Title of Invention	A STEPLESS CONTROL SYSTEM FOR ROLL BENDING AT FINISHING STAND OF TANDEM MILL.
Abstract	A control system adapted for roll bending and roll balancing and controlling the shape and flatness of the rolled strips and comprises plurality of cylinder means (13) located at the top of each bottom work roll chock, means (10) operatively connected to the said cylinder means (13) for supplying variable pressure to the said cylinder means, means (19) operatively connected to the valve means (10) for controlling the variable pressure to the cylinder means (13) and means adapted for maintaining defined preset pressure in the system.

Title of Invention

A STEPLESS CONTROL SYSTEM FOR ROLL BENDING AT FINISHING STAND OF TANDEM MILL.

Abstract

A control system adapted for roll bending and roll balancing and controlling the shape and flatness of the rolled strips and comprises plurality of cylinder means (13) located at the top of each bottom work roll chock, means (10) operatively connected to the said cylinder means (13) for supplying variable pressure to the said cylinder means, means (19) operatively connected to the valve means (10) for controlling the variable pressure to the cylinder means (13) and means adapted for maintaining defined preset pressure in the system.

Full Text	Field of the invention The present invention relates to a step-less control system for roll bending at finishing stand of tandem mill. In particular, the present invention relates to an improved work roll bending system with step-less control for improved control over hydraulic bending actuators. Background of the invention Achieving desired shape of cold rolled (CR) strips is of vital importance during cold rolling process at tandem cold mill (TCM). From the point of view of product quality the strip profile, shape and flatness are all important parameters. Shape of a strip is the manifest of the roll gap contour during cold rolling and a composite parameter defining both strip crown and flatness. The shape or flatness defects of CR strip develop due to mismatch of the profiles of the incoming strip and the active roll gap resulting in a transverse stress distribution and variation in elongation along the width of the strip. But, in a mill producing materials with various grades, width and thickness, it is not possible to select a pre-determined crown to match all the conditions. Therefore, to provide facilities to the operator to adjust the gap according to the profile of input material and thermal camber of work roll, control system is required to correct the roll gap contour for controlling shape and flatness defects of the strip. Correction of the roll gap profile like roll shifting, roll crossing, roll bending and differential roll cooling for controlling asymmetrical shape defect is required. Profile and Flatness Control Hardware The hardware called profile (shape) actuators either aim at compensating the effects of unavoidable roll bending effects on the roll gap profile by adjusting the effective roll crown in roll stacks or aim at eliminating or minimizing roll bending by altering the method of applying the load. The recent strip profile and shape control devices can be classified as : Multi-high Mills" > 6-High mills (Hitachi) - HCM, HCMW, UCM, UCMW, etc > Z-mills or cluster mills Roll Crown Control" > Parabolic ground crown > Variable crown - VC (Sumitomo) & IC (Blow Knox) back-up rolls provide lower order shape correction in the middle portion of the strip regardless of strip width > Pair Cross (Mitsubishi) - Work roll cross and Back-up roll cross mills provide parabolic corrections > Thermal crown rolls Roll Shifting Control: > Flat roll shifting - Work and Back-up roll shifting > Profiled roU shifting - CVC (SMS), UPC (MDS), K-WRS (KAWASAKI) Roll Bending Control > Single and Double chock work roll and back-up roll and intermediate roll bending in vertical plane - slender work rolls provide higher order shape corrections near the edges of the strip, in order to be effective at the central portion large diameter rolls should be used > Horizontal Roll bending (MKW, UC-2, UC-3, etc) Multipoint Bending Control by Special Back-up Rolls: > Divided backup roll benders or As-U rolls in cluster mills like Z-mill (Hitachi), KST mill (Kobe Steel), KT mill (Kobe Steel). These mills also use laterally shifted first intermediate rolls for shape control. Cluster type CR mill (Mitsubishi) uses divided back-up roll crown adjustment along with intermediate and work roll benders. Variable Back-up Roll Strength: > Self compensating back-up rolls (SC) > Tapered piston back-up rolls (TP) > Variable length rolls (VLR) Combination control: > 6-high CVC > 6-high UPC > Pair cross with work roll shifting > Work roll cross with work roll shifting The other methods of crown control include: > Tension Distribution Control (TDC) > Zonal Coolant Control Strip crown Strip shape in cold reduction is not a direct function of profile but is determined by roll camber. The finished strip profile is governed by hot band profile assumed to be uniform preferably with a slight crown and the shape control is realized by work roll camber controlled by the operator. Control is made by visual inspection. Thus to correct the hot band distortion and ensuing proper shape in cold mills, it is desirable to have a shape control system which permits instant and refined adjustment of roll camber. There are several means of obtaining or changing camber in the mill are like mechanical roll camber, thermal camber control, altering rolling loads and extreme mechanical control of roll bending. To reduce the strip thickness, the back-up roll is loaded as a simple beam and the work rolls will deflect by an amount directly proportional to the roll separating force. In practice, this bending is counter acted by grinding a mechanical crown in the roll initially either with a convex or a concave surface with parabolic or cycloid profile. There is no fixed mechanical crown that will suffice for the entire product range to be rolled. It is therefore necessary to apportion the product range to several groups of rolls with fixed crown of each group. This mechanical crown can be given to the back-up rolls, work rolls or a combination of both. Crown control In recent years, increased emphasis has been placed on the uniformity of gauge and flatness of both hot and cold rolled strip. The control of strip crown must be made where lateral spread can take place without up-setting the flatness of the product. Thus the control of strip crown and the control of strip shape or flatness can be achieved by proper control of mill crowns in the cold mills. In the cold mill adequate flatness can be maintained only using proper relationship of rolling pressure, strip tension and mill crowns. Various operating factors contribute to make this a difficult problem such as variable crown in the strip itself, non-uniformity of strip tension across the width of the strip as coil built up, changing of mill crowns after a shutdown and until thermal equilibrium of the rolls attained, variable wear on both the work and back-up rolls, schedule changes for width, chemistry etc, variable rolling pressure due to gauge variations, hard spots, speed changes, etc. The basic way to take care of crown changes with the use of proper shaped ground rolls compromising al factors. Proper crowns can be obtained by crown either or both work rolls, either or both back-up rolls or combination of work rolls and back-up rolls. It can be said that at no time it is really correct and the roller always try for rolling with a compromise crown. However there are three general methods of crown control are roll crossing, work roll bending and back-up roll bending by hydro-mechanical means. The need of instantaneous control is a direct result of users requiring better tolerances and shape. Roll bending Crown control by bending is based on the idea of roll bending of the support rolls by hydro-mechanical means. Crown control is achieved by producing a desired deflection in the roll by applying equal and opposite bending moments to each end of the roll. Crown control by roll bending works better than mechanical crowns. In reality, this method of control counteracts the bending due to rolling pressure with a reverse bending, where as mechanical crowning merely accommodates a deliberately misground roll shape to the offending bending deflection due to the rolling action. Conventionally hydraulic systems for control are used which are based on water hydraulic. In the conventional system 5% hydraulic oil is mixed 95% water to form the water hydraulic fluid in lieu of 100% oil hydraulics. The known water hydraulics system usually caters three drive units: > Auxiliary drive unit operating at system pressure of 64 kg/cm2 > Balancing & bending unit operating at pressure of 100 kg/cm2 > Back-up roll extraction unit operating at pressure of 200 kg/cm2 Auxiliary drive units are catered by four pumps (two pumps running and two pumps as stand by) to facilitate the feeding & guiding of strip to the mill and also helps in removal of coils from the tension reel of mill. The delivery pressure of these pumps is set at 64 kg/cm2 through a pressure relief valve at the discharge end. These hydraulic pressure operates the different distributor valves located at valve stand actuating total 250 numbers of hydraulic cylinders for different operations as mentioned above in feeding unit, in mill proper and in coil exit unit. The roll balancing and bending unit constitutes bending cylinders which are needed to create the positive bending as per requirement on the work rolls to counter the thermal crown developed on the roll during rolling. But no control is available on the positive bending (crown-in) of the work rolls. Therefore it is virtually impossible to get the right shape of the strip that is coming out of the mill. The conventional roll balancing and bending unit is catered by two pumps running. The discharge hydraulic pressure is maintained at 100 kg/cm2 through pressure relief valve and connected to the distributor valves of the four stands to actuate the work roll balancing cylinders for positive bending of work roll. To maintain the steady constant pressure and to compensate the surge pressure, the N2 Accumulator and load accumulator are connected to the pressure line to damp the pressure fluctuations. Presently mills have electromechanical screw-down system in all the stands for the purpose of adjusting the roll gaps. The hydraulic roll bending system is operating at a hydraulic pressure either 0 bar or 100 bar to control the shape deformities with the help of hydraulic roll force cylinders placed on the bottom work roll chocks. With the actuation of these cylinders facilitates the roll separating in two points (no separation - 0 bar and maximum separation - 100 bar). There is no provision for variation of the roll separating force in between (0 bar to 100 bar) by the operator to correct the shape as far as possible. During the period of Backup roll extraction additional pump is made to run and to develop 200 kg/cm2 pressure through a separate line with the help of pressure relief valve set at 200 kg/cm2. Therefore, the conventional water hydraulic system has the following deficiencies" > Incorrect roll bending and no continuous variable crown leading to symmetrical shape defects like long edges, center buckling, long bow, cross bow and quarter buckles of the rolled strip. > Decrease in roll force due to reduction in material hardness, lower stand reduction, increase in work roll thermal crown and change in incoming strip profile resulting to full center defect of the strip. > Sudden drop of hydraulic pressure in line due to external or internal leakages in valves, cylinders, hose pipes and load accumulator because of very high temperature, excess load, vibration and drop in viscosity of fluid. > Frequent failure of pumps and their glands due to excess loading on pumps because of operating total auxiliary drive units with hydraulic systems, resulting to improper control of roll bending system. Objects of the invention Thus, the basic object of the present invention is to provide for an improved roll bending system which would avoid the above discussed drawbacks / limitations of the known roll bending system presently available. Another object of the present invention is to provide for an improved roll balancing system which would facilitate reduction in shape defects of CR strips specially for TCM. Yet further object of the present invention is to provide for an improved roll balancing system which will create the variable positive bending as per requirement, on the work rolls to encounter the mismatch of the profiles of the incoming strip and the active roll gap resulting in a transverse stress distribution and variation in elongation along the width of the strip. Yet another object of the present invention is to provide for an improved roll balancing system with the facility to control the shape or flatness defects of CR strip occurring due to thermal crown developed on the roll during rolling. Yet further object of the present invention is to provide for an improved roll balancing system which provides the operator with the facility of a step-less control for roll bending at the finishing stand of TCM, to have a better control on shape and flatness of strip during rolling. Yet another object of the present invention is to provide for an improved roll balancing system operated in oil hydraulics for effective control over shape and flatness defects. Yet further object is to provide an improved method of roll balancing using the improved system of the present invention. Detailed description of the present invention By using variable hydraulic pressure instead of fixed pressure, a certain amount of crown control by bending the work rolls between the back-up rolls of a 4-Hi mill can be expected. Hydraulic shaping consists of supplementing mechanical and thermal crowning by applying hydraulic forces on the work roll chocks to encounter the roll gap contour variation during rolling. This forms the basis of the present approach. The system involves incorporation of proportionate pressure reducing and pressure relaxing valve in order to make the roll gap variable on a continuous basis. Thus, step-less control of roll bending is achieved. The system also provides flexibility to the operator to operate the variable roll balancing pressure as per requirement for controlling strip shape defects as the system is operated in a manual mode. The system is much less expensive compared to fully automatic systems and easy to maintain. The system of the present invention will now be described with reference to the following non-limiting embodiment and accompanying figures. Description of accompanying figures Figure 1 shows schematic drawing of roll balancing system of present invention Figure 2 shows the hydraulic power pack for bending of present invention Description of preferred embodiment The hydraulic power-pack has been installed to cater the Work roll bending actuators of last stand at TCM. The system (see Fig. 2) consists of four hydraulic cylinders (13) located in the top of each bottom work roll chock. These cylinders are actuated with the variable plunger stroke subjected to hydraulic force developed on each chock. These roll separating force will deflect the top work roll upward and the bottom work roll downward to create the desired work roll bending crown for ensuing proper profile of the strip at the roll gap. This roll balancing system is used to correct the shape defect as far as possible. The system consists of two nos. variable volume pump (3) controlled by a pressure compensator. The pressure compensator will maintain the preset pressure to 160 bar by varying the volume of oil discharged by positioning a wobbling plate on the drive shaft of the pump piston. The delivery pressure of the pump ranges from 0 to 320 bar. The pump suction is connected to a 3.0M3 hydraulic oil reservoir (l) with the help of shut-off valve (2) for individual run of the pump. The pressurized hydraulic oil (160 bar) is then passed through non-return valve (4) for unidirectional flow. After that the pressured hydraulic oil is allowed to pass through the duplex type pressure filter (5) for ensuing 5µ filtration level of hydraulic oil to the subsequent valves. The duplex type filter is introduced for continuous run of the system without any stoppage for maintenance of filter element due to jamming. The filtered oil is then passed through a pressure relief valve (6) set at 100 bar with on/off solenoid valve for the protection of the system and to deliver hydraulic oil at constant pressure of 100 kg/cm2, in the delivery line a N2 filled bladder type accumulator (8) is fitted to control the surge pressure and pressure fluctuation during operation. The glycerin filled pressure gauges (7) are connected to the different locations for the display of the hydraulic pressure. The directional control valve (9) helps to connect the path for flowing of oil through port and being controlled by the spool movement with the actuation of pilot pressure actuated with the help of on/off solenoid valve. When the directional control valve is not in use, the oil returns to tank through drain port. In the return line of oil to the reservoir, it is allowed to pass through shell and tube type heat exchanger (ll) to take out the heat of the oil and finally before draining to the reservoir, it has to pass through a return filter (12) of 20µ filtration level of the returned hydraulic oil. The pressurized hydraulic in then passed through a proportionate pressure relieving and reducing relief valve (10) for supplying variable pressure from 18 bar to 100 bar to the hydraulic cylinders in the work roll chock. The discharged pressure of the proportionate pressure relieving and reducing relief valve is being varied with the spool movement and opening of different ports as the voltage regulation of the built-in electronic voltage transducer. This electronic transducer is connected to the potentiometer by electrical cable for voltage regulation. To get a variable pressure ranges from 18 bar to 100 bar, the voltage regulation is needed from 0 to 2.0 volt in the potentiometer which is being installed at operator"s control desk. As per the requirement of roll balancing pressure for shape correction of the strip through visual observation, the operator regulates the voltage in the potentiometer and subsequently the variable pressure in the hydraulic cylinder (13) of work roll chock are obtained. The hydraulic pressure of the actuating cylinders at bottom work roll chock are being varied for the roll separating force as per the requirement for controlling the shape defects (center buckles / edge waviness) of the strip. The modified work roll balancing system for step-less control of hydraulic actuators has been installed and commissioned at the Stand # 4 of TCM to control the shape correction as far possible. Analysis of various parameters after installation of the system of the present invention lead the followings results — > The system is one of the easy way to control the shape defects as far as possible. > The system has helped to correct the edge waviness and center buckles on the strip. > It has helped to improve strip flatness by 20%. > System is easy to operate and retro-fitted to existing system. > It has minimized roll crown provisions > It has improved the shape and profile of the strip. > It has also improved customer services by reduced complaints by 15%. > The system is easy to maintain and less expensive than any other shape control hardware. Field Of The Invention The present invention relates to a step-less control system for roll bending and balancing at finishing stand of tandem mill. More particularly, it relates to an improved work roll bending and balancing system comprising proportionate step-less control pressure reducing and relieving cylinders for improved control over hydraulic bending actuators and thus controlling the shape and flatness of the rolled strips. Background Of The Invention Achieving desired shape of cold rolled (CR) strips is of vital importance during cold rolling process at tandem cold mill (TCM). From the point of view of product quality the strip profile, shape and flatness are all important parameters. Shape of a strip is the manifest of the roll gap contour during cold rolling and a composite parameter defining both strip crown and flatness. The shape or flatness defects of CR strip develop due to mismatch of the profiles of the incoming strip and the active roll gap resulting in a transverse stress distribution and variation in elongation along the width of the strip. But, in a mill producing materials with various grades, width and thickness, it is not possible to select a pre-determined crown to match all the conditions. Therefore, to provide facilities to the operator to adjust the gap according to the profile of input material and thermal camber of work roll, control system is required to correct the roll gap contour for controlling shape and flatness defects of the strip. Correction of the roll gap profile like roll shifting, roll crossing, roll bending and differential roll cooling for controlling asymmetrical shape defect is required. Profile and Flatness Control Hardware The hardware called profile (shape) actuators either aim at compensating the effects of unavoidable roll bending effects on the roll gap profile by adjusting the effective roll crown in roll stacks or aim at eliminating or minimizing roll bending by altering the method of applying the load. The recent strip profile and shape control devices can be classified as: 1. Multi-high Mills: (i) 6-High mills (Hitachi) - HCM, HCMW, UCM, UCMW, etc (ii) Z-mills or cluster mills 2. Roll Crown Control: (i) Parabolic ground crown (ii) Variable crown - VC (Sumitomo) & IC (Blow Knox) back-up rolls provide lower order shape correction in the middle portion of the strip regardless of strip width (iii) Pair Cross (Mitsubishi) - Work roll cross and Back-up roll cross mills provide parabolic corrections (iv) Thermal crown rolls 3. Roll Shifting Control: (i) Flat roll shifting - Work and Back-up roll shifting (ii) Profiled roll shifting - CVC (SMS), UPC (MDS), K-WRS (KAWASAKI) 4. Roll Bending Control: (i) Single and Double chock work roll and back-up roll and intermediate roll bending in vertical plane - slender work rolls provide higher order shape corrections near the edges of the strip, in order to be effective at the central portion large diameter rolls should be used (ii) Horizontal Roll bending (MKW, UC-2, UC-3, etc) 5. Multipoint Bending Control by Special Back-up Rolls: (i) Divided back-up roll benders or As-U rolls in cluster mills like Z-mill (Hitachi), KST mill (Kobe Steel), KT mill (Kobe Steel). These mills also use laterally shifted first intermediate rolls for shape control. Cluster type CR mill (Mitsubishi) uses divided back-up roll crown adjustment along with intermediate and work roll benders. 6. Variable Back-up Roll Strength: (i) Self-compensating back-up rolls (SC) (ii) Tapered piston back-up rolls (TP) (iii) Variable length rolls (VLR) 7. Combination control: (i) 6-high CVC (ii) 6-high UPC (iii) Pair cross with work roll shifting (iv) Work roll cross with work roll shifting 8. The other methods of crown control include: (i) Tension Distribution Control (TDC) (ii) Zonal Coolant Control Strip crown Strip shape in cold reduction is not a direct function of profile but is determined by roll camber. The finished strip profile is governed by hot band profile assumed to be uniform preferably with a slight crown and the shape control is realized by work roll camber controlled by the operator. Control is made by visual inspection. Thus to correct the hot band distortion and ensuing proper shape in cold mills, it is desirable to have a shape control system which permits instant and refined adjustment of roll camber. There are several means of obtaining or changing camber in the mill like mechanical roll camber, thermal camber control, altering rolling loads and extreme mechanical control of roll bending. To reduce the strip thickness, the back-up roll is loaded as a simple beam and the work rolls will deflect by an amount directly proportional to the roll separating force. In practice, this bending is counter acted by grinding a mechanical crown in the roll initially either with a convex or a concave surface with parabolic or cycloid profile. There is no fixed mechanical crown that will suffice for the entire product range to be rolled. It is therefore necessary to apportion the product range to several groups of rolls with fixed crown of each group. This mechanical crown can be given to the back-up rolls, work rolls or a combination of both. Crown control In recent years, increased emphasis has been placed on the uniformity of gauge and flatness of both hot and cold rolled strip. The control of strip crown must be made where lateral spread can take place without up-setting the flatness of the product. Thus the control of strip crown and the control of strip shape or flatness can be achieved by proper control of mill crowns in the cold mills. In the cold mill adequate flatness can be maintained only using proper relationship of rolling pressure, strip tension and mill crowns. Various operating factors contribute to make this a difficult problem such as variable crown in the strip itself, non-uniformity of strip tension across the width of the strip as coil built up, changing of mill crowns after a shutdown and until thermal equilibrium of the rolls attained, variable wear on both the work and back-up rolls, schedule changes for width, chemistry etc, variable rolling pressure due to gauge variations, hard spots, speed changes, etc. The basic way is to take care of crown changes with the use of properly shaped ground rolls compromising all factors. Proper crowns can be obtained by crown either or both work rolls, either or both back-up rolls or combination of work rolls and back-up rolls. It can be said that at no time it is really correct and the roller always try for rolling with a compromise crown. However there are three general methods of crown control are roll crossing, work roll bending and back-up roll bending by hydro-mechanical means. The need of instantaneous control is a direct result of users requiring better tolerances and shape. Roll bending Crown control by bending is based on the idea of roll bending of the support rolls by hydro-mechanical means. Crown control is achieved by producing a desired deflection in the roll by applying equal and opposite bending moments to each end of the roll. Crown control by roll bending works better than mechanical crowns. In reality, this method of control counteracts the bending due to rolling pressure with a reverse bending, whereas mechanical crowning merely accommodates a deliberately misground roll shape to the offending bending deflection due to the rolling action. Conventionally hydraulic systems for control are used which are based on water hydraulic. In the conventional system 5% hydraulic oil is mixed 95% water to form the water hydraulic fluid in lieu of 100% oil hydraulics. The known water hydraulics system usually caters three drive units such as auxiliary drive unit operating at system pressure of 64 kg/cm , balancing & bending unit operating at pressure of 100 kg/cm2 and back-up roll extraction unit operating at pressure of 200 kg/cm2. Auxiliary drive units are catered by four pumps (two pumps running and two pumps as stand by) to facilitate the feeding & guiding of strip to the mill and also helps in removal of coils from the tension reel of mill. The delivery pressure of these pumps is set at 64 kg/cm2 through a pressure relief valve at the discharge end. These hydraulic pressure operates the different distributor valves located at valve stand actuating total 250 numbers of hydraulic cylinders for different operations as mentioned above in feeding unit, in mill proper and in coil exit unit. The roll balancing and bending unit constitutes bending cylinders, which are needed to create the positive bending as per requirement on the work rolls to counter the thermal crown developed on the roll during rolling. But no control is available on the positive bending (crown-in) of the work rolls. Therefore it is virtually impossible to get the right shape of the strip that is coming out of the mill. The roll balancing and bending unit used conventionally is catered by two pumps running. The discharge hydraulic pressure is maintained at 100 kg/cm2 through pressure relief valve and connected to the distributor valves of the four stands to actuate the work roll balancing cylinders for positive bending of work roll. To maintain the steady constant pressure and to compensate the surge pressure, the N2 Accumulator and load accumulator are connected to the pressure line to damp the pressure fluctuations. Presently mills have electro-mechanical screw-down system in all the stands for the purpose of adjusting the roll gaps. The hydraulic roll bending system is operating at a hydraulic pressure either 0 bar or 100 bar to control the shape deformities with the help of hydraulic roll force cylinders placed on the bottom work roll chocks. With the actuation of these cylinders facilitates the roll separating in two points (no separation - 0 bar and maximum separation - 100 bar). There is no provision for variation of the roll separating force in between (0 bar to 100 bar) by the operator to correct the shape as far as possible. During the period of Back-up roll extraction additional pump is made to run and to develop 200 kg/cm2 pressure through a separate line with the help of pressure relief valve set at 200 kg/cm . The main disadvantages of the conventional water hydraulic system are incorrect roll bending and no continuous variable crown leading to symmetrical shape defects like long edges, center buckling, long bow, cross bow and quarter buckles of the rolled strip. Another disadvantage is that on decreasing the roll force due to reduction in material hardness, lower stand reduction, increase in work roll thermal crown and change in incoming strip profile the full center of the strip becomes defective. Further disadvantage of the conventional system is that there is sudden drop of hydraulic pressure in line due to external or internal leakages in valves, cylinders, hose pipes and load accumulator because of very high temperature, excess load, vibration and drop in viscosity of fluid. The other disadvantage is the frequent failure of pumps and their glands due to excess loading on pumps because of operating total auxiliary drive units with hydraulic systems, resulting in improper control of roll bending system. Thus there is a need to provide for a system for improved work roll bending and balancing of the rolled strips and proportionate step-less control pressure reducing and relieving cylinders for improved control over hydraulic bending actuators and thus controlling the shape and flatness of the rolled strips. Objects Of The Invention Thus, the basic object of the present invention is to provide for an improved roll bending system which would avoid the above discussed drawbacks / limitations of the known roll bending system presently available. Another object of the present invention is to provide for an improved roll balancing system which provides the operator with the facility of a step-less control for roll bending at the finishing stand of TCM, to have a better control on shape and flatness of strip during rolling. Further object of the present invention is to provide for an improved roll balancing system, which would facilitate reduction in shape defects of CR strips specially for TCM. Yet further object of the present invention is to provide for an improved roll balancing system which will create the variable positive bending as per requirement, on the work rolls to encounter the mismatch of the profiles of the incoming strip and the active roll gap resulting in a transverse stress distribution and variation in elongation along the width of the strip. Yet another object of the present invention is to provide for an improved roll balancing system with the facility to control the shape or flatness defects of CR strip occurring due to thermal crown developed on the roll during rolling. Yet another object of the present invention is to provide for an improved roll balancing system operated in oil hydraulics for effective control over shape and flatness defects. The other object is to provide an improved method of roll balancing using the improved system of the present invention. Summary Of The Invention Thus according to the main aspect of the present invention there is provided a control system adapted for roll bending and roll balancing and controlling the shape and flatness of the rolled strips, the said system comprises: (i) plurality of cylinder means located at the top of each bottom work roll chock; (ii) means operatively connected to the said cylinder means and adapted for supplying variable pressure to the cylinder means; (iii) means adapted for controlling the variable pressure to the cylinder means and operatively connected to the means for supplying variable pressure to the cylinder means and; (iv) means adapted for maintaining defined preset pressure in the system. Detailed Description Of The Present Invention The system comprises plurality of cylinder means comprising four hydraulic cylinders located at the top of each bottom work roll chock. Hydraulic force developed on the roll chock generates variable plunger stroke, which actuates the cylinders. Thus roll separating force generated deflects the top work roll upward and the bottom work roll downward to create the desired work roll bending crown for ensuing proper profile of the strip at the roll gap. This hydraulic shaping consists of supplementing mechanical and thermal crowning by applying hydraulic forces on the work roll chocks to encounter the roll gap contour variation during rolling. This forms the basis of the present approach. In the roll bending and balancing system of the present invention a hydraulic power-pack has been installed to cater the work roll bending actuators of last stand at the tandem cold mill (TCM). The system comprises means operatively connected to the said cylinder means and adapted for supplying variable pressure to the cylinder means. The said means for supplying variable pressure to the cylinder comprises proportionate pressure reducing and pressure relaxing valve adapted for making the roll gap variable on a continuous basis and achieving step-less control of roll bending. Thus the system also provides flexibility in operating roll 9 balancing under variable pressure condition as per requirement for controlling strip shape defects as the system is operated in a manual mode. The system is much less expensive compared to fully automatic systems and easy to maintain. The system comprises two variable volume pump controlled by a pressure compensator. The pressure compensator is adapted for maintaining the preset pressure to 160 bar by varying the volume of oil discharged by positioning a wobbling plate on the drive shaft of the pump piston. The delivery pressure of the pump ranges from 0 to 320 bar. The pump suction is connected to a 3.0M3 hydraulic oil reservoir by a shut-off valve for individual run of the pump. Non-return valve fitted to the delivery line of the pump is adapted for unidirectional flow of the pressurized hydraulic oil (160 bar). The discharge end of the pump comprises the duplex type pressure filter adapted for the pressured hydraulic oil to pass through ensuing 5µ filtration level of hydraulic oil to the subsequent valves. The duplex type filter is introduced for continuous run of the system without any stoppage for maintenance of filter element due to jamming. The present system of the invention comprises a pressure relief valve set at 100 bar with on/off solenoid valve adapted for carriage of filtered oil and protection of the system and to deliver hydraulic oil at constant pressure of 100 kg/cm2. N2 filled bladder type accumulator is fitted to in the delivery line adapted to control the surge pressure and pressure fluctuation during operation. The glycerin filled pressure gauges are connected to the different locations for the display of the hydraulic pressure. The directional control valve is adapted to connect the path for flowing of oil through port and being controlled by the spool movement with the actuation of pilot pressure actuated with the help of on/off solenoid valve. When the directional control valve is not in use, the oil returns to tank through drain port. In the return line of oil to the reservoir, it is allowed to pass through shell and tube type heat exchanger to take out the heat of the oil and finally before draining to the reservoir, it has to pass through the return filter of 20u filtration level of the returned hydraulic oil. The pressurized hydraulic is then passed through a proportionate pressure relieving and reducing relief valve for supplying variable pressure from 18 bar to 100 bar to the hydraulic cylinders in the work roll chock. The discharged pressure of the proportionate pressure relieving and reducing relief valve is varied by means operatively connected to the valve and the said means comprises the spool movement and opening of different ports of the voltage regulation 10 of the built-in electronic voltage transducer. This electronic transducer is connected to the potentiometer by electrical cable for voltage regulation. To get a variable pressure ranges from 18 bar to 100 bar, the voltage regulation is needed from 0 to 2.0 volt in the potentiometer which is being installed at operator"s control desk. As per the requirement of roll balancing pressure for shape correction of the strip through visual observation, the operator regulates the voltage in the potentiometer and subsequently the variable pressure in the hydraulic cylinder of work roll chock are obtained. The variable pressure instead of fixed pressure is adapted in the hydraulic cylinder for obtaining crown control by bending the work rolls between the back-up rolls of a 4 - Hi mill. The hydraulic pressure of the actuating cylinders at bottom work roll chock are being varied for the roll separating force as per the requirement for controlling the shape defects (center buckles / edge waviness) of the strip. The system of the present invention will now be described with reference to the following non-limiting embodiments and accompanying figures. Brief Description Of The Accompanying Figures Figure 1 illustrates schematic drawing of roll balancing system of present invention. Figure 2 illustrates the hydraulic power pack for roll bending in present invention. Figure 3 illustrates the positive bending of work rolls. Figure 4 illustrates schematic diagram of conventional roll balancing system. Figure 5 illustrates the schematic diagram of the interfacing layout of the system in the present invention. Detailed Description Of The Accompanying Figures Figure 1 illustrates schematically the roll balancing system adapted to correct the shape defect as far as possible in the present invention. As shown in figure 1 the system consists of four hydraulic cylinders (13) located at the top of each bottom work roll chock. The system consists of two variable volume pump (3) controlled by a pressure compensator. The suction pump (3) is connected to a 3.0M3 hydraulic oil reservoir (1) with the help of shut-off valve (2) for individual run of the pump (3). Non-return valve (4) fitted to the delivery line of the pump (3) is adapted for unidirectional flow of the pressurized hydraulic oil (160 bar). The discharge end of the pump (3) comprises the duplex type pressure filter (5) adapted for the pressured hydraulic oil to pass through ensuing 5µ filtration level of hydraulic oil to the subsequent valves. The present system of the invention comprises a pressure relief valve (6) set at 100 bar with on/off solenoid valve adapted for carriage of filtered oil and protection of the system and to deliver hydraulic oil at constant pressure of 100 kg/cm2. N2 filled bladder type accumulator (8) (also shown in figure 2) is fitted to in the delivery line (14) adapted to control the surge pressure and pressure fluctuation during operation. The glycerin filled pressure gauges (7) are connected to the different locations for the display of the hydraulic pressure. The directional control valve (9) between the proportionate pressure relieving and reducing relief valve (10) and pressure relief valve (6) is adapted to connect the path for flowing of oil through port and being controlled by the spool movement with the actuation of pilot pressure actuated with the help of on/off solenoid valve. In the return line (15) of oil to the reservoir (1), it is allowed to pass through shell and tube type heat exchanger (11) to take out the heat of the oil and finally before draining to the reservoir (1), it has to pass through a return filter (12) of 20(x filtration level of the returned hydraulic oil. Proportionate pressure relieving and reducing relief valve (10) adapted to pass the pressurized hydraulic for supplying variable pressure from 18 bar to 100 bar to the hydraulic cylinders in the work roll chock. As per the requirement of roll balancing pressure for shape correction of the strip through visual observation, the operator regulates the voltage adapting potentiometer and subsequently the variable pressure in the hydraulic cylinder (13) of work roll chock is obtained. In figure 2 the hydraulic power pack for roll bending in present invention is illustrated. The rolling stands (SI, S2, S3 and S4) is connected to the proportionate pressure relieving and reducing relief valve (10), N2 filled bladder type accumulator (8), 3.0JVT hydraulic oil reservoir (1) and pumps (3) through the delivery line (14). In figure 3 the positive bending of work rolls is illustrated. In the figure the top work roll (16) is deflected upward and the bottom work roll (17) is deflected downward by roll separating force thus producing desired work roll bending crown ensuing proper profile to the roll strip at the roll gap. In figure 5 the schematic diagram of the interfacing layout of the system in the present invention is illustrated. The power supply (18) connected to the proportionate pressure relieving and reducing relief valve (10) provides 24 volt to the proportionate pressure relieving and reducing relief valve (10). The potentiometer mill operating post (19) connected to the proportionate pressure relieving and reducing relief valve (10) for control operation of the valve (10). The system of the present invention provide the following main advantages - 1. The system is one of the easy ways to control the shape defects as far as possible 2. The system helps to correct the edge waviness and center buckles on the strip. 3. It helps to improve strip flatness by 20%. 4. System is easy to operate and retro-fitted to existing system. 5. It has minimized roll crown provisions. 6. It has improved the shape and profile of the strip. 7. The system is easy to maintain and less expensive than any other shape control hardware. We Claim 1. A control system adapted for roll bending and roll balancing and controlling the shape and flatness of the rolled strips, the said system comprises: (i) plurality of cylinder means located at the top of each bottom work roll chock; (ii) means operatively connected to the said cylinder means and adapted for supplying variable pressure to the cylinder means; (iii) means adapted for controlling the variable pressure to the cylinder means and operatively connected to the means for supplying variable pressure to the cylinder means characterized in that said means for controlling variable pressure comprises transducer means for spool movements and opening of different ports of voltage regulation and; (iv) means adapted for maintaining defined preset pressure in the system. 2. A system as claimed in claim 1, wherein the plurality of cylinder means comprises four hydraulic cylinders adapted for producing hydraulic force on the roll chock. 3. A system as claimed in claim 1, wherein the means adapted for supplying variable pressure comprises proportionate pressure relieving and reducing relief valve. 4. A system as claimed in claim 3, wherein the proportionate pressure relieving and reducing valve adapted for supplying pressure ranging from 18 bar to 100 bar to the hydraulic cylinders. 5. A system as claimed in claim 1, wherein the transducer means comprises electronic voltage transducer. . 6. A system as claimed in claim 1, wherein the means for maintaining defined preset pressure comprises two variable volume pumps for shaping of rolls through roll balancing and roll bending. 15 7. A system as claimed in claim 6, wherein the two variable volume pump comprises pressure compensator adapted for maintaining the defined preset pressure to 160 bar. 8. A system as claimed in claims 6 and 7, wherein the variable volume pump comprises the duplex type pressure filter adapted for the pressured hydraulic oil to pass through ensuing 5µ filtration level and for continuous run of the system without any stoppage for maintenance of filter element due to jamming. 9. A system as claimed in any of the preceding claims, optionally comprises pressure relief valve at 100 bar with on/off solenoid valve adapted for carriage of filtered oil and protection of the system delivering hydraulic oil at constant pressure of 100 kg/cm2. 10. A control system adapted for roll bending and roll balancing as herein described and illustrated with reference to accompanying figures. A control system adapted for roll bending and roll balancing and controlling the shape and flatness of the rolled strips and comprises plurality of cylinder means (13) located at the top of each bottom work roll chock, means (10) operatively connected to the said cylinder means (13) for supplying variable pressure to the said cylinder means, means (19) operatively connected to the valve means (10) for controlling the variable pressure to the cylinder means (13) and means adapted for maintaining defined preset pressure in the system.

Full Text

Field of the invention
The present invention relates to a step-less control system for roll bending at finishing stand of tandem mill. In particular, the present invention relates to an improved work roll bending system with step-less control for improved control over hydraulic bending actuators.
Background of the invention
Achieving desired shape of cold rolled (CR) strips is of vital importance during cold rolling process at tandem cold mill (TCM). From the point of view of product quality the strip profile, shape and flatness are all important parameters. Shape of a strip is the manifest of the roll gap contour during cold rolling and a composite parameter defining both strip crown and flatness. The shape or flatness defects of CR strip develop due to mismatch of the profiles of the incoming strip and the active roll gap resulting in a transverse stress distribution and variation in elongation along the width of the strip. But, in a mill producing materials with various grades, width and thickness, it is not possible to select a pre-determined crown to match all the conditions. Therefore, to provide facilities to the operator to adjust the gap according to the profile of input material and thermal camber of work roll, control system is required to correct the roll gap contour for controlling shape and flatness defects of the strip. Correction of the roll gap profile like roll shifting, roll crossing, roll bending and differential roll cooling for controlling asymmetrical shape defect is required.
Profile and Flatness Control Hardware
The hardware called profile (shape) actuators either aim at compensating the effects of unavoidable roll bending effects on the roll gap profile by adjusting the effective roll crown in roll stacks or aim at eliminating or minimizing roll bending by altering the method of applying the load. The recent strip profile and shape control devices can be classified as :
Multi-high Mills"
> 6-High mills (Hitachi) - HCM, HCMW, UCM, UCMW, etc
> Z-mills or cluster mills
Roll Crown Control"
> Parabolic ground crown
> Variable crown - VC (Sumitomo) & IC (Blow Knox) back-up rolls provide lower order shape correction in the middle portion of the strip regardless of strip width
> Pair Cross (Mitsubishi) - Work roll cross and Back-up roll cross mills provide parabolic corrections
> Thermal crown rolls
Roll Shifting Control:
> Flat roll shifting - Work and Back-up roll shifting
> Profiled roU shifting - CVC (SMS), UPC (MDS), K-WRS (KAWASAKI)
Roll Bending Control
> Single and Double chock work roll and back-up roll and intermediate roll bending in vertical plane - slender work rolls provide higher order shape corrections near the edges of the strip, in order to be effective at the central portion large diameter rolls should be used
> Horizontal Roll bending (MKW, UC-2, UC-3, etc)
Multipoint Bending Control by Special Back-up Rolls:
> Divided backup roll benders or As-U rolls in cluster mills like Z-mill (Hitachi), KST mill (Kobe Steel), KT mill (Kobe Steel). These mills also use laterally shifted first intermediate rolls for shape control. Cluster type CR mill (Mitsubishi) uses divided back-up roll crown adjustment along with intermediate and work roll benders.
Variable Back-up Roll Strength:
> Self compensating back-up rolls (SC)
> Tapered piston back-up rolls (TP)
> Variable length rolls (VLR)
Combination control:
> 6-high CVC
> 6-high UPC
> Pair cross with work roll shifting
> Work roll cross with work roll shifting
The other methods of crown control include:
> Tension Distribution Control (TDC)
> Zonal Coolant Control
Strip crown
Strip shape in cold reduction is not a direct function of profile but is determined by roll camber. The finished strip profile is governed by hot band profile assumed to be uniform preferably with a slight crown and the shape control is realized by work roll camber controlled by the operator. Control is made by visual inspection. Thus to correct the hot band distortion and ensuing proper shape in cold mills, it is desirable to have a shape control system which permits instant and refined adjustment of roll camber. There are several means of obtaining or changing camber in the mill are like mechanical roll camber, thermal camber control, altering rolling loads and extreme mechanical control of roll bending. To reduce the strip thickness, the back-up roll is loaded as a simple beam and the work rolls will deflect by an amount directly proportional to the roll separating force. In practice, this bending is counter acted by grinding a mechanical crown in the roll initially either with a convex or a concave surface with parabolic or cycloid profile. There is no fixed mechanical crown that will suffice for the entire product range to be rolled. It is therefore necessary to apportion the product range to several groups of rolls
with fixed crown of each group. This mechanical crown can be given to the back-up rolls, work rolls or a combination of both.
Crown control
In recent years, increased emphasis has been placed on the uniformity of gauge and flatness of both hot and cold rolled strip. The control of strip crown must be made where lateral spread can take place without up-setting the flatness of the product. Thus the control of strip crown and the control of strip shape or flatness can be achieved by proper control of mill crowns in the cold mills. In the cold mill adequate flatness can be maintained only using proper relationship of rolling pressure, strip tension and mill crowns. Various operating factors contribute to make this a difficult problem such as variable crown in the strip itself, non-uniformity of strip tension across the width of the strip as coil built up, changing of mill crowns after a shutdown and until thermal equilibrium of the rolls attained, variable wear on both the work and back-up rolls, schedule changes for width, chemistry etc, variable rolling pressure due to gauge variations, hard spots, speed changes, etc. The basic way to take care of crown changes with the use of proper shaped ground rolls compromising al factors. Proper crowns can be obtained by crown either or both work rolls, either or both back-up rolls or combination of work rolls and back-up rolls. It can be said that at no time it is really correct and the roller always try for rolling with a compromise crown.
However there are three general methods of crown control are roll crossing, work roll bending and back-up roll bending by hydro-mechanical means. The need of instantaneous control is a direct result of users requiring better tolerances and shape.
Roll bending
Crown control by bending is based on the idea of roll bending of the support rolls by hydro-mechanical means. Crown control is achieved by producing a desired deflection in the roll by applying equal and opposite bending moments
to each end of the roll. Crown control by roll bending works better than mechanical crowns. In reality, this method of control counteracts the bending due to rolling pressure with a reverse bending, where as mechanical crowning merely accommodates a deliberately misground roll shape to the offending bending deflection due to the rolling action.
Conventionally hydraulic systems for control are used which are based on water hydraulic. In the conventional system 5% hydraulic oil is mixed 95% water to form the water hydraulic fluid in lieu of 100% oil hydraulics. The known water hydraulics system usually caters three drive units:
> Auxiliary drive unit operating at system pressure of 64 kg/cm2
> Balancing & bending unit operating at pressure of 100 kg/cm2
> Back-up roll extraction unit operating at pressure of 200 kg/cm2
Auxiliary drive units are catered by four pumps (two pumps running and two pumps as stand by) to facilitate the feeding & guiding of strip to the mill and also helps in removal of coils from the tension reel of mill. The delivery pressure of these pumps is set at 64 kg/cm2 through a pressure relief valve at the discharge end. These hydraulic pressure operates the different distributor valves located at valve stand actuating total 250 numbers of hydraulic cylinders for different operations as mentioned above in feeding unit, in mill proper and in coil exit unit.
The roll balancing and bending unit constitutes bending cylinders which are needed to create the positive bending as per requirement on the work rolls to counter the thermal crown developed on the roll during rolling. But no control is available on the positive bending (crown-in) of the work rolls. Therefore it is virtually impossible to get the right shape of the strip that is coming out of the mill.
The conventional roll balancing and bending unit is catered by two pumps running. The discharge hydraulic pressure is maintained at 100 kg/cm2
through pressure relief valve and connected to the distributor valves of the four stands to actuate the work roll balancing cylinders for positive bending of work roll. To maintain the steady constant pressure and to compensate the surge pressure, the N2 Accumulator and load accumulator are connected to the pressure line to damp the pressure fluctuations. Presently mills have electromechanical screw-down system in all the stands for the purpose of adjusting the roll gaps. The hydraulic roll bending system is operating at a hydraulic pressure either 0 bar or 100 bar to control the shape deformities with the help of hydraulic roll force cylinders placed on the bottom work roll chocks. With the actuation of these cylinders facilitates the roll separating in two points (no separation - 0 bar and maximum separation - 100 bar). There is no provision for variation of the roll separating force in between (0 bar to 100 bar) by the operator to correct the shape as far as possible.
During the period of Backup roll extraction additional pump is made to run and to develop 200 kg/cm2 pressure through a separate line with the help of pressure relief valve set at 200 kg/cm2.
Therefore, the conventional water hydraulic system has the following deficiencies"
> Incorrect roll bending and no continuous variable crown leading to symmetrical shape defects like long edges, center buckling, long bow, cross bow and quarter buckles of the rolled strip.
> Decrease in roll force due to reduction in material hardness, lower stand reduction, increase in work roll thermal crown and change in incoming strip profile resulting to full center defect of the strip.
> Sudden drop of hydraulic pressure in line due to external or internal leakages in valves, cylinders, hose pipes and load accumulator because of very high temperature, excess load, vibration and drop in viscosity of fluid.
> Frequent failure of pumps and their glands due to excess loading on pumps because of operating total auxiliary drive units with hydraulic systems, resulting to improper control of roll bending system.
Objects of the invention
Thus, the basic object of the present invention is to provide for an improved roll bending system which would avoid the above discussed drawbacks / limitations of the known roll bending system presently available.
Another object of the present invention is to provide for an improved roll balancing system which would facilitate reduction in shape defects of CR strips specially for TCM.
Yet further object of the present invention is to provide for an improved roll balancing system which will create the variable positive bending as per requirement, on the work rolls to encounter the mismatch of the profiles of the incoming strip and the active roll gap resulting in a transverse stress distribution and variation in elongation along the width of the strip.
Yet another object of the present invention is to provide for an improved roll balancing system with the facility to control the shape or flatness defects of CR strip occurring due to thermal crown developed on the roll during rolling.
Yet further object of the present invention is to provide for an improved roll balancing system which provides the operator with the facility of a step-less control for roll bending at the finishing stand of TCM, to have a better control on shape and flatness of strip during rolling.
Yet another object of the present invention is to provide for an improved roll balancing system operated in oil hydraulics for effective control over shape and flatness defects.
Yet further object is to provide an improved method of roll balancing using the improved system of the present invention.
Detailed description of the present invention
By using variable hydraulic pressure instead of fixed pressure, a certain amount of crown control by bending the work rolls between the back-up rolls of a 4-Hi mill can be expected.
Hydraulic shaping consists of supplementing mechanical and thermal crowning by applying hydraulic forces on the work roll chocks to encounter the roll gap contour variation during rolling. This forms the basis of the present approach.
The system involves incorporation of proportionate pressure reducing and pressure relaxing valve in order to make the roll gap variable on a continuous basis. Thus, step-less control of roll bending is achieved. The system also provides flexibility to the operator to operate the variable roll balancing pressure as per requirement for controlling strip shape defects as the system is operated in a manual mode. The system is much less expensive compared to fully automatic systems and easy to maintain.
The system of the present invention will now be described with reference to the following non-limiting embodiment and accompanying figures.
Description of accompanying figures
Figure 1 shows schematic drawing of roll balancing system of present
invention
Figure 2 shows the hydraulic power pack for bending of present invention
Description of preferred embodiment
The hydraulic power-pack has been installed to cater the Work roll bending
actuators of last stand at TCM. The system (see Fig. 2) consists of four
hydraulic cylinders (13) located in the top of each bottom work roll chock. These cylinders are actuated with the variable plunger stroke subjected to hydraulic force developed on each chock. These roll separating force will deflect the top work roll upward and the bottom work roll downward to create the desired work roll bending crown for ensuing proper profile of the strip at the roll gap. This roll balancing system is used to correct the shape defect as far as possible.
The system consists of two nos. variable volume pump (3) controlled by a pressure compensator. The pressure compensator will maintain the preset pressure to 160 bar by varying the volume of oil discharged by positioning a wobbling plate on the drive shaft of the pump piston. The delivery pressure of the pump ranges from 0 to 320 bar. The pump suction is connected to a 3.0M3 hydraulic oil reservoir (l) with the help of shut-off valve (2) for individual run of the pump. The pressurized hydraulic oil (160 bar) is then passed through non-return valve (4) for unidirectional flow. After that the pressured hydraulic oil is allowed to pass through the duplex type pressure filter (5) for ensuing 5µ filtration level of hydraulic oil to the subsequent valves. The duplex type filter is introduced for continuous run of the system without any stoppage for maintenance of filter element due to jamming. The filtered oil is then passed through a pressure relief valve (6) set at 100 bar with on/off solenoid valve for the protection of the system and to deliver hydraulic oil at constant pressure of 100 kg/cm2, in the delivery line a N2 filled bladder type accumulator (8) is fitted to control the surge pressure and pressure fluctuation during operation. The glycerin filled pressure gauges (7) are connected to the different locations for the display of the hydraulic pressure. The directional control valve (9) helps to connect the path for flowing of oil through port and being controlled by the spool movement with the actuation of pilot pressure actuated with the help of on/off solenoid valve. When the directional control valve is not in use, the oil returns to tank through drain port. In the return line of oil to the reservoir, it is allowed to pass through shell and tube type heat exchanger (ll) to take out the heat of the oil and finally before draining to the reservoir, it
has to pass through a return filter (12) of 20µ filtration level of the returned hydraulic oil. The pressurized hydraulic in then passed through a proportionate pressure relieving and reducing relief valve (10) for supplying variable pressure from 18 bar to 100 bar to the hydraulic cylinders in the work roll chock. The discharged pressure of the proportionate pressure relieving and reducing relief valve is being varied with the spool movement and opening of different ports as the voltage regulation of the built-in electronic voltage transducer. This electronic transducer is connected to the potentiometer by electrical cable for voltage regulation. To get a variable pressure ranges from 18 bar to 100 bar, the voltage regulation is needed from 0 to 2.0 volt in the potentiometer which is being installed at operator"s control desk. As per the requirement of roll balancing pressure for shape correction of the strip through visual observation, the operator regulates the voltage in the potentiometer and subsequently the variable pressure in the hydraulic cylinder (13) of work roll chock are obtained. The hydraulic pressure of the actuating cylinders at bottom work roll chock are being varied for the roll separating force as per the requirement for controlling the shape defects (center buckles / edge waviness) of the strip.
The modified work roll balancing system for step-less control of hydraulic actuators has been installed and commissioned at the Stand # 4 of TCM to control the shape correction as far possible. Analysis of various parameters after installation of the system of the present invention lead the followings results —
> The system is one of the easy way to control the shape defects as far as possible.
> The system has helped to correct the edge waviness and center buckles on the strip.
> It has helped to improve strip flatness by 20%.
> System is easy to operate and retro-fitted to existing system.
> It has minimized roll crown provisions
> It has improved the shape and profile of the strip.
> It has also improved customer services by reduced complaints by 15%.
> The system is easy to maintain and less expensive than any other shape control hardware.
Field Of The Invention
The present invention relates to a step-less control system for roll bending and balancing at finishing stand of tandem mill. More particularly, it relates to an improved work roll bending and balancing system comprising proportionate step-less control pressure reducing and relieving cylinders for improved control over hydraulic bending actuators and thus controlling the shape and flatness of the rolled strips.
Background Of The Invention
Achieving desired shape of cold rolled (CR) strips is of vital importance during cold rolling process at tandem cold mill (TCM). From the point of view of product quality the strip profile, shape and flatness are all important parameters. Shape of a strip is the manifest of the roll gap contour during cold rolling and a composite parameter defining both strip crown and flatness. The shape or flatness defects of CR strip develop due to mismatch of the profiles of the incoming strip and the active roll gap resulting in a transverse stress distribution and variation in elongation along the width of the strip. But, in a mill producing materials with various grades, width and thickness, it is not possible to select a pre-determined crown to match all the conditions. Therefore, to provide facilities to the operator to adjust the gap according to the profile of input material and thermal camber of work roll, control system is required to correct the roll gap contour for controlling shape and flatness defects of the strip. Correction of the roll gap profile like roll shifting, roll crossing, roll bending and differential roll cooling for controlling asymmetrical shape defect is required.
Profile and Flatness Control Hardware
The hardware called profile (shape) actuators either aim at compensating the effects of unavoidable roll bending effects on the roll gap profile by adjusting the effective roll crown in roll stacks or aim at eliminating or minimizing roll bending by altering the method of applying the load. The recent strip profile and shape control devices can be classified as:
1. Multi-high Mills:
(i) 6-High mills (Hitachi) - HCM, HCMW, UCM, UCMW, etc (ii) Z-mills or cluster mills
2. Roll Crown Control:
(i) Parabolic ground crown
(ii) Variable crown - VC (Sumitomo) & IC (Blow Knox) back-up rolls provide
lower order shape correction in the middle portion of the strip regardless of
strip width (iii) Pair Cross (Mitsubishi) - Work roll cross and Back-up roll cross mills provide
parabolic corrections (iv) Thermal crown rolls
3. Roll Shifting Control:
(i) Flat roll shifting - Work and Back-up roll shifting
(ii) Profiled roll shifting - CVC (SMS), UPC (MDS), K-WRS (KAWASAKI)
4. Roll Bending Control:
(i) Single and Double chock work roll and back-up roll and intermediate roll bending in vertical plane - slender work rolls provide higher order shape corrections near the edges of the strip, in order to be effective at the central portion large diameter rolls should be used
(ii) Horizontal Roll bending (MKW, UC-2, UC-3, etc)
5. Multipoint Bending Control by Special Back-up Rolls:
(i) Divided back-up roll benders or As-U rolls in cluster mills like Z-mill (Hitachi), KST mill (Kobe Steel), KT mill (Kobe Steel). These mills also use laterally shifted first intermediate rolls for shape control. Cluster type CR mill (Mitsubishi) uses divided back-up roll crown adjustment along with intermediate and work roll benders.
6. Variable Back-up Roll Strength:
(i) Self-compensating back-up rolls (SC)
(ii) Tapered piston back-up rolls (TP) (iii) Variable length rolls (VLR)
7. Combination control:
(i) 6-high CVC
(ii) 6-high UPC
(iii) Pair cross with work roll shifting
(iv) Work roll cross with work roll shifting
8. The other methods of crown control include:
(i) Tension Distribution Control (TDC) (ii) Zonal Coolant Control
Strip crown
Strip shape in cold reduction is not a direct function of profile but is determined by roll camber. The finished strip profile is governed by hot band profile assumed to be uniform preferably with a slight crown and the shape control is realized by work roll camber controlled by the operator. Control is made by visual inspection. Thus to correct the hot band distortion and ensuing proper shape in cold mills, it is desirable to have a shape control system which permits instant and refined adjustment of roll camber. There are several means of obtaining or changing camber in the mill like mechanical roll camber, thermal camber control, altering rolling loads and extreme mechanical control of roll bending. To reduce the strip thickness, the back-up roll is loaded as a simple beam and the work rolls will deflect by an amount directly proportional to the roll separating force. In practice, this bending is counter acted by grinding a mechanical crown in the roll initially either with a convex or a concave surface with parabolic or cycloid profile. There is no fixed mechanical crown that will suffice for the entire product range to be rolled. It is therefore necessary to apportion the product range to several groups of rolls with fixed crown of each group. This mechanical crown can be given to the back-up rolls, work rolls or a combination of both.
Crown control
In recent years, increased emphasis has been placed on the uniformity of gauge and flatness of both hot and cold rolled strip. The control of strip crown must be made where lateral spread can take place without up-setting the flatness of the product. Thus the control of strip crown and the control of strip shape or flatness can be achieved by proper control of mill crowns in the cold mills. In the cold mill adequate flatness can be maintained only using proper relationship of rolling pressure, strip tension and mill crowns. Various operating factors contribute to make this a difficult problem such as variable crown in the strip itself, non-uniformity of strip tension across the width of the strip as coil built up, changing of mill crowns after a shutdown and until thermal equilibrium of the rolls attained, variable wear on both the work and back-up rolls, schedule changes for width, chemistry etc, variable rolling pressure due to gauge variations, hard spots, speed changes, etc. The basic way is to take care of crown changes with the use of properly shaped ground rolls compromising all factors. Proper crowns can be obtained by crown either or both work rolls, either or both back-up rolls or combination of work rolls and back-up rolls. It can be said that at no time it is really correct and the roller always try for rolling with a compromise crown.
However there are three general methods of crown control are roll crossing, work roll bending and back-up roll bending by hydro-mechanical means. The need of instantaneous control is a direct result of users requiring better tolerances and shape.
Roll bending
Crown control by bending is based on the idea of roll bending of the support rolls by hydro-mechanical means. Crown control is achieved by producing a desired deflection in the roll by applying equal and opposite bending moments to each end of the roll. Crown control by roll bending works better than mechanical crowns. In reality, this method of control counteracts the bending due to rolling pressure with a reverse bending, whereas mechanical crowning merely accommodates a deliberately misground roll shape to the offending bending deflection due to the rolling action.
Conventionally hydraulic systems for control are used which are based on water hydraulic. In the conventional system 5% hydraulic oil is mixed 95% water to form the water hydraulic fluid in lieu of 100% oil hydraulics. The known water hydraulics system usually caters three drive units such as auxiliary drive unit operating at system pressure of 64 kg/cm , balancing & bending unit operating at pressure of 100 kg/cm2 and back-up roll extraction unit operating at pressure of 200 kg/cm2.
Auxiliary drive units are catered by four pumps (two pumps running and two pumps as stand by) to facilitate the feeding & guiding of strip to the mill and also helps in removal of coils from the tension reel of mill. The delivery pressure of these pumps is set at 64 kg/cm2 through a pressure relief valve at the discharge end. These hydraulic pressure operates the different distributor valves located at valve stand actuating total 250 numbers of hydraulic cylinders for different operations as mentioned above in feeding unit, in mill proper and in coil exit unit.
The roll balancing and bending unit constitutes bending cylinders, which are needed to create the positive bending as per requirement on the work rolls to counter the thermal crown developed on the roll during rolling. But no control is available on the positive bending (crown-in) of the work rolls. Therefore it is virtually impossible to get the right shape of the strip that is coming out of the mill.
The roll balancing and bending unit used conventionally is catered by two pumps running. The discharge hydraulic pressure is maintained at 100 kg/cm2 through pressure relief valve and connected to the distributor valves of the four stands to actuate the work roll balancing cylinders for positive bending of work roll. To maintain the steady constant pressure and to compensate the surge pressure, the N2 Accumulator and load accumulator are connected to the pressure line to damp the pressure fluctuations. Presently mills have electro-mechanical screw-down system in all the stands for the purpose of adjusting the roll gaps. The hydraulic roll bending system is operating at a hydraulic pressure either 0 bar or 100 bar to control the shape deformities with the help of hydraulic roll force cylinders placed on the bottom work roll chocks. With the actuation of these cylinders facilitates the roll separating in two points
(no separation - 0 bar and maximum separation - 100 bar). There is no provision for variation of the roll separating force in between (0 bar to 100 bar) by the operator to correct the shape as far as possible.
During the period of Back-up roll extraction additional pump is made to run and to develop 200 kg/cm2 pressure through a separate line with the help of pressure relief valve set at 200 kg/cm .
The main disadvantages of the conventional water hydraulic system are incorrect roll bending and no continuous variable crown leading to symmetrical shape defects like long edges, center buckling, long bow, cross bow and quarter buckles of the rolled strip.
Another disadvantage is that on decreasing the roll force due to reduction in material hardness, lower stand reduction, increase in work roll thermal crown and change in incoming strip profile the full center of the strip becomes defective.
Further disadvantage of the conventional system is that there is sudden drop of hydraulic pressure in line due to external or internal leakages in valves, cylinders, hose pipes and load accumulator because of very high temperature, excess load, vibration and drop in viscosity of fluid.
The other disadvantage is the frequent failure of pumps and their glands due to excess loading on pumps because of operating total auxiliary drive units with hydraulic systems, resulting in improper control of roll bending system.
Thus there is a need to provide for a system for improved work roll bending and balancing of the rolled strips and proportionate step-less control pressure reducing and relieving cylinders for improved control over hydraulic bending actuators and thus controlling the shape and flatness of the rolled strips.
Objects Of The Invention
Thus, the basic object of the present invention is to provide for an improved roll bending system which would avoid the above discussed drawbacks / limitations of the known roll bending system presently available.
Another object of the present invention is to provide for an improved roll balancing system which provides the operator with the facility of a step-less control for roll bending at the finishing stand of TCM, to have a better control on shape and flatness of strip during rolling.
Further object of the present invention is to provide for an improved roll balancing system, which would facilitate reduction in shape defects of CR strips specially for TCM.
Yet further object of the present invention is to provide for an improved roll balancing system which will create the variable positive bending as per requirement, on the work rolls to encounter the mismatch of the profiles of the incoming strip and the active roll gap resulting in a transverse stress distribution and variation in elongation along the width of the strip.
Yet another object of the present invention is to provide for an improved roll balancing system with the facility to control the shape or flatness defects of CR strip occurring due to thermal crown developed on the roll during rolling.
Yet another object of the present invention is to provide for an improved roll balancing system operated in oil hydraulics for effective control over shape and flatness defects.
The other object is to provide an improved method of roll balancing using the improved system of the present invention.
Summary Of The Invention
Thus according to the main aspect of the present invention there is provided a control system adapted for roll bending and roll balancing and controlling the shape and flatness of the rolled strips, the said system comprises:
(i) plurality of cylinder means located at the top of each bottom work roll chock;
(ii) means operatively connected to the said cylinder means and adapted for supplying
variable pressure to the cylinder means; (iii) means adapted for controlling the variable pressure to the cylinder means and
operatively connected to the means for supplying variable pressure to the cylinder
means and; (iv) means adapted for maintaining defined preset pressure in the system.
Detailed Description Of The Present Invention
The system comprises plurality of cylinder means comprising four hydraulic cylinders located at the top of each bottom work roll chock. Hydraulic force developed on the roll chock generates variable plunger stroke, which actuates the cylinders. Thus roll separating force generated deflects the top work roll upward and the bottom work roll downward to create the desired work roll bending crown for ensuing proper profile of the strip at the roll gap. This hydraulic shaping consists of supplementing mechanical and thermal crowning by applying hydraulic forces on the work roll chocks to encounter the roll gap contour variation during rolling. This forms the basis of the present approach.
In the roll bending and balancing system of the present invention a hydraulic power-pack has been installed to cater the work roll bending actuators of last stand at the tandem cold mill (TCM). The system comprises means operatively connected to the said cylinder means and adapted for supplying variable pressure to the cylinder means. The said means for supplying variable pressure to the cylinder comprises proportionate pressure reducing and pressure relaxing valve adapted for making the roll gap variable on a continuous basis and achieving step-less control of roll bending. Thus the system also provides flexibility in operating roll
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balancing under variable pressure condition as per requirement for controlling strip shape defects as the system is operated in a manual mode. The system is much less expensive compared to fully automatic systems and easy to maintain.
The system comprises two variable volume pump controlled by a pressure compensator. The pressure compensator is adapted for maintaining the preset pressure to 160 bar by varying the volume of oil discharged by positioning a wobbling plate on the drive shaft of the pump piston. The delivery pressure of the pump ranges from 0 to 320 bar. The pump suction is connected to a 3.0M3 hydraulic oil reservoir by a shut-off valve for individual run of the pump. Non-return valve fitted to the delivery line of the pump is adapted for unidirectional flow of the pressurized hydraulic oil (160 bar). The discharge end of the pump comprises the duplex type pressure filter adapted for the pressured hydraulic oil to pass through ensuing 5µ filtration level of hydraulic oil to the subsequent valves. The duplex type filter is introduced for continuous run of the system without any stoppage for maintenance of filter element due to jamming. The present system of the invention comprises a pressure relief valve set at 100 bar with on/off solenoid valve adapted for carriage of filtered oil and protection of the system and to deliver hydraulic oil at constant pressure of 100 kg/cm2. N2 filled bladder type accumulator is fitted to in the delivery line adapted to control the surge pressure and pressure fluctuation during operation. The glycerin filled pressure gauges are connected to the different locations for the display of the hydraulic pressure. The directional control valve is adapted to connect the path for flowing of oil through port and being controlled by the spool movement with the actuation of pilot pressure actuated with the help of on/off solenoid valve. When the directional control valve is not in use, the oil returns to tank through drain port. In the return line of oil to the reservoir, it is allowed to pass through shell and tube type heat exchanger to take out the heat of the oil and finally before draining to the reservoir, it has to pass through the return filter of 20u filtration level of the returned hydraulic oil. The pressurized hydraulic is then passed through a proportionate pressure relieving and reducing relief valve for supplying variable pressure from 18 bar to 100 bar to the hydraulic cylinders in the work roll chock. The discharged pressure of the proportionate pressure relieving and reducing relief valve is varied by means operatively connected to the valve and the said means comprises the spool movement and opening of different ports of the voltage regulation
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of the built-in electronic voltage transducer. This electronic transducer is connected to the potentiometer by electrical cable for voltage regulation. To get a variable pressure ranges from 18 bar to 100 bar, the voltage regulation is needed from 0 to 2.0 volt in the potentiometer which is being installed at operator"s control desk. As per the requirement of roll balancing pressure for shape correction of the strip through visual observation, the operator regulates the voltage in the potentiometer and subsequently the variable pressure in the hydraulic cylinder of work roll chock are obtained. The variable pressure instead of fixed pressure is adapted in the hydraulic cylinder for obtaining crown control by bending the work rolls between the back-up rolls of a 4 - Hi mill. The hydraulic pressure of the actuating cylinders at bottom work roll chock are being varied for the roll separating force as per the requirement for controlling the shape defects (center buckles / edge waviness) of the strip.
The system of the present invention will now be described with reference to the following non-limiting embodiments and accompanying figures.
Brief Description Of The Accompanying Figures
Figure 1 illustrates schematic drawing of roll balancing system of present invention. Figure 2 illustrates the hydraulic power pack for roll bending in present invention. Figure 3 illustrates the positive bending of work rolls. Figure 4 illustrates schematic diagram of conventional roll balancing system.
Figure 5 illustrates the schematic diagram of the interfacing layout of the system in the present invention.

Detailed Description Of The Accompanying Figures
Figure 1 illustrates schematically the roll balancing system adapted to correct the shape defect as far as possible in the present invention. As shown in figure 1 the system consists of four hydraulic cylinders (13) located at the top of each bottom work roll chock. The system consists of two variable volume pump (3) controlled by a pressure compensator. The suction pump (3) is connected to a 3.0M3 hydraulic oil reservoir (1) with the help of shut-off valve (2) for individual run of the pump (3). Non-return valve (4) fitted to the delivery line of the pump (3) is adapted for unidirectional flow of the pressurized hydraulic oil (160 bar). The discharge end of the pump (3) comprises the duplex type pressure filter (5) adapted for the pressured hydraulic oil to pass through ensuing 5µ filtration level of hydraulic oil to the subsequent valves. The present system of the invention comprises a pressure relief valve (6) set at 100 bar with on/off solenoid valve adapted for carriage of filtered oil and protection of the system and to deliver hydraulic oil at constant pressure of 100 kg/cm2. N2 filled bladder type accumulator (8) (also shown in figure 2) is fitted to in the delivery line (14) adapted to control the surge pressure and pressure fluctuation during operation. The glycerin filled pressure gauges (7) are connected to the different locations for the display of the hydraulic pressure. The directional control valve (9) between the proportionate pressure relieving and reducing relief valve (10) and pressure relief valve (6) is adapted to connect the path for flowing of oil through port and being controlled by the spool movement with the actuation of pilot pressure actuated with the help of on/off solenoid valve. In the return line (15) of oil to the reservoir (1), it is allowed to pass through shell and tube type heat exchanger (11) to take out the heat of the oil and finally before draining to the reservoir (1), it has to pass through a return filter (12) of 20(x filtration level of the returned hydraulic oil. Proportionate pressure relieving and reducing relief valve (10) adapted to pass the pressurized hydraulic for supplying variable pressure from 18 bar to 100 bar to the hydraulic cylinders in the work roll chock. As per the requirement of roll balancing pressure for shape correction of the strip through visual observation, the operator regulates the voltage adapting potentiometer and subsequently the variable pressure in the hydraulic cylinder (13) of work roll chock is obtained.

In figure 2 the hydraulic power pack for roll bending in present invention is illustrated. The rolling stands (SI, S2, S3 and S4) is connected to the proportionate pressure relieving and reducing relief valve (10), N2 filled bladder type accumulator (8), 3.0JVT hydraulic oil reservoir (1) and pumps (3) through the delivery line (14).
In figure 3 the positive bending of work rolls is illustrated. In the figure the top work roll (16) is deflected upward and the bottom work roll (17) is deflected downward by roll separating force thus producing desired work roll bending crown ensuing proper profile to the roll strip at the roll gap.
In figure 5 the schematic diagram of the interfacing layout of the system in the present invention is illustrated. The power supply (18) connected to the proportionate pressure relieving and reducing relief valve (10) provides 24 volt to the proportionate pressure relieving and reducing relief valve (10). The potentiometer mill operating post (19) connected to the proportionate pressure relieving and reducing relief valve (10) for control operation of the valve (10).

The system of the present invention provide the following main advantages -
1. The system is one of the easy ways to control the shape defects as far as possible
2. The system helps to correct the edge waviness and center buckles on the strip.
3. It helps to improve strip flatness by 20%.
4. System is easy to operate and retro-fitted to existing system.
5. It has minimized roll crown provisions.
6. It has improved the shape and profile of the strip.
7. The system is easy to maintain and less expensive than any other shape control hardware.

We Claim
1. A control system adapted for roll bending and roll balancing and controlling the shape and flatness of the rolled strips, the said system comprises:
(i) plurality of cylinder means located at the top of each bottom work roll chock;
(ii) means operatively connected to the said cylinder means and adapted for supplying variable pressure to the cylinder means;
(iii) means adapted for controlling the variable pressure to the cylinder means and operatively connected to the means for supplying variable pressure to the cylinder means characterized in that said means for controlling variable pressure comprises transducer means for spool movements and opening of different ports of voltage regulation and;
(iv) means adapted for maintaining defined preset pressure in the system.
2. A system as claimed in claim 1, wherein the plurality of cylinder means comprises four hydraulic cylinders adapted for producing hydraulic force on the roll chock.
3. A system as claimed in claim 1, wherein the means adapted for supplying variable pressure comprises proportionate pressure relieving and reducing relief valve.
4. A system as claimed in claim 3, wherein the proportionate pressure relieving and reducing valve adapted for supplying pressure ranging from 18 bar to 100 bar to the hydraulic cylinders.
5. A system as claimed in claim 1, wherein the transducer means comprises electronic voltage transducer. .
6. A system as claimed in claim 1, wherein the means for maintaining defined preset pressure comprises two variable volume pumps for shaping of rolls through roll balancing and roll bending.
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7. A system as claimed in claim 6, wherein the two variable volume pump comprises pressure compensator adapted for maintaining the defined preset pressure to 160 bar.
8. A system as claimed in claims 6 and 7, wherein the variable volume pump comprises the duplex type pressure filter adapted for the pressured hydraulic oil to pass through ensuing 5µ filtration level and for continuous run of the system without any stoppage for maintenance of filter element due to jamming.
9. A system as claimed in any of the preceding claims, optionally comprises pressure relief valve at 100 bar with on/off solenoid valve adapted for carriage of filtered oil and protection of the system delivering hydraulic oil at constant pressure of 100 kg/cm2.
10. A control system adapted for roll bending and roll balancing as herein described and illustrated with reference to accompanying figures.
A control system adapted for roll bending and roll balancing and controlling the shape and flatness of the rolled strips and comprises plurality of cylinder means (13) located at the top of each bottom work roll chock, means (10) operatively connected to the said cylinder means (13) for supplying variable pressure to the said cylinder means, means (19) operatively connected to the valve means (10) for controlling the variable pressure to the cylinder means (13) and means adapted for maintaining defined preset pressure in the system.

Documents:

00154-kol-2004-abstract.pdf

00154-kol-2004-claims.pdf

00154-kol-2004-correspondence.pdf

00154-kol-2004-description (complete).pdf

00154-kol-2004-description (provisional).pdf

00154-kol-2004-drawings.pdf

00154-kol-2004-form 1.pdf

00154-kol-2004-form 18.pdf

00154-kol-2004-form 2.pdf

00154-kol-2004-form 3.pdf

00154-kol-2004-form 5.pdf

00154-kol-2004-letter patent.pdf

00154-kol-2004-pa.pdf

154-KOL-2004-(01-02-2012)-FORM-27.pdf

154-KOL-2004-FORM 27.pdf

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Patent Number

211265

Indian Patent Application Number

154/KOL/2004

PG Journal Number

43/2007

Publication Date

26-Oct-2007

Grant Date

24-Oct-2007

Date of Filing

31-Mar-2004

Name of Patentee

STEEL AUTHORITY OF INDIA LIMITED

Applicant Address

BOKARO STEEL CITY, BOKARO- 827001 JHARKHAND.

Inventors:

#	Inventor's Name	Inventor's Address
1	N/A	N/A

PCT International Classification Number

B21B 37/08

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA