Interstand strip gauge and profile conrol

Method and means for maintaining uniform thickness and profile of a metal strip during hot rolling in a multi-stand hot rolling mill, comprising a two-position looper located between adjacent mill stands and movable between a strip threading position above the rolling pass line of the mill and a rolling position substantially in the plane of the rolling pass line, and a thickness and profile gage movable out of operative relationship with the looper when the looper is in a threading position and into operative relationship with the looper when the looper is in a rolling position, the gage adapted to project a measuring X-ray beam from the thickness and profile gage onto the strip during rolling and at an angle to the strip of substantially 90.degree., thereby minimizing measurement error due to variable angularity between the X-ray beam and the strip during rolling.

FIELD OF THE INVENTION 
This invention relates to the hot rolling of metal strip and more 
particularly to means and methods for maintaining the uniformity of strip 
gauge and profile during hot rolling, 
BACKGROUND OF THE INVENTION 
To provide good thickness control, as well as good strip profile and 
flatness control, intermediate feedback signals have to be provided 
indicative of these parameters as they are measured between mill stands. 
In a cold tandem mill, such measurements and feedback control are a common 
practice. 
In hot strip mills, the installation of thickness and profile gages is 
difficult due to the action of loopers which are a part of the strip 
interstand tension control mechanism. In the state of the art, as 
represented, for example, by H. Harakei et al., Hot Strip Mill Gage 
Control Using Interstand Thickness Meter, Iron and Steel Engineer, August, 
1992, pages 54-59, special correction for change in angularity of the 
strip in respect to an X-ray beam is provided. However, such correction 
cannot be perfect, so it increases an error of measurement, and also makes 
control more complicated. 
The most appropriate position for an intermediate profile gage would be 
after stand F3 of a 6-stand mill and after stand F4 of a 7-stand mill. 
This is due to decreased value of allowable changes in relative crown at 
this point. Therefore, the desired crown-to-thickness ratio (relative 
strip crown) has to be obtained after those respective stands, as 
illustrated in FIGS. 1 and 2. 
SUMMARY OF THE INVENTION 
An objective of this invention is to stabilize the strip position in 
respect to an X-ray beam projected by a thickness and profile gage and to 
make an angle between the X-ray beam and the strip surface approach as 
close as possible to 90.degree.. 
To achieve that objective, an interstand tension control is provided with a 
two-position looper. During threading of the strip between the mill stand 
rolls, a thickness and profile gage is retracted from the pass line and 
the looper operates at the height which is optimum for threading. After 
threading of the strip is completed, the looper is lowered to the rolling 
position and the thickness and profile gage is moved into operative 
relationship with the strip to be rolled. See FIGS. 3 and 4.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
In FIGS. 3 and 5 a two-position looper is designated generally by the 
numeral 20, located between two 4-high mill stands 1 and 2, and is 
effective to raise a strip 3 into a threading position and to lower the 
strip 3 into a rolling position in operative association with a thickness 
and profile gage 7. The looper, which includes a roller 21 engageable with 
the underside of the strip 3, is raised and lowered by means of an 
hydraulic cylinder/piston assembly 6 and is provided with a position 
sensor B. 
FIG. 5 shows the two-position looper control in a threading position. In 
this position, switches 11 and 18 are in an "A" position. In such 
position, threading looper height reference H.sub.thr is compared with 
actual looper position signal H.sub.a generated by position sensor 8 which 
is connected with looper cylinder 6. After comparison by a position 
controller 9, the error signal is input into a main drive speed regulator 
12 which adjusts mill stand 1 speed in respect to mill stand 2 speed so 
that a desired looper height is maintained. 
At the same time, a strip tension reference S.sub.ref is compared with 
actual strip tension signal S.sub.a. An error signal generated by tension 
regulator 15 is fed into a servovalve controller 16 which regulates oil 
flow into and out of the hydraulic cylinder 6 through a servovalve 17. 
Thus a desired strip tension S.sub.ref is maintained when the looper 20 is 
maintained at the desired height H.sub.thr. The actual strip tension 
S.sub.a is calculated by the processor 14 based on pressure inside 
cylinder 6 as measured by a pressure transducer 13 and on actual looper 
height H.sub.a as measured by position sensor 8. 
After threading, switches 11 and 18 are set in the "B" position shown with 
dotted lines (FIG. 5). In that case, a position controller 10 becomes 
operative and the rolling looper height reference H.sub.rol is compared 
with actual looper position signal H.sub.a. The position error signal 
generated by the position controller 10 is input into servovalve 
controller 16 which, through servovalve 17, controls oil flow in and out 
of the looper cylinder 6. As a result the looper 20 is lowered into the 
rolling position. At the same time, speed regulator 12 is fed by a strip 
tension error signal generated by tension regulator 15, so a desired strip 
tension is maintained while the looper is set at the elevation which is 
optimum for strip profile control. 
By such means, the position of the thickness and profile gage 7 is 
maintained at essentially a 90.degree. angle to the strip 3 during rolling 
thereby minimizing measurement errors due to variable angularity between 
the X-ray beam of the thickness and profile gage 7 and the strip.