Low-carbon structural steels 139of fine ferrite grains on air cooling to ambient temperature. However, as indi-
cated in the Overview, the costly process of normalizing has now been largely
superseded by controlled rolling, whereby lower temperatures are employed in
the finishing stages of hot rolling in order to produce a fine austenite grain size
which transforms subsequently to a fine-grained ferrite microstructure.
In general, alloy additions are not employed specifically in structural grades in
order to produce solid solution strengthening. However, solid solution strength-
ening effects will arise from the presence of carbon and nitrogen in solution
and also from silicon and manganese, which are added primarily for deoxidation
and sulphide control purposes. On the other hand, structural steels can contain
manganese contents up to 1.5% which also result in substantial strengthening
due to the depression of the austenite to ferrite transformation and the conse-
quent refinement of the ferritic grains.
As indicated later in this chapter, precipitation strengthening reactions are of
major importance in the production of high-strength structural steels, particularly
those involving the carbides or nitrides of elements such as niobium, vanadium
and titanium. In this context, these elements are called micro-alloying elements
and are taken into solution in the austenite phase during the reheating stage,
but form compounds such as Nb(CN), V4C3 and TiC on transformation to
ferrite. However, the metallurgy of these high-strength low-alloy (HSLA) steels
is complex, requiring detailed consideration of solubility/temperature effects at
the reheating stage and precipitate size/cooling rate effects on transformation to
ferrite. A further consideration is that these micro-alloying elements are also
employed to retard the recrystallization kinetics of steels which are subjected
to controlled rolling. This involves the strain-induced precipitation of Nb(CN)
or TiC in the austenitic condition at a temperature below 950~ which retards
recrystallization, producing an elongated, pancake morphology. On cooling
to ambient temperature, the deformation substructure in the austenite grains
produces a fine ferritic structure with higher strength and toughness than that
achieved by normalizing.
There is obviously a limit to the strength that can be developed in
ferrite-pearlite microstructures through grain refinement, solid solution and
precipitation strengthening and for yield strength levels greater than about
500 N/mm 2, transformation strengthening is employed. Thus the alloy content
and cooling rate must be sufficient to produce a martensitic structure on quenching
which in turn must be tempered in order to provide an adequate balance of
strength and ductility/toughness. This invokes the concepts of hardenability and
tempering resistance which are essentially those involved in Engineering steels
which are discussed in Chapter 3.
Strengthening mechanisms in structural steels
Major research effort has been devoted to the detailed understanding of factors
affecting the properties of low-carbon structural steels. Whereas considerable cost
savings accrued from the use of lighter sections in higher strength steels, there was
also the need to maintain, or indeed improve upon, other important properties