AN IMPROVED PROCESS FOR THE PRODUCTION OF SOFT DOUBLE
REDUCED COLD ROLLED STEEL STRIPS
The present invention relates to an improved process for the production and
manufacture of Soft double reduced cold rolled steel strip by the Cold Roll Mill
6H.I. More Particularly the present invention deals with different steps for
manufacture of steel-based sheet products, yet having a superior deep-forming
machineability, in a highly simplified and economical manner and also the
present invention recognizes the steps adopted for a fast and reliable DCR rolling
in a DCR mill beyond the existing capability by modifying the conventional
processes.
PRIOR ART
The present invention relates to a method and system for rolling a pickled metal
strip using a cold-rolling procedure, which is followed by degreasing and an
annealing section followed by temper mill section. It is difficult to have a product
of the hardness range between 62-67 hardness in Rockwell 30T scale with the
present capabilities of our annealing and 4HI mill.
An US Patent publication No. US5706690 discloses a cold rolling mill and
method for cold rolling. The cold rolling mill includes at least two tandem four-
high reversing mills with at least one tension reel on each side of the tandem
mills. Bridle roll units may be positioned on each side of the tandem, reversing
mills to allow the cold rolling mill to be utilized as a temper mill.
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Another Japanese Patent publication JP2001259703 discloses a method and
equipment for temper rolling by which the generation of cross buckle is prevented
even in the case that the outflow angle of a steel strip from a temper rolling mill is
hard to secure. The rolling is performed by controlling the distance between the
contact points of a work roll for the steel strip and auxiliary roll within 100 mm by
pushing up or down the auxiliary roll which is provided on the outlet side of the
work rolls of the temper rolling mill.
Yet another Japanese Patent publication JP2005230900 discloses a temper
rolling method and a temper rolling mill which can manufacture a steel strip
excellent in surface property by suppressing the occurrence of roll marks on the
surface of the steel strip by preventing the occurrence of both dint flaws and the
adherence of foreign matters onto the surfaces of work rolls. The steel strip is
temper-rolled by using the work rolls and back-up rolls respectively taking the
Shore hardness A (Hs) of the work roll and the Shore hardness B (Hs) of back-up
rolls as the ranges A >= 90, B > 75 and (A-20) < B <= (A-10).
SUMMARY OF THE INVENTION
Such as herein disclosed an improved process of existing capabilities for the
production and manufacture of double reduced cold rolled steel strip by the
temper mills due to the non availability of required input hot rolled coil of required
grade i.e. 1.6mm -1.8mm HRC (TMBP2) having T61 temper. More Particularly
the disclosure deals with different steps for manufacture of rimmed steel-base
sheet products, yet having a superior deep-forming machinability.
Therefore the improved process as herein disclosed for the production and
manufacture of soft double reduced cold rolled steel strip by the Cold Roll Mill
6H.I comprising the steps of:- procuring commercial grade steel strip; removal of
the oxide layer by pickling; cold rolling at 6HI mill; degreasing the steel strips
rolling; annealing the steel strips; secondary reduction at temper / DCR mill (4
HI); recoiling of the steel strips.
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Therefore it is an object of the present invention to provide a process by which
the desired material hardness i.e. T61 temper may be achieved precisely. In so
doing, it is also desirable to achieve the desired value for the thickness of the
metal strip at the outflow from the temper pass section of the mill. It is also an
object of the present invention to provide a process which is fast, efficient,
economical and reliable.
As per yet another object of the present invention there is provided an improved
cold rolling method of strip, wherein the foreign objects on the surface of the rolls
are effectively removed, and carried away from the mills.
Using these measures relating to precise modeling of the relationship between
hardness of the metal strip and the degree of thickness reduction of the metal
strip, the desired setpoint hardness is achieved particularly well. The method
according to the present invention is used particularly advantageously in
achieving a constant material hardness over the entire length of the metal strip,
thereby preventing scrap to a great extent.
In an advantageous embodiment of the present invention, the thickness of the
metal strip is reduced by at least, 14-16%, in particular by at least 16%, in the
temper pass section. Furthermore, it is particularly advantageous to reduce the
thickness of the metal strip by 14 - 16% in the temper pass section.
Therefore the existing process is so modified to achieve T61 temper with the
available HRC i.e. 2.0 mm and above (TMBP2) and conventional grade such as
Grade D.
These and further objects, novel features and advantages will become apparent
as the description proceeds in reference to the accompanying drawings, in
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which:
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 is a schematic view showing the different oxide layers formed on the
surface of base metal as per the present invention;
Figure 2 is a detailed schematic illustration of the pickling line as per the
embodiment of the present invention;
Figure 3 is a detailed schematic illustration of the production line as per the
embodiment of the present invention;
Figure 4 is the comparison chart showing the physical and mechanical properties
of single reduced / double reduced strips as per the embodiment of the present
invention.
DETAILED DESCRIPTION
The tinplate business starts with a steel maker (like Tata Steel) who supplies the
Hot rolled coils which is processed into Tin Mill Black Plate (TMBP) and finally
coated with tin to produce tinplate sheets by tinplate maker (Like The Tinplate
Company Of India Limited). The tinplate sheets are fabricated into the cans and
other applications by fabricators (Like Hindustan Tin Works, Kaira Cans etc.)
which are subsequently filled up by the fillers.
The Hot rolled coils are processed through the various units at TCIL through the
Cold Rolling Mill Complex (CRM) and the Electrolytic Tinning Plant (ETP). The
inventive step for the present invention starts from the selection of the Grade of
the Hot rolled coils. The Grade used is Type MR continuous cast Aluminium
killed steel.
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The Grade of the hot rolled coil procured for the purpose was TMBP - 2 coil
(from Tata Steel), a tailor made product for TCIL with the following chemistry :

Grade C Mn S P Si Al N
TMBP- 2 0.08-0.11 0.3-0.4 0.02max 0.02max 0.03max 0.021-0.05 90
Whereas, as per the present invention the process is aimed at producing tinplate
from a commercial grade like Grade D which is easily available and can be
processed to suit the needs of soft DR. The chemistry of Grade D is as follows :

Grade C Mn S P Si Al N
Grade D 0.02-0.08 0.2-0.4 0.05max 0.05max 0.08max 0.02-0.07 90
To combat with the challenges and impact, the existing capabilities of our Cold
Rolling Mill Complex is developed by modifying the normal standard operating
practice. A product which has mechanical properties suitable for the end use that
of T61 application with the existing supply of the thickness and grade i.e 2.0mm
and above (of Hot Rolled Coil(HRC) and Grade D) has been developed and
being disclosed herein with details.
The different processes adopted for the preparation of the desired steel strip are
explained below in detail.
PICKLING
In the HOT rolling process the hot slabs are rolled into coil form. During the hot
rolling process the coil (iron) surface comes in contact with air (oxygen) and thus
forms an oxide layer over the coil surface. The oxide layers are of mainly three
types depending on amount of oxygen the iron (base metal) comes in contact
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with (detail shown in figure with different oxide layers). These oxide layers also
act as coating on the coil surface, which stops further oxidation. That is the
reason why the hot rolled coils can be stored for a longer time.
Before any further rolling thereby reducing thickness it is essential to remove the
oxide layer first. The process called Pickling does the oxide removal. In case we
don't remove the oxide layer before cold rolling, then while rolling the oxide
layers (scales) will separate out and will generate surface imperfection. The
rolled coils thickness will not be uniform and during rolling it may also damage
the rolls of the cold rolling mill.
Pickling: Pickling is the process for removing oxide layers from the surface of hot
rolled coils (H.R. coils) by the reaction with the inorganic acid (HCL and H2 SO4).
In the present case the pickling is done using HCL.
Hot rolled coils have even iron oxide layers (FeO, Fe2 O3, Fe3 O4) on the surface
(Fig 1). Prior to cold rolling they are removed in pickling through pickling process.
This makes the coil surface defect free and is also required to avoid pinching /
skidding at the mill.
Pickling line has three sections, entry section, wet process section and exit
section as shown in figure 2. In Pickling process the coil is fed with the help of
uncoiler, passed through three pickling tanks containing hot HCI (which is
indirectly heated through steam heat exchanger) with concentration from 3% to
10%. Then it is washed in 5 rinse tanks with soft water and dried in air dryer
having temperature of 110 deg Celcius (min) and finally recoiled in a recoiler.
COLD ROLLING PROCESS :
Cold rolling is one of the most important methods of metal working and the
process that makes high quality of flat strip in a sheet. Cold rolling gives the
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metal all those characteristics that make it unbelievably strong for applicable
daily use. By cold rolling, we reduce the metal to the desired thickness that we
want and to get its desired metallurgical and physical properties. The strip
coming out from the cold mill will be flat.
Cold rolling is done after the Hot rolled steel and strip have passed through the
pickling process to make the steel, free from rust and other defects. In cold rolling
maximum reduction can be taken up to 95%. Since making strip is a sense of
operation a mistake anywhere along the rolling operation can cause problem to
downstream users for example if the strip is having poor shape will create
problem in rolling at temper mill and in Tin coating line. As per one of the
exemplary embodiment of the present invention it is advantageous that the
thickness of the metal strip is reduced by at least 16%, in the temper pass
section. Furthermore, it is particularly advantageous to reduce the thickness of
the metal strip by 14 to 16% in the temper pass section
Our mill is called 6Hi, wherein there are 3 rolls above and 3 rolls below the strip.
The two rolls that come in contact with the strip are called work rolls. Two rolls
having larger diameter are called backup rolls and the rolls between the work roll
and backup roll are called intermediate rolls having the variable crown across the
barrel length.
Work rolls are driven by motor through spindles and other rolls are friction driven.
Gauge detectors provided at entry and exit side of mill continuously monitor
thickness of strip. Each end of the roll that neck is fitted with chock having
bearing inside of it and these chocks are fitted in the mill housing. During the
rolling of strip, emulsion is sprayed on the rolls and strip and which act as a
coolant and lubricant.
In the conventional process since in the Temper Mill only skin pass is given,
hence maximum reduction is to be given in the Cold rolling stage. Since the
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Hardness of the Hot Rolled coil i.e. TMBP - 2 grade is much higher, hence to get
the required reduction percentage the required number of passes required is 06
which not only results in higher cost of processing but also loss in throughput.
As per the present invention, the process of production of Soft DR there is a
secondary reduction given in the Temper Mill to the level of 13 % -1(3 %, hence
the reduction at Cold rolling mill is lesser. Added to that, since Grade D is a much
softer coil , hence the desired reduction can be achieved in 05 passes only
which means lower cost as well as higher throughput.

Conventional Approach Innovative Approach
Six Hi Reduction : 88.5 % Max Reduction : 87.75 % Max.
No. Of Passes : 06 No. Of Passes : 05
During the rolling process strip gets longer not wider and this is due to frictional
constraint at the rolling bite where reduction takes place due to friction from the
roll force and tensions provided by revering reel and the metal takes the path of
least resistance, which is forward and the frictional constraint prevents the metal
from widening.
The reduction of strip takes place due to three main forces acting on the strip.
1. The vertical force exerted by the rolls.
2. The transverse force or the one across the strip due to friction at the
roll and strip contact.
3. The tension applied to the strip in the rolling direction.
From the above lines we can conclude that one of the main things is proper roll
force and tensions are required to get correct deformation i.e. reduction i.e.
thickness of the strip.
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In the reversing mill overall reduction is taken about 90% max with equal pass
reduction in each pass. We can also give increasing or decreasing reduction in
different passes. The reduction is divided into 5 or 6 passes. The calculation of
the reduction is as follows.
Per pass reduction = (Entry thickness - Exit thickness)*100/Entry Thickness.
While the taking reduction Roll force, Roll bending, mill speed, tensions and
coolants must be given high preference to get good quality. In the present case
all these systems are being controlled by automated systems and the operators
can over control these during rolling for better control of strips.
Rolling Parameters:
The main parameters which influence the cold rolling operation are Roll Force,
Tension, Coolant and Incoming strip shape and gauge, Gauge control system.
Cleaning is considered to be most important pretreatment operation in metal
finishing. The function of the cleaning operation is removal of undesired foreign
materials from the metal surface to prepare it for subsequent operation.
The electrolytic cleaning line is designed to remove the lubricant oil, rolling oil,
grease, dirt, carbon deposits etc from the surface of the cold rolled strips. Fig 5
shows the detailed cleaning process as per the embodiment of the present
invention.
The annealing cycle is divided into several phases. The following steps below
briefly illustrate the annealing process.
1. Stacking of the coils/ setting of the inner cover
2. Inner cover clamping
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3. Cold tightness test (checking H2 valves)
4. Nitrogen pre-purge of Inner cover
5. Placement of heating hood and firing
6. Heating and soaking / hot tightness test / Nitrogen-Hydrogen Exchange
7. Cooling with Heating Hood
8. Radiation cooling / Heating hood removal
9. Cooling Hood setting / Blower cooling
10. Bypass / Rapid cooling Final Purge / Cooling hood removal
11. Declamping of inner cover
12. Base discharge
The annealing process in the art which is commonly carried out in the open coil
annealing furnace, in place of the batch type furnace, for improving the
processing efficiency and for obtaining a more even distribution of specified
temperature. Even in this case, however, part of the coil may frequently be
overheated to such a degree as to roughen the crystal grains in the
corresponding area thus affected.
The input hardness for T61 is at the minimum level of 58 on the Rockwell 30 T
scale. For this the minimum set Gas temperature (Tg) in the Annealing process
is at a level 560 Degree Centigrade. The process of soaking is continued to a
minimum duration of 12 Hours and at a base temperature (Tb) of 540 Degree
Centigrade (AT i.e. Tg - Tb = 20 Degree Centigrade). The Cooling with
Heating Hood is 30 minutes. Whereas, as per the disclosed invention the input
hardness for Soft DR is at the minimum level of 54 - 57 on the Rockwell 30 T
scale. For this the minimum set Gas temperature (Tg) in the Annealing process
is at a level of 580 Degree Centigrade. The process of soaking is continued for a
minimum duration of 12 Hours and at a base temperature (Tb) of 560 Degree
Centigrade (AT i.e. Tg - Tb = 25 Degree Centigrade). The Cooling with Heating
Hood is 2 Hrs. 30 minutes.
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Conventional Approach Innovative Approach
Cycle: Cycle:
Gas Temperature : 560 Deg C Gas Temperature : 580 Deg C
Annealing Base Temperature :540 Deg C Base Temperature : 555 Deg C
Delta T : 20 Deg C Delta T : 25 Deg C
Cooling with Heating Hood ; 30 Cooling with Heating Hood : 150
mins mins
TEMPER ROLLING AT TEMPER / DCR MILL (4 HI):
In temper rolling of the strip, the finished strip is a final product. Therefore, utmost
care should be taken that there is no contamination or defect generation of the
strip surface. To evade above mentioned problem, first of all, the complete
removal of the foreign objects on the surface of the back up roll and more
preferably work roll and back up roll of the mill The foreign objects are removed
from the surface of the rolls of the mill, and carried away out of the temper mill
through the exhaust ducts.
The primary purpose of temper rolling is to suppress the yield point extension
characteristic of most low-carbon annealed steels so that material may be
subsequently formed without the formation of Luder lines (Ref stress strain
curves of low carbon steels),
However the other reasons for Temper rolling include:
1. Attainment of the required metallurgical properties (Hardness and Yield
strength).
2. Provision of the desired surface finish to the product (in terms of average
or ELA roughness values and the peak counts per inch). It impacts the desired
surface finish, like the textured finish for appliance wrappers. This transfer of the
surface finish from rolls to the strip is improved by increasing the rolling force,
using large diameter rolls.
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3. Creation of a desired degree of flatness (and shape)
4. Attainment of desired magnetic properties (low core loss and high
permeability) in material intended for use as motor lamination stock ; and
5. Break up spangles and the elimination of curtains on galvanized and other
coated products.
Temper rolling is used extensively for flattening strip and correcting shape
defects incurred during cold reduction. In this practice, however non uniform
residual stress are induced in the temper rolled product which may be
troublesome in subsequent slitting and forming operations with the reappearance
of the original shape defects. It is therefore important that the cold rolled and
annealed strip brought to the temper mill be of satisfactory flatness.
In the conventional working the mode of rolling is Dry where a skin pass is given
to the annealed coil to eliminate the yield point phenomenon with a 1.2 - 2 %
offset. The roll used in Stand # 1 is shot blasted with bright rolls in stand # 2. The
details are given in table below.
As per the present invention the mode of rolling is wet where a reduction is at the
level of 13 % -16 % . To enable the rolling process ground rolls are used in both
the stands with emulsion sprayed in stand # 1. The details of the roll force ,
schedule are given in table below.

TemperMill Conventional Approach Innovative Approach
Mode of Rolling : Dry rolling Mode of Rolling : Wet rolling
Roll DetailsStand 1 (Shot Blasted): 110 + /-3Stand 2 ( Bright Rolls) : 15 + / -3 Roll DetailsStand 1 (Bright Rolls) : 32 + / - 3Stand 2 (Bright Rolls) : 38 + / - 3
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Emulsion: No Emulsion Emulsion Concentration : 1.2 % + / -6
Roll Force :Stand 1 : 750 - 800 TonsStand 2 : 650 - 700 Tons Roll Force : (With Auto Gaugecontrol ON)Stand 1 : 550 - 600 TonsStand 2 : 650 - 700 Tons
Tension values:Entry: 1000-1300 KgInterstand : 1700 - 2100 KgExit: 893 Kg Tension values:Entry : 2300 - 2600 KgInterstand : 2200 - 2500 KgExit: 770 Kg
No Rust Inhibitor used Rust Inhibitor used : Cone. 3 %
Reduction % age : 1.2 - 2 %Offset Reduction % age : 13 -16 %
After the tempering operation, the rolls / strips are coiled and transferred for the
electrolytic coating or other processes.
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.
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We Claim :-
1. An improved process for the production and manufacture of soft double
reduced cold rolled steel strip by the Cold Roll Mill 6H.I comprising the steps of:-
procuring commercial grade steel strips;
removal of the oxide layer by pickling;
cold rolling at 6HI mill;
degreasing the steel strips rolling;
annealing the steel strips;
secondary reduction at temper / DCR mill (4 HI);
recoiling of the steel strips.
2. A process for the production and manufacture of soft double reduced cold
rolled steel strip as claimed in claim 1 wherein the steel strip comprises of
composition such as carbon - 0.02 to 0.08%, Mn - 0.2 to 0.4%, S - 0.05% Max,
P - 0.05% max, Si - 0.08% max, Al - 0.02 to 0.07% and N - 90%.
3. A process for the production and manufacture of soft double reduced cold
rolled steel strip as claimed in claim 1 wherein the process of cold rolling further
comprising the steps of :-
secondary reduction to the level of 13 % -16 % in the temper mill, thereby
a reduction of total 87.75% by cold rolling mill with 5 passes..
4. A process for the production and manufacture of soft double reduced cold
rolled steel strip as claimed in claim 1 wherein the annealing process further
comprising the steps of :-
setting the gas temperature at the level of 580 degree Celsius and the
base temperature to 555 degree Celsius;
soaking is continued to a minimum duration of 12 hours at a base
temperature of 560 degree Celsius; thereby
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Cooling with Heating Hood is 150 minutes
5. A process for the production and manufacture of soft double reduced cold
rolled steel strip as claimed in claim 1 wherein the secondary reduction at temper
/ DCR mill (4 HI) further comprising the steps of :-
wet rolling with a reduction is at the level of 13 % - 16 %,where the
ground rolls are used in both the stands with emulsion of concentration 12% +/- 6
% is sprayed in stand 1.
6. A process for the production and manufacture of soft double reduced cold
rolled steel strip as claimed in claim 5 wherein the roll force applied at stand 1 is
550 - 600 tonnes and at stand 2 with 650 - 700 tonnes.
7. A process for the production and manufacture of soft double reduced cold
rolled steel strip as claimed in claim 5 wherein the temper mill is applied with a
tension of 2300 - 2500 Kg at entry and 770 Kg at the exit.
8. A process for the production and manufacture of soft double reduced cold
rolled steel strip as claimed in any of the preceding claims wherein the steel strip
produced is of T61 temper grade and of the thickness 2.0 mm and above.
9. A soft double reduced cold rolled steel strip using the method as claimed
in any of the preceding claims.
10. An improved process for the production and manufacture of soft double
reduced cold rolled steel strip by the Cold Roll Mill 6H.I, substantially as herein
described with particular reference to the accompanying drawings.

An improved process for the production and manufacture of soft double reduced
cold rolled steel strip by the Cold Roll Mill 6H.I comprising the steps of:-
procuring commercial grade steel strips;
removal of the oxide layer by pickling;
cold rolling at 6HI mill;
degreasing the steel strips rolling;
annealing the steel strips;
secondary reduction at temper / DCR mill (4 HI);
recoiling of the steel strips.