Tin Free Steel
Tin Free Steel
Tin free steel (TFS) is an electrolytic chrome plated steel consisting of a thin layer of chromium and a layer of chromium oxide deposited on a cold rolled steel base (black plate steel) which gives it a beautiful, lustrous metallic finish on both sides. It is also known as electrolytic chromium coated steel (ECCS). TFS offers outstanding corrosion resistance, lacquer adhesion as well as printability. It is an economical and high quality replacement for tinplate. It is not suitable for soldering and can only be used for welding after edge cleaning. ECCS must be lacquered on both surfaces.
TFS has been developed to meet economic requirements, and excels tinplate in paintability, paint adhesion and economy. It is widely used for making beverage can and 18 litre cans. It is also used for making photographic film cases and as a protective material for optical fiber cables. The cross section of tin free coating on steel sheet is given in Fig 1.
Fig 1 Cross section of tin free coating on steel sheet
Features of tin free steel
The main features of tin free steel are given below:
- Paint adhesion – TFS has excellent paint adhesion properties which is far better than those of tinplate. The external surfaces of cans should be painted to prevent corrosion. The internal surfaces should also be painted to prevent corrosion except when the content is motor oil or cooking oil.
- Heat resistance – High temperature baking (around 400 deg C) causes neither discoloration nor deterioration in material properties in TFS steel.
- Resistance to black sulphide stain – TFS steel has excellent resistance to black sulphide stain. Due to it TFS is the most suitable material or making fish cans.
- Appearance – TFS steel provides the unique surface luster characteristic of metallic chromium even when the same surface finish as in the case of tinplate, is applied to the substrate steel sheet.
- Corrosion resistance – TFS steel has very good corrosion resistance after painting. It is normally used with both surfaces painted. It can also be used with the internal surface unpainted depending on the contents. TFS tends to rust in a humid atmosphere hence it should be used as soon as possible after unpacking.
- Solderability – TFS is not compatible with soldering.
- Weldability – The metallic coating layer of TFS has a high elctrical resistance. When welding TFS, it is necessary that the metallic coating layers in the welded area is removed in advance. Weldability of TFS is inferior to tinplates
- TFS does not provide sacrificial protection like tinplate. Due precautions should be taken not to cause scratches on the surface of TFS after it is formed into cans and painted.
- Since the hardness of chrome is higher than that of tin, TFS is more resistant to the cutting than electrolytic tinplate and this exposes the dies to higher wear. Consequently, lubrication is needed during cutting operations
The TFS are specified as per the steel base, extent of tempering, the coating weight, annealing method and the surface finish.
The base steel is continuously cast and aluminum killed. The base steel can be single reduced or double reduced. The base steels are of the following three types.
- Type MR – These base steels are low in residual elements and have good corrosion resistance properties. These steels are widely used in general applications
- Type L – In this type the base steel has extremely low residual elements (Cu, Ni, Co and Mo). These steels have very good corrosion resistance to certain types of food products.
- Type D – In D type aluminum killed base steel is used. These types are used in applications involving deep drawing or other types of severe forming that tend to give rise to Lueder’s lines.
TFS is produced with the minimum and maximum average coating weights for metallic chromium layers of 50 mg/Sq m and 150 mg/Sqm respectively and the minimum and maximum average coating weights for chromium oxide layers of 5 mg/Sq m and 35 mg/Sqm respectively. TFS is normally produced in coil form in thickness ranges of 0.15 mm to 0.60 mm and width range of 700 mm to 970 mm. TFS is also being supplied in sheet from 508 mm to 1050 mm.
TFS is produced in the following surface finishes.
- Bright – in this type of finish the surface roughness is aimed at o.25 Ra. Bright finishes are normally for general use.
- Stone – In this type of finish the surface roughness aimed is 0.40 Ra. This type of finish makes printing and can making scratches less conspicuous.
- Matte – In this type of finish the surface roughness aimed is 1.00 Ra. This is dull type of finish and mainly used for making crowns.
- Silver – This type of finish is also called satin finish. This is rough dull finish mainly used for making artistic cans
The temper grades of TFS are as follow. Hardness values are given in HR 30 T. When the tinplates are produced from continuous annealed material then the symbol of CA is added to the temper grade such as T-5 CA
- Single reduced
- T-1 temper grade – It has hardness value of 49 +/- 3. It is used in applications involving deep drawing that require large flexibility.
- T-2 temper grade – It has hardness value of 53 +/- 3. It is used in application that need moderate drawability and some stiffness.
- T-2.5 temper grade – It has a hardness value of 55 +/-3. It is used in application that requires the drawability of T-2 temper grade in combination with the stiffness of t-3 temper grade.
- T-3 temper grade – It has a hardness value of 57 +/-3. It is used in application that requires stiffness to prevent buckling.
- T-4 temper grade – It has a hardness value of 61 +/-3. It is used in application that requires large stiffness such as can ends and crowns etc.
- T-5 temper grade – It has a hardness value of 65 +/-3. It is used in application that requires very good buckling resistance.
- Double reduced – These are thin steel materials with high strength and produced by cold reducing the steel material after the process of cold reducing and annealing.
- DR -8 temper grade – It has a hardness value of 73 +/-3. It is used in application that requires stiffness and strength.
- DR-9 temper grade – It has a hardness value of 76 +/-3. It is also used in application that requires stiffness and strength.
- DR -9 M temper grade – It has a hardness value of 77 +/-3. It has similar application as DR 9.
- DR – 10 temper grade – It has a hardness value of 80 +/-3. It is used in special application that requires strength.
Tin free steel is used mainly for those applications where tin is not required as a sacrificial element. It has the following applications:
- Can ends
- Drawn re drawn cans
- Beverage cans, bottle crown caps, metal caps
- 18 litre cans
- Candy tin
- Spray can
- Home electronic product exterior part
- Electrical and electronic components
- Photographic film cases
- As a protective material for optical fiber cables