Low-carbon strip steels 3all strip steel was cast into ingots about 500 mm thick. After cooling and removal
from the moulds the ingots were reheated to about 1250"C and rolled to slabs
about 200-250 mm thick and allowed to cool. The surface was then scarfed to
remove surface defects.
With the development of continuous casting, mainly in the 1970s, the ingot-
rolling process was eliminated and the slabs were cast directly to a thickness of
200-250 mm as a continuous process and allowed to cool. The first hot-rolling
stage then consisted of reheating to temperatures up to about 12500C and rolling
in two linked stages. The first stage, called roughing, reduced the thickness to an
intermediate gauge, usually in the range 30-45 mm, and the second stage, called
finishing, reduced the thickness to the final hot-rolled gauge required, often in
the range 1-2 mm up to 5-12 mm, depending on details of the mill employed.
The roughing section may consist of a single reversing stand through which
the steel passes backwards and forwards, usually five or seven times, or it may
consist of several non-reversing stands through which the steel passes once. It
may, however, consist of a combination of both reversing and non-reversing
stands. Whichever combination is used, the steel completely leaves one stand
before it enters the next one. Critical matching of rolling speeds is not, therefore,
required.
A finishing train usually contains seven stands which are positioned close
together. The front end of the strip exits the last stand well before the back end
of the strip enters the first stand. Exact matching of the speeds of each stand
is required, therefore, depending on the reductions in gauge in each stand. The
steel usually exits the last stand at a finishing temperature up to and above 900~
depending on grade, which ensures that all the deformation takes place in the
single-phase austenite region of the phase diagram. It is then cooled with water
on a run-out table before coiling at a coiling temperature that is usually close to
600~ but almost always in the range 200-7500C, depending on the metallur-
gical needs of the grade. Not all hot mills would, however, have the capability
of covering this entire range. The finishing temperature may be controlled by
using a suitable slab reheat temperature, usually in the range 1100-12500C, by
adjusting the speed of rolling through the mill and, if necessary, by introducing
delays between roughing and finishing to increase temperature loss or by inter-
stand cooling using water sprays. The temperature drop between roughing and
finishing may, however, on certain mills be reduced by the use of radiation shields
or by forming the steel into a coil to reduce the surface area for heat loss.
Some of the hot-rolled coil, coveting the complete gauge range, is sold for
direct use and some, usually in the gauge range up to about 5 mm, is cold rolled
to thinner gauges.
An oxide-free surface is required for cold reduction and this is achieved by
passing the steel through a pickle line. Traditionally, the acid used was sulphuric
acid, but most pickle lines now use hydrochloric acid.
The cold rolling is usually carried out using a tandem mill containing five
stands, with controlled front and back tension. Each stand usually contains four
rolls consisting of two work rolls and two back-up rolls, but six high stands,
each containing four back-up rolls, may be used in one or more positions if a
particularly high cold reduction is required.