2 Low-carbo n structu ral steels
Overview
Although open to wide interpretation, the term structural steels is commonly used
to identify the predominantly C-Mn steels, with ferrite-pearlite microstructures,
which are used in large quantities in civil and chemical engineering. The steels
are produced in plates and sections, sometimes up to several inches thick, and
generally with yield strength values up to about 500 N/ram 2. However, structural
steels also include low-alloy grades which are quenched and tempered in order
to provide yield strengths up to about 700 N/mm 2.
As illustrated in this chapter, these steels are used in a wide and diverse
range of applications, including buildings, bridges, pressure vessels, ships and
off-highway vehicles. More recently, structural steels have been used extensively
in very demanding applications such as offshore oil and gas platforms and the
associated pipelines that often operate in extremely cold and chemically aggres-
sive environments.
A major feature of most forms of construction in structural steels is the high
level of welding employed and the requirement for high-integrity welds. Welding
began to replace riveting as the principal joining process in the 1940s but, at
that time, structural steels were characterized by high carbon contents and were
therefore prone to cold cracking. The requirement for lower carbon grades with
improved weldability was illustrated very dramatically in the construction of the
first all-welded merchant ships (Liberty ships) during World War II. However, the
break-up of these vessels on the high seas also led to the recognition of a further
major property requirement in structural steels, namely toughness as opposed to
ductility.
In the early 1950s, the work of Hall and Petch revolutionized the design of
structural steels with the concept that refinement of the ferrite grains led to an
increase in both the yield strength and toughness of ferrite-pearlite steels. Thus
steels with yield strength values up to about 300 N/ram 2 could be produced
in aluminium-grain-refined compositions, with good impact properties and with
good welding characteristics. Ferrite grain refinement remains the single most
important metallurgical parameter in the make-up of modem structural grades but
the demand for higher strength steels required a further strengthening mechanism,
namely precipitation strengthening. Thus small additions of niobium, vanadium
and titanium were added to structural steels to raise the yield strength up to a
level of about 500 N/mm 2. Since they were added in levels of up to only 0.15%,
these additions became known as micro-alloying elements and the compositions
were designated High-strength Low-alloy (HSLA) steels.
The late 1950s and 1960s represented a period of major research on the
structure-property relationships and fracture behaviour of structural steels.
However, it also heralded the introduction of an important new technique in