Abstract:
A method of presetting the roll nip profile of a roll stand for rolling a rolled strip is provided. The roll nip profile is influenced by output values for the roll nip profile and the tensile stress distribution being set over the roll nip profile. The output values for the roll nip profile is determined by using a roll nip profile model which calculates the roll nip profile. The calculated roll nip profile or an equivalent quantity is linked to a correction value, in particular by addition or multiplication, to form a corrected calculated roll nip profile, so that the roll nip profile model is adapted to the actual roll nip profile of the roll stand by using the correction value.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and a device for presetting the flatness of a rolled strip by presetting the roll nip profile of a roll stand for rolling a rolled strip, with the roll nip profile being influenced by output values for the roll nip profile and the tensile stress distribution being set over the roll nip profile, with the output values for the roll nip profile being determined by using a roll nip profile model which calculates the roll nip profile. 
     BACKGROUND INFORMATION 
     To prevent unevenness in rolling, in particular in cold rolling, influences that would interfere with the required roll nip profile must be counteracted by appropriate setting of the flatness control elements. Until the flatness control used for this purpose has reached a steady state, a lower-quality rolled product known as deviation is produced. 
     SUMMARY 
     To minimize this deviation and to make the rolling operation more reliable, the object of the present invention is to adjust the mill train so that the rolled strip will have the proper flatness from the beginning. To do so, a presetting function is required. This should determine the cumulative crown, i.e., the roll nip profile, as accurately as possible in advance at the start of a roll pass, i.e., when the strip to be rolled is entering the roll stand, and it should adjust the flatness control elements accordingly. 
     This object is achieved according to the present invention by providing a method and a device for presetting the roll nip profile of a roll stand for rolling a rolled strip, where the roll nip profile is influenced by output values for the roll nip profile. The tensile stress distribution over the roll nip profile is influenced. The output values for the roll nip profile are determined by using a roll nip profile model which calculates the roll nip profile, with the calculated roll nip profile or an equivalent quantity being linked to a correction value to form a corrected calculated roll nip profile. The roll nip profile model is adapted to the actual roll nip profile of the roll stand by using a correction value. It has been found that especially accurate presetting of the roll nip profile is achieved by using this method. 
     In an example embodiment of the present invention, the corrected calculated roll nip profile and the actual roll nip profile are compared, a new updated correction value being determined on the basis of this comparison, in particular by weighting with a learning function. 
     In another example embodiment of the present invention, the actual roll nip profile is determined from values, in particular measured values, for the tensile stress distribution. This determination of the actual roll nip profile from the tensile stress distribution is an especially suitable method of determining the roll nip profile. 
     In a particularly example embodiment of the present invention, the roll nip profile is first set when the rolled strip enters the stand according to the output values for the roll nip profile calculated by using the roll nip profile model. The roll nip profile is then set according to output values for the roll nip profile calculated by a flatness control. The flatness control assumes the function of setting the output values after measured values for the tensile stress distribution are available and after the flatness control has reached a steady state. Although the roll nip profile is set by the flatness control, the correction factor for the roll nip profile model is calculated anew. The same measured values are used for the tensile stress distribution as for the flatness control. In this way, the roll nip profile model can be corrected without any additional measured values. Another advantage is that many measured values are available for correction of the roll nip profile model, so that an especially good correction of the roll nip profile model is achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a functional sequence of the method according to the present invention for presetting the roll nip profile. 
     FIG. 2 shows a flatness control according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows the sequence of the method according to the present invention for presetting the roll nip profile. On feed of a rolled strip into roll stand  1 , the roll nip profile is at first set according to output values c actuator  for the roll nip profile. Output values c actuator  are determined by using roll nip profile model  5  as a function of a given strip profile c p  and of setpoints Δσ* for the tensile stress distribution. To achieve an especially accurate presetting of the roll nip profile, roll nip profile model  5  is adapted to the actual roll nip profile. To do so, first tensile stress distribution Δσ is measured. An instantaneous strip profile c actual  is determined from tensile stress distribution Δσ by using a strip profile determination  2 . This instantaneous strip profile is compared by a comparator  3  with a corrected calculated roll nip profile c sum  determined by using roll nip profile model  5 . The function value of comparator  3  is a value Δσ which is a measure representing how a correction value k for adapting roll nip profile model  5  to the actual roll nip profile is adapted. Correction value k is adapted by using a correction value determination  4 . 
     The data flows shown with dotted lines in FIG. 1, i.e., c p , Δσ* and c actuator  are based on the presetting of roll stand  1 . 
     The data flows shown with solid lines, i.e., data flows for Δσ, c actual , c sum , Δk and k, are based on adaptation of roll nip profile model  5 . This training takes place when the roll nip profile is controlled. FIG. 2 illustrates such a control. In particular, FIG. 2 shows a roll stand  6  whose roll nip profile is adjusted according to output values c actuator,control  as a function of tensile stress distribution Δσ and setpoint tensile stress distribution Δσ* by using flatness control  7 . 
     The function sequences shown in FIGS. 1 and 2 are explained below in greater detail. 
     The object of flatness control is to adjust all the actuators that influence the roll nip profile in such a way as to achieve a strip stress distribution over the width of the strip corresponding as closely as possible to the required setpoint curve. The various influencing factors, also known as actuator efficiencies, of the individual actuators on the roll nip profile are also be taken into account. 
     In addition to the actuators, there are several other influencing factors whose effect on the roll nip should be compensated by the actuators. These influences include: 
     mechanical crown c m , i.e., the mechanical camber due to roll grind 
     wear crown c w , i.e., roll abrasion 
     temperature crown c t , i.e., deformation of the rolls due to a change in thermal condition 
     crown c fr  of the set of rolls due to the roll separating force, i.e., the sagging of the rolls due to the roll separating force acting on the supporting rolls 
     predetermined strip profile c p , i.e., the cross-sectional shape of the hot strip. 
     Some of these can be determined only by approximation. The sum of these values, taking into account the different signs due to the direction of the effect acting in the roll nip, added to or multiplied by a correction value k, yields the crown model used according to the present invention. It is essentially true that all the components are additively superimposed in the roll nip, and a modeled roll nip c sum  is obtained accordingly by the equation: 
     
       
         
           c 
           mod 
           =−c 
           m 
           +c 
           w 
           −c 
           t 
           +c 
           fr 
           +c 
           p 
           +k 
         
       
     
     where 
     c m  is the mechanical crown (roll grind) which is, a constant between two roll changes; 
     c w  is the wear crown (roll abrasion) which depends depending on the rolled strip length and the roll separating force; 
     c t  is the temperature crown (deformation due to change in thermal condition) as a function of time; 
     c fr  is the crown of the set of rolls due to the roll separating force (deformation due to the roll separating force acting on the supporting rolls) as a function of time; 
     c p  Is the predetermined strip profile (cross-sectional shape of the hot strip). 
     An approximate value c Sum,ps  for the crown to be preset can be calculated from the setpoint curve and the system influences determined by approximation. This preset crown is referred to below as the sum crown. 
     
       
         
           c 
           sum 
           =c 
           sp 
           +c 
           mod 
           =c 
           sp 
           −c 
           m 
           +c 
           w 
           −c 
           t 
           +c 
           fr 
           +c 
           p 
           +k 
         
       
     
     where c sp  is the crown from the setpoint curve. It represents the desired stress distribution of the strip. The setpoint curve does not contain the strip shape correction, i.e., c sp =f(Δσ*). 
     In this equation, c sum  is used as preset value c actuator  for setting the flatness control elements. This manipulated variable is the same as that used with active control for this function. 
     With c actuator  the output values for the flatness control elements are determined. Relationships of the following form hold: 
     
       
         
           c 
           acatuator 
           =ef 
           1,ps 
           * sp 
           1,ps 
           +ef 
           2,ps 
           +sp 
           2,ps 
           + . . . +ef 
           n,ps 
           *sp 
           n,ps 
         
       
     
     where 
     ef i,ps  are the efficiency factors of the actuators at the time of the presetting; these are to be determined from instantaneous strip data, 
     sp i,ps  are the output values to be preset. 
     A c actuator  can be achieved at i&gt;1 through an infinite number of combinations of output values. 
     Therefore, algorithms that yield the output value combinations with expedient strategies are used to find suitable output value combinations to achieve c actuator . The rapid actuator for bending is thus set at a positive value so that this actuator has sufficient reserve control in the direction of both positive and especially negative bending. This value is abandoned only if the crown to be set, i.e., the roll nip profile, cannot be achieved in this way. 
     As in presetting of the crown to be set, the actual crown can be calculated as the sum crown during the rolling operation. This sum crown depends on time t and on the thermal condition of the roll stand: 
     
       
           c   sum ( t )= c   sp ( t )+c mod ( t )= c   sp ( t )− c   m   +c   w ( t )+ c   t ( t )+ c   fr ( t )+ c   p   +k ( t ) 
       
     
     where 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 c m   
                 the mechanical crown (roll grind) is constant 
               
               
                   
                   
                 between two roll changes 
               
               
                   
                 c w (t) 
                 the wear crown (roll abrasion) depends on the rolled 
               
               
                   
                   
                 strip length and the roll separating force 
               
               
                   
                 c t (t) 
                 the temperature crown (deformation due to a change 
               
               
                   
                   
                 in thermal condition) as a function of time 
               
               
                   
                 c fr (t) 
                 the crown of the set of rolls due to the roll 
               
               
                   
                   
                 separating force (deformation due to the roll 
               
               
                   
                   
                 separating force acting on the supporting rolls) as 
               
               
                   
                   
                 a function of time 
               
               
                   
                 c p   
                 the predetermined strip profile (cross-sectional 
               
               
                   
                   
                 shape of the hot strip) is constant during a pass 
               
               
                   
                 k(t) 
                 the previous correction value. 
               
               
                   
                   
               
             
          
         
       
     
     With active control, the actuators are constantly re-optimized by the flatness control. Instantaneous output values sp i (t) are known. From these, sum crown c actuator  (t) set instantaneously by the controller can be determined by using the equation: 
     
       
           c   actuator ( t )= ef   1 ( cd*sp   1 ( t )+ ef   2 ( cd )* sp   2 ( t )+ . . . + ef   n ( cd )* sp   n ( t ). 
       
     
     The actual roll nip crown is determined on the oasis of the instantaneous stress distribution in the strip, which is measured constantly by a stress measurement device. The equation for this determination from the strip stress distribution is:            c   actual          (   x   )       =         Δσ        (   x   )       *       h   strip     E       +       c   p          (   x   )                                
     where 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 x 
                 the position of a stress measurement point over the 
               
               
                   
                   
                 strip width 
               
               
                   
                 Δσ(x) 
                 strip stress deviation at point x of the strip width 
               
               
                   
                   
                 at the average tensile stress 
               
               
                   
                 h strip   
                 strip thickness 
               
               
                   
                 E 
                 modulus of elasticity of the strip material 
               
               
                   
                   
               
             
          
         
       
     
     Thus, the actual crown is available as a vector c actual . The value determined with the sum crown can then be compared with the actual crown. This yields the error in the model crown. 
     
       
           Δk=c   actual ( t )− c   sum ( t ) 
       
     
     Correction value k contained in c sum ( t ) can thus be optimized. To obtain values for k that are independent of measurement errors and as reliable as possible, k is learned gradually with the help of an integral controller: 
     
       
         
           k=k+V 
           LEARN 
           *Δk 
         
       
     
     V LEARN  is the setting parameter for the learning rate. V LEARN  is preferably 0.01 to 0.1. 
     Learning of the correction value is performed approximately every ten seconds, for example, in active control. 
     Correction value k turns out very differently as a function of such strip and roll stand data as strip thick strip width, working roll diameter and roll separating force. However, the exact functional relationships are not known, so k(t) must be learned for a number of fixed individual supporting values; interpolation values are used for values between these learned values. 
     Conversely, correction value k(t) for the closest supporting values is to be learned for intermediate values. This must take place with weighting according to the distance from the intermediate value to the supporting value. Thus, interpolation is performed in both learning and querying. 
     The relationships, variables and equations presented here are each based on a position x over the width of the material strip, so they are a function f(x).