b shows the influence of these spots on the thermal cycle and recrystallization kinetics. The annealing process in accordance with this invention is based on thermal transport, recrystallization/ grain growth kinetics and rnicrostructure-properties correlation using a processor and mathematical calculations of the optimum time required to achieve the desired characteristics of the annealed coil. Based on the process inputs (e.g.: coil dimensions, furnace dimensions, operating parameters, thermal cycle), the mechanical properties of the coil undergoing annealing are predicted and the optimum, heating, soaking and cooling time is predicted. The soaking time is then set and is not dependent on the difference between the thermocouples. Calculation of the relevant times during the annealing process comprises three modules: (i) thermal, (ii) microstructural and (iii) mechanical property modules. The transient temperature cycle at different locations in the coil is calculated by the thermal module, which serves as an input to the microstructural module, where grain size is calculated on the basis of recrystallization and grain growth kinetics. Finally, the mechanical properties are estimated from the established microstructure-property correlation. The formulation and customization of the BAFSIM simulator which is used to carry out the calculations required for setting the times required for the process in the batch-annealing furnace in accordance with this invention is seen in Figure 4 of the drawings which is a block diagram for the creation of the BAFSIM simulator. The application of the simulator of this invention in the furnace is seen in Figure 5 of the invention which is a block diagram of the method of this invention showing the variables which are inputted and the simulation results received. Figure 6 of the accompanying drawings illustrates a typical method in accordance with this invention showing how the BAFSIM simulator is practically applied in the process of this invention, the block diagram boxes being self-explanatory. FORMULATION OF THERMAL MODULE: The thermal module forms the core of the simulator, where thermal interactions among different components of the furnaces are considered. It considers all the three modes of heat transfer (conduction, convection and radiation) to determine the transient temperature variation in coils and different furnace components, such as flue gas, furnace wall, protective cover, cooling hood, inert gas and convector plate. Output of this module is the complete transient temperature history during heating and cooling cycles, at different locations of the coil. In addition, temperatures of other components are also made available for analysis. In the following subsections, equations and solution strategy for some of the components is described. Thermal Model for the Coils The thermal profiles of the coils are obtained through the solution of conduction equation in cylindrical coordinates. Due to the cylindrical symmetry of the coil, only the radial (r-z) plane is analyzed. As the temperature varies along the axial direction of the furnace, all the four (or five, if desired) coils stacked in the furnace are analyzed. The energy equation for the coil can be represented by: where pm is the density, Cm and kz are temperature dependent specific heat and conductivity and Tm is the temperature of the coil. The radial conductivity of the coil (kr) depends on the sheet thickness and air gap between sheets. The boundary conditions used for the above equation are: where 'e' and 'a' are emissivity and absorptivity, *F is the shape factor, Tc is the cover temperature, Tgi and Tg0 are the hydrogen gas temperature in the inner core and outer annuli between coil and cover and o is the Stefan-Boltzmann constant. Eqn. l.a initializes the coil temperature to the ambient (Tamb), Eqn. (l.b) considers the convection with hydrogen gas and radiation with the cover hood at the outer surface of the coil, while Eqns. (l.c-d) consider the convection at the inner core of the coil. Heat transfer coefficients at the coil outer (ho) and inner (hi) surfaces, are obtained from the correlation for the flow through annuli and cylindrical tube while the heat transfer coefficient for the flow through convector plates (ht/b) is obtained from the correlation for flow through converging rectangular ducts. The heat transfer coefficient (h) is estimated using the following equation: (I.e.) where, Nu is the Nusselt number, kg is the gas thermal conductivity and De is the characteristic diameter. The Nusselt number for the cylindrical tube is expressed as: In the case of cylindrical annuli, the Nusselt number and characteristics diameter are given by: (lg) Whereas, in the case of rectangular ducts, the Nusselt number and characteristics diameter are given by: (l.h.) where Reynolds (Re) and Prandtl (Pr) numbers are; where μg is the viscosity, pg the density, cp the specific heat at constant pressure, kg the thermal conductivity, and the velocity of the gas. The nonlinear radiation boundary condition (1 .b) is split into linear and nonlinear terms for robustness and faster convergence of the equations. The above two-dimensional equation is implicitly solved using control volume formulation by line-by-line method, which is a combination of Tridiagonal Matrix Algorithm (TDMA) and Gauss-Seidel method. Thermal Model for the Furnace Hood The governing equation for the furnace wall is taken as the one dimensional conduction equation, given by: (2) where pw is the density, Cw and Kw are temperature dependent specific heat and conductivity and Tw is the temperature of the furnace wall. The variation of temperature in the axial direction is effected by the variation in flue gas temperature. The boundary conditions used for the above equation are: (2.a) where V and 'α' are emissivity and absorptivity, 'F' the shape factor, subscripts 'c' and T refer to the cover and flue gas respectively. The symbols 'hwo and Twi' refer to the heat transfer coefficients for outer and inner wall surfaces, with outer (Dwo) and inner diameters (Dwi,) respectively. Eqn. (2.a) is for the outer furnace surface and Eqn. (2.b) is for the inner furnace surface, considering flue gas radiation (Tf), radiation between cover and wall, and convection between flue gas and wall. The axial variation in temperature is effected by the flue gas variation and solving the one-dimensional equation for several grid points in the axial direction. This one-dimensional conduction equation is implicitly solved using control volume formulation by TDMA method. The heat transfer coefficient between cover and furnace (hWi) is taken for the flow through cylindrical annuli. Thermal Model for the Cover Hood The cylindrical cover is treated as one-dimensional shell as it is very thin and the temperature gradient across the thickness can be neglected. The governing equation for the cover can be given by following ordinary differential equation (ODE): where pc is the density, Cp,c are temperature dependent specific heat and conductivity of the cover, and / is the cover thickness. The above equation considers following interactions of cover with different furnace components: Tadiation with furnace wall, radiation and convection with flue gas, radiation with coil outer surface, and convection with hydrogen gas. This ODE is solved using fourth order Runge-Kutta technique. Thermal Model for the Flue gas The one-dimensional axial variation in flue gas temperature is obtained using the following ODE, which considers convection and radiation with furnace walls and cover hood: where Uf is the flue gas velocity, and Pf is the perimeter and Af is the cross sectional area between the furnace inner wall and cover outer wall. The adiabatic flame temperatures are assigned at the burner location and are taken as the initial values for the entire furnace module. The above ODE is also solved using fourth order Runge-Kutta technique. Thermal model for Hydrogen Gas One dimensional variation in axial temperature of hydrogen gas is considered for the gas flowing between cover hood and outer coil surface and within the inner coil core. The corresponding ODE are given by: where Ug is the hydrogen gas velocity, and Pg is the perimeter and Ag is the cross sectional area between the coil outer diameter and cover wall. The variation in hydrogen gas temperature across the convector plate in the radial direction is obtained by interpolating the outer and inner hydrogen gas temperatures. These ODEs are also solved using fourth order Runge-Kutta technique. The coils are divided into grids in the radial and axial directions. Grids for the gases, furnace wall and cover hood in the axial direction are replicated from the axial grids of the stacked coils. The governing equations for all the components are solved in sequence and iterated until a global convergence of temperatures is achieved. The transient profile is achieved by marching in time. EVOLUTION OF MICROSTRUCTURB AND FINAL MECHANICAL PROPERTIES In the batch annealing operation the microstructural changes in the coil are (a) recrystallizarion and (b) grain growth. In addition, in the case of A1K steels, A1N precipitates and coarsens during the batch annealing. The transient temperature variation at different coil locations, obtained from the thermal module is used to estimate the grain size using the recrystallization and grain growth kinetics. The grain size distribution is finally used to predict mechanical properties, through microstructure-property correlations. A1N Precipitation In the A1K steels, precipitation and growth of A1N is an important phenomenon controlling the favorable texture and normal anisotropy (r) in the product. This strong influence of A1N precipitates is effected through the precipitation-recrystallization interaction, resulting in the pancake shape grains. In order to achieve this fruitful interaction, precipitation is suppressed during the hot rolling stage by maintaining a low coiling temperature. The formation of fine precipitates during the slow batch annealing process and its interaction with nucleation and growth of recrystallized grains, result in preferential growth of grains having higher density and grain boundary energy. The precipitation and growth of A1N is a thermally activated process and its kinetics follows a characteristic C-curve with a maxima around 800 °C. The optimum heating rate for A1K steels during the batch annealing process for precipitation to precede the recrystallization is given by: (6) where, OHR is the optimum heating rate in Kh'1, CR is the %cold reduction and [Al], [N], and [Mn] are the weight % solute content in steel. Recrystallization Kinetics Recrystallization kinetics of steel has been conventionally examined using Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory. In the JMAK theory the fraction recrystallized is given by: (7) where X is the volume fraction recrystallized after time (t), krex is the temperature dependent constant and nrex is the Avrami exponent. The above equation can be rewritten as: (7a) Therefore, from the plots of ln[ln(l/l-X)] versus ln(t), the exponent 'nrex' and temperature dependent constants Krex can be obtained. The value of exponent 'nrex' should be unique for a single mechanism to be operative at the entire temperature range. The intercepts of these plots gave Ln Krex at three temperatures, while the slope yielded the Avrami exponent 'nrex'. The temperature dependent constant Krex' depends on the nucleation and growth rates, which follows Arrhenius type correlation: where, Ko is the pre-exponent constant Qrex is the empirical activation energy, characteristic to the transformation process. The activation energy can be obtained from the slope of inverse of absolute temperature [1/T] versus (l/nrex) In Krex plot. Grain Growth Kinetics The average grain growth rate is conventionally represented by the well-established Beck-type correlation: The pre-exponent constant (Ko) and activation energy of grain growth (Qgg) is obtained from the inverse of temperature versus ngg ln Kgg plot. where 'dgg' is the mean grain size and 'T' is the annealing time. The temperature dependent coefficient 'Kgg' (from the intercepts) and the exponent 'ngg' (from the slope) are determined from the ln-ln plot of annealing time versus grain size. The temperature dependent constant, Kgg follows Arrhenius type correlation: Mechanical Properties The end customer is ultimately interested in the mechanical properties of the product. Therefore it is important to convert the microstructural information (grain size) into mechanical properties. The relevant properties in a cold rolled-batch annealed steel sheet are: normal anisotropy (r), planar anisotropy (Δr), strain hardening exponent (n), tensile strengths (σyiaurs), and percent elongation (%E1). Out of the above properties, r and Δr have strong dependence on the history of the sheet being annealed (e.g. soaking and coiling temperature at the hot rolling stage). Therefore it is difficult to predict f and Ar without a complete tracking of the sheet flow and an extensive texture modeling and is not a part of this model. The remaining properties can be estimated using this simulator. At low temperatures, the grain boundaries act as obstacles to dislocation motion and provide strengthening through stress concentrations due to pileup of dislocations at the grain boundaries. The effect of grain size on yield strength of steel is given by the famous Hall-Petch relationship: (H) where σYs is the yield stress, σ0 is the frictional stress required to move dislocations and, khP is the Hall-Petch slope, and D is the grain size. For the calculation of UTS, correlation analogous to the Hall-Petch equation is used and equivalent frictional stresses and Hall-Petch slopes are computed. The power of mathematical modeling lies in its potential application for the process optimization for reduction in energy consumption, enhancement in productivity, improvements in process efficiency and product quality. Instead process optimization can easily be carried out using the mathematical model. With the availability of good computer speed at very affordable price, model based optimization provides an attractive alternative to the conventional trial and error methods. Using the process model, optimization of the various process parameters - such as temperature-time cycle, and throughput - can be carried out over large number of simulations. The common aims for process optimization in batch annealing operations are enhancement in productivity and reduction in energy consumption without impairing the product quality. Once the cycle has been designed using the simulator in accordance with this invention, can be deployed on the actual furnace as seen in Figure 6 of the accompanying drawings. Practical application of the method of the invention for batch annealing: Four interstitial free grade cold rolled steel coils, with 575 mm of core diameter, 1750 mm of outer diameter, 1 mm sheet thickness and 1125 mm as coil width, were stacked for batch annealing. The end specification for the annealed coil required the ultimate tensile strength to be less than 300 MPa and the grain size to be more than 18 microns for the entire coils. At first the annealing was carried out in conventional method. The annealing operation comprised of three segments, i.e. heating, soaking and cooling segments. In the heating segment, the control temperature was at first raised from room temperature (30°C) to 650°C in 6 hours, subsequently, and subsequently from 650°C to 725°C in 5 hours. In the soaking segment, the control temperature was maintained at 725°C till the desired specifications of mechanical properties and grain size were achieved. However, as during the conventional batch annealing operation, the operator is unable to see the microstructure and mechanical properties during the progress of annealing, the end of annealing is based on temperature criterion, which is monitored by thermocouples. In this case, when the temperature difference between the base thermocouple and control thermocouple was less than 30°C, the soaking phase of the annealing was deemed to be complete and cooling cycle was initiated. The temperatures of the two thermocouples are shown in Fig. 7. As can be readily seen that after around 26 hrs of annealing (heating + soaking time) the required temperature difference of 30°C is achieved. The total cycle time using this methodology is 46 hrs (11 hrs of heating, 15 hrs of soaking and 20 hrs of cooling). Batch annealing of an identical set of coils was carried out using the approach of this invention, where grain size and ultimate tensile strengths were monitored using the mathematical model in the computer attached to the annealing furnace. As described earlier, during the annealing of cold rolled steel, the grain size increases due to grain growth and the ultimate tensile strength decreases due to softening. The major advantage of directly monitoring the grain size and ultimate tensile strength, rather than indirectly through temperature, lies in the ability to end the soaking segment as soon as the specifications are reached. The variation in grain size and ultimate tensile strength with annealing time is shown in Figs. 8 and 9. It can be seen that by using this methodology, the required specification of grain size larger than 18 microns is achieved for annealing time (heating + soaking) is only 22 hrs and 30 minutes, whereas the specification of ultimate tensile strength lower than 300 MPa is achieved after 23 hrs and 30 minutes. Therefore the soaking cycle can be ended only after 23 hrs and 30 minutes, when both the specifications of grain size as well as ultimate tensile strengths are achieved. As can be readily observed, in this case study, the usage of the methodology of this invention, resulted in 2 hrs and 30 minutes of saving in annealing time, which is over 10% improvement in the annealing time (heating + soaking time). This will directly reduce the furnace energy consumption by -10% per cycle and enhance the furnace productivity by -5%. These processes being highly energy intensive with productivity being a major constraint, such improvements will translate into significant reduction in process cost and enhancement of furnace efficiency. Depending on the size of the coil and current operating practice (differential temperature) around 10-20% reduction in energy consumption and 5-10% improvement in productivity can be achieved using this methodology. Advantages of using the process in accordance with this invention: 1. It is for the first time an integrated batch annealing model with predicting capability extended to microstructure and mechanical property of the coil being annealed is available. 2. Design of optimal annealing cycles results in lower energy consumption, and higher productivity, while meeting the quality requirements. 3. Considerable reduction in plant trials and development time for designing cycles for new grades can be achieved by using the method of this invention. 4. As opposed to the conventional process, where temperature differential was used to decide the soaking time, here directly on the basis of grain size and mechanical property specifications, the optimal cycle can be designed resulting in higher furnace efficiency. 5. The process can be used to monitor the temperature, microstructure and mechanical property at various locations in the coils, during the course of annealing, which is otherwise not possible to monitor. Therefore, better preventive actions can be taken under abnormal situations. 6. In case of intermediate breakdown, the coils can be salvaged by designing a suitable cycle for re-annealing using the process. 7. The furnace can be controlled by online deployment of the process. This will result in smoother operation with reduced process variability, reduction of operator's fatigue as well as improved process efficiency. 8. This process can be used in coil annealing operations of steel rods, as well as nonferrous metal sheets and rod coils. As will be readily appreciated, numerous variations and combinations of the features set forth above can be utilized without departing from the present invention as set forth in the above description of the invention. Such variations are not regarded as a departure from the spirit and scope of the invention, and all such modifications are intended to be included within the scope of the above description. We Claim: 1. A process of batch annealing of coils in a cold rolling mill which comprises stacking a plurality of coils in a furnace, and heating the coils in the furnace in a cycle determined by data including coil dimensions, furnace dimensions operating parameters and thermal cycle to obtain coils having predetermined grain size and tensile strength, said cycle defining the temperature and time to be set for heating, soaking and cooling of the coils in the furnace feeding and processing the data into a simulator comprising three modules[i] thermal module; [ii] microstructural module and [iii] mechanical property module as herein describep and obtaining the optimum temperatures and time to be set for the annealing cycle for the predetermined grain size and tensile strength and setting the furnace to heat soak and cool the coils stacked therein in accordance with the set temperatures and time to obtain annealed coils. 2. A process of batch annealing of coils in a cold rolling mill as claimed in claim 1, in which processing data in the thermal module comprises calculating the transient temperatures cycles at different locations in the coils. 3. A process of batch annealing coils in a cold rolling mill as claimed in claim 2, in which processing data in the micro structure module comprises inputting the processed results from the thermal module and calculating grain size on the basis of recrystallization and grain growth kinetics as herein described. 4. A process of batch annealing of coils in a cold rolling mill as claimed in any one of the preceding claims in which processing data in the mechanical property module comprises estimating the mechanical properties of the annealed coils based on the microstructure property correlation as herein described. 5. A process of batch annealing of coils in a cold rolling mill as claimed in any one of the preceding claims in which calculating the transient temperature cycle in the coils includes determining the thermal profiles of the coils obtained through the solution of conduction equation in cylindrical coordinates. 6. Apparatus for batch annealing of coils in a cold rolling mill comprising a furnace having an enclosure in which coils to be annealed can be stacked; burners in the furnace for heating stacked coils; temperature setting and adjusting means which can be operated to set, vary and maintain a temperature within the furnace to subject the coils therein through a heating, soaking and cooling cycle; a computing and processing means cooperating with the furnace into which data relating to the coils and the furnace and the predetermined desired grain size and tensile strength can be inputted and the optimum temperature and time to be set and adjusted in the temperature setting and adjusting means can be computed and obtained on line to obtain annealed coils of the required grain size and tensile strength. 7. A process for batch annealing of coils as described herein with reference to the accompanying drawings. Dated this 18th day of January 2002 Mohan Dewan OfRKDewan&CO Applicants' Patent Attorneys

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