Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 5

Temperature Temperature

Batch annealing time (days) Continuous annealing time (minutes)

Figure 1.2 Schematic annealing cycles for batch and continuous annealing

and rapid cooling is illustrated in Figure 1.2. Clearly, as will be seen below, the
details of the cycles to be used depend on the type of steel to be processed.
It is useful to note that top batch-annealing temperatures are rarely much above
700~ whereas continuous annealing temperatures are usually above 700~ for
strip grades and often above 800~ These latter temperatures compensate for the
large differences in time between batch and continuous annealing. Most contin-
uously annealed steels for tinplate applications, however, are annealed below
700~ It is usual but not essential to produce hot dip coatings on lines which
also provide continuous annealing.
The last stage in the sequence for processing uncoated steel, prior to surface
inspection, is called temper rolling or skin passing, which usually provides a
cold reduction of 0.5-1.5%. This process is usually needed to remove the yield
point in the tensile curve to ensure that stretcher-strain markings are not formed
on pressing, but it also imprints a surface roughness that makes the steel surface
suitable for its application and may improve the steel flatness.
Annealed and temper-rolled steel may be coated with a metallic coating using
an electrolytic process on a separate coating line, but as indicated earlier, it
is usual to combine a hot dip process with continuous annealing on a single
combined line. Organic coatings may also be applied on top of either hot dip or
electrolytic metallic coatings.
It is useful to note that when initially introduced, all the rolling stages in the
processing of strip steel were intended merely to change the thickness of the steel
and hot processing was used because it was easier to reduce the thickness of a
thick material when it was hot and soft than when it was cold and hard. Each
stage is now regarded, however, as a metallurgical process with closely controlled
reductions, temperatures, cooling rates and times. The controlled processing
sequence now enables desirable metallurgical structures and hence properties
to be obtained at each stage that make the material either suitable for direct use
or for proceeding to the next stage in the process.
As mentioned previously, an important recent development in steel processing
has been the introduction of thin slab casting. Two main processes are involved,
but in each case, the casting process is linked directly to a hot mill. With the
CSP (compact strip production) process as practised at Nucor, 3 the slab is cast