58 Steels: Metallurgy and Applicationsplasticity. The steels contain a ferrite matrix with islands of separate phases that
usually contain bainite and metastable-retained austenite. Under the action of a
forming strain, the austenite transforms to martensite and imparts a high work
hardening coefficient and hence elongation to the steel.
Other steels may contain a substantially complete bainitic or martensitic
microstructure to give very high strength but these steels have low ductility.
A steel with a completely bainitic structure has, however, been used for certain
strapping applications for many years. A small quantity of steel is, however,
supplied in the relatively soft condition for forming and is then heat treated
to give a substantially martensitic structure and a tensile strength up to about
1600 N/mm 2. Further details concerning all these steels are given in the remainder
of this section.
It is useful to note that most higher-strength steels have yield stresses and
tensile strengths that are noticeably higher, by 10 or 20 N/mm 2 or more, in the
transverse direction than in the rolling direction. It is important, therefore, that
the direction of testing should be specified if a minimum strength requirement is
to be satisfied.
Micro-alloyed, high-strength, low-alloy (HSLA) Steel
Micro-alloyed steels are essentially low-carbon manganese steels alloyed with
additions of the strong carbide- or nitride-forming elements niobium, titanium or
vanadium, separately or together and are often known as HSLA steels. In the
hot-rolled condition, they usually have values of yield stress in the range from
300 up to 500 or 600 N/ram 2, but the greater tonnage tends to lie towards the
middle of this range. The upper limit of the potential yield stress range is usually
lower for a cold-rolled and annealed product, depending on the processing given.
The alloying elements have widely differing effects 97 due to the different solu-
bilities of their carbides and nitrides in both austenite and ferrite, and due to their
different precipitation kinetics. They increase strength by grain refinement and
precipitation effects when sufficient carbon (and nitrogen for vanadium steels)
is present in the steel, but the grain refinement itself may arise from several
mechanisms.
The addition of alloying elements may restrict austenite grain growth at the
slab-soaking stage through the presence of undissolved particles such as niobium
carbonitride or titanium nitride, as illustrated in Figure 1.62. It is seen that
niobium is more effective in restricting grain growth than vanadium, but there
can be an even more marked effect of titanium due to the formation of titanium
nitride.
A proportion of any precipitates present is also taken into solution during
slab reheating and this can lead to strain-induced reprecipitation on cooling
during hot deformation. This, in turn, as mentioned previously, retards austenite
recrystanization. When the final rolling is at a sufficiently low temperature,
called the recrystallization stop temperature, the inhibition of recrystallization
is complete. Subsequent transformation to ferrite then occurs from an unrecrys-
tallized austenite which in turn leads to a very fine-grained ferrite. Figure 1.63
illustrates for steels with one base composition how the recrystallization stop
temperature varies with increasing amounts of the several alloy additions. It is