Challenges in Reheating Furnaces – Session 4, Reheating and Process 28-29 October 2002, The Institute of Materials, London, UK

STEELTEMP® FOR TEMPERATURE AND HEAT TRANSFER ANALYSIS OF HEATING FURNACES WITH ON-LINE APPLICATIONS

Bo Leden, MEFOS, Luleå, Sweden

ABSTRACT The STEELTEMP programs are very comprehensive and can be used for analysis of many different steel processes. For reheating furnaces, detailed 2D, 3D and CFD studies can be made. In the reheating furnace models, the heating curve of the stocks and oxide scale forma-tion can be calculated, either from specified furnace temperatures - the simple heating model - or from the geometrical and thermal description of the furnace, fuel and combustion air flows, etc., using the complex heating model or the dynamic heating model. Models for induction heating of stocks for rolling are available as well. Combustion calculations can also be per-formed using the program STEELTEMP 2D. The three-dimensional finite-element (FEM) code is capable of taking into account non-uni-form heating of stocks caused by radiation shadowing from the skid pipes, the contact be-tween the wearer bars and the stocks, baffles in the furnace and end effects in the stocks. Special software for calibration of the heating models in the FOCS furnace control systems has been implemented. During the simulation, STEELTEMP 2D/3D reads and interprets the log files created by the FOCS system during the trial. The temperatures calculated with STEELTEMP 2D/3D can be compared, either with the corresponding temperatures calcu-lated on-line by the heating model of the FOCS system, the so-called 99-test, or with the measured temperatures of a test stock. Large deviations in 99-test are obtained if the FOCS logging is not working properly, the FOCS code is erroneous or the parameters in the input file of STEELTEMP 2D/3D disagree with the corresponding parameters in the furnace spe-cific database of the FOCS system. When the 99-test is working properly the calibration is performed, which means that the parameters of the heating model are adjusted to obtain an optimal fit between the temperatures calculated by STEELTEMP 2D/3D and the measured temperatures. On-line applications are presented for heating of Orvar Supreme ∅1.250*2.573 m ingot in bogie hearth furnace # 1 at Uddeholm Tooling AB. In the heating model of FOCS-BNF the most outer region of the ingot is filled with oxide scale, growing in accordance with the growth of oxide scale on the surface of the ingot. Another application deals with slab pusher furnace # 2 at SSAB Oxelösund’s heavy plate mill. For this furnace the 99-test is first performed showing that the system is quality assured. Then a recalibration of the heating model of FOCS-RF is made for the wall biases of the top and bottom heating zones using the optimising program STEELOPT . This program runs STEELTEMP 2D in a batch mode, during the calibration procedure. After the recalibration a good agreement is obtained between calculated and measured slab temperatures.

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1 INTRODUCTION

In 1976 the development of the finite-difference program, STEELTEMP 2D, for temperature and heat transfer analysis in steel works commenced. It became apparent that a tool was needed to simulate several consecutive steel plant operations accurately in order to be able to optimise the whole production in the work. Today, the following editions of the STEELTEMP software are available: • STEELTEMP 2D for 2D temperature calculations in steel works • STEELTEMP 3D for 3D (FEM) temperature calculations in reheating furnaces • STEELTEMP GR for 2D (FEM) temperature calculations during groove rolling • STEELTEMP CFD for CFD flow and temperature calculations in reheating furnaces The development of the STEELTEMP programs since 1977 is illustrated in Figure 1. First, the STEELTEMP 2D [1] edition was developed for temperature and heat transfer analysis during teeming, cooling, stripping, heating, flat rolling and open die forging. Based on the rolling model in STEELTEMP 2D the STEELTEMP GR edition for groove rolling was developed in the late nineteen eighties. Ten years later, the first version of a 3D heating model, STEELTEMP 3D, capable of calculating the temperatures and skidmarks on slabs heated in reheating furnaces appeared. In early 2000, the first version of STEELTEMP CFD was released for computational fluid dynamics. Flue gas flows and flue gas temperatures in a furnace can be calculated with the aid of this program. From the very start, STEELTEMP 2D was run on main frame computers using punch cards. During the nineteen eighties, most applications were on Digital VAX/Alpha and Nord com-puters.

Figure 1 – Illustration of the development of the STEELTEMP programs since 1977.

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From 1989 to 1995, PCs with the operating system Dos were used in parallel with Digital VAX/Alpha computers. Since 1995, all STEELTEMP applications are run on PC with the operating system Windows. The capacity of the computers used to run STEELTEMP has increased by a factor of more than 1000 since 1980. 2 MATHEMATICAL MODELS

2.1 2D and 3D reheating furnace models

In the reheating furnace models [2], the heating curve of the stocks and oxide scale formation [3] can be calculated, either from the measured furnace temperatures - the simple heating model [2] - or from the geometrical and thermal description of the furnace, fuel and air flows, etc., using the complex heating model [2] or the dynamic heating model [4]. Non-uniform heating of stocks caused by radiation shadowing effects from the skid pipes, contact between the wearer bars and the stocks, baffles in the furnace and end effects in the stocks can be analysed using STEELTEMP 3D, cf. [5-6]. 2.2 CFD reheating furnace model

In STEELTEMP CFD a dynamic grid of the furnace with stocks, updated from the FOCS log files, is generated. For an arbitrary time 3D images, showing the exact position of the stocks including gaps between stocks, can be plotted and the flue gas flows and flue gas tem-peratures in the furnace calculated. 2.3 Software for calibration of the heating models in FOCS systems

Special software has been developed for calibration of the heating models in the following FOCS systems [7-8]: • FOCS-RF (Compaq VAX/Alpha (Open VMS) and PC (Windows NT)) • FOCS-BNF (Compaq VAX/Alpha (Open VMS) and PC (Windows NT)) • AF200FOCS (ABB Master Piece 200) • RFC200FOCS (ABB Advant Controller) During the simulation, STEELTEMP 2D/3D reads and interprets the log files created by the FOCS system during the trial. The gas and wall biases of the thermocouples used to calculate the furnace temperature above the test stock are stored in a calibration file. These biases can be changed by an optimising program, STEELOPT, running STEELTEMP 2D in a batch mode, during the calibration procedure to obtain an optimal fit between the measured and cal-culated heating curve of the test stock. The temperatures recorded from the test stock are con-verted and stored in a measured data file. The temperatures calculated with STEELTEMP 2D/3D can be compared, either with the corresponding temperatures calculated on-line by the heating model of the FOCS system, or with measured temperatures in a test stock. The first comparison, the so-called 99-test, is done to ensure that the same parameters are used both in STEELTEMP 2D/3D and the FOCS system. The second comparison is used when calibrating the FOCS system.

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3 APPLICATIONS

3.1 Two-dimensional calculations of temperatures and oxide scale on an Orvar Supreme ∅1.250*2.573 m ingot heated in bogie hearth furnace # 1 at Uddeholm Tooling AB

3.1.1 Bogie hearth furnace

In the forging plant of Uddeholm Tooling AB four bogie hearth furnaces are used for high temperature annealing and heating of ingots with weights up to 45 tonnes. The instrumenta-tion and modelling of bogie hearth furnace # 1 is shown in Figure 2.

Figure 2 – Instrumentation and modelling of bogie hearth furnace # 1 at Uddeholm Tooling AB. 3.1.2 Experimental procedure

A circular Orvar Supreme ∅1.250*2.573 m test ingot was heated in the oxyfuel fired bogie hearth furnace # 1 at the forging shop of Uddeholm Tooling AB. The ingot was charged close to the charging door. Further into the furnace three other ingots resting on the hearth were heated. Above these ingots other ingots were piled to created a realistic heating situation. The test ingot was charged at 10:07 a.m. on the 27th of June 2001 and was discharged from the furnace at 10:30 p.m. the following day. The test ingot was equipped with six thermocouples drilled into the ingot and connected to the furnace control system FOCS-BNF for logging. The locations of the thermocouples are given below. Channel 1: Surface, 12 o’clock, 40 mm depth Channel 2: Half radius, 12 o’clock Channel 3: Surface, 9 o’clock, 40 mm depth

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Channel 4: Centre Channel 5: Surface, 3 o’clock, 40 mm depth Channel 6: Surface, 6 o’clock, 40 mm depth The test conditions are summarised below. Test ingot: ∅1.250*2.573 m, type Orvar Supreme Position: Y-direction 2.448 m, z-direction 1.588 m Initial ingot temperature: Surface 597 °C, middle 651 °C, centre 676 °C Fuel: Oxyfuel (Propan 95%) Homogenisation temperature: 1300 °C Forging temperature: 1240 °C Residence time in furnace of test ingot: 36 hr 23 min During the trial, log files were created by the Furnace Optimising Control System for Batch Normalising Furnaces, FOCS-BNF. Ingot data, charging, scale breaking times, etc. were stored in an event file. The measured gas and wall/roof temperatures, the fuel and air flows to the control zones, etc. were stored in a signal file. The measured temperatures in the test ingot were stored in a measured file. These three files can be retrieved by the calibration model of STEELTEMP 2D for calibration to recreate the furnace operations during the trial. 3.1.3 Results

The circumference of the cross section of the ingot is divided into four 90ο regions in ϕ-direc-tion according to Figure 2. In r-direction the cross section is also divided into four regions of sizes 50, 25, 15 and 10 % of the ingot radius. Initially these regions are all filled with Orvar Supreme steel. When an oxide scale of thickness 1 mm is formed on the surface of the ingot the outer 10 % region is filled with oxide scale instead of steel and its thickness is set to 1 mm. Thereafter, the size of this region will grow in accordance with the growth of oxide scale on the surface of the ingot. The upper part of Figure 3 displays the history plot of the calculated ingot temperatures at the measurement points located at the surface (40 mm depth) 12 o’clock, half radius 12 o’clock and centre as well as the calculated gas and wall/roof temperatures above the top region (180ο- 270ο) and outer regions (90ο- 180ο, 270ο- 360ο) around the circumference of the ingot. Good agreement is obtained between the calculated and measured temperatures in the three selected measurement points. The noticeable deviations are explained below. Deviations in the surface temperature (black curve): The ingot was charged at 10:07 a.m. The temperature calculations in FOCS-BNF were first restarted at 11:05 a.m., which caused the furnace temperature, read by STEELTEMP 2D from the log files, to be constant between 10:07 a.m. and 11:05 a.m. During this period, the actual furnace temperature increased considerably. When the oxide scale formed on the ingot surface exceeds 1 mm the outer region in r-direc-tion is filled with oxide scale of thickness 1 mm. The position of the node point just below the surface at 12 o’clock is then automatically moved towards the surface and will be

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repositioned 1 mm below the surface. Therefore, the temperature at this node point is sud-denly increased. This occurs at 3:36 p.m., ca. 20 000 s after that the ingot has been charged. The corresponding measurement point is located at 40 mm depth the whole time. Deviation in the centre temperature (green curve): Approximately 45 000 s after that the test ingot has been charged a disturbance occurs in the measurement of the centre temperature of the ingot. This causes the centre temperature to deviate from the corresponding temperature calculated by STEELTEMP 2D during the last part of the heating. At the end of heating the thickness of the oxide scale is 12 mm. The total amount of formed oxide scale is 498 kg, corresponding to 2 % of the weight of the ingot.

Figure 3 – Upper part: Calculated ingot temperatures at the measurement points located at the surface (40 mm depth) and the half radius at 12 o’clock, at the centre and measured/calculated furnace temperatures of bogie furnace # 1 at Uddeholm Tooling AB. Lower part: Deviations between calculated and measured ingot temperatures.

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3.2 Calibration of the heating model of the furnace control system FOCS-RF of pusher furnace # 2 at SSAB Oxelösund AB

3.2.1 Pusher furnace

In the plate mill at SSAB Oxelösund AB traditional steel grades, weldable extra-high-strength construction steel and abrasion resistant steel are heated in one 55 and one 80 tonne/hr pusher furnace. Furnace # 2 consists of a top and bottom-fired heating zone, and a top-fired soaking zone with plane hearth. The heating zones are equipped with six front burners in the top zone and five front burners in the bottom zone, while the soaking zone is equipped with six side burners, three on each side wall of the furnace. The furnace can be fired both with coke oven gas and heavy oil. Didier-Werke AG of Germany supplied the furnace. The instrumentation and mathematical modelling of the furnace are shown in Figure 4. The top and bottom heating zones are each divided into two dark zones and two and three burner zones, respectively. In the figure these zones are identified as zones 1-4 and 9-13, respec-tively. The soaking zone consists of two dark zones and two burner zones, zones 5-8. From a control point of view, the furnace is divided into three control zones. The wall/roof tempera-tures are measured with conventional thermocouples in all zones. In dark zone 9, the gas tem-perature is measured with a conventional thermocouple in the waste-gas flue.

Figure 4 – Instrumentation and modelling of 55 tonne/hr slab pusher furnace # 2 at the heavy plate mill at SSAB Oxelösund AB.

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3.2.2 Short description of STEELOPT

STEELOPT is a program for optimisation of the gas and wall biases of the indicating and controlling thermocouples of the heating model of the FOCS-RF system. For each test slab experiment a complete set of calibration files should be prepared. These sets should reference the same Opt.cal file for the biases of the thermocouples and Plotc.plt file for the calculated slab temperatures and their deviations from the measured temperatures. STEELOPT’s principal way of working may be described as follows: 1. Change bias values and write them to the Opt.cal file. The first iteration uses the initial values supplied by the user in the Opt.cal file 2. Spawn a new batch STEELTEMP 2D process that uses the new bias values. When spawning the new process STEELOPT grants a handle to STEELTEMP 2D that it will eventually use for signalling STEELOPT when its execution is finished 3. When STEELTEMP 2D has finished its execution, it signals STEELOPT to read the Plotc.plt file. STEELOPT calculates its sum of errors squared from Plotc.plt as L = wA * ∆θA

2 + wB *∆θB 2 + wC * ∆θC

2 where wX = wheights of differences ∆θX for the measurement points X = A, B, C ∆θX =differences between calculated and measured slab temperatures for X = A, B, C If a minimum in error in the error-sum-of-squares L has not been found and the maximum number of iterations is not reached the procedure is repeated from step 1. In STEELOPT there are several available commands. Variables can be set to fixed values, a window in the length direction of the furnace can be specified for which the optimisation should be done and the weight factors wA , wB , wC set to different values. 3.2.3 Calibration procedure

Special software for calibration of the heating models in the FOCS furnace control systems has been implemented. During the simulation STEELTEMP 2D/3D reads and interprets the log files created by the FOCS system during the trial. The temperatures calculated with STEELTEMP 2D/3D can be compared, either with the corresponding temperatures calcu-lated on-line by the heating model of the FOCS system, the so-called 99-test, or with the measured temperatures of a test stock. Large deviations in 99-test are obtained if the FOCS logging is not working properly, the FOCS code is erroneous or the parameters in the input file of STEELTEMP 2D/3D disagree with the corresponding parameters in the furnace spe-cific database of the FOCS system. When the 99-test is working properly the calibration is performed which means that the parameters of the heating model are adjusted to obtain an optimal fit between the temperatures calculated by STEELTEMP 2D/3D and the measured temperatures. Figure 5 displays STEELTEMP 2D run-time plots for the 99-test showing the furnace, slab and furnace temperatures generated from the log files and slab temperature differences be-tween temperatures calculated by STEELTEMP 2D and FOCS-RF. The furnace tempera-tures in the middle part of the figure are the temperatures above the selected test slab, used by

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the heating model of FOCS-RF to calculate the slab temperatures on-line. The furnace tem-peratures are recreated by STEELTEMP 2D from the log files generated by the FOCS-RF system. These files also contain the slab temperatures calculated on-line by FOCS-RF. From the lower part of the figure, it can be seen that the FOCS-RF system is quality assured, i.e., the temperature deviations between the temperatures calculated by STEELTEMP 2D and FOCS-RF are negligible.

Figure 5 - The results of the STEELTEMP 2D quality assurance test of the heating model of the FOCS-RF system for the pusher furnace #2 at SSAB Oxelösund AB. Upper part: The furnace and its zones. Middle part: Calculated slab temperatures at the measurement points located at 25, 50 and 75 % depth of slab thickness and furnace temperatures (red/blue straight/broken curves corresponds to gas/wall temperatures in the upper/lower zones, respectively). Lower part: Deviations between calculated and measured slab temperatures. In Figure 6 the results of a recalibration of the above heating model of FOCS-RF for SSAB’s pusher furnace # 2 are shown. In this case, only the wall biases of the top and bottom heating zones have to be recalibrated. To take into account the cooling effect from the slab ends on the side wall mounted thermocouples in the bottom heating zone, negative fixed wall biases are used for thermocouple TI10, TI11 and TC12, respectively. Using STEELOPT

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Figure 6 – Window SteelOpt used for entering commands when calibrating the heating

model of the FOCS-RF system of pusher furnace # 2 at SSAB Oxelösund AB.

Figure 7 – Window SteelOpt_Graph used for presentation of optimisation results obtained

with STEELOPT.

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the optimal values of the wall biases of the roof mounted thermocouples TI2, TI3 and TC4 are calculated. During the optimisation, all other biases were fixed. In the list of variables in the SteelOpt window in Figure 6 the fixed variables are marked with an * behind the numbers of the variables. The upper part of Figure 7 displays the history plot of the calculated slab temperatures at the measurement points located at 25, 50, 75 % depth of slab thickness, as well as the measured/ calculated furnace temperatures. Temperature deviations between the calculated and meas-ured slab temperatures are shown in the lower part of the figure. In the upper part of this figure the positions of the indicating and controlling thermocouples can also be seen. These positions are indicated as square red and blue buttons for the gas and wall biases of the ther-mocouple, respectively. If the variable is fixed, the button appears to be pressed into the window, otherwise, it does not. As can be seen from the lower part of the figure, the agree-ment between the calculated and measured temperatures is good for the recalibrated model. 4 REFERENCES

[1] Leden, B.: ” STEELTEMP - A program for temperature analysis in steel plants”, Scand. J. Metallurgy, 15 (1986), p. 215-223.

[2] Leden, B.: ”Mathematical reheating furnace models in STEELTEMP”, Proceedings

of the International Conference SCANHEATING ’85, MEFOS, Luleå, June 1985. [3] Jarl, M. and Leden, B.: ”Oxide scale formation on steel in fuel fired reheating fur-

naces”, Proceedings of the International Conference SCANHEATING ’85, MEFOS, Luleå, June 1985.

[4] Leden, B., Rensgard, A. and Korteniemi, M.: ”Analysis of hot charging process for

slabs by advanced simulation program STEELTEMP at Outokumpu Polarit’s hot strip mill in Tornio, Finland”, La Revue de Métallurgie-CIT, 91 (1994), No. 4,

p. 541-548. [5] Leden, B., Lindholm, D. and Nitteberg, E.: "The use of STEELTEMP software in

heating control", La Revue de Métallurgie-CIT, 96 (1999), No. 3, p. 367-380. [6] Lindholm, D. and Leden, B.: “A finite element method for solution of the three-dimen-

sional time-dependent heat-conduction equation with application for heating of steels in reheating furnaces”, Numerical Heat Transfer, Part A: Application, 35 (1999), No. 1,

p. 155-172. [7] Leden, B.: "A control system for fuel optimisation of reheating furnaces", Scand. J.

Metallurgy, 15 (1986), No. 1, p. 16-24. [8] Norberg, P.-O. and Leden, B.: "New developments of the computer control system

FOCS-RF - Application to the hot strip mill at SSAB Domnarvet", Proceedings of the International Conference Scanheating II, MEFOS, Luleå, June 1988.