IMPROVED METHOD FOR IMPROVEMENT OF SHAPE OF HOT ROLLED COILS IN
HOT STRIP MILL
FIELD OF INVENTION
The present invention relates to a method for the improvement of shape of the hot rolled
coils in hot strip mill. More particularly, the present invention relates to improvement of
the hot rolled strip profile and reduce wedge, thereby introducing the modified reduction
schedule at roughing stands to reduce width variation and also modified reduction
regimes at finishing stands to reduce gauge variation
BACKGROUND ART
Increasing demand is being faced by the hot strip producer to provide accuracies in the
geometrical parameters of profile, flatness, thickness (gauge) and width. The variation
of these parameters along the length of the strip leads to deterioration in its quality level
thereby affecting the downstream processing at CRM and the marketability of the coils.
Considering the downstream application of a hot rolled strip, profile plays a very
important role. Defect in strip profile introduced during hot rolling is very difficult to be
eliminated during subsequent processing. Thus strip profile influences the final profile
of cold rolled strip. Therefore, it becomes imperative to try to control strip profile during
hot rolling stage itself. Strip flatness is another important parameter which influences
steady processing in later stages and quality requirements of customers. While it is
always aimed at achieving the best flatness as regards profile, a definite positive crown
has to be maintained in the strip depending on further processing.
Gauge variation in HR coils is basically due to longitudinal strip temperature gradient
and non - homogeneous material properties along the coil. The Automatic Gauge
Control (AGC) system installed in the finishing stands minimises this variation. The coil
box is another tool to reduce this variation.
Width variation in HR coils results from lateral mass flow during the deformation process
in roughing stands. It may also result from necking caused by loppers in the finishing

stands. All the modern mills have Automatic Width Control (AWC) systems which can
control width up to a narrow range.
In the prior art, a Japanese specification JP 10263692 discloses method for controlling
coiling shape of hot rolled steel strip on temper-rolling line. The tension set value Ts of
a tension reel and the measured value T of the telescope quantity detected by a coil
shape sensor are inputted in a shape control device. The rotation of a tension reel drive
motor is controlled so that the tension of the tension reel becomes the set value Ts
when not telescope is caused. The tension is detected by a tension sensor and
feedback-controlled. When the telescope is generated, the shape control device
controls the rotation of the tension reel drive motor so as to increase the tension of the
tension reel by &alpha (T) based on the measured value T of the telescope quantity.
Generation of the telescope can be prevented irrespective of the strip speed or the
operation speed by changing the tension of the tension reel according to the telescope
quantity.
Another Japanese specification JP 2001137931(A) discloses manufacture method of
hot-rolled steel sheet excellent in uniformity of crown and shape of steel sheet. In the
manufacturing method of the hot-rolled steel sheet by which the crown and shape of the
steel sheet are controlled by heating a rough bar using an induction heating device
having plural induction coils which are provided between a roughing mill and a finishing
mill, by measuring the temperature of the rough bar between the roughing mill and the
induction heating device, determining the amount of necessary temperature rise to the
target temperature from this measured value of temperature and executing feed forward
control to the electric power for each induction coil, the hot-rolled steel sheet excellent in
the uniformity of the crown and shape is manufactured.
HSM, RSP, was modernized during 1995-96 and presently the total material processed
here is through the continuous casting route. The mill consists of 3 Roughing stands
and 6 Finishing stands. The first roughing stand (RoA/0) is a combination horizontal
stand and a vertical stand. The other two roughing stands (Ri and R2) are 4 high
horizontal stands. There is a delay table after R2 stand, one coil box and a crop shear at

the end of the delay table. There are six numbers of 4 high finishing stands (Fi to F6)
and two hydraulic down coilers. A schematic diagram of the mill is shown in Fig.1.
No modern actuator is available for control of strip profile due to which the HR strip
profile at RSP remains inconsistent and unsatisfactory. Lot of variation also exists in the
gauge and width front.
OBJECT OF THE INVENTION
Therefore the primary object of the present invention is to provide recommendations on
cycle over tonnage for finishing stand rolls by investigations on wearing behaviour of
rolls which would reduce i) Strip crown from 80|xm to 50>m and ii) Strip wedge from
0.15mm to 0.05mm
Further object of the present invention is to provide optimisation of reduction regimes to
reduce gauge and width variations by at least 10%
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig 1 illustrates the layout of Hot Strip Mill in the Steel Plant in accordance with the
present invention;
Fig 2 illustrates the wearing Pattern of Finishing Stand Rolls before innovation in
accordance with the present invention;
Fig 3 illustrates the correlation obtained between strip crown and tonnage rolled in
accordance with the present invention;
Fig 4 illustrates the wearing Pattern of Finishing Stand Rolls after innovation in
accordance with the present invention;
DETAILED DESCRIPTION
Improper profile and wedge, high gauge variation and negative width bring down the
yield of hot rolled (HR) coil during its downstream processing at Cold Rolling Mill (CRM)

and also affect the marketability of the product. Investigations at Hot Strip Mill (HSM),
Rourkela Steel Plant (RSP) revealed that roll wear, cycle over tonnage, thermal
conditions of roll and reduction schedule have important bearings on generation of
shape defects in HR coils. Scientific investigations were carried out and the following
improved method steps suggested for the improvement in strip shape:
• Cycle over tonnage for finishing stand rolls to be < 2000t to improve strip profile
and reduce wedge
• Modified reduction schedule at roughing stands to reduce width variation
• Modified reduction regimes at finishing stands to reduce gauge variation
With implementation of these measures, improvement in strip shape was observed as
shown below:

Investigations on roll crown and wedge:
In the absence of any actuator in the mill, the initial roll crown imparts the biggest
influence on roll gap contour and hence strips profile. With increased tonnage of rolling,
the roll gap changes as wearing of rolls takes place. Roll wear measured at different
tonnages indicated that wearing rate progressively increases with increased tonnage of
rolling. The roll wear pattern obtained at different tonnages of rolling also indicated that
after 2300 to 2500 tons of rolling, wearing is rapid (Fig.2). It was also evident that at

higher tonnages, a taper exists from one side of the roll to the other which is the reason
for wedge in strips.
The high wear of rolls has adverse effect on strip profile. Strip crown measurement
carried out for coils of different sizes in pickling lines and correlated with their tonnage of
rolling (Fig. 3).
The figure suggested that to achieve a strip crown of < 50>m, the cycle over tonnage
should be restricted to 2000 tons. With this cycle over tonnage, at least the no. of roll
changes in a week, starting from Saturday to Thursday works out to be 11 (6 days).
This is allowed with a continuous production rate of < 2000t per rolling campaign.
With decrease in cycle over tonnage to <2000t and with at least 11 roll changes in a
week, the improvement in strip profile was observed as shown in Fig.4. Roll wear
pattern observed at 2000t showed low wear rates particularly for stands F1 and F2.
Investigations on gauge variation:
Longitudinal strip temperature gradient and non-homogeneous material properties along
the coil are responsible for gauge variation in HR coils. A difference of 80 - 100°C exists
from head end of the coil to the tail end before entry in the finishing stands. This gives
rise to variation in roll separating force and consequent difference in gauge along the
length of the coil.
In order to improve strip shape and reduce gauge variation, modified reduction regimes
at finishing stands were formulated for different sizes of the product as shown in Table
1. This is based on investigations carried out on the existing pattern of reduction as
given for different sizes of HR coils. Analysis of data had revealed that non-uniform
reduction pattern existed which led to generation of gauge variation in coils. The
modified reduction regime has been designed such that a gradual reduction starting
from Stand F1 to Stand F6 is to be given to the coil to arrive at the targeted size. The
individual %reduction at each stand for each size was primarily targeted to achieve
reduction in gauge variation. The highest % reduction (within allowable loading limits of
the mill) would be at Stand F1 and gradually come down to the last stand. At stand F6 it

is intended to give a skin passing effect on the strip surface by restricting the mill load to
16.6%. Implementation of this modified reduction regimes has led to reduction of gauge
variation and improvement in shape of the coil.

Investigations on width variation:
Necking in finishing stands was found to be the primary reason for width variation in HR
coils. Width measurement of coils carried out at pickling lines revealed that negative
width occurred in 11 % cases of observation. To reduce the occurrence of necking and
negative width, the bar width coming out of R2 was increased by 10 mm than the
planned width. This has been given in Table 2. This was achieved by modification in
drafting schedule at R1 whereby an increase in spread was obtained. Optimization of
schedule was done through trials based on feedback of finished width from coiler pit.


Table 2: Modified Reduction Regimes at Roughing Stand (R1) for width control
Although the foregoing description of the present invention has been shown and
described with reference to particular embodiments and applications thereof, it has
been presented for purposes of illustration and description and is not intended to be
exhaustive or to limit the invention to the particular embodiments and applications
disclosed. It will be apparent to those having ordinary skill in the art that a number of
changes, modifications, variations, or alterations to the invention as described herein
may be made, none of which depart from the spirit or scope of the present when
interpreted in accordance with the breadth to which they are fairly, legally, and equitably
entitled.

We Claim
1. An improved method for improvement of shape of hot rolled coils in hot strip mill
comprising the steps of:-
fixing of cycle over tonnage for finishing stand rolls at < 2000t to improve strip
profile and reduce wedge;
modifying the reduction regimes at Finishing Stands for reduction of gauge
variation;
modifying the reduction schedule at Roughing Stand for reduction of width
variation.
2. An improved method for improvement of shape of hot rolled coils as claimed in
claim 1, further comprising the steps of making at least 11 changes of the rolls in a
week for the improvement in strip profile and lowering of the wear rates for stands ( F1
and F2) under continuous production.
3. An improved method for improvement of shape of hot rolled coils as claimed in
claim 2, wherein the continuous production rate is of less than or equal to 2000t per
rolling campaign.
4. An improved method for improvement of shape of hot rolled coils as claimed in
claim 1, wherein the gradual reduction starting from Stand F1 to Stand F6 is given to the
coil to arrive at the targeted size.
5. An improved method for improvement of shape of hot rolled coils as claimed in
claim 1, wherein the individual percent reduction at each stand for each size is primarily
targeted to achieve reduction in gauge variation.
6. An improved method for improvement of shape of hot rolled coils as claimed in
claim 1, wherein the highest percent reduction (within allowable loading limits of the
mill) is at Stand F1 and gradually come down to the last stand.

7. An improved method for improvement of shape of hot rolled coils as claimed in
claim 1, wherein at stand F6 gives a skin passing effect on the strip surface by
restricting the mill load to 16.6%.
8. An improved method for improvement of shape of hot rolled coils as claimed in
claim 1, wherein the step of modifying the reduction regimes at finishing stands is
carried out as per Table 1.
9. An improved method for improvement of shape of hot rolled coils as claimed in
claim 1, wherein the step of modifying the reduction schedule at Roughing Stand for
reduction of width variation is carried out as per Table 2.

The present invention relates to an improved method for improvement of shape of hot
rolled coils in hot strip mill comprising the steps of fixing of cycle over tonnage for
finishing stand rolls at ≤ 2000t to improve strip profile and reduce wedge; modifying the
reduction regimes at Finishing Stands for reduction of gauge variation; modifying the
reduction schedule at Roughing Stand for reduction of width variation.