Low-carbon strip steels 71the steel with an Nb/C ratio of 0.7 increases its bake-hardening effect up to an
annealing temperature of 900"C. The r values of the 0.3 ratio steel are very much
less than those of the 0.7 ratio steel.
Figure 1.76(b) also shows similar relationships for titanium-bearing steels
where Ti* is the residual titanium content after the amount that is tied up with
nitrogen or sulphur is taken into account. The titanium steel, however, leads to
a lower bake hardening than the niobium steel for the same atomic ratio. In
addition, the titanium steel would require a higher annealing temperature if a
combination of good bake hardening and the highest possible r value is required.
Ultra-low-carbon, bake-hardening steels are often produced using a combined
titanium and niobium addition and, in principle, with no titanium or niobium
addition. It is found, however, that the latter steels may develop relatively large
grains in the hot band structure which lead to high levels of planar anisotropy. 12~
Recent work 65 has indicated that ultra-low-carbon steels containing titanium
and vanadium could also be made suitable for bake-hardening applications,
Further research, however, needs to be carried out on this system.
Transformation-strengthened steels
Transformation-strengthened steels are steels that develop high strength by
containing a proportion of transformation products such as martensite or bainite
and retained austenite in their microstructure. When these relatively hard phases
are distributed throughout a matrix of ferrite, favourable combinations of strength
and ductility may be obtained when the proportion of the hard phases is
up to about 20% or more. The ultra-high-strength steels, however, contain
a high proportion of bainite, martensite or retained austenite. The dual-phase
steels exhibit continuous yielding which is followed by a high work-hardening
rate. They possess, therefore, a low yield stress to tensile strength ratio and
good cold formability for their strength. Steels containing ferrite, bainite and/or
martensite, but also containing retained austenite (TRIP steels), provide even
higher formability than dual-phase steels, as illustrated in Figure 1.77. The
benefits of the TRIP effect were first recognized in steels containing high levels
of chromium, nickel and molybdenum, 121 but it is now well known that steels
containing manganese and silicon or aluminium are also suitable.
Both dual-phase and TRIP steels, with tensile strengths up to above 600 and
800 N/mm 2 respectively, may be manufactured in the hot-rolled condition by
controlling the cooling conditions in relation to the chemistry, but they may also
be obtained by cold rolling and annealing to give higher strength. The annealing
must, however, be continuous because the long slow cool associated with batch
annealing makes it impossible to generate the necessary microstructures with
this method. Ultra-high-strength steels with a tensile strength of up to 1000 or
1200 N/mm 2 or more may only be manufactured at present in cold-rolled gauges,
using continuous annealing.
Dual-phase steels
Williams and Davies 122 in 1963 were the first to reveal the beneficial effects of a
ferrite-martensite structure, but it was not until the 1970s that practical interest