STRIP EDGE SHAPE CONTROL

Transcription

1 STRIP EDGE SHAPE CONTROL Gary Boulton, Tino Domanti, Terry Gerber, Glen Wallace Industrial Automation Services The control of shape in the strip edge region remains a significant challenge for shape control systems. A number of additional difficulties exist, commencing with the requirement to obtain a reliable and accurate measurement through to the need to generate localised changes to the rollgap profile at the strip edge. In high speed aluminium rolling mills, strip edge flatness may manifest itself as very tight edges which leads to the requirement to limit rolling speed to avoid strip breaks. In steel mills rolling thin hard product, strip edge flatness may manifest itself as pie crust edges which have the potential to pinch when rolled. This paper presents both an analytical and a practical summary of the current state of the art. The major contributors to the rollgap error at the strip edge, namely the work roll thermal camber, the strip edge profile and the work roll flattening are examined for different rolling situations. The benefits of work roll heating outside the strip edge and side shifting work rolls are investigated and the different approaches compared. INTRODUCTION The control of strip edge flatness commences with reliable measurements. This can present challenges due to the uncertainty in determining the exact location of the edge. When a measurement zone is only partially covered by the strip, the signal from its transducer must be adjusted to take account of the partial coverage. This cannot be done if the strip edge positions are not known accurately, so measurement of the position of one or both edges is essential if the shapemeter is to be used for edge shape control. In addition, the requirement, for better measurement resolution in the vicinity of the strip edges, has led to the availability of shapemeters with narrower zones or with narrower zones in the common strip edge regions. A range of mechanical actuators are available for control of strip shape, including work roll bending, roll tilt, work roll side-shifting, intermediate roll shifting, roll crossing and inflatable or segmented backup rolls. Most of these effect the flatness over the entire width of the strip and are hence suitable for controlling only one or two of the common shape disturbances. For shape defects occurring in a narrow band near the strip edge, the only practical actuators are the coolant sprays, roll shifting and SCR (Special Crown Roll) rolls [2]. CAUSE OF STRIP EDGE FLATNESS The strip shape near the edge is driven by variation in the sum of three main variables: the strip thickness profile, the work roll thermal expansion, and the flattening of the work roll surface. All these factors change rapidly near the strip edge and consequently the strip shape here is particularly sensitive to their values. As this paper focuses on the flatness in the edge region it will not overly concern itself with the bending of the roll stack, but rather focus upon the changes in the three factors described above. Fig. 1 provides an example of the values of the strip thickness profile, work roll thermal expansion and work roll flattening for a high speed aluminium mill rolling hard product. The thermal camber shown is for the situation of a uniform coolant applied over the roll width and no spray control is being performed. It can be seen that the thermal camber dominates the axial gradient of the rollgap error at the strip edge. In such a situation the lower reduction at the strip edge would lead to a high localised tension stress.

2 .2 Change in Roll Radius (mm) Coolant Heat Transfer: 9.35 kw/deg/m 2 Heat Input: 1.12 MW/m Rolling Force: 3.4 MN/m Element Width: 25mm Exit Thickness.5mm Work Roll Flattening Strip Thickness Edge Drop Uncontrolled Thermal Camber Deviation of Rollgap Profile from Strip Profile Fig. 1 Rollgap Error and Components for Uniform Coolant Flow Case 1 In contrast, Fig. 2 shows the strip thickness profile, work roll thermal expansion and work roll flattening for a high speed steel tandem mill rolling hard, thin tinplate product. The higher rolling force and lower thermal camber result in a rollgap error profile which initially decreases before rising at the edge. The rollgap error profile presents a dilemma for the flatness control system. The rapid change from tight strip inboard of the strip edge to a loose edge means that any correction action possible by non-localised actuators will intensify one of the flatness errors. Hence, what commonly occurs in a shape control system is that the bending increases to remove the general parabolic form of the flatness error intensifying the loose edge leading to pie crust edges Change in Roll Radius (mm) Coolant Heat Transfer: 2 kw/deg/m 2 Heat Input:.93 MW/m Rolling Force: 7.7 MN/m Element Width: 25mm Exit Thickness.5mm Work Roll Flattening Strip Thickness Edge Drop Uncontrolled Thermal Camber Deviation of Rollgap Profile from Strip Profile Hot Edge Sprays Fig. 2 Rollgap Error and Components for Uniform Coolant Flow Case 2 As described earlier, tight edges arise when the high heat input into the roll associated with large thickness reductions and fast rolling speeds is combined with low heat removal such as occurs with kerosene based lubricants to create a large thermal camber. The severity of the hot edges can be reduced by the judicious use of the spray coolant control. Fig. 3 shows how, by reducing the coolant applied to work roll at the strip edge region, the thermal camber there can be increased to effectively correct the rollgap mismatch.

3 .1.5 Flattening + Edge Drop Cancel Change in Roll Radius (mm) Thermal Camber Profiles Thermal Camber for Constant Heat Transfer (Controlled) Coefficient Thermal Camber -.25 (Uncontrolled) Flattening and Edge Drop Change to Thermal Camber via Changes to Heat Transfer Profile Fig. 3 Rollgap Components with Optimised Thermal Camber - No Hot Edge Sprays To illustrate how this was accomplished Fig. 4 shows the spray heat transfer coefficient and the roll surface temperature required to cancel the rollgap mismatch as shown in Fig. 3. The spay heat transfer coefficient is the mathematical representation of the spray coolant change. Higher on times result in higher heat transfer coefficients Work Roll Surface Temperature Rise (oc) WR Surface Temperature HTC Spray Heat Transfer Coeffiicent (W/m 2 / o C) Fig. 4 Heat Transfer Coefficient and Roll Surface Temperature for Optimised Thermal Camber No Hot Edge Sprays The rollgap mismatch at the strip edge has been eliminated by decreasing the coolant applied to the work roll at the strip edge (which is presented as a decrease in the spray heat transfer coefficient). This increases the roll temperature at the strip edge increasing the thermal camber there. It is however necessary to limit the maximum work roll temperature and for the current example the work roll at the strip edge reaches its limit of 5 o C above the coolant temperature. To generate the necessary work roll temperature difference to eliminate the roll gap mismatch once the maximum work roll surface temperature has been reached requires the center of the work roll to be cooled. Hence the central region of the work roll has a surface temperature of approximately 3 o C above the coolant temperature and this requires a maximum heat transfer coefficient of approximately 9.7 kw/m 2 / o C. Hence the ability to remove a tight edge under the constraint of a maximum work roll surface temperature is limited by the available heat transfer from the coolant sprays. In practice the available heat transfer may be severely limited when using kerosene lubricants.

4 Recently a small number of mills have installed hot sprays, which can be turned on just outside the strip edges, in order to better control the thermal gradients and thereby achieve better control of strip edge shape. Fig. 5 shows the same rolling conditions as Fig. 3. However two hot edge sprays per side operating at 5 o C above the basic coolant temperature with a heat transfer coefficient of 1 kw/m 2 / o C have been added to heat the region 5 mm outside each strip edge. A comparison of Fig. 5 with Fig. 3 shows that the uncontrolled thermal camber is larger and flatter at the strip edges due to the presence of the hot edge sprays. As the rollgap mismatch is smaller the required correction to the thermal camber is smaller. This is highlighted in Fig. 6. which shows the roll surface temperature and spray heat transfer coefficients required to correct the rollgap mismatch shown in Fig. 5. The work roll surface temperature in the central region of the work roll must only be reduced to 37.5 o C above the coolant temperature and the maximum heat transfer coefficient is 7.3 kw/ C/m 2 compared to 9.7 kw/m 2 / o C when no not edge sprays were present. The ability to reduce the occurrence of tight edges can be seen to be directly related to the maximum coolant heat transfer which can be effectively used to reduce the thermal camber..1.5 Flattening + Edge Drop Cancel Change in Roll Radius (mm) Change to Thermal Camber via Changes to Heat Transfer Profile -.2 Thermal Camber Profiles Thermal Camber for Constant Heat Transfer (Controlled) Coefficient Thermal Camber -.25 (Uncontrolled) Flattening and Edge Drop Fig. 5 Rollgap Components with Optimised Thermal Camber - With Hot Edge Sprays 6 12 Work Roll Surface Temperature Rise (oc) WR Surface Temperature HTC Spray Heat Transfer Coeffiicent (W/m 2 / o C) Fig. 6 Heat Transfer Coefficient and Roll Surface Temperature for Optimised Thermal Camber With Hot Edge Sprays

5 Several cases where the thermal camber was larger than the roll flattening were investigated. Constraints were imposed on the maximum allowable work roll surface temperature (limited to 5 C above the roll coolant temperature) and the minimum heat transfer coefficient for the coolant sprays. Fig. 7 shows the minimum value of the maximum coolant heat transfer coefficient required to control the roll surface temperature and completely remove the rollgap profile error for various heat input rates. Higher heat input rates, which correspond to more reduction or faster strip speeds, require higher heat transfer coefficients to completely eliminate the rollgap profile error and maintain the roll temperature within acceptable limits. The hot edge sprays are seen to increase the ability to control the temperature and completely remove the rollgap profile error. 24 Heat Transfer Coefficient (kw/ o C/m) Centreline Heat Transfer Coefficient Centreline Heat Transfer Coefficient, no Hot Edge Sprays Heat Input (MW/m) 2.5 Fig. 7 Centreline Coolant Heat Transfer Coefficient for Optimal Thermal Camber and Various Work Roll Energy Input As an example, if a heat transfer coefficient of 8 KW/m 2 / C represents the desired maximum heat transfer coefficient (allowing some dynamic range for control) and the roll temperature was limited to 5 C above the coolant temperature then the maximum heat input which still allows control of the strip shape without the use of hot edge sprays is.95 MW/m while the maximum heat input with hot edge sprays would be 1.32 MW/m. On a typical schedule this would correspond to a 4 percent increase in rolling speed. Two major design parameters of a hot edge spray system are the hot edge spray flow rate and its temperature. This paper does not allow a full discussion of these parameters however further details are provide in [1]. Some the main points are covered below. If the hot spray temperature substantially exceeds the maximum roll surface temperature then it is possible to generate larger thermal cambers outside of the strip edge than exist inside the strip. In the current situation these are counter productive and result in additional cooling just inside the strip edge to compensate. A hot edge spray temperature slightly above the maximum roll surface temperature results in the average roll coolant flow being relatively insensitive to the hot edge spray heat transfer coefficient. The optimum choice of hot edge spray temperature and heat transfer coefficient aims to provide a smooth temperature transition across the strip edge boundary. In general this can be achieved by using higher flow rates with lower temperature hot sprays or lower flow rates with higher temperatures. While lower flow rates are desirable as they require smaller capacity equipment, they are more sensitive to flow rate variations and to rolling conditions and require higher hot spray temperatures. Excessive temperatures are undesirable for kerosene because of its flash point and for water because of the boiling point.

6 Work Roll Side Shifting An almost opposite problem to that of tight edges occurs when the rolling forces are high and the thermal camber is moderate. In such circumstances the steep work roll flattening results in a rapid transition in the strip shape at the strip edge, and pie crust edges, as highlighted in Fig. 2 may occur. These are very narrow (possibly 25 to 5 mm wide) loose strip edge which lie outside of a tight region just inboard of the edge and can result in subsequent pinching during rolling. In these circumstance side shifting rolls can be used to advantages Shape (MPa) Fig. 8 Strip Shape vs Intermediate Roll Offset for Constant Roll Bending Fig. 8 highlights how the flatness at the strip edge can be altered by shifting parallel ground intermediate rolls with opposing outboard tapers on the top and bottom rolls. The sideshift represents the distance from the intermediate roll taper start to the strip edge, with positive values lying outside the strip edge. The current figure examines the impact of the side shift alone and the roll bending has not been changed as the side shift is altered. Even so it can be noted that when the intermediate roll taper is far outside the strip edge a very loose edge is produced with the outer edge element nearly 1 MPa more compressive than its neighbour. As the intermediate roll taper is moved towards the strip the loose edge is reduced and for the current example a side shift of 5 to 1 mm produces the optimum flatness. It is interesting to note that this agrees with previous observations on four high mills where it is recommended that the backup roll chamfer should lie at least 75 to 1 mm outside the maximum strip width edge to avoid poor edge flatness. As the start of the intermediate roll taper is moved further towards the strip edge a tight edge is produced.

7 IR-WR Load (kn/mm) Fig. 9 Work Roll Intermediate Roll Contact Pressure for Various Side Shift Values Fig. 9 highlights how the contact pressure on the top work roll is impacted by the intermediate roll axial position. In this case the start of the intermediate roll taper moves from -125 to -1 mm from the mill centreline, the latter value corresponding to the strip edge. As the intermediate roll is moved towards the strip edge, the moment applied to the work roll outside the strip edge is significantly reduced, thereby opening the gap and creating a tighter strip edge. An equal but opposite pressure reaction acts on the bottom work roll. 15 Steel - Tinplate 1 Aluminium 5 Strip Edge Shape (MPa) Side Shift (mm) Fig. 1 Strip Edge Flatness vs Intermediate Roll Sideshift Fig. 1 shows the sensitivity of the strip edge shape to sideshift, for fixed roll bending. The steel mill is the same one discussed in Figs. 7 to 9. The sideshift as observed previously can cause significant changes in the strip edge flatness. For comparison, the effect of side shift on the edge flatness of an aluminium mill rolling thin hard product is also shown. It can be observed that the side shift has little impact on the strip edge flatness. The reason can be ascertained by contrasting the rollgap error components shown in Figs. 1 and 2. Here it can be seen that the work roll thermal camber dominates the rollgap error for the aluminum rolling example. As such the contact between the work roll and the intermediate roll is lost near the strip edge where the thermal camber change is significant. Thus while the start of the intermediate roll taper remains outside the strip edge, the movement of the intermediate roll does not change the contact region between the work and intermediate roll. A tight edge is obtained similar to that which occurs when the intermediate roll taper is moved to the strip edge for the steel case.

8 Conclusion This paper has examined the parameters which generate the shape at the edge of the strip. These are the strip thickness profile, the work roll thermal camber and the work roll flattening. All change rapidly in the strip edge region making the shape here particularly sensitive to their values. The normal mill actuators used to control strip shape do not allow sufficiently localized control to compensate for the rollgap errors in this region. Two actuators which have been successfully employed in practice to modify the shape at the strip edge are side shifting intermediate rolls and hot edge sprays. The paper presents an analysis of these two actuators showing how they interact with the strip shape determining parameters. Two examples are presented to asses the potential benefits of each of these actuators. The first case is that which might be experienced by a mill rolling hard products and using kerosene based lubricants which results in a large thermal camber. Here the hot edge sprays have significant potential to improve the edge flatness or the maximum rolling speed. The second case is where the rolling force and consequently the roll flattening is large as may be experienced on a steel tinplate tandem mill. The benefits of side shifting intermediate rolls for this case are studied. References [1] Bates, M., Domanti, T., Johnston, McInnes, C., and Thomas, P. Hot Sprays and Strip Edge Shape Control, AISE October 22 pp [2] Mannesmann Demag Sack Advances in Aluminum Rolling, Lahnstein/Koblenz, Germany 1993