Steels_ Metallurgy and Applications, Third Edition

286 Steels: Metallurgy and Applications

rolling conditions (reheat 1200"C, finish 1150~ the vanadium addition produces

the better impact properties (lower FAIT) and the beneficial effect appears to

increase slightly with increasing strength. Controlled rolling (reheat l l00~

finish 800"C) produces a major improvement in toughness in both types of steel

but the niobium steel displays the better combination of properties, maintaining an

FATI' of about - 60~ over the range of tensile strength from 625 to 850 N/mm 2.

The better performance of the niobium steel under controlled rolling conditions

is probably related to the fact that niobium has a greater effect than vanadium

in suppressing austenite recrystallization during hot rolling which leads to the

production of a finer grain size. However, in situations where controlled rolling

cannot be employed, then vanadium steels offer the more attractive properties.

High-carbon wire rod

Although tensile strengths >2000 N/mm 2 are normally associated with lightly

tempered martensites, or with maraging grades, these strength levels can also

be achieved very readily in wire products by cold-drawing rods with a fine

pearlitic microstructure. The demand for such products in the UK amounts to

about 230000 tonnes/annum and some of the more important applications are

listed below:

9 Wire ropes

9 Prestressed concrete wire

9 Tyre cord reinforcement

9 Bridge suspension cables

9 High-pressure hose reinforcement

9 Helical springs (bedding and seating)

9 Core wire for electrical conductor cables

9 Piano strings

Rod rolling and conditioning

All wire is produced from hot-rolled rod and in the context of high-carbon rod
this involves steels with carbon contents in the range 0.5-0.9%. Depending upon
the carbon content, this results in the formation of a mixed ferrite-pearlite or
a completely pearlitic microstructure. However, natural cooling in air from a
high finishing temperature results in the generation of coarse pearlite which is
unsuitable for severe cold-drawing operations. Traditionally, high-carbon rod is
reheated to a temperature just above Ac3, in order to reaustenitize the mate-
rial, and then quenched into a lead bath at 450-500"C. The steel is therefore
allowed to transform isothermally at a relatively low temperature to form a fine
lamellar pearlite. Such a treatment is termed patenting and develops a high-
strength structure which is also capable of extensive cold drawing. Patenting is
still employed to a limited extent but most high-carbon steels are now drawn
directly from as-rolled rod which has been subjected to an in-line cooling opera-
tion at the end of rod rolling. This produces a microstructure which is similar to
that developed by the costly patenting process, but, on average, controlled cooled