Manufacturing Facilities -
Stages of Production
All tinplate originates as in the steel-making
furnace (Tata Steel), where the proper chemistry for steel is obtained to meet the
specific needs of the end user. All tin mill products start their production process in a
Basic Oxygen Furnace (Tata Steel).
Hot Rolled
Coil Manufacture
Once the steel is produced, the continuous casting process
transforms molten steel into slabs in one continuous operation. Continuous casting is a
highly energy- and cost-efficient process that produces a slab of excellent quality and
consistency. Continuous casting methods are now used in virtually all carbon steel
production facilities to produce products of consistent, high quality. Next, the slab goes
into the Hot Strip Mill (HSM) of Tata Steel where its thickness is reduced and it is wound
in a down coiler into coil form. This is where the Hot Rolled Coil (HRC) is procured as
the basic raw material by TCIL from Tata Steel.
CRM Process Flow
Diagram
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Pickling Process
To remove the scales adhering to the hot rolled
coils Tinplate Company uses its Push-Pull shallow bath Hydrochloric Acid Pickling Line.
The line has 3 pickling tanks and 5 rinse tanks. The Pinch Roll # 1 in the entry section
pushes the strip that is "paid off" and the Pinch Roll # 3 in the delivery
section pulls it thro the process section of the line. The line has a cascading system for
rinse water and online trimming facility.
Quality check
Incoming coils are checked for its shape and
steel composition.
Acid baths are checked for free acid levels
Rinse water is checked for chlorides
Pickled coils are checked for rolled in scale,
sliver, pickling quality and surface finish.
If all checks conform to standards of delivery to
next process the coil is cleared for processing in cold rolling mill.
6 Hi Cold Rolling Mill
Tinplate, as a packaging material, continues to
be most preferred, both in packaging food products like processed food, vanaspati, ghee,
etc., and also where criticality of application is important, as in the case of battery
jackets and pesticide cans, to name a few select areas.
Understanding the need of increased
indigenisation, TCIL commissioned its state-of-the-art 120,000 tonnes per annum Cold
Rolling Mill having the benefit of synergy with TATA STEEL, from where raw material is
available in the form of Hot Rolled Coils for the manufacture of TMBP coils.
Process
The mother mill of the complex is a 6 High,
continuous variable crown-reversing mill. The pickled strip is "paid off" from a
Pay-Off Reel and fed into the cold rolling mill. Depending on the input thickness, desired
output gauge and pass schedule designed by the mill operator, the hydraulic system sets
the draught. After the first pass that has run between the POR and Reversing Reel # 2, the
tail end is fed into the Reversing Reel # 1. The strip is run reverse and forward between
RR1 and RR2, thro the CR mill, with the thickness being reduced in each pass. The
thickness gauges at the entry and the exit side of the mill gives real time feed back thro
the automation system to the mill's hydraulic system for online correction of roll force
and hence the thickness.
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Quality checks
Shape of CR strip
Gauge tolerance or conformity
Surface finish
Surface defects.Full hard CR material from here
is routed to Coil Preparation Line for trimming or directly packed and sent to the
customer.
Full hard material
Full hard steel is made without the usual
subsequent annealing step. The resulting product has a particularly high stability. Due to
the elimination of the grain recrystallization process associated with annealing, the
material retains high internal stresses. Its deformability is therefore restricted, i.e.,
very high forces must be employed in any downstream metalforming process.Classic
applications for this material include products such as corrugated roofing sheet and some
white goods (furniture items).
Electrolytic Cleaning
Process
During the process of cold rolling cooling oil is
used to keep the temperature of the roll and the strip within limits. This cooling oil
mixed with water in an emulsion has to be removed for further processing. This is achieved
in the Electrolytic Cleaning Line. The line uses alkaline dunk and ortho-silicate
electrolytic cleaning to remove the emulsion oil coating. The strip is then scrubbed,
rinsed, dried and rewound on a reel into coils. These coils are then tilted 90o and taken
for annealing.
Quality checks
Plate out on strip to check for remnant oil.
Annealing
Process
Annealing is done to relieve the stresses that
are built into it during the process of cold forming. Cold rolling of the steel strip
makes it hard and brittle. Annealing is done using the batch annealing process.
Degreased coils are stacked four high on a base.
An annealing cover is placed over this stack and pure hydrogen is pumped in. Then the
heating hood is placed and programmed to heat the coils to predesigned temperatures. This
follows a heating - soaking - cooling cycle with cut off temperatures and times depending
on the hardness required by the customer. After the process that takes 32 - 38 hours, the
coils are discharged from the base.
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Quality checks
In the base the temperature and cycle of
heating, soaking and cooling.
Annealed hardness of the coil.
Temper Mill
Process
The mill is a four high two-stand tandem mill
with a facility to give a further reduction of upto 30%. After the process of annealing
the strip becomes soft. To put it to further use the surface has to be given the proper
finish and hardness. For this purpose the strip is passed thro the temper mill. Normally a
skin pass is given (80% of product). This is dry temper rolling; where the hardness and
shape is controlled in the first stand and the surface finish in the second stand. In the
rest (20%) of cases a post annealing cold reduction of upto 30% is given by controlling
the draught in the first stand. This process is known as Double Cold Reduction. This
process also uses a cooling emulsion for roll and strip cooling. This imparts higher
hardness and strength to the strip enabling thinner sections to be put to use in place of
thicker plates
Quality checks
Shape of coil with regard to bowness in the
strip.
Thickness.
Surface finish and defects.
Temper (hardness).
Coil Preparation Line
Process
Full hard and / or tempered coil before being tin
plated has to be trimmed to the required width as per the customer's order. For this
purpose the coil is passed thro' the CPL with edge trimming facility. The coil is
"paid off" and passed thro' the trimmer to cut off both sides to the ordered
width of the end product and then rewound. While rewinding the edge guide system thro a
continuous feedback process ensures that the two sides of the coil winding is without any
projected edges. This is done to facilitate handling of the coils. The full hard cold
rolled coils are packed and shipped out of this mill to customers.
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Quality checks
Shape of coil - edge waviness and bow
Edge burr on the strip.
Electrolytic
Tinplating Line
The unique properties of tin provide a coating on
the steel that protects the contents, while providing the steel with an attractive
appearance, corrosion resistance and ease in bonding, welding and painting. Tin is applied
to both sides of the black plate coil through an electrolytic process. The thickness of
the tin coating (called coating weight) is dependent upon the end-use application. Typical
applications for electrolytic tinplate include food and beverage containers, paint trays,
battery tops and paint, varnish and aerosol cans.
The Tinplate Company of India Limited pioneered
the Indian Industry when they put up the country's first tinning line in 1922. The growth
of TCIL has been a continuous process of development and modernization to keep Indian
Tinplate at par with international standards.
The technological advances had led to the
introduction of Electrolytic Tinplate (ETP), while the spiraling and prohibitive cost of
tin resulted in the manufacture of a suitable substitute, namely Tin Free Steel (TFS) or
Chromium coated steel. To keep pace with these developments, TCIL was the first to set up
a combination line capable of producing both ETP and TFS. The plant was commissioned in
1978 and has an installed capacity of 90,000 tonnes per annum. The line can be run upto a
speed of 247 m/min and is capable of processing coils ranging from 0.17 mm to 0.60 mm in
thickness and 600 mm to 965 mm in width.
Process flow Diagram of Electrolytic
Tinplating plants
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Process
The Electrolytic Tinplating line is the only one
of its kind in India. It has a facility to coat the line with tin or with chromium /
chromium oxide (Tin Free Steel). The ETP uses the FEROSTAN process, where in an acidic
bath of Stannous Sulphate is the electrolyte. The strip is "paid off" and passed
thro the looping tower - which stores material to enable the line to run continuously,
even while the entry is stopped for welding the tail end of one coil to the head of the
next. The strip then passes into the cleaner solution to remove any remnant traces of oil
or emulsion. Then it goes thro the pickling bath where the Sulphuric acid removes any
oxide that may have formed in the CRM complex. This acid also etches the plate, to prepare
it for the electro-deposition of tin. After the surface is fully prepared for plating it
passes into the plating section. Here the strip enters the bath with pure tin being used
as anode and the strip itself as the cathode. By passing the required direct current -
this is dependant on the coating weight required by the customer - in the bath the plate
is coated. Thereafter it passes into the melting furnace where the deposited tin is melted
to give it a brighter finish and also make it adhere to the base material better. After
this the strip is chemically treated to reduce the action of atmosphere on tin and then
coated with oil to facilitate handling.
In case the order is for tin free steel, the
strip is passed directly from the pickling bath to the chrome plating section. Here by
using chromic acid bath and inert anodes a layer each of chromium and chromium oxide is
deposited on the strip. Since this coated steel does not have any tin plating on its
surface it is called tin free steel.
The strip is then passed thro the online
inspection to segregate any sheet that falls outside the acceptable range of thickness,
sheets with pinholes in them and then cut up as per order and piled into packets of about
1500 KGs. This is then shipped out to customers in trucks or containers, the latter mainly
for overseas customers.
Quality checks
Cut length and width of sheet
Surface finish and defects3. Temper and other
physical properties of the sheet
Coating weight
Tin-free Steel/Electrolytic Chrome Coated
Steels (ECCS)
Unlike the multiplicity of coating weights with
tin, only one standardized chromium-coated product is manufactured. The chromium coating
process was developed in the 1960s as an alternative to tin coatings for packaging
products cost-effectively, with desirable material properties. Chromium offers excellent
lacquer adhesion, storage properties and strong resistance to food corrosion with proper
applications of lacquer. Chromium-coated steels are most frequently used for can tops,
screw and lug caps, snap and press-on closures and shallow-drawn food cans. Packaging
steels are coated with different protective and decorative coatings, depending on their
intended use.
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