Differences in the transverse thickness of the shape (i.e. changes in the shape of the section) and changes in the shape of the plate are caused by changes in the shape of the roll gap and rolling mill.
6. Uneven thermal expansion of roll
In the rolling process, the heating and cooling conditions of the roll are unevenly distributed along the roll body. In most cases, the temperature in the middle of the roll body is higher than that in the edge (but sometimes the opposite happens), and generally, the roll temperature on the transmission side is slightly lower than that on the operation side. In the diameter direction, the temperature of the roll surface and the roll core is also different. In the stable rolling stage, the temperature of the roll surface is higher, but the opposite situation will also occur due to the rapid cooling of the roll surface during the stop rolling.
7. Roll wear
The friction between workpiece and work roll and between backup roll and work roll will make the roll wear uneven and affect the shape of roll gap. However, because there are too many factors affecting roll wear, it is difficult to calculate the roll wear in theory. We can only rely on a large number of actual measurements to obtain the wear laws of various rolling mills, so as to take corresponding methods to compensate roll wear.
8. Elastic deformation of roll
This mainly includes elastic bending and elastic flattening of rolls. The elastic flattening of the roll is unevenly distributed along the length of the roll body, which is mainly due to the uneven distribution of unit pressure. In addition, the flattening near the edge of the rolled piece should also be smaller, so that there is a thinning area at the edge of the rolled piece. With the decrease of roll diameter, the thinning area at the edge also decreases.
Generally, although this area is not very large, it also affects the yield. Uneven elastic flattening also occurs between the work roll and the support roll, which directly affects the bending deflection of the work roll. The elastic bending deflection of roll is generally the most important factor affecting the shape of roll gap.
It is considered that the deflection of the supporting roll is equal to the deflection of the supporting roll for a long time, so the deflection of the supporting roll can be assumed mainly according to the analysis of the deflection ratio of the supporting roll to the working roll. Therefore, it is considered that the roll body deflection difference of support roll can be used to replace the roll body deflection difference of work roll in roll shape design. But in fact, this is incorrect. Both theory and experiment show that the actual deflection of work roll is much larger than that of support roll. This is mainly because there is elastic flattening deformation between the work roll and the support roll. As a result, the part of the work roll outside the plate width is subjected to the cantilever bending of the support roll, which greatly increases the deflection of the work roll itself.
The smaller the width of the rolled piece, the greater the deflection of the work roll. Therefore, when designing the roll profile, if we do not consider the elastic deformation characteristics of the work roll, but only rely on the calculation of the deflection difference of the support roll body to deal with the problem, the result must be inconsistent with the reality. That is, the bending deflection of the work roll of the four high mill depends not only on the bending deflection of the backup roll, but also on the deflection caused by the uneven elastic flattening between the backup roll and the work roll.