DEVELOPMENT OF AUTOMATIC CONTROL SYSTEM OF HOT-STRIP MILL PROCESS PARAMETERS

It is observed that the contemporary trend of rolling development is widening of hot-strip mills assortment because of ultrathin hot-rolled strips production instead of cold-rolled ones. The advisability of the automatic control system of process parameters (such as tension, gauge, and roll speed) update is demonstrated on the examples of wide-strip mills 2000 and 2500 at Magnitogorsk Iron & Steel Works. Control algorithms for electric drives speed rates are examined, which provide proportional position control of finishing rolling speed under control and perturbation actions with corrective action transmission against rolling direction. Corrected digital algorithms of tension dynamic control enabling longitudinal grow-back decreasing due to accuracy increase of looper electric drive torque and loop size in interstand space computation are presented. The streamlined structure of automatic control system of strip gage (ACSG). The system realizes control method on base of indirect control with sensor values averaging over a specified period of time combined with direct control following exit gage signal and external correction providing perturbation actions compensation and draft redistribution in finishing trains. The developed system of automatic head end gage correction by moving rollers apart before strip enters mill with their subsequent return to specified position for main strip end rolling is demonstrated. The results of improved ACSG application investigations on operating rolling mill are presented. The method and the system of interactive tension and strip gage control are proved and examined whereby automatic correction of hydraulic roll screwdown position of the previous stand under perturbation and control actions caused by change of looper or roll screwdown of the next stand positions is implemented. The results of research and commercialization of the developed systems confirm technical-and-economic efficiency provided with reduction in input of materials, improvement in the quality of production and steadiness of the processing.


Introduction
The rolling mills automation level defines the accuracy of process parameters control that is the main factor affecting the quality and competitiveness of iron and steel works product.
Research conducted by experts from International Iron and Steel Institute (IISI), (since 2008  World Steel Association, Worldsteel) allowed to state the main requirement which should be met in XXI century that is high quality, low cost and small-lot steel delivery in a short space of time [1].
In two recent decades, there is a growing demand for thin hot-rolled 0.8-1.2mm strip being a final market product that is not to be treated in cold position.Nevertheless, it is objectively held that conventional highly productive mill guarantees good quality when the gauge of the plate is not less than 1.5 mm.While attempting to produce strip plate thinner than named, there are technical and technological difficulties connected with overranging the permissible speed and temperature [2].
It is evident that full compatibility of quality characteristics for hot-rolled and cold-rolled strips with the same gauge is reachless.However, for the significant part of assortment it is enough to provide the necessary geometrical dimensions and their high-precision distribution along the length.

Автоматизированные системы управления технологическими процессами
Historically, the domestic steel industry produced all hot-rolled strip products on highly productive hot-strip mills (HSM) that changed several generations in their improvement.Such mills at Magnitogorsk Iron & Steel Works are mills 2000 and 2500 originally designed for producing strips made of commercial steel with a gauge from 4 to 16 mm with production output about 4 million tons per year.
A similar situation exists at the other steel works where the second and the third generation mills are used.Thereby, the market needs in thin hot-rolled strip must be provided with these very mills.
Automation level of domestic HSM is high enough, but to produce the strips of broadened assortment it needs in improving the main automatic control systems of such process parameters as loop tension and height (ACST&H), strip gage (ACSG) and speed rate control (SSRC).In such case, a conceptually new problem is to provide interactive control of these parameters in steady-state and dynamic conditions in the same way that it is realized on cold-rolled mills.
The main HSM process unit defining capacity and product quality is continuous (finishing) train.The continuous train of mill 2000 consists of seven four-high stands (Fig. 1).Besides, complex includes hydraulic roll screwdowns as a part of ACSG, and driving loopers in interstand spaces that really work as tension sensors.The quantity of stands depends on rolling strip section: thin strips are rolled with all finishing stands, and thicker strips with five or six ones.The equipment configuration of finishing mill 2500 group is similar to shown one.The difference is that only four last stands are supplied with HRS.During hot rolling, the main perturbations affecting thickness deviation (gage interference) are changes of temperature, chemical and mechanical heterogeneity of strips, changes of interstand tensions, roll speed changes, backup and working roll bounce, roll wear, etc. Simultaneous action of all or most of perturbations gives a very complicated picture of thickness deviation.However, the result gage interference of hot-rolled strip can be presented very tentatively as a sum of thickness changes caused by temperature wedge, skid notches and interstand tensions fluctuations (Fig. 2) [3].
To decrease the influence of each named factor on strip gage interference, complex of technical solutions was developed and implemented on mills 2000 and 2500 at Magnitogorsk Iron & Steel Works.The major developments are given below.

Development of the speed rate control system
The distinction of thin strips rolling is a tension strong effect on the thickness that usually takes place in cold-rolling mills.In HSM finishing group, tension control in conjunction with ACST&H is implemented due to speed control system of interrelated stands electric drivers.In fact, it is the system of automatic speed matching under combined rolling.
To increase tension control accuracy, a system and algorithm of cascade speed correction of previous finishing stand electric drivers [4,5] are developed.The gist of the matter is in correction actions summation and transmission against rolling direction.The last rolling stand is used as a supporting one according to constant strip speed requirements when emerging from the finishing group.Fig. 3 gives scheme of cascade design for Fx and Fx-1 stands where Fx is a supporting stand.
While correcting Fx stand speed during the rolling, the necessary condition is to keep preset strip tensions in all interstand spaces.For this purpose, the circuit transmitting percent actions of Fx stand speed changes to all the previous stands.These actions are transmitted both under manual intervention and under looper perturbation actions or roll gaps changes when rolling.

Fig. 3. Scheme of speed control system with corrective action transmission against rolling direction
The aim of such speed control method implementing is to increase the accuracy of the loop height setting and control in interstand space.For this purpose, the system of loop size control, which corrects the speed of stand before the looper, is developed (Fig. 4).Loop size controller effecting on speed setting equalizes the difference between the preset and real strip lengths.It allows to keep the loop size constant.The controller has proportional-plus-integral property.EMF of looper drive is used as a stabilizing feedforward of looper location correction circuit.Controller setting is the height of metal loop set h above the mill pass line (Fig. 5).The operating looper lifting angle according to preset loop height is calculated using the formula (the indications are on  Looper length is evaluated by formula: Preset strip length in interstand space is calculated on base of preset loop height set h : F (i+1) r In the system, operating (actual) looper lifting angle Act  is used as feedback signal.Actual strip length is calculated from it: The reviewed technical solutions are implemented in commercial operation in finishing mill 2500 group automatic process control system.Simatic S7-400 controllers and Simatic ET-200M remote input/output modules are used as base hardware, and herein interface channels PROFIBUS and Ethernet TCP/IP are used for communication.In this case, the changes made in speed control system refer only to industrial sequence control software and require no additional hardware including additional process parameters sensors.
The results of experimental studies of improved implemented SSRC are given in [6,7].Long-term operation confirmed its advantages consist in improved accuracy of rolling strip preset tension support.

Improvement of strip gage automatic control system
In ACSG of the most modern HSM strip gage is regulated in each stand output with indirect Sims-Golovin formula evaluation [1].Furthermore, the output gage at the time of ACSG switching on is stored.Further, it is maintained up to the end of the lot or manual intervention.After that, the new value is stored.Output gage correction allows obtaining the preset lengthway gage strip.This correction is relatively "slow" because of transport lag.
The functional chart explaining ACSG design concept is given on Fig. 6.
A setting of roll screwdown (RS) position can be conditionally divided in static correction signals responsible for required strip gage and dynamic correction signals responsible for gage stabilizing along the strip.Static correction signals are permanent for the whole lot of metal (if there is no manual intervention).Dynamic correction signals are calculated for each strip and nulled when the strip gets out of the stand.

Fig. 6. Signals of ACSG assignment
Gage setting signal h set at the stand output is added together with correction signal h cor and compared with the current strip gage.Gage deviation ∆h is fed to controller of dynamic correction 1 of ACSG position.Output signal X ACSG of this controller is a task to remove RS for received gage deviation compensation.Then static setting X stat of hydraulic RS position is added to the ACSG dynamic correction signal X ACSG and compared with a current roll screwdown position X cur .The value of roll screwdown position deviation ΔХ is an input signal for hydraulic cylinder (HC) position controller 2. The following is a U ~ I transducer 3 generating a signal for servovalve 4 which controls oil flow F in hydraulic cylinder.

Dynamic gage correction
The functional chart of dynamic correction system is given on Fig. 7 [8].The input signal Δh of position controller is composed as following: .
The set gage h set is calculated when gage-stabilizing mode is on under the condition that metal is treaded into the next stand, i.e. the steady mode of rolling takes place.According to the developed algorithm, the position sensor readings and roll force are averaged in preset time (≈400 ms), and the strip gage at the stand output is calculated and stored as a setpoint.Then the following correction signals are added to the setpoint:  h gage -fine correction from output gage.It is an integrated value of gage deviation from set gage at the finishing group output;  h next -correction from the next stand.Its function is to reassign control to the previous stands when dynamic correction of this stand RS gets close to limit;  h ext -external correction.The possible corrections can be such as a compensation of oil film thickness in roll bearings, rolls thermal-compensation, roll ware compensation, forward correction, etc.
Feedback signal h cur (the current strip gage at this stand output) is subtracted from the obtained sum.The obtained deviation Δh is entered into proportional-plus-integral position controller with the limited integral part.
The calculated output signal of dynamic correction X ACSG is added to the static setting of RS position: , where K p , K i -coefficients of proportional and integral parts of controller; K M , K S -stiffness factors of metal and stand.Thus, the hydraulic RS position setpoint is determined by a sum of static active (average) position, ACSG dynamic correction and coarse correction from output gage X CCOG: Along with the presented technical solutions, the improved ACSG, which provides draft redistribution to finishing stands.It prevents the saturation of last stand RS position controller under large corrective signals coming from thickness gage [9].It gives an opportunity to limit integral correction on the last stands and so to prevent gage control circuit cutting.

The results of the improved ACSG experimental investigations
Deviations of the thickness are shown on Fig. 8.These deviations are measured with output thickness gage set after the stand No.13 of the mill 2000 when this improved ACSG is working.Oscillograph trace is obtained during 4.7 mm thick strip rolling.Neither temperature wedge no definite skid notches are present.The strip grow-back on the main section is not more than 80 µm that is tolerance.The experimental results prove that using of ACSG, in which the proposed technical solutions are implemented, makes it possible to annul the temperature wedge, to decrease amplitude of skid notches by factor of 1.8 and to reduce the length of grow-back sections.Generally, the system enables to reduce longitudinal grow-back and as a result, to enhance the quality of output product.
However, total exclusion of thickness variations caused by run-out of backup rolls is not fully succeeded.Use of low-frequency filter on the measuring unit output provides it with the features of relaxation circuit and adversely affects the ACSG ability to exclude perturbations like skid notches.Besides, Fig. 8 shows decreasing thickness deviations in the course of strip head end rolling which is not typical for the other hot-rolling mills.
To exclude this type of grow-back, the method of parametric control of strip head end thickness by means of forward roll gap increase before the strip enters the stand with subsequent screwdown return to initial position for the main section rolling [1012].The functional chart of the control system implementing the method of rolling with roll gap automatic change (correction) in strip length function [13,14].
For gap change parameters setting, the experiments for testing the strip rolling with different correction parameters in stands were carried out [15].In addition, an influence of the roll gap change on strip thickness deviations was examined by means of mathematical modeling techniques [16].As a result, averaged thickness correction parameters during head section rolling are founded for strips of different assortment.
On Fig. 9 there are signals of the mill 2000 diagnostic system received during the rolling with the proposed algorithm implementation.In this experiment the program setting of screwdown positions of stands No. 7-10 (stand No. 10 was the last rolling one) is done.Besides, the signal of metal presence in the stands is fixed (full lines).In the bottom of the figure, there are oscillograph traces of thickness deviations from the set one, which are received by a signal from output gage.Oscillograms prove that the grow-back along the whole strip is within the tolerance limits.The thickness deviations of the head section are fully compensated.Presented results confirm the efficiency of improved ACSG using the developed strategies of gage control.

The development of the electromechanical system of strip tension automatic control
Strategy of tension indirect control with the use of drive (floating) loopers in interstand spaces was implemented in ACST&H of mills 2000 and 2500.The description of ACST&H for mill 2500 is given in [4].In the place cited, it is proved that it doesn't provide preset parameters of tension control accuracy during rolling the strips of expanded assortment.
The key requirement to ACST&H during thin strips rolling is supporting the specific tensions in range of 0.517 N•mm -2 in all dynamic modes and herewith, acceptable deviations should not exceed ±10 %, and grow-back should not exceed ±5 % [17].
As it is shown in [18,19], meeting the increased requirements for dynamic criteria can't be provided only due to computational accuracy increase and process parameters control.The problem aroused how to increase response speed by means of using the resource of thickness control channel implemented on base of hydraulic RS.The most functional is the development of the system of interrelated tension and thickness control, as it is done on cold-rolling mills.
According to theoretic and experimental research, the specific tensions in the interstand spaces of the finishing group of mill 2500 under dynamic mode exceed the steady state value by a factor of 1.51.9%.It leads to the thickness deviation by 5.58.7 %.In accordance with specified requirements, this value shouldn't exceed 5 %.Besides, the grow-back caused by tension changes is proved to increase with decreasing of strip thickness.For example, during rolling the strips 4mm thick, the thickness deviations on the initial and final sections are 3.54.5 %, and during rolling the strips less than 2 mm thick, it is 1112.5 %.
The principle of interactive control of the rolling process-dependent parameters in the continuous group of wide-strip mill was set as follows.It is support of the constancy of strip volume per second in interstand spaces under steady-state and dynamic modes is to be provided by simultaneous automatic control of hydraulic screw-down position and the speed of the next stand rolls under control and perturbation actions changing the looper position.
This principle was suggested for implementing together with traditional loop-forming control.The similar principle termed as "Mass control" exists in foreign publications but it concerns the continuous cold-rolling mills [1].The differences of the study stands group from such type of mills are the presence of loopers and the absence of tension direct gaging sensors in interstand spaces.These features define the fundamental differences of interrelations and, thereafter, the necessary methods and algorithms of control.
To provide technical implementation of the suggested principle, the method of tension and gage interrelated control in the continuous group of HSM [20].The distinguishing feature is the additional corrective effect on hydraulic RS simultaneously with affecting the speed of the previous stand engine during the looper position change.The functional chart of tension and loop indirect control system implementing the method is given on Fig. 10 [21].
The developed ACST&H consists of fast-active loop of the loop size control with effect on i-stand hydraulic RS position, loop control system with effect on speed of i-stand electric driver and "traditional" indirect control system of strip tension in i-space.Additional loop allows to improve tension control accuracy due to roll gap correction during looper position changes.As a result, grow-back decreases along the whole strip.This has positive effects such as quality improvement, reduction of consumption factor connected with decrease of top and bottom scrap, etc.
Mathematic simulation proved that fast-active loop implementation decrease the looper gear lifting time in half (from 0.53 to 0.26 sec) [22].The preset tension comes into 10 % tolerance zone in 0.13 sec (in 0.57 sec in the existing system), tension over-shoot reduces in 1.7 times (from 37.5 to 22 %), and thickness deviation decreases by factor of 1.4 (from 4.2 to 3 %).
The ACST&H algorithms implemented in controllers of mill 2500 software are studied by experiment.On Fig. 11 there are the strip gage deviation oscillograms received from thickness gauge signals in finishing group output.

Fig. 2 .
Fig. 2. Result strip gage interference and its components

Fig. 7 .
Fig. 7. Signals and structure of dynamic gage correction system

Fig. 8 .
Fig. 8. Thickness deviations along the strip during the improved ACSG work

Fig. 10 .
Fig. 10.The functional chart of the developed ACST&H: TC, TCC, CC, CCC, CS, CSC, SS, SSC, LSC and SSR thyristor converters, current controllers, current sensors, speed sensors, looper speed controllers and s MEB -moment evaluation block; LCS and LCT hydraulic roll screwdown; SSS -system signals setter; HPC and HPS and position sensor; IP -indicator of metal presence in the rollers; M, MS

Fig. 10 .Fig. 11 .Fig. 11 .
Fig. 10.The functional chart of the developed ACST&H: TC, TCC, CC, CCC, CS, CSC, SS, SSC, LSC and SSR thyristor converters, current controllers, current sensors, speed sensors, looper speed controllers and stand speed regulators; LPS -looper position sensor; moment evaluation block; LCS and LCT -controllers with effect on stand and with effect on system signals setter; HPC and HPS -hydraulic screwdown position indicator of metal presence in the rollers; M, MS -motors of looper and stand; SG -rolling stand gear