Note: Descriptions are shown in the official language in which they were submitted.
.1 15~529
1 -- 21 DSS-2;37
TEMPER~TURE CONTROL IN HOT STRIP MILL
_
Cross-ReEerence to Related Patents
STRIP TEMPERATURE CONTROL SYSTEM, U.S.
Patent No. 3, 905, 216, issued September 16, 1975 to
E.N. Hinrichsen, here the "Runout Table Cooling Patent~.
METHOD OF REVISING WORKPIECE TEMPERATURE
ESTIMATES OR MEASUREMENTS USING WORKPIECE DEFORMATION
BEHAVIOR, U.S. Patent No. 3,628,358, issued December 21,
1971 to D~3. Fapiano the A.S. Norton Jr., here the
"Temperature Calculation Patent".
Background of the Invention
Field of the Invention
The present invention relates to the rolling
of metal strips and, more particularly, to a technique
for accurately controlling the temperature of a strip
during the rolling process.
Description of the Prior Art
_
Sheet metal is produced by rolling sl.abs,
bars or other relatively massive workpi.eces into
elongate,thin strips. Although finish rolling often is
done under room temperature conditions, the initial
reduction of the workpiece from its bulk form is done
at elevated temperatures in a facility known as a hot
strip mill. In a hot strip mill the workpieces are
heated in a reheat furnace to a temperature of around
2200 degrees Fahrenheit (F).
~ .
~k
1 1S5~2g
21 DSS-2537
-- 2
The reason the workpieces are heated to such an
elevated temperature is that the temperature of
the workpiece influences the resistance to deformation
of the workpiece. That is, a hot workpiece has a
lower resistance to deformation than a cold workpiece
and, accordingly, requires less roll force before it
will be deformed by a given amount than a cold
workpiece of similar composition and dimensions. In
short, deforming a workpiece maintained at an elevated
temperature may be done easier and faster than one
maintained at a lower temperature.
The temperature at which the rolling
process is commenced is not maintained -throughout
the mill. As the strip passes from one stand of
rolls to another, heat losses caused by radiatlon
and/or strip-to-roll conduction reduce the temperature
of the strip to about 1500F to 1700F, depending
in part upon the thickness of the strip. After
the strip leaves the last mill stand, it must be
cooled further prior to being coiled and banded.
The Runout Table Cooling Patent describes a
particularly effective technique for reducing the
temperature of a hot rolled strip between the time
the strip leaves the last mill stand and before it
is coiled.
In addition to the obvious goal in a hot
strip mill of reducing a workpiece to a desired
finish thickness, it also is important that the
temperature drop from the initial elevated temperature
to the finish temperature and to the temperature at
which the strip is coiled be controlled as much as
possible. As explained more completely in the
Runout Table Cooling Patent, metallurgical properties
of hot rolled strip metal are dependent not only
upon the composition of the metall but also upon the
1 155~213
21 DSS-2537
-- 3 --
temperatures at which the final thickness reductions
take place, the temperature at which the strip is
coiled, and the rate at which the temperature of
the strip changes during the final coo]ing process.
When steel is the material under consideration, the
final thickness reductions normally take place above
1600F and the s-trip is cooled on -the runout tables
to approximately 1200F. While finishing and
coiling temperatures are of principal importance,
the temperature at reductions preceding the final
reduction may be important in achieving certain
metallurgical properties. For these metallurgical
grades, it is desirable to maintain constant temperature
not only at the final rolling stands but at one or
more of the preceding rolling stands. A secondary
consideration relates to the control of strip
flatness during rolling. In modern, computer-
controlled hot strip mills, one objective of the
reduction schedule is to assign reductions in
successive stands which will produce roll separating
forces and associated strip crowns which are
compatible with good flatness. These strategies
are well known and are described, for example, in
"Automatic Shape Control - Hoogoven's 88 - In. Hot
Strip Mill" by F. Hollander and A. G. Reinen,
Iron and Steel Engineer, April, 1976. Strip flatness
control will be improved where the rolling force at
each stand is maintained more nearly constant
throughout the strip length. This would o~ course
require that intermediate as well as final rolling
temperatures be held essentially constant. In short,
by maintaining temperature at all stands essentially
constant in the presence of variations in incoming
strip temperature and variations in rolling speed,
both metallurgical qualities and strip flatness can
~ 1 55~29
21 DSS-2537
be enhanced.
A significant problem in maintaining the
temperature of a metal strip at desired predetermined
levels relates to "skid marks." Skid marks are
sections of a strip at temperatures significantly
below the average temperature of the strip, often
by as much as 100F. Skid marks are caused because
the workpieces are pushed through the reheat furnace
on skids or other supports. The skids are water
cooled and, thus, are at a lower temperature than
the temperature of -the rest of the furnace.
Accordingly, small sections of the workpiece in
direct contact with the skids will not be heated as
much as other portions of the workpiece. The
temperature deviation of the areas of the workpiece
in contact with the skids is carried throughout the
remainder of the rolling process, even though the
great initial temperature deviation may be largely
attenuated by the time the rolling process is complete.
In any event, the existence of the skid marks
causes a temperature variance in the strip along
the length of the strip. This has made it difficult
to control the temperature of all portions of the
stxip with a great deal of accuracy.
Another important consideration influencing
the temperature of the strip is that of rolling
speed. Modern high speed rolling mills thread the
initial portion of the workpiece through the mill
and coiler at a relatively low speed and accelerate
3Q rapidly to higher speeds where most of the rolling
is done. All of the heat transfer phenomena in
the rolling process are time dependent. Strip
temperature loss through radiation and conduction
to the work rolls is reduced at higher speeds, while
energy input to the strip may be slightly increased
1 ~ S5S29
21 DSS-2537
-- 5
due to strain-rate related increases in deformation
resistance. ~t the same time, strip temperature
on entering the finishing train may be decreasing
due to radiation loss. Additionally, the cooling
effect of interstand sprays is dependent upon strip
speed not only because of the reduced cooling time
at higher speeds, but also because of interaction of
the closely spaced sprays comprising a group due to
incomplete recovery of surface temperature on
entering successive spray regions. These considerable
variations make it very difficult to predict with any
degree of accuracy the temperature which the strip
will attain as i~ passes through the various mill
stands.
On mills not equipped with interstand
cooling sprays, control of strip finishing temperature
has been achieved by adjustments to the finishlng
- speed. The necessary adjustment has, in some cases,
been precalculated to exactly compensate for the
variation in temperature of the strip entering the
first rolling s-tand. The temperature achieved in
this manner can then ~be sensed by means of a
pyrometer located immediately downstream of the
last mill stand. If the temperature exiting the
last mill stand is too high, the mill can be slowed
down; if the temperature is too low, the mill can
be accelerated. A major disadvantage with this
method is that the maximum speed and, therefore, the
production rate are determined by the -temperature of
the incoming strip. A second disadvantage is that
the correction technique is very slow and large
portions of the strip may be finished at incorrect
temperature.
The potentially most effective technique
to control the temperature of a strip as it is
~ 1 5~529
21 DSS-2537
-- 6
being rolled is to provide a number of individually
controllable water sprays between adjacent mill
stands. If the sprays are positioned above and
below the strip and across the width of the
s-trip, effective coolin~ of the strip can be
accomplished. ConsequentIy, higher rolling speeds
are made possible and the temperature increases
caused by the higher rolling speeds can be corrected
through the use of water sprays~
The single most important problem with
the water spray approach has been ln properly
sensing the temperature of the strip and thereafter
controlling operation of the water sprays. Although
a pyrometer positioned downstream of the last mill
stand has been very effective as a monitoring
device, presently available pyrometers and other
temperature sensors have not been sufficiently
accurate to permit a reliable indication of interstand
strip temperature. Another consideration is that of
transport lag, that is, the problem of sensing the
temperature of the strip at one downstream location,
providing an upstream temperature correction, and
having to wait for the results of the temperature
correction to become known to the downstream
temperature sensor. Because of the stand spacing and
total elongation between the first interstand spray
and the downstream pyrometer, an error or a correction
in strip temperature at the first interstand spray
may not be evident until 300 or 400 feet of strip
have passed the interstand spray location.
In an attempt to overcome the foregoing
problems associated with water spray control,
variGus prior art proposals have been made, all
without much success. One of these proposals has
been to calculate in advance a temperature profile
~ 155~2g
21 DSS-2537
for a strip of different thicknesses, rolling speeds,
and so forth. As the mill accelerates to its
desired rolling speed, water sprays are commenced
at predetermined intervals. ~he sprays are activated
first near the last mill stand and are activated
se~uentially in an upstream direction. During mill
deceleration, the sprays are deactivated in the
reverse sequence. A fundamental problem with this
approach is that most of the cooling takes place
toward the final mill stands. This means that the
temperature corrections tend to be concentrated
toward the end of the rolliny process rather than
distributed through the rolling train where they
actually occur. This changes the temperatures at
which intermediate reductions occur, and, as a resu:lt,
also changes the rolling forces associated with
these earlier reductions. These changes may
adversely affect both metallurgical properties and
strip flatness. A second, practical problem is
that errors in the predictive calculations, due for
example to changes in cooling spray effectiveness,
are not known unless sensed by a downstream pyrometer,
which is subject to the previously described delay
problems.
Other "predictive" approaches are possible,
but they all suffer from the drawback of not being
able to accurately account for all situations which
will be encountered during operation of a hot strip
mill. In short, predictive approaches have serious
shortcomingsl but no completely acceptable adaptive
control system exists either.
Summar~ of the Invention
The present invention overcomes the ~oregoing
and other drawbacks of prior art temperature control
proposals and provides an effective, controllable
1 ~5~29
21 DSS-2537
technique to roll me-tal workpieces in a hot strip
mill at desired temperature levels. Essentially, the
invention comprises calculating at each mill stand,
as a function of s-trip deformation resistance, the
difference between strip temperature in a first
strip region and strip temperatures at successively
later strip regions and, in response to the calculated
temperature differences, controlling the operation
of water sprays located i~mediately downstrearn of
each mill stand so as to hold essentially constan~
the temperature at which strip enters the subsequent
stand. A pyrometer located on the exit side of
the last mill stand provides a control on the
absolute temperature of the strip by continuously
updating the temperature references, or 1l lock-on"
values, of one or more of the later mill stands.
In a preferred embodiment, the temperature
change at each mill stand is calculated by determining
the change in deformation resistance of the workpiece
from measurements of roll separating force and
correlating the deformation resistance change to the
temperature change of the workpiece. More specifically,
a load cell included as part of each mill stand
senses:
(a) a first roll separating force at or
near the strip head end; and
(b) changes of roll separating force from
the first roll separating force.
Entry and delivery thicknesses at each
mill stand are calculated from the load cell force
readings, the roll positions, and the mill modulus
according to a known method for controlling strip
gauge. Deformation resistance is determined from
the ratio o force to reduction at each mill stand.
Pre-stored empirical data are used to correlate the
1 155529
21 DSS-2537
g _
change in deformation resistance with changes in
temperature. The empirical data are determined in
advance for different rolling condi-tions. Corr~ctions
for the change in the deformation resistance resulting
from changes in strain rate are included.
By continuously monitoring the load cells,
in effect a continuous reading on strip temperature
change is obtained without the use of interstand
pyrometers. Where hydraulic cylinders are used to
adjust roll gap, hydraulic pressure may be used as
an indication of roll separating force, instead
of load cells. The temperature control system
automatically accomodates variations in spray
effectiveness due to water pressure, water temperature,
nozzle conditions, and other causes. A more uniform
temperature trajectory through successive rolling
stands is made possible by feeding required
temperature corrections from each mill stand to
water sprays immediately downstream of that mill stand.
Description of the Drawings
Figure l is a schematic view of a temperature
control system according to the invention incorporated
in a hot strip mill;
Figure 2 i5 a plot of temperature versus
mill stand location, curves being plotted for different
locations along the length of the strip and for
heating effects due to rolling speed;
Figure 3 is a plot of relative resistance
to deformation with respect to temperature or one
grade of workpiece material;
Figure 4 is a plot of per unit change in
relative deformation resistance per degree F, with
respect to temperature; and,
Figure 5 is a plot of per unit change in
relative deformation resistance with respect to
~ 1 5~529
21 DSS-2537
-- 10 --
s-train ra-te for one grade of workpiece material.
Description of the Preferred Embodiment
In a hot strip mill, the initial reductions
of the thickness of a metal slab are taken in a set
of tandem mill stands known collectively as a
roughing train. Figure l shows in greatly simplified
form the last stand RL of a roughing train along with
other components in a hot strip mill. As the slab
emerges from the stand RL, i-t moves across a mill
table 20 toward a finishing train 22 consisting of
mill stands Fl, F2, F(n-l), and F(n) arranged in
tandem. In a typical hot strip mill, seven mill
stands are provided in the finishing train 22. The
final reductions in thickness are taken in the
finishing train 22 to produce a metal strip which
may be 1,000 or more feet in length, two to seven
feet in wi.dth, and 0.040 to 0.50 inches in thickness.
During its passage through the roughing
train and the finishing train 22, the strip gradually
20 is cooled -from it initial temperature of about
2200F. By the time the strip reaches stand F(n) it
has cooled to around 1500-1700F. As the strip emerges
from the last stand F~n) of the finishing train 22,
it traverses a cooling or runout table 24 before
being wound by a coiler 26. Strip tension during
the coiling operation may be maintained by a pair
of pinch rolls 28, 30 located at the coiler end of the
runout table 24. A number of individually controlled
water sprays, one of which is designated by the
numeral 32, are located above and below the runout
table 24 to form a cooling zone 34 in which the
strip is cooled to a proper temperature for coiling,
usually on the range of 850-1,300F. Reference is
made to the Runout Table Cooling Patent for a
description of a preferred technique for cooling
1 1 $5529
21 DSS-2537
-- 11 --
the strip.
Each stand iTI the finishing train 22
includes an upper work roll 40 and a lower work
roll 42. Upper and lower backup rolls 44, 46
are pressed against -the upper and lower work
rolls 40, 42, respectively, during a rolling
operation to prevent excessive distortion of the
work rolls 40, 42. This configuration is known-as a
four-high mill. Each mill stand includes roll-
adjusting screws 48 to regulate the opening betweenthe upper and lower work rolls 40, 42. The roll
opening may be determined as a functi.on of screw
position. One appropriate means for accomplishiny
this function is illustrated in Figure 1 by shaft
encoders 41 which provide feedback signals to bus 43.
A load cell 50 is positioned intermediate the roll-
adjusting screws 48 and the upper backup rolls ~0
to provide an indication of the compressive force
exerted between the upper and lower work rolls 40,
42. Variations in rolling force exerted by a strip
passing between the rolls 40, 42 will be sensed by
the load cell 50. The work rolls 40, 42 are positioned
by a screwdown control system 52 which controls the
position of the roll-adjusting scres 48. The thickness
of the strip after entry is maintained essentially
constant by the automatic gauge control system in a
manner well known in the art, and the thickness of
the strip as it exits the stand can be determined
from unloaded roll opening, roll separating force
and mill modulus in accordance with known practices;
e.g., U.S. Patent 2,726,541, issued December 13, 1~55.
The work rolls 40 and 42 are driven by suitable
motors (not shown) the speeds of which are sensed
by appropriate sensing means such as tachometers 45
which provide speed feedback signals to a bus 47.
:~ 1S5~29
21 DSS-2537
- 12 -
In the prefPrred embodiment, individually
controllable water sprays 54, 56, 58 are located
above the strip and are positioned intermediate
adjacent mill stands Fl, F2, ... F(n)~ Individually
controllable water sprays 60, 62, 64 are located
below the strip and are positioned intermediate
adjacent mill stands F1, F2, ...F(n). Taken together,
the individual sprays 54, 56, 58, 60, 62, 64 may be
referred to collectively in appropriate contexts as
cooling sprays 66. A water spray control system 68
is connected to the sprays 66 to control operation of
the sprays 66. (An alternate to the use of individually
controlled discrete sprays would be the proportiona:L
control of spray flow. The important function here
desired is the variation in amount of coolant.) A
calculator 70 receives inputs from the load cells 50,
encoders ~l (bus 42) and tachometers 45 (bus 47)
and, after appropriate analysis as will he described
subsequently, sends a control signal to the water
spray control 68. A pyrometer 72 is positioned
upstream of the first mill stand Fl. A second
pyrometer 74 is positioned a short distance downstream
of the last mill stand F(n). The pyrometers 72, 74
sense the temperature of the strip as the strip enters
and exits the finishing train 22.
During the rolling process, the strip loses
heat through radiation, through conduction to the
work rolls, through convection to the air, and it is
heated by the energy required in deformation. The
first three phenomena are directly dependent on time,
while the deformation energy is slightly dependent
upon deformation rate. Thus the temperature change
experienced by the strip is related to the speed at
which the mill is run and the degree of reduction to
which the strip is subjected. Under modern rolling
1 1~552~3
21 DSS-2537
- 13 -
conditions, the strip may pass through the finishing
train 22 at such a speed that the temperature of the
strip exiting the last mill stand F(n) may be sufficiently
high that -the cooling capacity of the cooling zone 34
may be exceeded. The cooling sprays 66 thus provide
a capability to cool the strip sufficiently during its
passage through the finishing train 22 that maximum
rolling speed may be attained and yet the cooling
capacity of the cooling zone 34 will not be exceeded.
A significant concern, however, is that the supplementary
cooling be applied in a manner which introduces the
least possible disturbance to the temperatures at
which all finishing reductions occur. Desirably,
changes in strip temperature will be corrected in the
region where they occur.
The foregoing considerations can be understood
more clearly by referring to Figure 2 which assumes
a seven stand mill. Curve A is a plot of temperature
versus mill stand location for the head end of a strip
as it passes through the finishing train 22. The
initial temperature Ti is that sensed by the upstream
pyrometer 72. The final temperature Tf is that
sensed by the downstream pyrometer 74. Target
temperatures have been identified for predetermined
temperatures which the head end of the strip desirably
will attain at mill stands F5, F6 and F7. Cur~e A
accordingly defines a desired temperature profile of
the head end of a strip. Curbe B is the temperature
profile of the tail end of a strip run at low mill
speed and without use of the water sprays 66. During
most of its processing through the finishing train 22,
the tail end of the strip will be at a temperature
less than desired.
Curve C is the temperature profile of the
tail end of a strip rolled with just sufficient
1 1 S552~
21 DSS-2537
- 14 -
finishing train acceleration to achieve a constant F7
exit temperature. Curve C represents modern practice
on mills not equipped with interstand cooling sprays.
The maximum attainable rolling speed is limited at all
times by the target delivery temperature Tf and the
lnitial strip temperature Ti. Curve D is a temperature
profile of the tail end of a strip processed under
higher rolling speeds with the use of water sprays.
The preferred use of water sprays would be one in
which Curve D is made to conform as closely as
possible to curve A. For the conditions illustrated,
appropriate use of cooling sprays beginning with those
immediately downstream of F4 could be used to restore
strip temperatures at F5 through F7 to their initial
or head end values. For some products, it is
desirable to "bias" the spray selection strategy;
that is, one or more of the group of sprays in each
interstand location can be turned on upon strip entry,
where there is sufficient heat capacity in the strip
to achieve the target final temperature for the strip
head end. Then, as strip entry temperature drops
approaching the strip tail end, sprays in the early
interstand spaces would be turned off in order to
restore strip temperatures to their head end values
while sprays in the later interstand spaces would be
turned on to compensate for temperature increases
there.
The most important factor in maintaining a
constant strip temperature "profile" as it passes
through the finishing train 22 is accurate measurement
of strip temperature changes in the region of the
interstand sprays, where these changes must be
corrected. Even with temperature variations due to
skid marks, if an accurate indication of temperature
change can be had, then the individual water sprays
1 ~ ~5529
21 DSS-2537
- 15 -
can be controlled to eliminate the temperature variation
as soon as it appears. The pyrometers 72, 74 placed
upstream of the first mill stand Fl and downstream of
the last mill stand F(n) may provide acceptable
continuously available temperature information.
Pyrometers are not presently practical between mill
stands because temperature measurements in these
locations are influenced adversely by steam, spray,
and standing water deposited on the strip by the water
sprays 66. Furthermore, the strip surface is subject
to abrupt "chilling" in its passage through the work
rolls 40, 4~ rendering inaccurate temperature
measurements made before adequate recovery time has
elapsed~ The downstream pyrometer 74, while
lS producing acceptable temperature measurements, provides
only the aggregate temperature change and no
information concerning how this change is distributed
over the finishing train 22. Furthermore, as stated
earlier, this information is available only after
the region of strip being measured may be 300 or 400
feet past the point at which the temperature correction
should have been made.
The foregoing problems are overcome by the
present invention in which temperature changes at
each mill stand are determined by interpreting the
output o~ the load cells 50. It has been found that
changes in roll separating force associated with the
strip thickness ~eductions can be correlated to
changes in temperature of the strip, p:^ovided the
change in deformation resistance associated with
chan~es in rollin~ speed are taken into account.
Reference is made to the Temperature Calculation Patent
for d description of a technique for relating force
to temperature. In that patent, corrections for
changes in rolling speed were not considered since
2 9
21 DSS~2537
- 16 -
large changes in speed were not typical in the
applications for which the invention was originally
made. In modern hot strip mills, however, speed
changes within the coil may exceed 2:1, requiring
adjustments for the associated strain rate effects
be~ore accurate temperature inferences can be drawn.
When a temperature change has been
observed in the foregoing manner at a particular
mill stand, the temperature change can be used in
a feed-forward strategy to control the water sprays 66
immediately downstream of that mill stand. For
example, if a temperature deviation exists at
mill stand Fl, the water sprays 54, 56, etc.
immediately downstream of the mill stand Fl can be
activated to attempt to correc'c the temperature
deviation. This process can be carried out for each
of the other mill stands. Any temperature deviations
not corrected between mill stancls Fl and F2 will be
sensed by the temperature calculation at mill stand
F2 and individual water sprays 54, 56, etc. downstream
of the mill stand F2 can be activated as needed in
an attempt to correct the temperature deviation.
As the strip progresses through the finishing-
train 22, increasingly effective temperature correction
will be attained in a uniform manner so that a
particular desired temperature at mill stand F(n) will
be reached.
If a desired high threading speed will produce
higher than desired strip temperature, certain of the
water sprays 66 can be activated in advance of the
initial temperature determinations. Use of pre-
activated sprays has other advantages as mentioned
earlier. Deactivation of previously activated sprays
as the colder tail end approaches the early stands,
while activating additional sprays in the later stands,
~ ~ 5S~29
21 DS5-2537
- ~7
may permit closer conformance -to the desired
temperature profile. Additionally, pre-activation
permits response to skid marks near the strip head
end by deactivating sprays as the colder skid mark
regions pass throuyh the early stands.
An important advantage of -the foregoing
-technique is that the need for a transport lag
compensation largely is avoided, provided the
system can function ~ast enough. Even though
presently existing rolling mill speeds are quite
high (on the order of 3000 feet/minute), water
spray reaction time is sufficiently fast (0.50 -
0.75 seconds) that temperature correction action
can be taken immediately downstream of each mill
stand without excessive error. In effect, each
mill stand temperature computation is independent
of upstream temperature computations and temperature
- corrections, and acts to hold temperatures in its
region of control as constant as possible.
The described technique is a so-called
lock-on system in that the temperature computation
and the water spray activation is based on temperature
deviations, rather than on absolute temperature
measurements. The upstream pyrometer 72 permits
initial set-up calculations to achieve the desired
head end temperature and desired temperature
profile. The downstream pyrometer 74 permits
absolute temperature to be monitored and further
corrective actions to be applied. For example, if
the downstream pyrometer 74 senses that the
temperature of the strip is, say, 20F too high, an
error signal can be fed upstrQam to the temperature
calculation at mill stand F(n-l). Additional water
spray immediately downstream of mill stand F(n-l)
then could be activated. If the temperature
1 1~5S29
21 DSS-2537
- 18 -
deviation were great enough that the spray capacity
downstream of mill stand F(n-l) were exhausted,
then the "excess" temperature deviation signal could
be sent upstream to the preceding mill stand F~n-2)
Additional water spray immediatley downstream o~
mill stand F(n-2) then could be activated. In this
manner, a closed loop control system on the absolute
temperature of the strip can be attained.
Alternative methods for applying the
corrections from the downstream pyrometer could
increase the number of upstream stands to which
the correction is applied. This reduces the
disturbance to the temperature "profile" resulting
from this feedback action, while increasing the
time for all corrective actions to become evident
at the pyrometer 74.
The use of feedback from a downstream
sensor, whether the pyrometer 7~ or the temperature
change sensed by the mill stand F(n) to control an
upstream spray, may introduce the need for
compensation for transport lag. Transport lag
compensation techniques already are known, and
principally require a knowledge of the speed of
the strip, the distances between temperature
sensing locations and water spray locations,
and the time available in which corrective action
can be taken. Because most of the corrective action
taken according to the present invention is by
eed-forward strategy, any transport lag compensation
problems are minimized greatly because they occur
only at the extreme downstream end of the finishing
t~ain 22.
In order to estimate a temperature change
at a given mill stand, forces, thickness reductions,
and rolling speeds are determined for successive
2 9
21 DSS-2537
-- 19 --
sampling time intervals. Strain rate, e, is defirled
as the rate at which strain occurs, given in per-unit
per second. After initial threading of the strip into
a mill stand and after a brief time delayl on the
order of 1 to 2 seconds, to allow the mill to recover
from impact speed drop and to permit tension transients
to subside, the ration of roll force to reduction
(F/~h) is developed and stored as a lock-on value of
deformation resistance. A lock-on value for strain
rate e also is developed. The strip temperature
change ~ T at lock-on is, by definition, zero. For a
scanning interval (i), the following equations can be
used to determine the estimated strip temperature change:
{L(F/~ h)i ~(F/~h)O~ -~ - {L~n (ei/eO)]X ~ Ih~ne~
~ T = ~ _~T
~ r(~ h) i / ~Tl FB
L(~F/~h)o /
Where F = foxce sensed by the load cells 50;
A h = reduction in strip thickness;
ZO e = rate at which strip thickness reduction occurs;
TFB = temperature correction from the pyrometer 74;
O = conditions existing at a given starting point;
and,
i = conditions existing after a predetermined
time or strip length interval.
~ ~ R h ~ h2
Where V = work roll peripheral velocity;
R = work roll radius;
hl = thickness of strip entering rolls;
h2 = thickness of strip exiting rolls; and,
h = thickness reduction = ~hl - h2)
The first bracketed term in the numerator of
equation (1) represents the per-unit change in deformation
resistance during a scanning interval (i). The second
2 9
~1 DSS-2537
- 20 -
bracketed term in the numerator of equation (1)
represents that portion of the per unit deformation
resistance change which is attributable to changes in
rolling speed. The strain rate e is calculated from
equation ~2). The denominator of equation (1)
represents the change in relative deformation resistance
with temperature, and ~\TFB represents corrections
to the lock-on temperature value based upon subsequent
measurement by the pyrometer 7~. Equation (1) uses
the ratio of deformation resistance, (F ~ h)i/ (F/ ~ h)o,
which is necessary to give accurate results where the
reduction, ~ h, in a stand may change. Where gauge
control holds reductions in each stand constant, a
close approximation may be achieved by using the ratio
of forces, Fi/Fo.
The effect of the cooling sprays 66 can be
determined from the equation:
(3) ~ TSj = Kj (Ts w)
hv
Where j = identifying index of individual sprays
comprising an interstand group;
Kj = a variable (usually empirically derived)
dependent upon the axial extent and flow rate of the
water spray;the specific heat, density, and coefficient
of convection of the strip; and the number of activated
sprays in the group;
Ts = calculated temperature of the strip;
Tw = temperature of the water;
h = the thickness of the strip; and,
v = the velocity of the strip.
The decision to activate or deactivate a
water spray downstream of a given mill stand is made
by comparing the most recent calculation of temperature
change~ T for a particular zone, j, with the sum of
the temperature drops, f~Tsi~ due to presently actuated
~ 15$~2~
21 DSS-2537
- 21 -
sprays~ A deadband is provided to prevent excessive
cycling of the water sprays. Control of the water
sprays is determined as follows:
~=n
_
(4) If ~T~ ~ ~ Tsj + D, turn of spray if available.
~=o
j=n
(5) If ~ T< ~ ~ T - D, turn off spray.
s~
j=o
Where n = the number of individually
controllable water sprays between adjacent mill stands;
and,
D = a predetermined temperature increment,
corresponding approximately to one-half the temperature
drop from one spray.
Certain of the factors of equations (1)
and (3) may be yenerated off-line, and stored to be
called upon during actual on-line processing of a
strip. The graph of Figure 3 represents predetermined
relationships between temperature and relative
deformation resistance for typical steel materials.
In this Figure, deformation resistance is shown
relative to that at 2000F, for one material grade.
The point of reference is immaterial, since the curves
are used only to determine the per unit change in
deformation resistance per degree of tempera-ture
difference. At 1650F, for example, this change would
be approximately -.0025 per unit per degree F; that is,
the deformation resistance at 1651F would be about
0.25% less than the deformation resistance at 1650F.
Another way of relating per unit deformation
resistance change to temperature change is shown in
Figure 4. Figure 4 is developed by dividing the
slope at each point along the curve of Figure 3 by
the relative deformation resistance at that point.
1 ~ 5~529
21 DSS-2537
- 22 -
Again it ~ seen that deformation resistance at
165~ F changes -.25% per degree F increase in temeperature.
Other representations may be used to yield mathematically
equivalent results.
The graph of Figure 5 represents, or vaious
strain rates, the rate of change of relative deformation
resistance with the natural logarithm of strain rate.
The logarithm of strain rate is used simply because it
provides a more linear relationship. Figure 5 showsl
for example, that forstrain rates of approximately 100
per unit per second which are typical of hot strip
finishing, a 10 percent change in rolling speed, and
thus in strain rate, would produce an approximate 2%
change in rolling force. This relationship, sometimes
referred to as "strain-rate sensitivityl can be determined
by either laboratory or production rolling mill tests.
A family of such curves is generated for the actual
operating conditions encountered during normal operation
of a mill. Appropriate curves can be called upon,
depending upon the type of material to be processed.
The curves of Figures 3 and 4 are based on actual
operating experience for a particular steel grade.
The effectiveness of force change as an
indicator of temperature change varies over the rolling
temperature range, requiring typically from 3F to
7F to produce a 1% force change. Since speed changes
of only 10~ may produce force changes of the same order,
it is clearly necessary to correct for speed differences
where they are significant. With proper care in force
interpretation, it is practical to discern force
changes of 1~ or less, corresponding to temperature
changes as small as 3F at some mill stands.
A consideration influencing the accuracy of
the temperature determination is the presence of
eccentricity in the backup rolls 44, 46. If the roll
:~ ~ $~29
21 DSS-2537
- 23 -
bodies are eccentric with respect to their journals,
then the strip reduction will vary during the rotation
of the backup rolls resulting in variation in the roll
separating orce. The effect of eccentricity on
temperature determination can be minimized by employing
a sampling period long enough to average force readings
over one or more backup roll revolutions. In general,
eccentricity is a potential problem only at the center
stands of the finishing train 22. At the initial mill
stands where the eccentricity is much smaller than the
thickness reduction, the associated force variations
are negligible. For example, an eccentricity at mill
stand Fl o~ .002 inches typically will produce a force
variation of 0.4 percent or less. At the delivery end
of the finishing train 22, eccentricity as a percent of
thickness reduction is large enough to be of concern,
but the backup roll rotational rate is high enough to
provide acceptable averaging during an interval of 1 or
2 seconds. At the intermediate mill stands, thickness
reduction is perhaps 0.1 i.nches and rotational speed is
still fairly low, perhaps 1~2 revolution per second.
Here, the sampling intervals should be on the order of
2 or 3 seconds in order to provide acceptable eccentricity
averaging. In general, a sampling interval of about
2 seconds also will be sufficient to reduce temeprature
variations in the strip due to skid marks while providing
acceptable force averaging.
Some modern rolling mill systems employ band
pass filter techniques to remove eyelic components in
the sensed force due to roll eccentrieity. Where these
techniques are in use, shorter force scanning intervals
and faster response may be practical.
Another consideration whieh must be taken
into account is incoming thickness variations to the
first mill stand Fl. These variations, which may
1 ~S5~29
21 DSS-2537
- 24 -
result from eccentricity in the roughing train or from
skid marks which pass uncorrected through the roughing
train, may cause force readings which might be interpreted
as temperature changes. A solution is to reduce the
temperature change estimates derived from the force
changes, deliberately underestimating the required
corrections at mill stand Fl. After passing through
mill stand Fl, the incoming thickness variations will
be attenuated sufficiently that temperature readings
from mill stands F2 through F(n) can be employed without
adjustment.
By employing the present invention, it is
now possible to control quite accurately and without
long delays the temperature of a strip as it passes
through the finishing train 22. Because the temperature
of the strip can be controlled quickly and accurately,
the mill can be accelerated to the maximum permissible
speed at faster accelerating rates than would otherwise
be possible. Furthermore, the interstand cooling sprays
can be used to correct local variations such as skid
marks which otherwise would be impossible to correct
using the feedback techniques of prior art.
Maintenance of the planned temperature profile as well
as the planned final temperature permits not only
maximum production, but minimizes flatness variations
and changes in metallurgical properties which would
result from changes in the temperature and force
"profile" through the finishing train.
Although the invention has been described
in its preferred form with a certain degree of
particularity, it will be understood that the present
disclosure of the preferred embodiment has been made
only by way of example and that numerous changes may
be resorted to without departure from the true spirit
and scope of the invention as hereinafter claimed. It
:1 ~55529
21 DSS-2537
- 25 -
is intended tha-t the pa-tent shall cover, by suitable
expression in the appended claims, whatever feature
of patentable novelty exist in the invetnion
disclosed.
