Tin Free Steel
Tin Free Steel
Tin free steel (TFS) is an electrolytic chromium plated steel consisting of a thin layer of chromium and a layer of chromium oxide deposited on a cold rolled sheet steel base (black plate) which gives it a beautiful, lustrous metallic finish on both sides. It is also known as electrolytic chromium coated steel (ECCS). Chromium coating has primarily been developed as an economical and high quality alternative to the tinplate. TFS excels tinplate in paintability, paint adhesion, and economy. Steel producers introduced TFS in 1965.
TFS offers outstanding corrosion resistance, lacquer adhesion, and printability. The layer of chromium protects TFS against corrosion. It is an economical and high quality replacement for tinplate. Its appearance is less attractive (lower brightness) than tinplate. The absence of tin makes TFS non appropriate for acidic food (pH less than 4) because of lower corrosion resistance. TFS is not used without any biological protection for food products. This protection is frequently given through lacquer. TFS is excellent for lacquer adhesion and is always required to be lacquered on both the surfaces before use.
The coating of TFS consists of three components, excluding the steel base which represents the bulk volume of the tinplate. These three components are (i) chromium metal layer, (ii) chromium oxide layer, and (iii) layer of oil film. The function of each of the layer is given below.
The role of steel substrate base is similar to that of the electrolytic tinplate. It ensures the formability of TFS during its usage and machining and provides strength to the finished products.
Metallic chromium layer has strong passivation capability and good corrosion resistance, but as it is cathodic coating, it can only provide mechanical protection for steel base. Besides, due to strong passivation capability of metallic chromium, an extremely thin layer of passivation film is easy to produce chromium oxide.
Chromium oxide layer has relatively high chemical stability, and the electrode potential of the film is more positive than that of iron. When surrounded by ordinary corrosive media, it is a cathodic coating, requiring coating to protect the contents. As an amorphous layer, the layer can close pores of the metallic chromium layer and provide excellent coating adhesion. The combined thickness of the metallic chromium and chromium oxide layers is around 0.02 micro meters.
The oil film is applied to the strip to aid the de-piling of plates and to aid fabricability in can production. The thickness of the oil film layer is around 0.005 micro meters. The role of the oil film is similar to that of the electrolytic tinplate. Oil film is necessary to reduce friction and scratches while handling the sheets. The three most common oil types are (i) dioctyl sebacate (DOS), (ii) butyl stearate oil (BSO), and (ii) acetyl tributyl citrate (ATBC). Fig 1 shows these three layers of TFS.
Fig 1 Tin free steel coating layers
Production process for TFS
TFS is produced by an electrolytic treatment in a bath of chromic acid to deposit metallic chromium and chromium oxide on the black plate. Black plate coils are fed onto the TFS coating line, being loaded onto the two uncoilers needed to allow continuous operation. The tail end of the coil being processed is welded to the leading end of the next coil to be processed. This necessitates the two coils being stationary during welding. To avoid shut down during welding, lines are fitted with looping towers or accumulators which can hold varying amounts of uncoiled plate. Most of the TFS coating lines incorporate side trimmers after the accumulator to cut the strip to the correct width. Many of the TFS coating lines incorporate tension or stretch levellers, which apply controlled tension across the strip to remove distortions.
Chromium is deposited by electroplating in a bath of chromic acid to deposit metallic chromium and chromium oxide on the black plate. Plating is preceded by cleaning in a pickling and degreasing unit, followed by thorough washing to prepare the surface.
After coating, the plate is given a light oiling to help preserve it from attack and to assist the passage of sheets through container-forming machines without the damage of the coating layer. Finally the strips are sheared into sheets or coiled, and then packed for shipment to the can manufacturers. Fig 2 shows a flow diagram for the production of tin free steel.
Fig 2 Flow diagram for the production of tin free steel
TFS is a heterogeneous material with a stratified structure, formed by a low carbon steel sheet coated with chromium on both sides. Besides single reduced black plate, TFS is also produced with double reduced black plate, ultra thin black plate, and extra wide black plate. Double reduced TFS is produced by giving the steel a second cold reduction, of the order of 15 % to 50 %, following annealing. This operation replaces temper rolling (Fig 2). TFS is lighter in weight and is more cost effective. TFS produced with ultra thin black plate (around 0.15 mm) shows high heat tolerance and superior formability. Extra wide TFS is produced with width measurements upto 1,220 mm.
Double reduction gives TFS high levels of rigidity and strength which enables the thinner TFS to display an identical level of durability of the single reduced TFS. Double reduced product also has marked directional properties, i.e. its formability is very different in the rolling direction and transversely to it. For this reason, it is especially important to specify the rolling direction and to use the double reduced tinplate correctly.
TFS is produced with the minimum and maximum average coating weights for metallic chromium layers of 30 mg/sq m and 150 mg/sq m respectively and the minimum and maximum average coating weights for chromium oxide layers of 5 mg/sq m and 35 mg/sq m respectively. TFS is normally produced in coil form in thickness ranges of 0.15 mm to 0.60 mm and width range of 457 mm to 1,220 mm. TFS is also being supplied in sheet with length ranging from 450 mm to 1,206 mm.
Features of tin free steel
TFS has a beautiful, lustrous metallic finish on both the sides. Although the coating is very thin, it has excellent corrosion resistance. It has good reaction with paints, so that the surface is beautiful after painting. It is resistant to alkalines and neutral pH, so that it can be used without painting in certain situations. It complies with the statutory requirements needed for food packaging. It has several features. The main features are given below.
Paint adhesion – TFS has excellent paint adhesion properties which are far better than those of tinplate. This allows its use for making various painted cans or example DRD (Drawn redrawn) cans. Excellent paint adhesion makes TFS the most suitable material for making food cans, crowns, artistic cans, and normal cans with painting.
Heat resistance – TFS can undergo high temperature lacquer baking for short period with no change in its corrosion resistance. High temperature baking (around 400 deg C) causes neither discoloration nor deterioration in the coating properties in TFS steel.
Abrasion resistance – TFS is practically abrasion proof. Hence it is easier to handle and transport TFS than tinplate.
Resistance to black sulphide stain – TFS steel has excellent resistance to black sulphide stain. Due to it TFS is the most suitable material for making food cans with inside painting. Food cans made of TFS are not susceptible to sulphur staining and blackening.
Appearance – Although the same surface finish as that of the tinplate can be applied to TFS, the appearance after chromium coating is a metallic luster which is different from that of tinplate. Lacquered or printed, the surface of TFS takes on a pleasing tone not obtainable with the tinplate.
High workability after lacquering – Lacquer adhere to the TFS so tightly that the lacquered sheet can take considerable fabrication. The tough surface film has outstanding resistance to flaking and scratching. Also, because of its excellent non-aging properties, TFS retains its high after-lacquering workability for long periods.
Corrosion resistance – TFS offers superb chemical and corrosion resistance. It has very good corrosion resistance after painting. Lacquering and painting enhances these properties, protecting the steel even at the most severely fabricated points. TFS 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 is to be used as soon as possible after unpacking.
Solderability – TFS is not suitable for soldering.
Weldability – The metallic coating layer of TFS has a high electrical 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 are to be taken not to cause scratches on the surface of TFS after it is formed into cans and painted. Further, 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. As a result, lubrication is needed during cutting operations.
The TFS is specified as per the steel base, extent of tempering, annealing method, coating weights, and the surface finish. The base steel which is used for TFS 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 – This base steel is low in residual elements and has good corrosion resistance properties. This steel is widely used for general applications.
Type L – In this type the base steel has extremely low residual elements (Cu, Ni, Co, and Mo). This steel has improved corrosion resistance for certain types of food product containers.
Type D – In type D, aluminum killed base steel is used. This type of steel is used in applications involving deep drawing or other types of severe forming which tends to give rise to Lueder’s lines. This type of steel is needed for minimizing severe fluting and stretcher-strain hazards or for use in severe drawing application.
TFS is produced in the following surface finishes.
Bright – This surface finish is luster bright. In this type of finish the surface roughness (Ra) is aimed at 0.25 micro meters. Bright finishes are normally used for general use.
Stone – This surface finish is grind finish. In this type of finish the surface roughness (Ra) is aimed at 0.4 micro meters. This type of finish makes printing and can making scratches less conspicuous.
Super stone – This surface finish is rough grind finish. In this type of finish the surface roughness (Ra) is aimed at 0.6 micro meters.
Matte – This surface finish is dull finish. In this type of finish the surface roughness (Ra) is aimed at 1 micro meter. This finish is mainly used for making crowns and DI (drawn and ironed) cans.
Silver 1 – Silver finish is also called satin finish. This surface finish is dull finish. In this type of finish the surface roughness (Ra) is aimed at 1 micro meters. This finish is mainly used for making artistic canisters.
Silver 2 – This surface finish is rough and dull finish. In this type of finish the surface roughness (Ra) is aimed at 2.5 micro meters. This finish is mainly used for making artistic canisters.
Silver 3 – This surface finish is rough and dull finish. In this type of finish the surface roughness (Ra) is aimed at 3 micro meters. This finish is mainly used for making artistic canisters.
Base steel strength and workability depend on steel grade, oiling method used, and particularly the degree of temper rolling. TFS is produced in the same tempers as tinplate. These tempers are given below. There are seven tempers (T1, T2, T2.5, T3, T3.5, T4, and T5) in case of single reduced base steel and six tempers (DR7.5, DR8, DR8.5, DR9, DR9M, and DR10) in case of double reduced base steel.
T1 temper grade – It has aimed hardness value of 49 (Rockwell 30T value). It is used in the production extra deep drawn cans which need high ductility.
T2 temper grade – It has aimed hardness value of 53 (Rockwell 30T value). It is used in the production printed cans and average deep drawn cans which need ductility and surface hardness.
T2.5 temper grade – It has aimed hardness value of 55 (Rockwell 30T value). It is used in normal applications such as cans, bodies, and caps which need a fair degree of ductility.
T3 temper grade – It has aimed hardness value of 57 (Rockwell 30T value). It is used in normal applications such as cans, bodies, and caps which need a fair degree of hardness.
T3.5 temper grade – It has aimed hardness value of 59 (Rockwell 30T value). It is used in the production of general cans which need a fair degree of hardness and high degree of toughness.
T4 temper grade – It has aimed hardness value of 61 (Rockwell 30T value). It is used in the production of general cans which need comparatively a high degree of toughness.
T5 temper grade – It has aimed hardness value of 65 (Rockwell 30T value). It is used in the production of large cans which need high buckling resistance. It is also used in the production of caps and bodies of pressure vessels.
DR7.5 temper grade – It has aimed hardness value of 71 (Rockwell 30T value). It is used in the applications needing stiffness and strength such as production of large bodies for beer cans, carbonated beverage cans, and DRD cans etc.
DR8 temper grade – It has aimed hardness value of 72 (Rockwell 30T value). It is used in the applications needing stiffness and strength such as in the production of large bodies for beer cans, carbonated beverage cans, and DRD cans etc.
DR8.5 temper grade – It has aimed hardness value of 73 (Rockwell 30T value). It is used in the applications needing stiffness and strength such as in the production of large bodies for beer cans, carbonated beverage cans, and DRD cans etc
DR9 temper grade – It has aimed hardness value of 75 (Rockwell 30T value). It is used in the applications needing stiffness and strength such as in the production of tops and bottoms for beer cans, and carbonated beverage cans, and bodies DRD cans etc.
DR9M temper grade – It has aimed hardness value of 76 (Rockwell 30T value). It has similar application as DR 9 temper grade.
DR10 temper grade – It has a hardness value of 79 (Rockwell 30T value). It is used in special application which needs strength such as in the production of tops and bottoms for beer cans, and carbonated beverage cans, and bodies DRD cans etc.
Important aspects of TFS
TFS has many distinctive properties and provides a number of advantages, especially if it is used as printed or lacquered. There are some important aspects which are associated with TFS. The care needed for the use of TFS for best results is described below.
TFS sheets are to be carefully handled so that they are not scratched and not stained with finger-prints, sweat, or foreign matter. TFs sheets can undergo press drawing, seaming and other similar processes with satisfactory results, if they are applied in the proper manner with consideration given to the thickness of the sheet.
Oil and grease (for example lubricants used during drawing operations) are to be removed. This can be carried out easily with different types of organic solvents, emulsion cleaners, or light alkaline cleaners. Strong alkaline solvents and electrolytic cleaning are to be avoided.
The outside surface of TFS is required to be covered with lacquer. The inside surface is also to be covered with lacquer except for use as oil cans. TFS can be lacquered or printed with very good results by any of the conventional methods such as roller coating, spray coating, or dip coating. Coating materials suitable for TFS include baking or heat hardening lacquers and printing inks which are normally used for tinplate. It is important not to scratch the lacquered surface of the TFS during the can making.
TFS lends itself excellently to joining either with adhesives or by welding. The methods used are (i) bonding with organic adhesives such as seam cement, and (ii) bonding with seam welding or spot welding. The soldering of the TFS is very difficult. Further, for applying welding to TFS, the metallic and oxide chromium of both the sides is to be cleaned off because of its high resistance.
For the storage, TFS is to be kept is to be kept in a clean and dry place after it’s unpacking. TFS is not to be stored outside.
TFS is used mainly for those applications where it is not required as a sacrificial element. It is widely used for making beverage can, all purpose cans such as 18 litre cans and pail cans, etc., DRD cans, candy cans, spray cans, and various other containers. It is used (i) for bottle crown caps, and metal caps, (ii) for making photographic film cases, (iii) as a protective material for optical fiber cables, (iv) as printed circuit boards, (v) components of information communication apparatus, (vi) for home electronic product exterior part, and (vii) electrical and electronic components.