Shape Control Technologies in Hot Strip Mill

Shape Control Technologies in Hot Strip Mill

The shape of a rolled strip is characterized by its transverse section profile (crown) and by its flatness. The accuracy of the strip shape is an important parameter which determines the quality of the hot rolled strip. Strip shape is an indispensable factor in determining the market competitiveness of the rolled strip. Since the strip shape is a key indicator of quality, the shape control technology is the core technology of hot strip production. There are several factors which influences the shape of rolled strip in the hot strip mill (HSM).

One of the key objectives in rolling of strip in HSM is to achieve the target thickness with optimum quality in terms of crown and flatness of the exit strip. During thickness reduction, it is very important to have a constant reduction across the strip width in order to have a uniform elongation between the centre and the edge of the strip. If this condition is not met, an internal stress condition is generated, causing flatness defects (centre buckles or wavy edges). Fig 1 shows the concept of perfect flatness and flatness defects.

Fig 1 Concept of perfect flatness and flatness defects

With the increasing demand for higher strip dimensional tolerances, the need to maintain a uniform strip crown and a flat shape during the rolling of hot strip has become one of the most challenging technical tasks in the HSM. The shape control technologies which affect the shape quality of the hot rolled strips fall into three categories namely (i) shape control actuating devices for rolls, (ii) shape control model, and (iii) rolling process system. Shape control technology is also developing in the direction of integration. In this article, shape control actuating devices for rolls are described.

Shape control actuating devices for rolls

During rolling in HSM, the exit crown and flatness of the strip are influenced by several factors such as roll thermal profile, rolling force, roll wear, and strip width, etc. These factors change during the rolling campaign. For the compensation of these factors and to control strip crown and , it is essential to install a series of actuating devices which can continuously modify the roll bite shape. The three most popular technologies for roll actuating are (i) roll bending technology, (ii) roll shifting technology, and (iii) continuously variable crown (CVC) technology.

Roll bending technology – The available profile control range without visible flatness defects (centre buckle and edge waves) is correlated with strip exit thickness. The higher the strip exit thickness, the higher is the profile’s capability to change without generating a flatness defect. A typical diagram with the limitation of crown ratio change versus strip thickness is shown in Fig 2. The important factor in the design of the bending system is the impact range of the bending actuating device on crown and flatness control. When the rolled material is thick, the impact of bending on the strip crown ratio is limited, whereas when it is thin, the impact is very high.

Fig 2 shows an example of a bending control range which is capability of the mill to control the crown ratio without flatness defects. The bending control range depends on two main factors namely (i) the power of the crown control actuating device used on the stands, and (ii) the position along the mill stand where these actuating devices are installed. For increasing the crown and flatness control capability of HSM, it is necessary to install the actuating devices in the right position to optimize their efficiency.

Fig 2 Bending control range

The work roll bending (WRB) device because of its usability and easy-to-install structure, is the most commonly used shape control actuating mechanism for the hot strip rolling. Although the ideal WRB load capacity is a little less than one-tenth of the rolling load, it is in many cases difficult to achieve even if WRB is combined with negative bending to improve the total WRB load.

A double chock bending (DCB) device, equipped with two chocks at a roll neck of work roll, is an effective WRB method which is able to easily achieve a balanced design among the three strength constraint conditions (roll strength, bearing strength, and chock strength) and achieve a large bending capacity in a limited space. However, these types of rolls have roll axis ends which slightly protrude and cannot be easily installed by converting an existing mill.

WRB has a relatively simple structure, and is frequently installed with other shape control actuating devices. Fig 3 shows an example of the control characteristics achieved when DCB is installed with a variable crown (VC roll). In the figure, the mechanical centre crown value indicates on the vertical axis represents the difference in the strip thickness between the strip centre and the strip edge when the rolling force is distributed evenly in the width direction, while the mechanical quarter crown value indicates on the horizontal axis represents the difference in the strip thickness between the strip centre and strip quarter width position.

Fig 3 Work bending method for shape control

Since a highly responsive hydraulic system is needed to apply bending loads, the direct-drive servo valve is adopted in some cases. Although it is difficult to use negative bending with many rolling mills that frequently need threading and tailing out, the usability of negative bending in threading and tailing out can be improved by adopting a highly responsive servo valve with a short pipe length between valve and cylinder.

Small diameter work rolls are effective in reducing the rolling load in rolling extremely thin strips and hard materials. However, in the WRB device, by which a roll is bent at its end, the bending effect does not transmit well to the roll centre region. It is possible to additionally install support rolls, which come in contact with a work roll, to support it at the roll barrel length, in order to provide bending and support effects. One kind of bending (support) roll, in addition to the main support roll which vertically supports the rolling load, is designed to push the work roll in an oblique or horizontal direction. In some cases, a shape control method is adopted in which the specific portions of the work roll barrel are hydraulically and selectively pushed (FFC method) by the bending rolls, as shown in Fig 3.

For rolling mills with a long barrel, as a method of bending a work roll, an outboard bending mechanism is installed at the back-up roll to bend the large diameter back-up roll and indirectly bend a long barrel work roll. The estimated mechanical centre crown control effect of each rolled strip width with an outboard back-up roll bending (BURB) device for hot steel rolling in a plate mill (work roll diameter is 1,020 mm, back-up roll diameter 1,830 mm, and barrel length is 4,700 mm) is shown in Fig 3. The figure also shows the mechanical centre crown control effect of each rolled width with WRB. It can be seen from the figure that when the strip width is large, the control effect of WRB is greater than that of BURB. However, when the strip width is small, the control effect of BURB is greater than that of WRB.

In multi-high rolling mills (such as six-high rolling mill), it is possible to equip the large diameter intermediate roll with a vertical bending function to indirectly bend a work roll. The intermediate roll bending method and back-up roll bending method can be used without being affected by the complicated WRB pressure control required for changing work rolls, strip threading and tailing out.

Work roll double jack heavy bending system – In order to achieve the desired thickness profile in the roll gap and to control that gap during the rolling of the entire coil, despite the possible thermal and geometrical variations of the incoming work piece , a positive and negative heavy bending system for all the finishing stands has been developed. Work roll bending is used typically in dynamic and continuous strip crown and flatness control during rolling. When roll bending forces are applied in the direction of the rolling force, the bending is positive (crown in); when bending forces are applied in the direction opposite to the rolling force, the bending is negative (crown out) as shown in Fig 4.

Fig 4 Work roll bending system

The on-line control model calculates the sum of work roll bending forces as a function of roll separating force, strip width, roll diameter, work roll mechanical crown, work roll thermal crown, work roll wear and entry strip profile. This means that the work roll bending system settings can be changed quickly under load (dynamic control) in order to achieve strip crown within the target tolerances and optimum strip flatness. The positive bending actuating devices are located in the ‘Mae West’ blocks, while the negative bending cylinders are located inside the back-up roll (BUR) chocks (Fig 4). This system is capable of obtaining top-level performance without any interference with any other mill control, and without any increase in maintenance. Positive heavy bending upto 200 ton/chock and negative heavy bending upto 120 ton/chock can be achieved. These forces can be achieved without affecting bearing life because of the double-jack roll bending system, which keeps the bending force centred with respect to the bearing centre line, regardless of the axial position of the roll, according to the side shifting procedures.

Roll shifting technology – Different roll shift shape control technologies have been developed. These technologies basically consist of (i) a method by which the shape control effect can be improved by shifting rolls, reducing the contact portion between rolls outside the strip width and as a result to improve the deflection of the rolls, and (ii) a method by which the shape control effect can be improved by shifting specially shaped rolls, and producing the geometric roll gap distribution change effect in the width direction. This is shown in Fig 5. Out of various roll shift methods, the method based on the effects of geometrically shaped shift rolls has the advantages in that the shape control effect can be freely set by the geometric shape. A numerical control (NC) roll grinding machine makes it possible to more freely set the grinding roll curve, and more easily improve the effect of the optimized roll curve.

Fig 5 Roll shift methods for shape control

Basically, the same shift mechanism is used in the above mentioned two methods but the geometrically shaped shift rolls are more effective. The geometrically shaped shift rolls have a roll profile which has simple concave and convex combined curve (S-shaped curve). The shift rolls profile can be optimized and the optimally-shaped shift roll is also sometimes called as a ‘combined numerical profile’ (CNP) roll. Roll shift method is further aided by an automatic setting system which simultaneously calculates the setting values for the shift positions and roll bending pressures and provides commands, according to the roll shape.

Further, the shift roll shapes can be optimized for each rolling mill plant or rolling mill stand can be optimized, and the shape of shift rolls of a rolling mill stand can be optimized one by one so that the control effect for the strip width with a higher production ratio becomes higher. To thoroughly optimize the roll shape, the shape is frequently determined not by using specific functions, but by using numerical data. The work roll shift method can also be used for roll wear dispersion to reduce step wise uneven wear on rolls in hot rolling of the steel strip.

A shifting device can be installed at either the operation side or drive side, depending on the workability and serviceability needed in the rolling mill. Roll shift shape control actuating devices are used more frequently than before since they are able to produce flexible control effects depending on the selected roll shape, and they can be installed on a large sized hot rolling mill.

Shaped roll technology – In order to increase the strip crown and flatness range control, shaped roll technology can be applied to all the finishing stands. In this case shifting is used not only to control the distribution of the work roll wear, but also to control strip crown and flatness. Typically, the crown control capacity with shaped rolls is two to three times higher than the capacity of the bending system. A smooth profile shape for the work rolls is adopted in order to combine the crown and flatness control with the wear control function. The profile of the work roll is a curve consisting of an asymmetric sine function and a three-order multinomial function. The shifting system is dedicated to gap profile setting, thus it is used only to set the suitable work roll  gap profile (static control), while during rolling strip crown control (dynamic control) is performed by bending actuating device.

Inflating roll method (Variable crown roll method) – In case of the work roll with a longer roll barrel length, the effect of WRB is less likely to transmit to the strip width central region in WRB shape control. The strip shape control effect can be maintained by combining the WRB method with the method by which a back-up roll is partially inflated in the roll barrel direction for shape control. If the shape control performance can be maintained by using an inflating roll as a back-up roll, it is possible to eliminate the need to process the work roll to form a convex-curved shape in strip centre region. This type of roll reduces the chance of sharp uneven contact between the strip and the work roll, and is effective especially when the quality of the strip surface needs to always be superior.

For rolling mills for small rolling loads, a variable crown (VC) roll having a hydraulic chamber for inflation in the roll barrel length centre, and a WRB roll can be used at the same time. Fig 3 shows an example of the performance calculation with a four-high rolling mill (diameter480/diameter 1,220 × 1,950 mm) equipped with a VC roll having an inflation rate of 0.32 mm and a DCB. For shape control with a larger rolling force, taper-piston (TP) rolls (fig 6) have been developed.

Fig 6 Taper piston roll and surface displacement

Fig 6 shows a TP roll. It consists of an arbor, sleeve, and taper piston, and adjustments of the roll profile by hydraulically adjusting the taper piston position. The profile can be more freely adjusted by installing two pistons at each side. TP rolls are used for hot rolling mills whose back-up rolls have a large barrel length.

TP rolls can also easily be installed in 2-high rolling mills, in which shape control actuating devices cannot easily be installed because it is not possible to apply the work roll bending. Since TP rolls can be installed as work rolls, the size and inflation are smaller than those achieved with the back-up rolls in hot rolling mills. However, because strips are directly contacted and affected by the changes in the roll profile, the shape control effect becomes larger. Fig 6 shows an example of the measured inflation curve of a medium diameter TP roll with a diameter of 550 mm for 2-high mill.

Other methods

In wide strip mills for thin thickness, a pocket-like shape is more likely to occur because it is difficult to locally control shapes in random (asymmetrical) positions with the mechanical shape control actuating devices. In such cases, zone coolant or spot coolant (or heating) control is effective. Such control, which uses local thermal expansion and local contraction of a roll, is combined with a strip flatness sensor to form an automatic shape control system.

Work roll thermal crown (RTC) cooling headers – The work roll RTCs are two tilting headers (Fig 7) placed at the exit side of each finish rolling stand (top and bottom). The nozzles are placed on these headers along a parabolic path whose vertex is on the centre line of the rolling mill. Furthermore, each RTC header can be rotated by a hydraulic cylinder. The combination of spray nozzle distribution and header rotation angle has the capability to cool the roll centre more efficiently than the edges or vice versa. Thus, the cooling gradient along the roll barrel makes it possible to effectively control the work roll thermal crown by a suitable angular positioning of the RTC headers. The results of using an RTC system are (i) short transition to steady-state, (ii) effective work roll thermal crown control, and (iii) reduction of the bending force required to compensate for RTC, thus more bending force can be used to control strip crown and flatness.

Fig 7 Work roll thermal crown header and roll crossing system

Work roll and back-up roll crossing – Work roll and back-up roll crossing (flexible-crown and free-rolling), is a new concept which has been developed for a rolling stand. This concept makes possible roll crossing and shifting during rolling for continuous adjustment of strip crown within an extremely wide control range and independent control of the roll wear. The target is to meet the strip profile and flatness control requirements for the most demanding product mix, including ultra-thin gauges in a wide range of strip profiles, required for various final applications.

The production of ultra-thin strip in endless or semi-endless rolling needs a very wide strip crown control ranges (upto 1,700 micrometers) which are four to five times wider than the control range normally needed and achieved by conventional actuating devices. These efforts have led to the development of the work roll and back-up roll crossing stand which overcomes the limitations of all the technologies presently available. It features roll bending, shifting and crossing to control changes in roll wear and RTC independently, as well as rolling load effects on the strip profile. The work roll and back-up roll crossing system installed in a mill can have(i) work roll and backup roll crossing, (ii) dynamic control of the crossing angle, (iii) work roll positive and negative bending, (iv) work roll shifting system with no hysteresis, (v) crossing and shifting under rolling load, and (vi) independent use of crossing and shifting.

Crossing the rolls has the same effect on the strip which grinding a convex crown has on the work roll. The equivalent amount of roll crown is Ceq = Se-Sc = (L2 tan2A) / (2(Dw + Sc)) where L is the barrel length, Dw is the work roll diameter, Sc is the roll gap at roll centre, Se is the roll gap at roll edge, and A is the cross angle. The crossing actuating devices together with the bending actuating devices are able to modify the quadratic and quartic parts of the strip profile. Roll crossing is achieved by F shaped blocks traversing the mill window and bearing the oscillating liners on which the work roll chocks and back-up roll chocks slide. Each F shaped block is placed in a slot in the inner face of the housing post. The block is coupled to the slot by means of two special mechanical cam-type jacks and a hydraulic balancing system.

The advantages of the work roll and back-up roll crossing stand are (i) continuously adjustable strip crown within a wide control range, (ii) distribution of work roll wear and edge wear control performed through shifting under load, (iii) work roll wear and strip crown control are independent, (iv) uniform back-up roll wear, (v) increasing back-up roll life due to uniform contact pressure between work roll and back-up roll, (vi) schedule-free rolling capability, (vii) high efficiency in controlling thermal crown, and (viii) specifically designed for endless rolling.

Other aspects

Basically, shape control actuating devices for the hot strip rolling are operated and controlled based on the shape sensor outputs. Hence, even with high-performance actuating devices, shape errors can occur owing to the shape detection errors or feedback calculation errors. For preventing this, a tension leveler, which automatically and mechanically corrects strip flatness, can be adopted as a piece of final finishing equipment for thin strips. Furthermore, to increase its effect on extremely thin strips, a hydro tension leveler, which has a strip bending head with an extremely small curvature radius, can be adopted.

A differential speed rolling mill has a bottom roll which operates at a different speed from the top roll, reduces the rolling force, helping enhance productivity. It provides benefits such as (i) smaller minimum rolling thickness, (ii) improved strip shapes, and (iii) finer microstructure size. Making the differential speed ratio variable makes the rolling force variable thus produces the shape control effect. From such a comprehensive perspective, the differential speed rolling mill has a differential gear system with planetary gears. This makes it possible to select the appropriate differential speed ratio without increasing the total equipment motor power.

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