Preproduction Steps leading to Tinplating

Step 1: Continuously cast steel

Although ingot casting can be used, the steels for tinplate use are almost exclusively made from slabs produced by continuous casting. In this process the molten steel is cast directly in slab form by casting into a water cooled copper mould. An outer solidified skin on steel is formed and this is then drawn through the caster by water cooled rolls where further solidification progresses until a solid slab is formed.
The steel prior to casting is fully killed i.e. the excess oxygen from the steelmaking process is removed by the addition of an oxide former, usually aluminium.
The advantages of continuous casting over the previously used ingot route are the freedom from segregation and the internal cleanliness of the steel. This is of great importance for two piece can manufacture.

Principal Steps in Manufacturing Process

Step 2: Hot Rolling

  • Continuous casting machines deliver slabs directly to the Hot Mill ready for subsequent hot rolling. The finished slabs may be up to 1300mm wide x 250mm thick. They may be scarfed prior to hot rolling to remove surface defects which could have a harmful effect on subsequent processing.
  • The slabs are reheated to a soak temperature of between 1200 to 1300°C . The slabs are then hot rolled firstly through a roughing section where the slab is reduced from the initial 250mm to 35 - 45mm in thickness and then rolled through the finishing section where the final gauge is achieved. Coils for tinplate usage usually have a final gauge somewhere between 1.6mm - 3.0mm.
  • The roughing section may comprise four or five four-high non-reversing stands or a single four-high reversing mill, dependent on the mill design. The finishing train may have up to seven four-high rolling stands in tandem.
  • After rolling, the strip is cooled to a controlled temperature on a run out table using banks of water sprays. The strip is then coiled ready for further processing.
  • Control of the slab heating and the roughing, finishing and coiling temperatures is critical for obtaining the required mechanical properties. In a modern continuous hot strip mill the exit speed may be up to 16m/s.
  • During cooling the steel surface develops an oxide scale and this must be removed prior to subsequent cold rolling. Removal of the oxide is carried out by an acid treatment in a pickling line. The coil is fed into the pickling line via a “scalebreaker” which mechanically flexes the strip around the rollers and loosens brittle scale. This process is made continuous by welding the coils together at the entry end to the line.
  • The pickle line proper consists of a hot water preheat tank, acid pickling tanks (normally four), primary and secondary spray rinse tanks and a hot water dip tank. The tanks are enclosed to extract fumes. The pickling acid is 20% sulphuric acid at 900C or 10% hydrochloric acid at room temperature. Sulphuric acid has been traditionally used but there is a trend towards using hydrochloric acid which is quicker acting and gives a clean white surface after pickling.
  • In order to minimise attack on the steel itself inhibitors are usually included in the pickling solutions. Drag out between tanks is usually prevented by sets of wipers and wringer rolls.
  • At the exit of the line the strip edges can be trimmed if required and oil is applied to both surfaces prior to recoiling. The oil acts as a protection against corrosion during storage and as an initial lubricant in the subsequent cold rolling operation.

Step 3: Cold Rolling

  • All modern tinplate has a cold reduced steel base and almost all is rolled in a multi-stand mill. Most operators use a five-stand, four-high tandem mill for cold rolling, although there are some six- and four-stand mills. The feedstock for the cold reduction mill is the pickled and oiled hot rolled coil and normally individual coils are rolled separately. About 90% cold reduction is imparted. For conventional tinplate, as opposed to double-reduced, the steel is rolled roughly to the final ordered thickness in this cold rolling stage.
  • Modern cold reduction mills operate at speeds up to 2400 m/min. Many factors such as load, inter-stand tension, lubrication, cooling and roll contour are closely monitored and controlled in order to produce a high yield of prime product. Most producers have installed automatic control equipment.

Step 4: Cleaning

  • For cold reduction, lubrication is necessary and lubricating oil and water are applied to the strip and rolls either separately or as an emulsion. All traces of this lubricant must be removed from the strip before annealing, since residual oil can cause staining or discoloration of the strip, preventing good tinning. Material destined for batch annealing is treated in a cleaning line in which the strip passes through dip or spray tanks, followed by rinsing and scrubbing, then onto an electrolytic treatment section. The solutions used in strip cleaning lines are usually made up from proprietary mixtures containing alkaline phosphates or silicates together with sodium hydroxide and wetting agents. Finally the strip is rinsed and dried in a blast of hot air. Fume extraction facilities are essential. In the case of steel destined for continuous strand annealing a cleaning section is incorporated in the annealing line.

Step 5: Annealing

  • The mechanical working which the steel undergoes during cold rolling has a profound effect on the mechanical properties. In cold working, the grain structure is distorted and the metal becomes harder and stronger but less ductile. In order to soften the steel and restore its ductility, recrystallisation of the grains is required. This can be induced by a controlled heat treatment cycle (process annealing) to produce the required grain structure.
  • Two types of annealing may be used in tinplate manufacture, batch (or box) annealing (BA) or continuous annealing (CA)

Step 6: Temper Rolling/Skin Passing

  • After annealing, the steel strip is in a very soft condition called dead soft. It needs a very light rolling treatment, usually in a two-stand mill operated without strip lubrication to increase its tensile strength. This happens through process known as “temper rolling” or “skin pass rolling” This process defines the final gauge and imparts mechanical properties appropriate to the end use. It also improves the strip shape and produces the desired surface finish on the strip. The overall reduction is of the order of 0.5 to 4%
  • Temper rolling also removes the pronounced yield point noted on CA products. This allows ends and bodies to be produced without “Luders” lines/ bands or fluting being visible.
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