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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 Cold Rolling to produce TMBP Coils Electro- tinplating

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

1.     Incoming coils are checked for its shape and steel composition.

2.     Acid baths are checked for free acid levels

3.     Rinse water is checked for chlorides

4.     Pickled coils are checked for substrate related defects such as rolled in scale, sliver and process related 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/ETR and RR2/DTR, 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

1.     Shape of CR strip

2.     Gauge tolerance or conformity

3.     Surface finish

4.     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 metal forming 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

1.     Plate out on strip to check for the cleaning efficiency.

Annealing

Process

Annealing is done to relieve the stresses that are built into it during the process of cold rolling which makes the steel strip 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

1.     In the base the temperature and cycle of heating, soaking and cooling.

2.     Annealed hardness of the coil at the Temper Mill entry.

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 secondary 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

1.     Shape of coil with regard to bowness in the strip.

2.     Thickness.

3.     Surface finish and defects.

4.     Temper (hardness) for SR/Proof stress for DR

Coil Preparation Line (CPL)

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

1.     Shape of coil - edge waviness and bow.

2.     Edge burr on the strip.

3.     Width tolerance (0 to +2 mm)

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 spiralling and prohibitive cost of tin resulted in the manufacture of a suitable substitute, namely Tin Free Steel (TFS) or Electrolytic ally Chromium coated steel(ECCS). 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 179000 tonnes per annum. The line can be run upto a speed of 300 m/min and is capable of processing coils ranging from 0.14 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

1.     Cut length and width of sheet

2.     Surface finish and defects (Tin in Tinplate sheets and Cr in TFS)

3.     Temper and other physical properties of the sheet

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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  Pickling Line (Entry)
      
  Pickling Line (Exit)
  6 HI Pulpit
  
Front View of 6 HI Mill
Full Hard Material
Degreasing Line Process Section
  
Degraesing Line (Exit side)
Annealing Furnace Area
  
Inner cover being encapsulated byheating hood for recrystalization process
Temper Mill Automation
  
Temper Mill Full View
  
Coil Preperation Line
  
CPL Exit
Electro Tinplating (Process) SectionETP/TFS Line
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