138 Steels: Metallurgy and Applicationsthe production of structural steels, namely controlled rolling. In essence, this
enabled fine-grained steels to be produced in the as-rolled condition, thereby
eliminating the need for costly normalizing heat treatments. More importantly,
controlled rolling led to the generation of steels with properties far superior to
those that could be obtained in the normalized condition.
In the 1970s and 1980s, controlled rolling was augmented with controlled
cooling and the combination is now referred to as thermomechanical processing.
In its more severe form (direct quenching), controlled cooling is now used as
an alternative to reheat quenching for the production of quenched and tempered
grades.
Structural steels have therefore undergone very significant changes, each
change producing a substantial improvement in an important property such as
strength, toughness or weldability. Aspects such as improved cleanness and
inclusion shape control have also been adopted, leading to improvements in
fabrication and service performance. These factors, coupled with very favourable
cost comparisons, have meant that structural steels have remained virtually
unchallenged by competitive materials, other than reinforced concrete, in most
of their traditional applications.
Underlying metallurgical principles
Structural steels, in the form of plates and sections, are produced in the hot-rolled
condition and, in many respects, the underlying metallurgy is essentially similar to
that involved in the strip grades. However, structural steels are generally used in
thicker sizes and higher strengths, and toughness rather than ductility/formability
is the more important requirement. The higher levels of elements such as carbon
and manganese in structural steels also impose more detailed consideration of the
control of microstructure and maintenance of properties in the welded condition.
A vast amount of research has been carried out on factors affecting the strength
and toughness of structural steels, particularly those involving ferrite-pearlite
microstructures which account for the bulk production of these steels. However,
the most important issue is the control of ferdte grain size since refinement of
the ferrite grains leads to an increase in both yield strength and toughness. This
effect contrasts sharply with other strengthening mechanisms, such as solid solu-
tion strengthening and precipitation strengthening, which are accompanied by a
reduction in toughness. In conventional hot rolling, structural steels would be
reheated to a temperature of around 1250~ and rolling would be completed at
temperatures of the order of 1000*C. This will result in a fully recrystaUized,
coarse austenitic structure which transforms to a coarse ferrite-pearlite structure
on cooling to ambient temperature. In turn, this will yield material with a low
level of toughness and the additional process of normalizing is required to refine
the microstructure and improve the impact strength. This involves reheating the
hot-rolled product to a temperature of about 910"C and, during the heating cycle,
A1N is precipitated from solid solution in the ferrite which restricts the growth
of the austenite grains at the normalizing temperature and leads to the formation