EP4066952A1

EP4066952A1 - This invention relates to a method and a device for controlling lubrication during cold rolling of strip

Info

Publication number: EP4066952A1
Application number: EP21166765.4A
Authority: EP
Inventors: Leonardus Joannes Matheus JACOBS
Original assignee: Tata Steel Ijmuiden BV
Current assignee: Tata Steel Ijmuiden BV
Priority date: 2021-04-01
Filing date: 2021-04-01
Publication date: 2022-10-05

Abstract

This invention relates to a method and a device for controlling lubrication during cold rolling of strip, and to the use thereof.

Description

Field of the invention

This invention relates to a method and a device for controlling lubrication during cold rolling of strip.

Background of the invention

Cold rolling is a process in which material is usually fed horizontally through a set of rollers in a rolling mill stand and by exerting force on these work rolls in the vertical direction the material thickness is reduced. Due to the high speed and continuous or semi-continuous operation, the rolling process is the most widely used process in the steel industry to reduce the thickness of steel strip or sheet.
A rolling mill stand (also referred to as "mill stand" or "stand") consists of at least two work rolls (2-high configuration). In many cases the stand comprises four (4-high) or six rolls (6-high), of which two are the work rolls interacting with the steel strip or sheet, and 2 back-up rolls and, in case of a 6-high configuration, an intermediate roll on either side between the work rolls and the back-up rolls to support the work rolls. Through the use of larger back-up rolls, issues of work roll deflection can be avoided and smaller work rolls can be used to reduce the rolling forces, energy consumption and potentially achieve more consistent thickness.
An industrial high speed and (semi-)continuous cold rolling mill usually comprises more than one mill stand, and this plurality of mill stands is operated in a tandem configuration, meaning that the strip to be cold rolled is being cold rolled in more than one mill stand at the same time. Such a tandem cold mill may be a stand-alone cold mill with an uncoiling and a coiling device before and after the mill. Such a stand-alone configuration can be operated in a batch mode or in a continuous mode wherein consecutive coils are welded head to tail before the cold rolling mill and are separated after the cold rolling mill. Nowadays there are also tandem cold rolling mills directly linked to a pickling line, where the coils are welded before pickling and are only separated after cold rolling.
Lubrication plays an important role during cold rolling. The main reason for lubricating the cold rolling process is to control friction and surface quality. Low friction values reduce the rolling force and therefore enables a larger reduction. However, too low friction values are not desirable in view of process instabilities that may be the result. Controlling the lubrication is therefore very important to achieve a stable cold-rolling process and a good cold-rolled product. Lubrication is usually applied by means of spraying the lubricant onto the work rolls and/or into the roll gap. Homogeneity of lubrication of the top and bottom surface of the metal strip is important to obtain a homogeneous product. Unfortunately, the lubrication on the bottom surface is often less effective, which leads to differences of process conditions (such as friction) and differences in surface properties of the strip (such as surface topography and surface quality) and internal stresses in the strip, which may have an adverse influence during further processing of the strip or the final properties of the finished product. For instance during cold-rolling of tinplate heat scratches may occur. They usually first emerge at the bottom side of the strip, where the amount of lubricant entrained in the roll bite is less than at the top side of the strip. When rolling is continued, the defect progressively worsens: the top side of the strip becomes affected as well, and the work rolls may become damaged as well.
In the state of the art the problem of inhomogeneous lubrication is solved by means of applying different lubrication flows to the top and bottom strip surface. However, this is quite a laborious solution that requires additional hardware such as pumps, flow meters, pipes which cost money to install and maintain and moreover it is not always as effective as hoped.

Objectives of the invention

It is an objective of the present invention to actively influence the lubrication conditions on the bottom surface of a steel strip during cold rolling.
It is also an objective of the present invention to improve the homogeneity of the lubrication conditions between the top and bottom surface of the steel strip during cold rolling.
It is also an objective of the present invention to enable a controlled difference in lubrication conditions between the top and bottom surface of the steel strip during cold rolling.
It is also an objective of the present invention to provide an improved cold rolling device for actively controlling the lubrication conditions during cold rolling of a steel strip.

Description of the invention

One or more objectives of the invention are reached by a method for controlling the lubrication during cold-rolling of a metal strip in a cold-rolling mill comprising one or more rolling mill stands, wherein the cold strip is led to the roll-gap between the work rolls of one or more of the rolling mill stands, and wherein the cold strip is bent around an entry-angle control device (eacd) roll which is adjustably movable in a vertical direction before entering the roll-gap of the one or more of the rolling mill stands in that the strip is bent around the eacd-roll resulting in at least partial plastic bending of the strip thereby inducing residual tensile stresses in the outer fibres of the strip to enable adjusting the entry angle of the strip with respect to the pass-line of the one or more rolling mill stands.
The residual stresses induced by the eacd-roll are additional stresses to those that may already be present in the steel strip, for instance as a remnant of the coiling or the passing of the guide rolls after uncoiling. By positioning the horizontally oriented (its rotational axis being substantially parallel to those of the work rolls) eacd-roll in the correct vertical position, the incoming strip is bent around the eacd-roll to such an extent that the strip is led to the roll-gap by exactly following the pass-line (i.e. no residual stresses are present in the strip after the eacd-roll). This results in a symmetric situation, wherein the angle between the surface of the work roll and the surface of the material to be rolled is the same for the bottom work roll and the top work roll. This angle is called the bite angle α₀, and in this case α_top = α_bottom (see figure 9c). The bite angle depends on the roll radius of the work roll and the thickness reduction (h₁-h₂). If the cold strip that is led to the roll-gap does not follow the pass-line exactly (angle with the pass-line = 0), but enters the roll gap under an angle with the pass-line ≠ 0, then the bite angle α_top ≠ α_bottom. As this entry angle increases, then the roll bite angle α_top between the strip and the top work roll decreases and the roll bite angle α_bottom between the strip and the lower work roll increases with respect to α₀ (see figure 9b). If the strip approaches the roll gap from below the pass-line then the roll bite angle α_top between the strip and the top work roll increases and the roll bite angle α_bottom between the strip and the lower work roll decreases with respect to α₀. The principle behind the claimed invention is to control the entry angle by at least partial plastic bending of the strip thereby inducing residual tensile stresses in the outer fibres of the strip. This allows the operator or the process control model to influence the lubrication conditions on the top and bottom surface of the strip and thereby influence the properties like the surface properties of the strip (such as surface topography and surface quality) and internal stresses in the strip.
This method is based on the finding that the introduction of the plastic bending of the strip influences how the strip is fed into the roll-gap between the work roll. These residual stresses can cause a significant asymmetry at the roll-gap entry which influences the lubrication condition of the top and the bottom strip surface. The plastic deformation of at least the outer fibres of the strip persists in the strip even when it is rolled under tension. This manifests itself stronger when the strip is thicker. By influencing the residual stresses the asymmetry can be influenced or even removed, and as the degree of asymmetry influences the lubrication conditions, the ability to influence the residual stresses enables the cold-mill operator or the cold-mill process control model to control the lubrication conditions and even control differential lubrication if desired.
In a cold-rolling mill for cold-rolling strip the entry and exit tension is generated by the uncoiler and the coiler (or by the linked process step before or after the cold-rolling mill), or between the mill stands in a tandem mill. Guide rolls are used to bring the strip at the pass-line level. It should be noted that the pass-line level is not at a constant vertical position. As a result of e.g. changing roll diameters (wear, grinding, etc,) the pass-line varies. These guide rolls may also measure strip speed and strip tension. The entry-angle control device comprises a roll around which the strip is led and exerts a bending action on the strip. The roll diameter of the eacd is chosen such that bending of the strip around part of the roll's circumference ensures that at least the outer fibres of the strip are plastically deformed so that after leading the strip away from the eacd-roll directly towards the roll-gap there are still residual stresses in the strip as a result of this bending. The inventors found that by changing the amount of residual stresses by means of the eacd-rolls action that the asymmetry of the lubrication condition between the top and bottom surface of the strip can be controlled. This control can result in a more homogeneous lubrication top vs bottom, or, if it is desired, a controlled inhomogeneous lubrication top vs bottom.
The movement of the eacd-roll in the vertical direction (substantially up and down) is essential to control the amount of plastic deformation in the strip before entering the roll-gap. By moving the eacd-roll up or down, i.e. away from or to the pass-line, the strip is bent around a larger or smaller part of the roll's circumference, thereby affecting the residual stresses in the strip. The movement may be controlled by the operator or automatically by the control model, and adapted during rolling if the need arises, for instance to change the lubrication conditions during rolling.
It is noted that the method according to the invention allows the installation of an eacd before one of the rolling mill stands, before all of the rolling mill stands or before a selection of the rolling mill stands. In a 4-stand finishing mill there are 15 different combinations possible (see figure 10). One would expect the beneficial effect of an eacd to be the largest before the first rolling mill stand. However, that depends on the problem that needs to be solved. For instance, heat scratches tend to be caused during rolling in a later rolling mill stand. So to avoid those heat scratches it may be beneficial to install an eacd at least before that particular rolling mill stand. Optimum flexibility is obtained by an eacd before each rolling mill stand.
It is noted that in the method according to the invention the strip normally exits the roll gap in a symmetric manner. Interactions between rolling mill peripherals may introduce new length bow into the strip, which may need to be compensated by the next eacd or by one of the next eacd's in the rolling mill. Or, alternatively, differential lubrication conditions may be needed and the eacd may then be used to turn the symmetric situation in a deliberate asymmetric situation to achieve the desired differential lubrication conditions.
In a preferred embodiment the strip, after having been bent around the eacd-roll is fed into the roll-gap of the one or more rolling mill stands without any further mechanical interaction of the strip and the cold-rolling mill's peripherals, such as, but not limited to, damming rolls, guide rolls for guiding the strip in the cold-rolling mill, strip velocity rolls or tensiometer rolls, during the travel from the eacd-roll to the roll-gap.
This embodiment means that the bending action of the eacd's bending roll is effectively the last mechanical action upon the strip before entering the roll-gap. There may be interaction with the strip of another kind, for instance the application of a lubricant or a coolant, or a lubricant-coolant onto the strip on one or both sides by means of spray nozzles or spray heads or the like. This absence of any mechanical interaction means that the eacd is in the best possible position to effectively control the lubrication conditions in the roll-gap. In case there is mechanical interaction between leaving the eacd-roll and entering the roll gap, then the impact of that mechanical interaction must be taken into account by the operator or the control model.
In an embodiment the eacd comprises a twin roll configuration comprising two parallel rolls between which the strip is led.
The eacd with a twin-roll configuration is equally adjustably movable in a vertical direction as a single roll configuration and allows to increase or decrease the entry angle of the strip with respect to the pass-line of the said one or more later rolling mill stands by bending the strip around one of the two parallel rolls of the twin roll configuration of the eacd. In comparison to the single roll configuration the twin-roll configuration allows not only to push the strip down or up (depending if the eacd is mounted below or above the strip) but it can push the strip down and pull it up. One of the two rolls of the twin-roll configuration serves as the roll around which the strip is bent to achieve the at least partial plastic bending of the strip, and which one of the two depends on the position of the eacd.
In an embodiment the vertical position of the eacd-roll or rolls before a rolling mill stand is determined on the basis of one or more of the group of parameters comprising mechanical properties of the strip, strip thickness, strip tension and diameter of the eacd-roll before said rolling mill stand. It is noted that the mechanical properties, like Youngs modulus or yield strength are the properties of the strip before it enters the relevant mill stand before which the eacd is mounted. Also the initial or intermediate surface condition may be included (such as roughness values).
In a preferred embodiment the optimum vertical position of the eacd-roll is determined and controlled by means of a control model. If it is desired a feedback-control type system may be used so that the position of the roll may be automatically adjusted to maintain pre-set lubrication conditions in the roll-gap.
In an embodiment the position of the eacd-roll is chosen such that the strip enters the roll-gap at a zero angle with the pass-line (α_top = α_bottom). This results in a symmetric feeding of the strip into the roll-gap of the rolling mill stand.
Alternatively in another embodiment the position of the eacd-roll is chosen such that the strip enters the roll gap at a non-zero angle with the pass-line (α_top ≠ α_bottom) to create controlled differential lubrication conditions between the top and bottom surface of the strip.
According to a second aspect the invention is also embodied in a device for cold-rolling a metal strip comprising a cold-rolling mill comprising one or more rolling mill stands, strip supply means for supplying a strip to the cold rolling mill stand or stands, each mill stand comprising two work rolls, strip discharge means for discharging a cold-rolled strip from the cold rolling mill, lubrication means for lubricating the work rolls and the roll-gap between the work rolls, optionally one or more of the group of rolls comprising damming rolls, guide rolls for guiding a strip in the cold-rolling mill, strip velocity rolls or tensiometer rolls, wherein before one or more of the rolling mill stands an entry angle control device (eacd) is provided that comprises at least one eacd-roll for inducing, in use, residual tensile stresses in the outer fibres of the strip by bending the metal strip around the eacd-roll to control the entry angle of the strip in the roll gap of the rolling mill stand immediately following the eacd-roll.
In an embodiment the eacd comprises two parallel eacd-rolls between which, in use, the strip is led and wherein one of the parallel eacd-rolls induces the residual stresses in the outer fibres of the strip by bending the strip around that eacd-roll to control the entry angle of the strip with respect to the pass-line of the rolling mill stand. In a simpler form the eacd only comprises one roll acting as a bending roll, but the eacd according to this embodiment allows acting into both directions, i.e. above and under the pass-line of the rolling mill stand before which the eacd is mounted.
The invention is also embodied in a device wherein any one, more or all of the other rolling mill stands of the cold-rolling mill are provided with an eacd before the rolling mill stand to induce residual tensile stresses in the outer fibres of the strip to control the entry angle of the strip into the roll gap of the rolling mill stand immediately following the respective mill stand with respect to the pass-line of the rolling mill stand.
In an embodiment of the invention the optimum vertical position of the eacd is determined and controlled by means of a control model.
In a preferred embodiment the cold-rolling mill is directly coupled with the preceding pickling line, optionally with a looper tower in between, so the strip supply means comprise a pickling line.
According to a third aspect the method and device according to the invention are used to produce a cold-rolled strip. Due to the control of the lubrication conditions, the strip either is completely homogeneous as far as the surface conditions of the top and bottom surface are concerned. Or, alternatively, the potential to control the lubrication conditions has resulted in a strip with a differential lubrication with the associated potential to affect the surface quality of the strip or to counteract any other asymmetry in the lubrication conditions.

Figures

The invention is now further explained by means of the following, non-limitative figures.

Fig. 1a & b: Schematic drawing of a 2-high mill with uncoiler, coiler and guide rolls.
Fig. 2a: Schematic drawing of a 2-high mill with uncoiler and single roll eacd.
Fig. 2b: Schematic drawing of a 2-high mill with uncoiler and twin roll eacd.
Fig. 3: Schematic drawing of a 4 stand 4-high tandem mill with uncoiler and coiler.
Fig. 4: Experimental set-up for Digital Image Correlation measurements.
Fig. 5: Symmetric strip feeding conditions.
Fig. 6: Asymmetric strip feeding conditions.
Fig. 7: Measured oil film thickness (at exit roll bite).
Fig. 8: Schematic drawing of length bow.
Fig. 9a:Length bow leading to figure 9b configuration.
Fig. 9c: Symmetric strip feeding.
Fig. 9b: Asymmetric strip feeding.
Fig.10: Possible configurations in a 4-stand cold-rolling mill.

Figure 1a and b show schematic drawings of a single stand 2-high mill with uncoiler (U) and coiler (C) and two guide rolls (GR). It is noted that drawing is not to scale. Figure 1a shows the rolling mill without a strip being present in the roll gap. In this example the guide rolls are positioned in a horizontal position on the pass line (PL).
In figure 1b a strip is uncoiled from the uncoiler, led to the roll gap via the guide roller and then from the roll gap via another guide roller to the coiler. The rolling direction (RD) is indicated with an arrow. The strip, after having been led past the GR on the entry side possesses a so-called length bow. The strip (S) is led into the roll-gap under an angle (α_top < α_bottom) and deviates from the pass line (PL) of the rolling mill stand on the entry side. This is referred to as asymmetric feeding. After rolling the strip exits the roll gap in a straight line toward the GR on the exit side along the pass line.
Figure 2a shows an embodiment of the invention where the strip is pressed down by the eacd to compensate the length bow that is present in the strip on the entry side (as depicted in figure 1b) by plastically deforming the strip and thereby changing the angle under which the strip enters the roll gap. Figure 2b shows another embodiment where the eacd comprises a twin-roll configuration of eacd rolls between which the strip is guided to the roll gap wherein the twin-roll configuration can move up and down as indicated by the arrow drawn in the twin roll configuration. This embodiment allows a large variation of the entry angle. The embodiment in figure 2a allows decreasing the length bow present in the strip as depicted in figure 1b to achieve a symmetric feeding of the strip into the roll gap (α_top = α_bottom), or even the reverse of the initial length bow (α_top > α_bottom). It is noted that the angles shown in figures 2a and 2b are exaggerated for clarity and to show the principle of the invention, and the drawing is not to scale. The pass line is indicated in the drawing by the dash-dotted line. The dashed twin roll configuration shows an alternative position of the eacd and the line of travel of the strip, indicated by the dashed line, lies above the pass line.
Figure 3 shows a schematic drawing of a 4-stand 4-high tandem mill with uncoiler and coiler. The length bow before the first rolling mill stand is schematically shown. Each work-roll is backed-up by a back-up roll (BUR). The eacd according to the invention may be positioned before stand 1, or 2, or 3 or 4, or combinations thereof, so that a total of 15 different configurations are possible in a 4-stand tandem mill.
Figure 4 shows the experimental set-up for Digital Image Correlation (DIC) measurements as seen from above. The strip S having a width w is led through the roll gap of which only the bottom work roll (having a diameter D) is shown and images are taken from the side by the camera. The contrast is provided by graphite speckles which have been deposited onto the edge of the strip and the lights positioned next to the camera. Using the DIC-measurements the cold rolling process was studied. By aiming a camera on the strip in the rolling gap the displacement of the material during cold rolling can be analysed. By taking a sequence of images and analysing these the relevant parameters such as the location of the neutral point and the vertical and horizontal displacements of the points of the strip can be assessed.
Figure 5 shows a picture used for DIC-measurements. The entry of the strip on the top and bottom (*) and the exit of the strip on the top and bottom (**) is the same for the upper work roll (UWR) and the lower work roll (LWR), meaning that α_top = α_bottom.
In figure 6 an example is shown where the strip is led into the roll gap of the mill stand asymmetrically (as in figure 1b) and makes first contact with the upper work roll (α_top < α_bottom). The bite angle α is clearly smaller for the top work roll compared to the bottom work roll. It should be remarked that the strip exits the roll bite in a symmetric way. So asymmetric feeding results in a bite angle with the upper roll that is different from the bite angle with the bottom roll. The bite angle is particularly relevant when considering lubricant flow. Smaller bite angles result in thicker oil layers. By means of droplet tests this was confirmed experimentally. In Figure 6 the points where the rolls and strip come in contact are indicated with arrows. To test the effect of the asymmetric feeding the strip was fed into the roll gap according to the prior art by using the guide rolls to feed the strip over the pass line (α_top < α_bottom). This was compared to the results where the strip was led towards the roll gap over the eacd-roll. Other test conditions were exactly equal between the two variants of each test. The measured film thickness at the exit of the roll bite is given in Figure 7 for a strip thickness of 0.84 mm (figure 7a) and 1.40 mm (figure 7b). Figure 7 shows the impact of symmetric and asymmetric feeding conditions on the thickness of the oil film as measured on the top of the strip entering the roll-gap. It is clear that symmetric feeding improves the homogeneity of the lubrication conditions, because α_top = α_bottom. The measurements presented in figure 7a and b are performed by means of an oil droplet method and are presented as a function of the rolling speed. This method consists of rolling an oil droplet of a known volume, the area of the resulting spot after rolling is measured. The average oil film thickness at the exit of the bite is determined by taking the quotient of the volume and that area. As is common for these graphs, the log-log scale is used. Open symbols indicate asymmetric feeding conditions, closed symbols indicate the corresponding droplet measurements under symmetric feeding conditions.
To stress the impact of the eacd-roll on the feeding conditions, the series in figure 7 are named "asymmetric feeding" when the strip has been rolled in the configuration as depicted in figure 1 and 9a-b before being rolled and "symmetric feeding" when the strip has passed the eacd-roll before being rolled as depicted in figure 2 and 9c. By using the eacd-roll additional internal stresses are introduced in the strip which leads to the change in entry angle of the strip in the roll gap and thus to the change in lubrication conditions. The measured film thickness is consistently lower on the top when the strip is fed symmetrically in the roll bite. This leads to a more homogeneous lubrication.
Figure 8 shows the phenomenon of length bow.
Figure 9c and 9b show the principle of straight entry of the material to be rolled in the roll gap (c) and skewed entry (b).
Figure 10 shows an example of the different configurations in a 4-stand tandem rolling mill.

Claims

Method for controlling the lubrication during cold-rolling of a metal strip in a cold-rolling mill comprising one or more rolling mill stands, wherein the cold strip is led to the roll-gap between the work rolls of one or more of the rolling mill stands, and wherein the cold strip is bent around an entry-angle control device (eacd) roll, which is adjustably movable in a vertical direction, before entering the roll-gap of the one or more of the rolling mill stands in that the strip is bent around the eacd-roll resulting in at least partial plastic bending of the strip thereby inducing residual tensile stresses in the outer fibres of the strip to enable adjusting the entry angle of the strip with respect to the pass-line of the one or more rolling mill stand.
Method according to claim 1 wherein the strip having been bent around the eacd-roll is fed into the roll-gap of the one or more rolling mill stands without any further mechanical interaction of the strip and the cold-rolling mill's peripherals, such as, but not limited to, damming rolls, guide rolls for guiding the strip in the cold-rolling mill, strip velocity rolls or tensiometer rolls, during the travel from the eacd-roll to the roll-gap.
Method according to claim 1 or 2 wherein the eacd comprises a twin roll configuration comprising two parallel rolls between which the strip is led.
Method according to any one of the preceding claims wherein the vertical position of the eacd-roll or rolls before a rolling mill stand is determined on the basis of one or more of the group of parameters comprising mechanical properties of the strip, strip thickness, strip tension and diameter of the eacd-roll before said rolling mill stand.
Method according to any one of the preceding claims wherein the optimum vertical position of the eacd-roll is determined and controlled by means of a control model.
Method according to any one of the preceding claims wherein the position of the eacd-roll is chosen such that the strip enters the roll gap into the roll-gap at a zero angle with the pass-line (α_top = α_bottom).
Method according to any one of claims 1 to 5 wherein the position of the eacd-roll is chosen such that the strip enters the roll gap at a non-zero angle with the pass-line into the roll-gap (α_top ≠ α_bottom).
Device for cold-rolling a metal strip comprising a cold-rolling mill comprising one or more rolling mill stands, strip supply means for supplying a strip to the cold rolling mill stand or stands, each mill stand comprising two work rolls, strip discharge means for discharging a cold-rolled strip from the cold rolling mill, lubrication means for lubricating the work rolls and the roll-gap between the work rolls, optionally one or more of the group of rolls comprising damming rolls, guide rolls for guiding a strip in the cold-rolling mill, strip velocity rolls or tensiometer rolls, wherein before one or more of the rolling mill stands an entry angle control device (eacd) is provided that comprises at least one eacd-roll for inducing, in use, residual tensile stresses in the outer fibres of the strip by bending the metal strip around the eacd-roll to control the entry angle of the strip in the roll gap of the rolling mill stand immediately following the eacd-roll.
Device according to claim 8 wherein the eacd comprises two parallel eacd-rolls between which, in use, the strip is led and wherein one of the parallel eacd-rolls induces the residual stresses in the outer fibres of the strip by bending the strip around that eacd-roll to control the entry angle of the strip with respect to the pass-line of the rolling mill stand.
Device according to any one of claims 8 or 9 wherein any one, more or all of the other rolling mill stands of the cold-rolling mill are provided with an eacd before the rolling mill stand to induce residual tensile stresses in the outer fibres of the strip to control the entry angle of the strip with respect to the pass-line of the rolling mill stand.
Device according to any one of claims 8 to 10 wherein the optimum vertical position of the eacd is determined and controlled by means of a control model.
Device according to any one of claims 8 to 11 wherein the strip supply means comprise a pickling line.
Use of the method according to any one of claims 1 to 7 to produce cold-rolled strip.
Use of the device according to any one of claims 8 to 12 to produce cold-rolled strip.