4 Steels: Metallurgy and ApplicationsCold-rolled gauges are usually in the range 0.4-3.0 mm and generally involve
the use of 50-80% cold reduction, though tinplate mills provide up to 90%
cold reduction to give thinner gauges. Clearly the thinner cold-rolled gauges are
usually rolled from the thinner hot band gauges in order to limit the amount of
work required from the cold-rolling mill. When the application requires a gauge
that is common to both hot- and cold-rolled gauges, a cold-rolled and annealed
material would only be used if a good surface and a high degree of formability
were required. Otherwise, a hot-rolled material would be used for cost reasons.
Tinplate and certain other packaging applications require gauges down to less
than 0.2 mm and are rolled from the thinner hot-rolled gauges. Certain grades of
tinplate, however, involve two stages of cold reduction.
Cold rolling causes the steel to become hard and strong and with very little
ductility. For almost all applications, therefore, an annealing treatment is required
to reduce the strength and to give the formability that is needed for the final
application. Various metallurgical changes take place during annealing including
recovery, recrystallization and grain growth, and the formation, growth or disso-
lution of precipitates or transformation products. It is the complex controlled
interaction of these changes which provides the steel with its final properties.
Both batch or continuous annealing may be used for many types of steel, but
certain steels may only be processed by continuous annealing. The traditional
method was, however, batch annealing and this method in still in common use.
With this method, several tightly wound coils are stacked on top of each other
with their axes vertical. They are enclosed by a furnace cover which contains a
protective atmosphere which is recirculated to promote heat transfer between the
cover and the steel during both heating and cooling. Traditionally, ther protec-
tive atmosphere has been HNX gas, which is nitrogen with up to 5% hydrogen,
but furnaces designed for the use of 100% hydrogen were introduced during
the 1980s and now provide an appreciable proportion of the batch-annealing
capacity world-wide. Rapidly recirculated hydrogen has a higher heat transfer
coefficient than the HNX gas and this enables faster heating and cooling rates
to be achieved. The complete cycle may still, however, take up to several
days. Hydrogen annealing gives improved control over mechanical properties
and improved surface cleanliness. I
Continuous annealing for formable strip was first developed in Japan during
the late 1960s and early 1970s, though continuous annealing for higher-strength
steels for tinplate had been commonly used world-wide since the 1960s. The
original cooling method for continuous annealing was by recirculated HNX gas,
but other methods have now been developed. These include gas jet cooling with
HNX gas, cold-water quenching, hot-water quenching (called HOWAQ), roll
cooling, gas-assisted roll cooling, water-mist cooling and gas-jet cooling with
high concentrations of hydrogen in nitrogen. A review of heating and cooling
methods for continuous annealing has been given by Imose 2 and the importance
of fast cooling rates for continuous annealing will become clear later. Continuous
annealing cycles usually last for a few minutes depending on the type of steel
being processed. A typical continuous annealing cycle, consisting of heating,
holding, slow cooling, rapid cooling, very slow cooling or holding, slow cooling