Patent Publication Number: US-8539804-B2

Title: Method and device for controlling a roll gap

Description:
FIELD OF THE INVENTION 
     The invention relates to the production of substantially long and flat strips or sheets. In the following strips and sheets are used synonymously. For example, the strip is made of a metal such as copper, steel or aluminum. More particularly, the invention relates to a method and a device for controlling a roll gap when rolling a strip in a rolling mill including at least two rolls and at least two actuators that independently control the size of the roll gap. 
     The present invention is useful for hot rolling as well as for cold rolling. 
     PRIOR ART 
     During production of a metal strip it is common practice to roll the material to a desired dimension in a rolling mill. The rolling mill includes at least, two rolls and a thickness control system that controls the gap between the rolls, also denoted the roll gap, and thereby the thickness of the produced strip. According to common terminology, one side of the rolling mill is denoted an operator side and the other side is denoted a drive side. Each of the operator side and the drive side is provided with actuators, for example a mechanical actuator such as hydraulic actuator, for adjusting the distance between the rolls. Thus, the roll gap on the operator side and the drive side can be adjusted independently of each other. It is also known to use thermal actuators for adjusting the roll gap. A thermal actuator adjusts the roll gap by cooling or heating parts of the working rolls. Besides the thickness control, the rolling mill is also provided with a flatness control. 
     For control of the thickness, the thickness of the strip is measured at, at least, one point on the strip after rolling, i.e. after the strip has passed through the work rolls. Usually, the thickness is measured at a point in the center of the strip. This measurement is used as input to the thickness control, together with a desired value of the thickness of the strip. Thus, thickness control according to the prior art aims at a constant reduction of the strip across the width of the strip. 
     Sometimes it happens that a strip material, before rolling, has an asymmetric thickness profile. For example, hot rolled strips often have a thickness profile that is thickest at the center of the strip and is decreasing towards the sides of the strip. In some cases, the thickness profile of the strip material is tapering towards one of its ends, which means that the strip material is thicker in one of its end than in the other end, also denoted a wedge shaped strip. Strips materials with tapered thickness profiles are, for example, common in narrow cold rolling mills where a wide hot-rolled strip, having a thickness profile that is thickest at the center of the strip, is divided into two narrower strips before cold milling. 
     As long as the thickness of the strip before rolling is essentially constant over the width of the strip, the thickness control system works fine. However, if the strip before rolling has an asymmetric thickness profile, the thickness control system will create an asymmetric flatness error in the strip. This flatness error is due to the fact that the thickness reduction of the strip causes a relative elongation of the strip of the same amount as the relative thickness reduction. If, for example, one side of the strip before rolling is thicker than the other side, the relative elongation of the strip after rolling becomes smaller on that side than on the other side, which leads to flatness problem. After some time, this flatness error can be detected and corrected by the flatness control system. However, during this time the flatness of the strip will not be optimal. This flatness error is created even if the roll gap is perfectly adjusted to the incoming thickness profile of the strip. 
     Different forms of setup models are today used in order to match the roll gap to the thickness profile of the strip. However, as soon as a thickness correction is done, mainly in the beginning and end of the strip, the thickness correction will create an asymmetric flatness error when rolling a wedge shape strip. This is due to the fact that, according to prior art, thickness corrections are always done with the same amount on both operator and drive side of the mill. Flatness error may lead to part or parts of the strip having to be rejected. Thus, flatness problem is costly for the strip producer. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an attractive solution to the above problem. 
     According to one aspect of the invention this object is achieved with a method as defined herein. Such a method comprises receiving information on the amount of wedge shape in the strip thickness profile across the strip width, and based thereon controlling the actuators such that the relative reduction of the strip on both sides of the rolling mill becomes essentially the same. 
     The thickness control according to the invention is performed with regard to the fact that the workpiece before rolling may have different thickness profiles across its width. According to the invention, the thickness control is made with regard to the relative reduction of the strip, instead of with regard to the absolute reduction of the strip as in the prior art. A constant relative reduction across the width of the strip will cause a constant relative elongation across the width of the strip, and thus a flat strip. Thus, when rolling wedge shaped strips, i.e. strips that are thicker on one side compared to the other side, it is important that both sides have the same relative thickness reduction during rolling. 
     To be able to control the actuators so that the relative reduction on both sides of the rolling mill becomes essentially the same, it is necessary to have information on the amount of wedge shape in the strip thickness profile across the strip width. This information can be obtained in different ways. The information could be given directly from measurement of the strip thickness at least two points across the strip width, or indirectly via receiving the positions of the roll gap actuators on operator and drive side. This information is, for example, obtained from a preceding hot rolling process, or is measured, for example by means of scanning. During rolling it is normal to position the roll gap actuators to minimize the flatness error thus making the work rolls follow the thickness profile wedge of the strip. Therefore the positions of the actuators quite well reflect the wedge shape of the strip. It is also possible to estimate the wedge shape in the strip thickness profile. Alternatively, information on the thickness profile is determined based on a measured flatness error together with information of the roll gap actuators positions. 
     According to an embodiment of the invention, the rolling mill further includes a thickness control system, which calculates a thickness correction for the roll gap based on a desired strip thickness, and the method further comprises receiving information on the thickness correction to be done, and controlling the actuators, based on the thickness correction and the amount of wedge shape in the strip thickness, such that the relative thickness correction on both sides of the rolling mill becomes essentially the same. According to this embodiment, the control of the actuators, such that the relative reduction of the strip on both sides of the rolling mill becomes essentially the same, is achieved by controlling the actuators such that the relative thickness correction on both sides of the rolling mill becomes essentially the same. 
     The desired strip thickness and thereby the desired thickness reduction in the roll gap are commonly controlled with an automatic thickness control system (AGC). This system continuously calculates thickness corrections, which are fed to a roll gap actuator control system. The thickness control system comprises a thickness correction loop that repeatedly calculates the desired thickness correction for the roll gap based on a desired strip thickness and measurements of the actual strip thickness after rolling. 
     The method further includes receiving information from the thickness control system about the amount of thickness correction to be made. In order to achieve the same relative reduction on both sides of the mill, each correction output also has to give the same relative thickness correction on both sides of the mill. Applying the thickness correction symmetrically on both sides of the mill, as in the prior art, means to create a flatness error when rolling a wedge shaped strip. The relative reduction of the strip is equal to the sum of all relative thickness corrections made from the beginning of the rolling of the strip. If the roll gap is controlled such that the relative thickness correction on both sides of the rolling mill becomes essentially the same in each step of the thickness correction loop, a constant relative reduction across the width of the strip will be achieved. According to this embodiment of the invention, the thickness correction is distributed to the actuators on both sides of the mill so that the relative thickness corrections on both sides of the rolling mill become essentially the same, which results in the flatness error being minimized. An advantage with this embodiment is that it uses information on the thickness correction, which is already available from the thickness control system, in order to achieve the same total relative reduction on both sides of the mill. 
     The relative thickness correction is commonly defined as the quotient of the thickness correction from the thickness control system and the actual thickness of the strip, either before or after rolling. 
     According to another embodiment of the invention, the method comprises receiving information on the thickness of the strip before rolling the strip at least two points across the width of the strip, receiving information on the thickness of the strip after rolling the strip at least one point across the width of the strip, computing a relative reduction of the strip based on the thickness of the strip before and after rolling, and controlling the actuators based on the computed relative reduction of the strip and the information on the thickness of the strip before rolling the strip at least two points. 
     The relative reduction, also denoted the fractional reduction, of the strip is commonly defined as the difference between the incoming thickness of the strip, i.e. the thickness of the strip before rolling, and outgoing thickness of the strip, i.e. the thickness of the strip after rolling, divided by the incoming thickness of the strip: (H−h)/H, where H is the incoming thickness and h the outgoing thickness. 
     The relative reduction is determined at one point across the width of the strip, for example at the center of the strip or at one of its ends, and then the size of the roll gap, i.e. the distance between the rolls, is controlled in such way that the same relative reduction is achieved at least at another point across the width of the strip, and preferably across the whole width of the strip. The maximum number of control points across the width of the rolls depends on the number of actuators controlling the roll gap. For example, if the rolling mill has two actuators controlling the roll gap, it is possible to control the size of the roll gap at two points across the width of the rolls. 
     According to another embodiment of the invention, the roll gap actuators independently control the size of the roll gap on an operator side of the mill and on a drive side of the mill and the method comprises estimating a desired roll gap on the operator side of the mill based on the computed relative reduction of the strip and the thickness of the strip of the operator side before rolling and based thereon controlling the roll gap actuator on the operator side, and estimating a desired roll gap on the drive side of the mill based on the computed relative reduction of the strip and the thickness of the strip of the drive side before rolling and based thereon controlling the roll gap actuator on the drive side. 
     It is easy to realize that the method according to the invention, as defined in the appended set of method claims, is suitable for execution by a computer program having instructions corresponding to the steps in the inventive method when run on a processor unit. 
     According to a further aspect of the invention, the object is achieved by a computer program product directly loadable into the internal memory of a computer or a processor, comprising software code portions for performing the steps of the method according to the appended set of method claims, when the program is run on a computer. The computer program is provided either on a computer-readable medium or through a network. 
     According to another aspect of the invention, the object is achieved by a computer-readable medium having a program recorded thereon, when the program is to make a computer perform the steps of the method according to the appended set of method claims, and the program is run on the computer. 
     According to another aspect of the invention this object is achieved by a device as defined in claim  10 . Such a device is adapted to receive information on the amount of wedge shape in the strip thickness profile across the strip width, and the device is adapted to control the actuators, based on the information on the amount of wedge shape in the strip thickness profile, such that the relative reduction of the strip on both sides of the rolling mill becomes essentially the same. 
     The invention is particularly useful for controlling strip thickness in a cold rolling mill. This is because of the common use of slit strip in cold rolling mills. During hot rolling it is normal to control the strip thickness profile to a symmetric shape. 
     The invention is particularly useful for controlling a roll gap when rolling a wedge shaped strip in a rolling mill. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be explained more closely by the description of different embodiments of the invention and with reference to the appended figures. 
         FIG. 1  shows schematically a side view of a rolling mill including a device for controlling the thickness of a strip according to a first embodiment of the invention. 
         FIG. 2  shows a front view of the rolling mill shown in  FIG. 1 . 
         FIG. 3  shows a top view of the rolling mill shown in  FIG. 1 . 
         FIG. 4  shows a block diagram of a method for controlling the thickness of a strip in a rolling mill according to a first embodiment of the invention. 
         FIG. 5  shows schematically a side view of a rolling mill including a device for controlling the thickness of a strip according to a second embodiment of the invention. 
         FIG. 6  shows a block diagram of a method for controlling the thickness of a strip in a rolling mill according to a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
       FIGS. 1-3  show a rolling mill, including a device  14  for controlling the roll gap of the mill according to a first embodiment of the invention, from different views. The figures show a metal strip  1  passing through a rolling mill  2  in a direction shown by an arrow. The rolling mill includes two main rolls  3   a - b  and two supporting rolls  4   a - b . Two actuators  6 , 7 , in this case hydraulic actuators, determine the distance between the main rolls  3   a - b , also denoted the roll gap. The actuator  6  determines the distance between the rolls on a drive side  10  of the rolling mill and the actuator  7  determines the distance between the rolls on an operator side  11  of the rolling mill. The actuators  6 ,  7  independently control the size of the gap between the rolls on each side of the rolling mill. The rolling mill further includes a sensor  12  for measuring the thickness of the strip after the milling. The sensor  12  is located essentially at the center of the strip across the width of the strip, i.e. at essentially equal distance from both edges of the strip. The sensor  12  measures the thickness at one point  13  across the width of the strip.  FIG. 2  shows the size d C  of the roll gap in the centre of the mill, the size d OS  of the roll gap on the operator side, and the size d DS  of the roll gap on the drive side. 
     The device  14  is adapted to control the positions of the actuators  6 , 7 . The device  14  receives information on the thickness H of the strip before rolling. In this embodiment, the thickness information is received from a preceding hot rolling process. Alternatively, the information may be received from a scanner scanning the strip before it enters into the rolling mill. In this embodiment, information on the thickness of the strip before milling is needed at three points  15   a - c  across the width of the strip, as shown in  FIG. 3 . The points should be selected at a distance from each other in a direction perpendicular to the direction of the movement of the strip. In this embodiment, the first point  15   a  is located at the operator side  10  of the rolling mill, the second point  15   b  is located at the center of the width of the strip, i.e. in a corresponding location as the sensor  12 , and the third point  15   c  is located at the drive side  11  of the rolling mill.; and 
     The device  14  is adapted to compute a relative reduction 
               Δ   ⁢           ⁢   h     H         
of the strip based on the thickness of the strip before and after rolling the strip, i.e. before and after reduction of the size of the strip. In this embodiment the relative reduction
 
               Δ   ⁢           ⁢   hc     Hc         
of the center of the strip is calculated based on the strip thickness before reduction H C , measured at point  15   b , and the strip thickness after reduction h C , measured at point  13  according to the following:
 
Δ h   C   =H   C   −h   C   (1)
 
     The device  14  is also adapted to compute a desired roll gap dos on the operator side of the mill based on the computed relative reduction of the center of strip 
               Δ   ⁢           ⁢   hc     Hc         
and the thickness H OS  of the strip of the operator side before rolling, i.e. the thickness measured at point  15   a . The computation unit is also adapted to compute a desired roll gap d DS  on the drive side of the mill based on the computed relative reduction
 
               Δ   ⁢           ⁢   hc     Hc         
of the center of the strip, and the thickness H DS  of the strip of the drive side before rolling i.e. the measured thickness in point  15   c.  
 
     Alternatively, it is also possible to calculate the relative reduction divided by the strip thickness after rolling 
                 Δ   ⁢           ⁢   hc     hc     ,         
this will achieve about the same result as dividing by the strip thickness after rolling.
 
     The computation requires the use of some Arithmetic Logical Unit, ALU, but it can be implemented in either the digital circuitry of an FPGA, an ASIC, or a simple microprocessor. The device further comprises appropriate data processing means known in the art such as input and output means and memory means. 
     The device  14  is adapted to control the actuators  6 , 7  based on the computed desired roll gaps d OS  and d DS . The actuators adjust the distance between the rolls to the desired roll gaps on the operator and drive side. Thereby, the relative reduction across the width of the strip becomes essentially constant. 
       FIG. 4  is a flow chart illustration of the method and the computer program product according to a first embodiment of the present invention. It will be understood that each block of the flow chart can be implemented by computer program instructions. 
     Information on the strip thickness H C , H OS , H DS  before rolling the strip is received, block  20 . Information on the strip thickness h C  after rolling the strip is received, block  22 . The relative reduction of the center of the strip is computed, block  24 , based on the thickness of the strip before and after rolling: 
     
       
         
           
             
               
                 
                   
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       hc 
                     
                     Hc 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           H 
                           C 
                         
                         - 
                         
                           h 
                           C 
                         
                       
                       ) 
                     
                     / 
                     
                       H 
                       C 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     In order to achieve a constant relative reduction of the strip across the width of the strip the following relation shall be valid: 
     
       
         
           
             
               
                 
                   
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       hc 
                     
                     Hc 
                   
                   = 
                   
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           h 
                           OS 
                         
                       
                       
                         H 
                         OS 
                       
                     
                     = 
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           h 
                           DS 
                         
                       
                       
                         H 
                         DS 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Thus, the relative reduction on the operator side 
               Δ   ⁢           ⁢     h   OS         H   OS           
and the drive side
 
               Δ   ⁢           ⁢     h   DS         H   DS           
shall be the same as the relative reduction
 
               Δ   ⁢           ⁢   hc     Hc         
in the centre of the strip.
 
     The desired size d OS  of the roll gap on the operator side is calculated based on the following equation, block  26 : 
     
       
         
           
             
               
                 
                   
                     d 
                     OS 
                   
                   = 
                   
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         hc 
                       
                       Hc 
                     
                     * 
                     
                       H 
                       OS 
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     The desired size d DS  of the roll gap on the drive side is calculated based on the following equation, block  28 : 
     
       
         
           
             
               
                 
                   
                     d 
                     DS 
                   
                   = 
                   
                     
                       
                         Δ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         hc 
                       
                       Hc 
                     
                     * 
                     
                       H 
                       DS 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Thereafter, the actuator  7  on the operator side is adjusted until the roll gap on the operator side is equal to the calculated size dos, block  30 , and the actuator  6  on the drive side is adjusted until the roll gap on the drive side is equal to the calculated size d DS , block  30 . 
       FIG. 5  shows schematically a side view of a rolling mill including a device  40  for controlling the thickness of a strip according to a second embodiment of the invention. Components corresponding to those in  FIG. 1  have been given the same reference numerals, and will not be described in more detail here. As can be seen in the drawing, the rolling mill further includes a thickness control system  42 , which calculates a thickness correction POS add  for the roll gap based on a desired strip thickness h ref  and a measurement of the strip thickness h c  after rolling. The thickness correction is calculated as the difference between the actual thickness of the strip after rolling and the desired strip thickness. The thickness correction is in the order of μm. The device  42  is adapted to receiving the thickness correction POS add  from the thickness control system  42 , and to generate control signals to the actuators  6 , 7 , based on the thickness correction and the amount of wedge shape in the strip thickness, such that the relative thickness corrections on both sides of the rolling mill become essentially the same. 
     The thickness control system  42  continuously calculates thickness corrections POS add , which are fed to the device  40 . In order to achieve the same total relative reduction on both sides of the mill, each correction output also has to give the same relative correction on both sides of the mill. When rolling wedge shaped strips i.e. strips that are thicker on one side compared to the other side, it is important that both sides have the same relative thickness reduction during rolling. 
     The following equations are applied in order to ensure the same relative thickness reduction across the strip:
 
POS add =(POS addOS +POS addDS )/2  (6)
 
Where,
 
POS add =thickness correction calculated from the thickness control system
 
POS addOS =thickness correction to be applied to the roll gap actuator on the operator side of the mill
 
POS addDS =thickness correction to be applied to the roll gap actuator on the drive side of the mill
 
W=relative strip wedge profile is defined by:
 
 W =( H   DS   −H   OS )/ H   OS  
 
or (when using an automatic flatness control system):
 
 W =(POS actDS −POS actOS )/POS actOS   (7)
 
where,
 
H DS =incoming strip thickness on operator side
 
H OS =incoming strip thickness on drive side
 
POS actDS =actual position of roll gap actuator(s) on drive side
 
POS actOS =actual position of roll gap actuator(s) on operator side
 
     In order to get the same relative thickness correction on both operator side and drive side the following must apply:
 
POS addDS =POS addOS (1 +W )  (8)
 
     Solving these equations give:
 
POS addDS   =POS   addOS (1 +W )=(2POS add −POS addDS )*(1 +W )
 
POS addDS =(2POS add *(1 +W ))/(2 +W )  (9)
 
POS addOS =2POS add −POS addDS   (10)
 
       FIG. 6  is a flow chart illustration of the method and the computer program product according to a second embodiment of the present invention. It will be understood that each block of the flow chart can be implemented by computer program instructions. 
     Information on the amount of wedge shape in the strip thickness profile across the strip width is received, block  52 . This information is, for example, the actual positions POS actDS , POS actOS  of roll gap actuators on the drive side and the operator side. The relative strip wedge profile W is calculated according to equation 7, block  54 . The thickness correction POS add  is received from the thickness control system, block  56 . Thereafter, the thickness correction POS addOS  to be applied to the roll gap actuator on the operator side of the mill is calculated according to equation 10, block  58 , and the thickness correction POS addDS  to be applied to the roll gap actuator on the drive side of the mill is calculated according to equation 9, block  60 . Then, the actuators on the operator and drive sides are adjusted in accordance with the calculated thickness correction. 
     The term comprises/comprising when used in this specification is taken to specify the presence of stated features, integers, steps or components. However, the term does not preclude the presence or addition of one or more additional features, integers, steps or components or groups thereof. 
     The present invention is not limited to the embodiments disclosed but may be varied and modified within the scope of the following claims. For example, the relative strip wedge profile W can be calculated as W=(POS actDS −POS actOS )/POS actDS .