Steels_ Metallurgy and Applications, Third Edition

96 Steels: Metallurgy and Applications

Tinplate

Tinplate enjoys a pre-eminent position in the packaging industry, particularly in

relation to cans for food and beverages. Substantial amounts of tinplate are also

used in aerosol containers and for the packaging of paints and oils. This section

reviews briefly the manufacturing process for tinplate and the basic procedures

employed in canmaking. These specific aspects, together with other important

technical details relating to tinplate, are covered in a very definitive and compre-

hensive publication by Morgan. 15~

Method of manufacture

Virtually all tin mill products in the UK are produced from continuously cast

steel to the chemical composition shown in Table 1.11. Where special corrosion

resistance is required, a steel with 0.015% P max. and 0.06% Cu max. (type L)

is supplied.

The starting point in the manufacture of tinplate is hot-rolled strip with a typical

thickness of about 2 mm. This is pickled to remove the scale formed during hot

rolling and the material is then cold rolled to the required thickness in either a

single-stage or a two-stage operation. For single-reduced tinplate, the cold-rolling

reduction is of the order of 90% and the material is available in the thickness

range 0.16-0.6 mm. After cold rolling, the strip is softened by either batch

or continuous annealing in order to restore ductility, but the annealed material

is then given a light cold reduction, termed temper rolling, before the tinning

operation. Temper rolling improves the surface finish and flatness of the strip

and also provides the required mechanical properties for particular applications.

Thus single-reduced tinplate is available in a range of tempers with different

strengths and this is illustrated in Table 1.12.

In double-reduced tinplate, cold-rolled and annealed strip is subjected to a

second cold reduction of 30-40%. No further annealing is undertaken and the

material is substantially work hardened, exhibiting a marked directionality in

properties. The bulk of double-reduced tinplate is produced to the thickness range

0.16-0.18 mm but is available in the range 0.13-0.27 mm. Typical mechanical

properties are shown in Table 1.13. By virtue of its higher strength, double-

reduced tinplate provides the facility to reduce the cost of a can or other compo-

nents by decreasing the thickness of the material without loss of rigidity.

Following the cold-rolling sequences described above, the strip is coated with

tin. A small amount of material is still coated by hot dip tinning, similar to

that described earlier for galvanizing, but the Ferrostan process involving the

Table 1.11 Tinplate composition

Weight % max. (Type MR)

C Mn P S Si Cu Ni Cr Mo Al

0.13 0.6 0.02 0.05 0.03 0.2 0.15 0.1 0.05 0.02-0.10