STEELMAKING FURNACE WITH HUMIDITY CONTROL DEVICE

Abstract

A steel strip annealing furnace with a dew point control system. The furnace/control system can be more readily controlled to the desired dew point than the prior art control system and can handle the set point changes required as different types of steel coils are continuously run therethrough.

Claims

1-4. (canceled)

5. A steel strip annealing furnace with a dew point control system, the furnace including: a furnace having an upper region and a lower region; a furnace atmosphere injector configured to inject furnace atmospheric gases into an injection region in the upper region of the furnace; a steam generator coupled with the atmosphere injection system to mix steam into the furnace atmospheric gases and including a steam generator control unit to control generation of steam; a control system for controlling the steam generator to provide a desired dew point within the furnace; the control system including an input dew point (DP) set point signal generator generating a DP set point signal corresponding to a desired furnace DP; the control system further including two DP sensors measuring a local dew point and transmitting a signal representative of the measured local dew point; one of the DP sensors being an upper DP sensor positioned in the upper region of the furnace and adjacent the injection region; the other of the DP sensors being a lower DP sensor positioned in the lower region of the furnace, remote from the injection region; the control system further including two proportional-integral-derivative (PID) controllers configured in a cascaded loop configuration; the control system further including three signal convertors (SC), each SC designed to receive a DP input signal and convert the DP input signal into a partial pressure of steam (PPS) output signal; a lower of the PID controllers being connected to a first SC of the SCs, the first SC having an input DP set point signal from the DP set point signal generator, and an output PPS set point signal transmitted to the lower PID controller; the lower PID controller also connected to a second SC of the SCs, the second SC having an input lower feedback DP signal from the lower DP sensor and an output lower feedback PPS signal transmitted to the lower PID controller; the lower PID controller comparing the PPS set point signal and the lower feedback PPS signal and generating a lower PID error value; the lower PID error value being added to the PPS set point signal to generate a lower PID output PPS signal; the lower PID controller connected to the upper PID controller, the lower PID controller transmitting the lower PID output PPS signal to the upper PID controller, the lower PID output PPS signal becoming an upper input PPS set point signal for the upper PID controller; the upper PID controller also connected to a third SC of the SCs, the third SC having an input upper feedback DP signal from the upper DP sensor and an output upper feedback PPS signal transmitted to the upper PID controller; the upper PID controller comparing the upper input PPS set point signal to the upper feedback PPS signal and generating an upper PID error value added to the upper input PPS set point signal to generate an upper PID output signal; the upper PID controller connected to the steam generator control unit; the upper PID controller transmitting the upper PID output signal to the steam generator control unit thereby controlling the injection of steam into the furnace.

6. The annealing furnace with dew point control system as recited in claim 5 wherein the control system further includes a feed forward control unit.

7. The annealing furnace with dew point control system as recited in claim 6 wherein the feed forward control unit calculates an adjustment signal to be added to the upper PID output signal.

8. The annealing furnace with dew point control system as recited in claim 7 wherein the adjustment signal to be added to said upper PID output signal is calculated based on known upcoming changes in a steel grade or chemistry, line speed, and steel strip width.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0019] FIG. 1 is a schematic depiction of a hot dip galvanizing line;

[0020] FIG. 2 is a schematic depiction of a prior art system for controlling the atmosphere/humidity within an annealing furnace;

[0021] FIG. 3 is a plot of the dew point and steam generator output vs time for the prior art control system;

[0022] FIG. 4 plots the relationship between dew point in ° C. and percent water in the furnace gas;

[0023] FIG. 5 plots the relationship between partial pressure of water in Pa and the dew point in ° C.;

[0024] FIG. 6 is a schematic depiction of the inventive furnace with control structure;

[0025] FIG. 7 plots the dew point of the RTS furnace using the inventive control structure versus production time for a number of steel coils; and

[0026] FIG. 8 is a schematic depiction of the inventive furnace/control system which includes a feed forward module.

DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention is an annealing furnace for steel strip and control system that can be more readily controlled to the desired dew point and that can handle the set point changes required as different types of steel coils are continuously run therethrough.

[0028] In evaluating the limitations and flaws of the prior art furnace and control structure, the present inventors noted that the relationship between the dew point and the water concentration in the atmosphere is highly nonlinear. FIG. 4 plots the relationship between dew point in ° C. and percent water in the furnace gas. As can be seen, the relationship is highly non-linear, making the task of controlling the dew point very difficult. The inventors also noted that the relationship between dew point and partial pressure of water is relatively linear. FIG. 5 plots the relationship between partial pressure of water in Pa and the dew point in ° C. Therefore, the present inventors added a step to the control system wherein all dew point set points and dew point measurements are converted to partial pressures when input to the control structure.

[0029] The inventors also noted that the mixing time for water input to the furnace until the dew point sensor actually sensed the water is quite large. This again makes control of the dew point very difficult because of the large time lag between water input and sensor measurement. To help combat this, the inventors added a second dew point sensor closer to the steam injection point.

[0030] Finally, the inventors added an addition PID controller in cascade with the original one to improve control of the dew point.

[0031] FIG. 6 depicts a furnace with the new control structure. While only one furnace (RTH 3) is depicted, the same control structure was implemented for both the RTH 3 and the RTS 4. The new control structure retains the original dew point sensor 7 and the bottom of the furnace, and adds a new dew point sensor 7′ at the top of the furnace near the steam injection point. The control structure also includes dew point converters 12, 12′ and 12″ to convert the set dew point, and measured dew points into partial pressures of steam. Thus, the convertor 12 converts the set point dew point signal 10 into a set point partial pressure of water 10*. The convertor 12′ converts the measured dew point signal 10′ from the lower dew point sensor 7 into a partial pressure of steam 10′*. Finally, convertor 12″ converts the measured dew point signal 10′″ from the upper dew point sensor 7′ into a partial pressure of steam 10′″*.

[0032] The equations for conversion of dew point in ° C. to partial pressure of water in atmospheres is given by the following equations:

[00001]$P=10^{\min \left(6.28-\frac{2320}{dp+273.15},7.54\frac{2665}{dp+273.15}\right)}$$\mathrm{dp}=\max \left(2320/\left(6.28-{\log }_{10}P\right),2665/\left(7.54-{\log }_{10}P\right)\right)-273.15$

[0033] It should be noted that the conversion from atmospheres to Pa is 1 atm=101325 Pa.

[0034] The inventive control system now includes two PID controllers forming a cascaded control. The set point signal after conversion to partial pressure of steam 10* is input to the outer loop PID controller 8 this is compared with the measured dew point signal 10′ from the lower dew point sensor 7, which has been converted to a partial pressure of steam 10′*. Outer loop PID controller 8 uses the two signals 10* and 10′* to create an error signal which is added to the set point signal 10* to produce an input signal 10″* to the inner loop PID controller 8′.

[0035] This input signal 10″* is compared with the measured dew point signal 10′″ from the upper dew point sensor 7′, which has been converted to a partial pressure of steam 10′″*. Inner loop PID controller 8′ uses the two signals 10″* and 10′″* to create an error signal which is added to the input signal 10″* to produce an output signal 10′″* to the steam generator controller 6′ which adjusts the output of the steam generator 6.

[0036] These improvements to the control structure of the furnace results in a significant improvement in the dew point control within the furnace. FIG. 7 plots the dew point of the RTS furnace using the inventive control structure versus production time for a number of steel coils and includes a set point change. As can be seen, the dew point control of the furnace is significantly improved and is good enough for continuous production.

[0037] The inventors have further contemplated the possible need for a feed forward mechanism to the control structure. The feed forward signal would be generated based on the type of steel being processed (i.e. the carbon content thereof, reactivity with water vapor, etc), expected line speed changes, steel strip width changes and atmospheric changes to the system. FIG. 8 is a depiction of a furnace/control system which includes a feed forward module 14. A feed forward signal 10{circumflex over ( )} would be mathematically created based on these factors and it would be combined with the output signal 10″″* of the cascade control system to preemptively adjust the signal to the steam generator controller 6′ and ultimately to the steam generator 6. The feed forward signal 10{circumflex over ( )} may increase or decrease the amount of steam being injected into the furnace by the steam generator 6, depending on what the upcoming change involves.

[0038] If the steamer output (controlled ultimately by the inner loop PID 8′) is lower than 4% or higher than 100% (i.e. outside the physical limits of the steam generator 6), there is internal logic which prevents the integrator from windup. That same logical needs to be sent to the outer loop PID to place that integrator into a hold state to prevent windup.

[0039] The control system may also include dry out logic. This Logic will flood both the RTH and RTS furnaces with HNx (pure atmosphere with no added steam) should the steamer output be less than the threshold for steam injection and the error is such that there is too much water in the furnace. This is used when furnace dew point is very high and the steamer is at its lowest setting. Flooding the furnace with dry atmospheric gas from the atmospheric gas supply 5 will flush out the excess moisture very quickly. Once the excess moisture has been flushed from the furnace, the steam generator 6 can bring the furnace back to the proper desired dew point.