Roller bending technology and positive and negative bending
In strip production, the bending and shifting roller device is an indispensable part. The bending roll can improve the strip shape; the shifting roll is a horizontal movement of the flat roll, which improves the use efficiency of the roll and prolongs the life of the roll. However, in actual use, vicious phenomena such as oil leakage, internal leakage, piston rod strain, dust ring protrusion, shifting roller displacement during steel rolling, oil circuit block rotary joint pipeline breakage, etc. often occur in the bending and shifting roller device, which have adverse effects on equipment and production.
Roll bending uses mechanical force to bend a roll barrel to control strip crown (see strip crown) and flatness (see flatness control). Hydraulic roll bending has developed rapidly since its introduction in the early 1960s and has become essential equipment on many plate and strip mills.
A roll-bending cylinder consists of a cylinder barrel, cylinder head, piston rod, seals and other parts. The cylinders in the work-roll bending system also balance and clamp the work rolls. The fixed blocks are mounted on the inside of the mill housing windows, have replaceable slide plates and are secured with screws and wedge keys to support the moving blocks. The moving blocks are key components of the roll-bending and roll-shifting device. Each is fitted with vertically acting bending/balancing hydraulic cylinders, a work-roll shifting cylinder and a work-roll locking cylinder. Replaceable slide plates are also provided between them and the fixed blocks and work-roll chocks. The work-roll locking system uses hydraulic locking and is arranged horizontally on the operator side. It grips the upper and lower work-roll chocks and transmits axial movement to the work rolls. It must be released during roll changes.
Roll bending can improve the strip shape. When roll bending control is required, the hydraulic system is changed from the pressing position to the bending position, a positive roll bending force is applied to the work roll, and the roll gap of the rolling mill is changed to achieve the purpose of controlling the strip shape.
Common faults of bending and shifting rollers
(1) The roll bending cylinder often leaks oil, internal leakage, piston rod strain, and dust ring protrusion.
(2) The end cover of the shifting roller cylinder often leaks, the shifting roller lock nut and the piston rod screw thread are broken, and the shifting roller shifts during the steel rolling process.
(3) The pipeline of the rotating joint of the oil circuit block of the lower moving seat on the transmission side was broken, causing failure time and a large amount of oil leakage.
Hydraulic roll bending technology can be divided into:
·Work roll bending: the ratio of roll length L to diameter D L/D<3.5
·Backup roll bending roll: L/D≥3.5
Both roll bending methods include positive bending and negative bending.
Positive bending: The bending force causes the bending direction of the roll to be opposite to the bending direction caused by the rolling force, that is, the crown of the work roll increases during roll bending. For normal work roll bending, the hydraulic cylinder is usually installed on the lower work roll bearing seat, and the hydraulic roll bending force acts between the upper and lower bearing seats.
Negative bending: The bending direction of the roll caused by the roll bending force is the same as the bending direction caused by the rolling force, that is, the crown of the work roll decreases during roll bending. The work roll negative bending installs the hydraulic cylinder on the backup roll bearing seat, and the roll bending force acts between the work roll bearing seat and the backup roll bearing seat.
Negative bending of work rolls has 3 advantages:
(1) The roll bending force does not interfere with the plate thickness automatic control system;
(2) There is no need to disassemble the high-pressure oil supply circuit joint of the hydraulic cylinder when replacing the work roll;
(3) can prevent iron oxide scale, emulsion, etc. from invading the hydraulic cylinder. Adding a negative bending work roll can expand the adjustment range of the hydraulic bending roll.
Backup-roll bending Backup-roll bending is also widely used for strip shape control. Although both positive and negative backup-roll bending are available, the great majority of applications use positive bending; negative bending is less common. In this arrangement, the bending force is applied to the roll ends outside the bearing chocks, using the bearings as fulcrums to bend the backup roll. Its main advantage is the ability to adjust both longitudinal and transverse strip thickness differences simultaneously. Backup-roll bending devices provide high bending forces and sensitive roll-crown adjustment over a fairly wide range.
The effect of back-up roll bending is better than that of work roll bending, so it is widely used in large hot-rolling thick plate mills. It is also used in wide-band hot rolling mills and single-stand reversible hot rolling mills, and even in strip cold rolling mills.
The roll bending cylinder undertakes the dual tasks of roll bending and balancing. Low pressure is used for balancing and high pressure is used for roll bending control. The high-pressure circuit is controlled by an electro-hydraulic servo valve. The set value of the roll bending force is determined by the computer, and the corresponding voltage signal U0 is given. At the same time, it is compared with the feedback value U of the pressure sensor, and the difference △U is sent to the electro-hydraulic servo amplifier until the roll bending force is consistent with the set value.
Optimum roll-bending force: Before strip rolling, the force required to obtain good strip shape or transverse thickness accuracy is calculated in advance from the incoming material grade, width and thickness, the initial crown of the stock, the reduction, rolling force and original mill parameters. This is known as the optimum roll-bending force. It is calculated using a strip-shape prediction model, and the bending device is set to the corresponding position to ensure good strip shape and low strip crown after passage through the mill. This is the preset control of optimum roll-bending force in a strip mill.
The preset calculation of the optimal roll bending force is complex and is generally performed by a microcomputer. Due to the fast response speed of the system, it can timely calculate the optimal roll bending force value that should be set at each moment through the information obtained by continuously measuring the strip thickness and plate crown, and correct the roll bending force value at any time. Therefore, it can be used for online real-time control.
Effects and limitations of roll bending: Hydraulic roll bending is of great significance to flatness control and is an effective means to control strip flatness.
It has the following effects:
(1) The flatness of the strip steel is significantly improved;
(2) Lateral thickness non-uniformity is reduced by 20%~25%;
(3) The service life of the roll is increased by 15%~20%;
(4) The rolling mill productivity is increased by 5%~7%.
However, there are still some shortcomings in hydraulic bending rollers:
(1) The bending force of the roll is insufficient to eliminate the middle wave shape of the wide band steel;
(2) also cannot eliminate complex plate defects such as complex waves and local waves that appear in production;
(3) interferes with automatic plate thickness control.
(Source: Steel Rolling Home)