MODIFIED ROLL COOLING SYSTEM FOR ENHANCED ROLL LIFE AT HOT
STRIP MILL
FIELD OF INVENTION
The present invention pertains to a modified roll cooling system with improved
cooling efficiency. More particularly the present invention relates to a nozzle
arrangement for use in directing a flow of cooling fluid upon a heated sheet of
metal in a rolling mill.
BACKGROUND OF THE INVENTION
Rolling mills are commonly used for producing elongated metal sheets for
various applications. In general, such rolling mills receive a slab of metal which is
heated and is caused to pass between at least one pair of rollers of a roller
assembly in order to thin and lengthen the metal slab. In certain known types of
rolling mills, the slab is serially passed through various roller assemblies in a
heated state. Each of these roller assemblies have associated spacings through
which the metal slab passes which are progressively made smaller such that the
slab is continually thinned as it passes through the rolling mill until an elongated
metal sheet is produced. The elongated metal sheet can then be wrapped by
means of a coiler for various uses:
A typical apparatus for water-cooling the work piece in a mill making wire or rod,
hereinafter termed a wire mill, has a plurality of cooling units that are arranged in
a row in the travel direction of the wire and that each comprise a pair of support
shells that can interlock with one another and that hold guides for the wire and
feed channels for the cooling medium, normally water.

An apparatus of this type as described in U.S. Pat. No. 5,257,511 comprises a
series of semicircular bodies, positioned sequentially and constituting generally
semicylindrical half-shells that enclose a guide tube for the wire rod and feed
channels for the cooling medium. These apparatuses are very complex in terms
of construction, since the half-shells are held together by clamps positioned on
the outside and each require a coupling to the water infeed lines. This
construction makes it quite difficult to maintain and exchange components.
Rolls in hot rolling mills are required to sustain the arduous rolling conditions and
therefore their performance does not only depend on the materials they are
made of but also on the mill service conditions. It is expected from the rolls that
they are able to sustain an extended service campaign without excessive wear
and other damages or breakages giving increased overall roll life and decreased
production delays. In a hot rolling mill, the efficiency of roll cooling is one of the
main factors on which the quality of the rolled products and the productivity of the
mill depend. Improper or insufficient cooling of work rolls can not only lead to roll
breakage due to excessive differential expansions but can also significantly affect
the shape or crown of work roll and result in non-uniform shape of the rolled
product. Another critical aspect of inadequate roll cooling is that, it causes
detonation of the roll surface through wear. Effective roll cooling in a high
productivity rolling mill maintains roll temperature within limits, retain the
mechanical properties and wear resistance of roll and minimizes or controls
thermal expansion of the roll which, in turn, will help achieve product dimensional
tolerances and increased roll campaign life through precise and metered cooling
of the rolls for attaining flatness and dimensional accuracy of the product.
Many hot rolling mills producing flat products, have not given due consideration
to the importance of roll cooling, which is an essential and indeed vital aspect of
the rolling process. The efficient design of roll coolant application system can
add to the success of mill by allowing routine maintenance without sacrifice in
mill productivity and can reveal substantial operational and financial savings.

The HSM of BSL commissioned in 1970's with a capacity of 3.363 MT of
producing different grades of steel strips, catering mostly to the domestic market
of cold reducers. Different thicknesses ranging from 1.8 to 16.0mm with the width
range 900 to 1850 mm are being processed in the mill. In the recent past, the mill
capacity was enhanced to 3.99 MT, without any technological upgradation on roll
cooling system in the Roughing zone.
The previous roll coolant application system as shown in Fig 1 and 2 was
inadequate with respect to increased productivity. The previous spray headers
was having two rows of spray hole(6mm0) and each row was catering cooling of
work roll(WR) and back-up roll(BUR). This led to non-uniform distribution of flow
onto roll surface, over-flooding of rolls, low impinging coolant spray on rolls which
resulted in higher roll thermal crown, excessive generation of fire-cracks in roll
surface, higher grinding off-take and premature roll thermal failure. In order to
enhance cooling efficiency, a new modified roll coolant application system was
designed and commissioned to achieve effective heat transfer coefficient (HTC)
from work rolls to control thermal fatigue, reducing specific roll consumption,
higher roll campaign life and to produce improved quality of hot rolled strips from
HSM route.
SUMMARY OF THE INVENTION
Therefore such as herein described there is provided a modified roll cooling
system with improved cooling comprising of at least one cooling unit projecting a
plurality of cooling jets under pressure over the said roll; said cooling unit
comprise of a variable coolant flow distribution along the barrel length to cater the
uniform roll thermal crown.
As per one of the object of the present invention the coolant flow distribution is
made stand-wise and further to ensure aligned spray pattern flat jet spray nozzle
are introduced.

Another object of the present invention is to provide intense cooling of work roll
with specially designed double row spray concept
Yet another object of the present invention is to provide identification of spray
headers location to ensure higher heat removal rate
As per one of the exemplary embodiment of the present invention there is
provided arrangement for the overlap of spray to avoid dry spotting of rolls.
As per another embodiment of the present invention there is provided a manual
flushing arrangement in spray header.
Further object of the present invention is to provide a cooling system which is
configured for retrofitting to existing mill logistics, versatile, robustness to
withstand impact, ease of maintenance and inter-changeability and easy
maintenance and trouble free operation with low investment
Given the following detailed description, it should become apparent to the person
having ordinary skill in the art that the invention herein provides a novel
engineered tile and method permitting exploitation of significantly augmented
efficiencies while mitigating problems of the prior art
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 illustrates the roll cooling header design as prior art;
Figure 2 illustrates the spray header location as prior art;
Figure 3 illustrates the modified spray header design for stand # 1 in accordance
with the present invention;

Figure 4 illustrates the modified spray header design for stand # 2 & 3 in
accordance with the present invention;
Figure 5 illustrates the modified spray header design for stand # 4 & 5 in
accordance with the present invention;
Figure 6 illustrates the modified spray header location 1 in accordance with the
present invention;
Table 1: Modified Spray header design parameters 1 in accordance with the
present invention;
DETAILED DESCRIPTION
Roll is considered as backbone of any rolling mill. It is used as vital tool to
transfer the required deformation energy to the work-piece for producing steel in
the desired shapes and sizes. All the key performance parameters of any mill,
viz. productivity, product-quality and production-cost are highly dependent on
features of rolls being used in the mill. Rolls are required to retain their original
profile for longer campaign size of rolling to obtain higher mill productivity and
good dimensional accuracy of the rolled products. The wear rate of rolls is highly
dependent on its temperature. The stress generated due to cyclic heat load
promotes generation and propagation of micro-cracks in the rolls and may cause
roll peeling, banding or even their spalling. The heat picked up by the roll body
while in contact with heated materials during hot rolling need to be extracted by
cooling with adequate amount of water distributed judiciously over the roll body at
the required pressure. The quality of hot rolled products largely depends on the
surface condition of the rolls. Therefore, the need for efficient roll cooling
becomes critical for controlling the thermal condition of roll surface and
controlling surface detonation caused by friction as well as product quality.

The Hot Strip Mill (HSM) of Bokaro Steel Limited (BSL) was commissioned in
1970's and in the recent past, the mill capacity was enhanced to 3.99 MT. No
major technological upgradation on roll cooling system had taken place in the
Roughing Zone. The previous roll coolant application system was designed with
unscientific coolant spray with numerous drill holes in the spray headers and
found to be inadequate in many respect. The previous system led to non-uniform
distribution of coolant flow, over-flooding of coolant and low impinging coolant
spray onto roll surface due to excessive openings in the spray headers. It was
resulting to higher roll thermal crown, excessive generation of fire-cracks in roll
surface, higher grinding off-take and premature roll thermal failure.
Therefore, to enhance the efficiency of roll coolant application system, a new roll
cooling system was designed and commissioned at Roughing stands of HSM,
BSL, in order to achieve higher heat transfer coefficient (HTC) from work rolls for
controlling thermal fatigue, reducing specific roll consumption, higher roll
campaign life.
New Roll cooling system:
In the new roll cooling system, the following works had been carried out for the
improvement of previous roll coolant application system at Roughing zone of
HSM:
Designing of modified coolant system for achieving higher HTC.
Variable coolant flow distribution along the roll barrel length.
Modified coolant application header configuration retrofitting to existing
mill logistics and ease of maintainability.
Achieving maximum HTC:
The important criterions for achieving maximum HTC are proper nozzle selection
for coolant spray and the spray condition.

Nozzle selection : To maintain nearly constant HTC on the impact area the flat
jet nozzles were suitable for achieving maximum HTC with relatively low coolant
pressure and had been selected for the modified roll coolant application system.
Spray condition: Basically the spray condition depends upon three basic
parameters like coolant pressure, spray angle and the spray height. By reducing
total number of openings and introduction of nozzles in the spray header in the
new design, the coolant pressure had been increased from 2.5bar 3.5bar, which
clearly indicates the improved HTC with the modified coolant system. The lesser
spray distance results to higher HTC. In the new roll coolant system, the spray-
distance is restricted to 500mm for top WR cooling header and 400mm for
bottom WR cooling header, due to space constraint in the mill fixture. In the
system, the impinging spray angle has been kept at 15-20° for top WR and
10-15° for bottom WR for more surface coverage and also spray just after the
roll-bite at the exit side.
Coolant flow distribution:
The volume of coolant used for roll cooling varies, depending on the type of mill.
As the volume and pressure of the coolant are directly related to the impact
density of the spray and in turn to the heat transfer efficiency, a minimum of 3
bar is need to achieve any thermal response to the sprays. The coolant flow
distribution was optimized in accordance with stand speed, roll force and
percentage reduction pattern through off-line roll cool model. In the new roll
cooling system, coolant flow distribution was varied along the roll barrel length
with gradual increment of flow from edge to the centre of roll to take care of roll
thermal camber.
Roll coolant header configuration:
The coolant spray must impinge as precisely and accurately as possible onto the
roll-bite for maximized effectiveness, which directly depends on the positioning of

the headers within the mill fixtures. This optimal location had taken into account
existing mill furniture, access of services coolant piping to mill stand, rubber
hoses and ease of accessibility for maintenance. An effort had been given to
complete the flow calculations on the roll coolant system. Pipe size and routing
designed to eliminate coolant pressure losses. Spray headers were designed to
accommodate these flow rates without restrictions. The spray headers were
made up of 100NB seamless MS pipe. The number of nozzles at the central
zone was higher than the number of nozzles at the edge zones. The water
supply connections were provided from both the end to the spray header, to
maintain balanced and uniform flow rate through each nozzles, as because the
inlet connections to the each zones were having equal branching pipe lines of
150NB from main delivery pipe.
The back-up roll (BUR) spray headers were fitted with lesser number of nozzles
for mild cooling of BUR and located towards the bite of BUR and WR, in order to
arrest heat flow from WR to BUR due to friction. All the spray headers were
equipped with manual flushing arrangement for clog-free operation.
Roll coolant headers: Each spray headers for WR were having double row of
nozzle, where as the spray headers for BUR were having single row of nozzle.
The row of higher number of nozzle in the work roll coolant spray header was
oriented towards the work roll-bite to prevent the heat wave penetration to the
rolls from heated deformed stock. The variable coolant flow distribution was
carried out with different nozzle spacing at each zone to control the roll thermal
camber. All these roll coolant headers were provided with hose connections and
flushing arrangement for ease of periodic maintenance and made retrofitting to
pervious coolant supply line. All the nozzles are fitted to the header with an off-
set of 20° and minimum spray overlap of 40% to 60% in order to avoid dry spot
running of work rolls due to nozzle choking.
Nozzle configuration: The spray nozzles are selected of flat jet dove-tail type.
The dove-tail type ensures perfectly aligned spray pattern onto the roll surface

and facilitates periodic cleaning of nozzles. The nozzles are fitted to the roll
coolant spray header with an off-set angle of 20° in order to avoid interference of
the adjacent spray and to maintain perfectly aligned herringbone spray pattern
for effective cooling of rolls. The spray angle of nozzle was maintained at 45° for
one row and 60° for other row of work roll spray header, and 60° for back-up
rolls to obtain more spray width coverage.
The back-up roll cooling headers are provided with 21 nos. of nozzles with equal
space interval. For top and bottom work roll coolant header one row with 29
numbers of nozzles (9 nozzles at central zone, 12 nozzles at intermediate zone
and 8 nozzles at edge zone) and other row with 21 nos. of nozzles with equal
space interval were provided on each header. All the nozzles are welded to the
spray header and kept of same flow rate for ease of maintenance and inventory
Improvement in techno-economics indices:
The modified system helped to reduce the specific roll consumption from
0.0331 kg/t to 0.0226kg/t in R1 and from 0.164 to 0.104 kg/t in R2 to R5
Grinding off-take of rolls also reduced by 30% to 40 %
Average campaign life of roll increased by 10%
Higher Benefit to Cost ratio of 25: 1
Financial impact of Rs 350 Lakhs annual saving
Salient Features of the invention:
Modified roll cooling system with improved cooling efficiency
Model based coolant flow distribution on stand-wise
Introduction of flat jet dove-tail type spray nozzle to ensure aligned spray
pattern
Intense cooling of work roll with specially designed double row spray
concept
Identification of spray headers location to ensure higher heat removal rate

Overlap of spray to avoid dry spotting of rolls,
Manual flushing arrangement in spray header
Retrofitting to existing mill logistics
Easy maintenance and trouble free operation with low investment
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 invention. The particular embodiments and applications
were chosen and described to provide the best illustration of the principles of the
invention and its practical application to thereby enable one of ordinary skill in the
art to utilize the invention in various embodiments and with various modifications
as are suited to the particular use contemplated. All such changes, modifications,
variations, and alterations should therefore be seen as being within the scope of
the present invention as determined by the appended claims when interpreted in
accordance with the breadth to which they are fairly, legally, and equitably
entitled.

We Claim:-
1. A rolling mill cooling system including a roller assembly through which a
heated piece of metal is passed in order to produce a progressively thinned and
elongated metal sheet which is subsequently cooled in a cooling zone comprising
of
at least one cooling unit projecting a plurality of cooling jets under
pressure over the said roll;,
said cooling unit comprise of a variable coolant flow distribution along the
barrel length to cater the uniform roll thermal crown.
2. A rolling mill cooling system as claimed in claim 1, wherein the cooling unit
is configured for stand-wise coolant flow rate in consideration to reduction, heat
generation due to deformation and heat conduction from hot slab.
3. A rolling mill cooling system as claimed in claim 1, wherein the cooling unit
is further configured for intense cooling of work rolls and mild cooling for back-up
rolls.
4. A rolling mill cooling system as claimed in claim 3, wherein the cooling unit
comprise of double row of spray for intense cooling of work rolls and single row
of spray for mild cooling of back-up rolls.
5. A rolling mill cooling system as claimed in claim 1, wherein the cooling unit
is further configured for increased coolant pressure and higher impinging force
through reduction in openings in spray headers.
6. A rolling mill cooling system as claimed in claim 1, wherein the spray
comprise of dove-tail type flat jet nozzles for ensuring aligned coolant spray
pattern.

7. A rolling mill cooling system as claimed in claim 1, wherein the cooling unit
is configured for at least 40 - 60% overlap of spray to avoid dry spotting of roll
due to nozzle choking.
8. (optional) A rolling mill cooling system as claimed in claim 1, wherein the
cooling unit is versatile, robustness to withstand impact, ease of maintenance,
inter-changeable and capable of retrofitting to existing mill logistics.
9. A rolling mill cooling system as claimed in claim 1, wherein the cooling unit
further comprises of a manual flushing arrangement in spray header.
10. A rolling mill cooling system as claimed in claim 6, wherein all the nozzles
are fitted to the header with an off-set of 20°.
11. A rolling mill cooling system as claimed in claim 6, wherein the spray
angle of nozzle is maintained at 45° for one row and 60° for other row of work
roll spray header, and 60° for back-up rolls to obtain more spray width coverage.
12. A rolling mill cooling system as claimed in claim 1, wherein the coolant
pressure is increased from 2.5bar to 3.5bar.
13. A rolling mill cooling system as claimed in claim 1, wherein the spray
distance is restricted to 500mm for top WR cooling header and 400mm for
bottom WR cooling header, due to space constraint in the mill fixture.
14. A rolling mill cooling system as claimed in claim 6, wherein the impinging
spray angle is kept at preferably 15-20° for top WR and 10-15° for bottom WR
for more surface coverage and also spray just after the roll-bite at the exit side.

15. A rolling mill cooling system as claimed in claim 6, wherein the back-up
roil cooling headers are provided with 21 nos. of nozzles with equal space
interval and further for top and bottom work roll coolant header one row with 29
numbers of nozzles (9 nozzles at central zone, 12 nozzles at intermediate zone
and 8 nozzles at edge zone) and other row with 21 nos. of nozzles with equal
space interval were provided on each header.
16. A rolling mill cooling system as claimed in claim 6, wherein all the nozzles
are welded to the spray header and kept of same flow rate for ease of
maintenance and inventory.

The present invention relates to a rolling mill cooling system including a roller
assembly through which a heated piece of metal is passed in order to produce a
progressively thinned and elongated metal sheet which is subsequently cooled in
a cooling zone comprising of at least one cooling unit projecting a plurality of
cooling jets under pressure over the said roll; said cooling unit comprise of a
variable coolant flow distribution along the barrel length to cater the uniform roll
thermal crown.