1 Low-carbon strip steels
Overview
Sheet iron was first rolled during the seventeeth century. It was hot rolled by
hand as separate sheets and coated with tin to form tinplate. Cold rolling was
later introduced initially to give an improved surface, but cold-rolled and annealed
steel strip is now used for a number of reasons. It may be produced with good
shape and flatness and with close control of gauge and width, and it may be made
with a clean surface with a roughness that makes it very suitable for painting.
It has been found that formed components made of sheet steel may be easily
welded to form complete structures such as motor cars. These structures may
have a high degree of rigidity, partly as a result of good design, and partly also
as a result of the high elastic modulus of the steel itself. Sheet steel may be
coated with other metals, including zinc and aluminium, separately or together,
to provide enhanced corrosion and oxidation resistance. Alloy coatings have
also been developed to give enhanced properties. It is the ability of sheet steel,
however, to be economically and satisfactorily formed into a wide range of
complicated shapes without splitting, necking or wrinkling, as well as all its
other advantages such as low cost, which is ensuring its continued use as a
major engineering material.
The low cost of steel arises partly as a result of the nature of the extraction
process and the abundance and cheapness of the raw materials, and also as a
result of the continued development of the steelmaking process itself. The first
continuous mill to produce steel strip in coil form was commissioned in the
United States in 1923 and this was clearly cheaper than rolling individual sheets
by hand. The first mill outside the United States was commissioned at Ebbw
Vale in 1938. The introduction of oxygen steelmaking mainly in the 1960s also
enabled the refinement of impure iron into steel to be achieved much more rapidly
and cheaply.
The introduction of the continuous casting of slabs enabled the stage of hot-
rolling ingots to slabs to be eliminated, and this combined with vacuum degassing
enabled new and more consistent steel chemistries to be obtained. Close control
of the continuous casting process itself has led to a reduction in the number
of inclusions by several orders of magnitude. The result has been that steel
may be used for many, very thin, tinplate applications that could not have been
considered for ingot route-processed steel available previously. Other changes
that have reduced costs have been the linking of pickle lines with tandem cold
mills and the linking of cold mills to continuous annealing lines, both of which
eliminate the between-process handling costs.
A further means of reducing costs has been the introduction of thin slab casting,
which in one of its forms enables the roughing stage to be eliminated. Finally,
the development of strip casting will enable the finish hot-rolling sequence to be