Heavy Plate Department,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201900,China

Abstract: Owing to the frequent variation in steel grades,plate dimensions,mill pacing,and mill working conditions,the zero point of a plate mill fluctuates accordingly,potentially leading to incorrect plate thickness results.Using the actual measured thickness curve data,a zero-point correction model was developed based on lot classifications with respect to the steel grade,dimensions,and last-pass rolling force.The application of this model achieves enhanced variation prediction of the zero point and improved plate-thickness control.

Key words: plate mill; zero-point correction; lot classification; adaptation

1 Introduction

The target plate thickness accuracy is a critical factor in the plate-rolling process,as it directly affects the final prime yield of the plate production line and reduces the ability to meet the product requirements of customers.In the control system of the plate mill,the setup process is based on some hypotheses and is very complicated;therefore,the values obtained based on the aforementioned setup exhibit some deviation during the actual use.The adaptation of a zero-point correction (ZPC) is an important factor in the setup of calculation model of plate thickness.Only by using a highly accurate zero-point value,can the final plate thickness after rolling meet the requirements of customers.

2 Thickness calculation model

The plate thickness of each pass during plate rolling can be calculated using Equation (1):

h=s+Δhstand+Δhroll-ΔhMorgoil-

Δhthermal+Δhwear-s0

(1)

where,his the exit thickness;sis the setup gap;Δhstandis the change in plate thickness owing to the mill stretch;Δhrollis the change in plate thickness owing to the deformation,flattening,and roll contour of the roll sets;ΔhMorgoilis the change in plate thick-ness owing to the oil film on the back-up roll bearing;Δhthermalis the change in plate thickness owing to the thermal deformation of the roll sets;Δhwearis the change in thickness owing to the wear of the roll sets;ands0is the zero gap.

The mill stretch can be calculated using Equation (2):

(2)

where,Pis the rolling force;andMis the modulus of the mill.

By comparing the measured and setup thick-nesses,the deviation in the plate thickness can be obtained using Equation (3):

Δh=Δhmeas-(s+Δhstand+Δhroll-ΔhMorgoil-

Δhthermal+Δhwear-s0)

(3)

where,hmeasis the measured plate thickness.

In the calculation of thickness deviation,the measured rolling force,velocity,thermal defor-mation,and wear values are used.

Using the measured-thickness values,the ZPC value of the mill can be calculated using Equation (4):

Δs0=klearn(hmeas-h)

(4)

where,klearnis the adaptation value (ranging from 0 to 1 in the plate-rolling process).

3 Factors influencing the ZPC value

From Equation (1),the roll gap and final plate thickness can change owing to the fluctuations associated with many equipment items,including mill stretch,thermal deformation and wear of the roll sets,and a change in the roll contour.The rolling force can also be affected by a change in the dimensions of the rolling plate.One characteristic of plate rolling is the high changing frequency of the steel grade and dimensions,implying high ZPC uncertainty.In the actual plate production,the factors discussed in the subsequent subsections can severely affect the ZPC value.

3.1 Thickness of the final plate

When there is no change in the steel grade or dimensions of the plate,the ZPC value will remain constant[1].During normal plate rolling,to achieve the required dimensions,10-30 passes are required for each plate.The variation in each plate is also very large.The working point of the mill during the rolling process will change based on the plate thickness,and the ZPC value will fluctuate when rolling plates with different thicknesses.

3.2 Width of the plate

Limited by the testing conditions and mill com-plexity,the mill modulus is normally obtained based on the contact of the work rolls;however,the actual mill stretch is significantly related to the rolling plate width[2].In the studied mill,the plate width was determined to be 1.3-4.8 m,implying that the mill stretch will vary significantly.Hence,the plate width must be considered in the ZPC model.

3.3 Change in the rolling force

From Equation (1),the gap of the mill is directly related to its rolling force and modulus.In the control models of the plate mill,the modulus of the mill is considered to be linear.However,during the actual rolling process,the modulus of the mill is not constant,particularly when the rolling force is less than 3 000 t.During normal plate rolling,the rolling force of each pass and that of the final pass of different plates change significantly.During the setup process,the use of a constant modulus can result in large deviations.Because the modulus changes with changes in the rolling force,the rolling force must also be considered in the ZPC model.

3.4 Roll pacing of the plate line

There are many variations in the types of plate rolling,including controlled rolling and batch rolling.Further,any abnormal shutdown of the mainline can cause large fluctuations in the mill pacing.As the thermal conditions of the roll change,the ZPC value changes accordingly.

4 Problems of thickness control in the con-trol system

The thickness control system has only a short-term ZPC adaptation function.This short-term adaptation function is based on the deviation in the thickness of the previous plate,with no consider-ation of the differences in the plate dimensions and rolling force.In the normal plate-rolling process,when there is a big change in the current plate com-pared with the previous one with respect to the steel grade,plate dimensions,and rolling force,the operator manually inputs a ZPC value based on his/her work experience,which is always greater than 2 mm.This major change in ZPC causes instability in the rolling process and thickness fluctuations,such that sometimes the plate thickness does not meet the requirements of either the customers or the prime yield.The control system has no ability to compensate for the thickness tolerance produced by a change in the mill pacing.

Owing to the aforementioned problems of the thick-ness control system,the automatic ZPC function is always shut down by the operator to enable manual control of the thickness.This manually determined ZPC value is based on the work experience of the operator,which changes with each operator.There-fore,abnormal control results are often observed such that thickness control has become a major pro-blem in plate production.

5 New ZPC adaptation model

To solve the thickness control problem,the ZPC control model must be improved.Based on the influencing factors described in Section 2,a new ZPC adaptation model was developed.

5.1 Structure of the improved ZPC adaptation model

The new model is classified into short-term and long-term ZPC adaptation functions to meet the thickness control requirements of plates exhibiting the same and different dimensions.The structure of the model includes the plate thickness,draft of the last pass,rolling force of the last pass,and plate width,as shown in Table 1.

Table 1 Zero-point correction parameters

5.2 Procedure of the new ZPC model

According to Equations (1) and (4),the correc-tion of the ZPC value involves two procedures:update and application.The update and application pro-cedures are performed in the adaptation and pass schedule calculation models,respectively.Fig.1 shows the flowcharts of the update and application procedures.

Fig.1 Flowcharts of ZPC update and application procedures

5.3 Model running process

In the first stage of the new adaptation model,the main task is to produce an initial value,which is then refreshed based on the actual rolling data.By comparing the adapted value with the high-perfor-mance value,the model structure and parameter values are improved.When the fluctuation in the adapted values is within the required range,the new model can be implemented for actual use.

Fig.2 shows the human-machine interface of the pro-cess computer.In this figure,the red rectangular frame denotes the ZPC value calculated using the newly developed adaptation model and the blue rectangular frame denotes the ZPC value of the rolling plate.

Fig.2 Operation of the new ZPC model

After being operated for six months,the newly structured model achieved good results.A statistical analysis of the obtained ZPC values showed that their range of fluctuation was narrower than before the implementation of the new model.During January-March 2018,the fluctuation in the ZPC values manually determined by an operator was 0.509 mm.Using the new model during January-March 2019,the fluctuation in the ZPC value was determined to be 0.254 mm.A comparison of these fluctuation values revealed that the new model reduced the ZPC fluc-tuation by approximately 50%.

Fig.3 shows the ZPC fluctuation in the new model.

Fig.3 ZPC fluctuation in the new model

Note that each time a new set of working rolls is installed,the initial ZPC value is approximately -1.0 mm.Hence,the center value of the ZPC fluc-tuation is -1.0 mm.

6 Conclusions

Because of the changes in the ZPC value,a new ZPC adaptation model was developed based on a new structure.The application of this new structure,which considered the steel grade,plate dimensions,and rolling force,achieved good plate thickness control.By using this new model,the plate-rolling operation was easier to manage for the operators.