Low-carbon strip steels 61recrystallized austenite grain size and the amount of deformation given below
the austenite recrystallization temperature.
It is also useful to note, as mentioned earlier, that the final ferrite grain size
depends on the cooling rate after the steel leaves the last finishing stand due to
the influence of cooling rate on transformation temperature..One consequence of
this is that richer compositions are often needed to develop a given strength from
a relatively thick steel that cools slowly compared with a relatively thin steel that
may be cooled more quickly.
The formation of fine precipitates themselves may also lead to precipitation
strengthening when the precipitation occurs during the transformation from
austenite to ferrite or after the transformation is complete. These precipitates
may interfere with ferrite grain growth during or after the transformation and
may, therefore, have a further grain-refining effect. It is useful to note, however,
as pointed out by Picketing, 1~ that precipitates formed in the austenite do not
contribute directly to precipitation strengthening in the final hot-rolled ferrite
structure.
Hot-rolled, high-strength, micro-alloyed steels may be cold rolled and annealed
using either batch or continuous annealing to give steels that also have high
strength and it is generally found that for a given annealing method, there is a
correlation between the strength of the hot band and the strength of the cold-rolled
and annealed product. This is illustrated for batch-annealed niobium steels in
Figure 1.66. Figure 1.67 shows that a fairly similar relationship applies to vana-
dium steels and also shows that the strength of a continuously annealed vanadium
steel is higher than that of a batch-annealed steel of the same composition. A
similar relationship applies to niobium steels, as illustrated in Figure 1.68, and
comparison between Figures 1.67 and 1.68 shows that a given niobium addition
may develop greater strength than the same vanadium addition.
Hayami and Furukawa 95 showed that the strength increase to be obtained from
a niobium or titanium addition may be empirically related to the square root of
the composition. Their results are in qualitative agreement with Figures 1.67 and
Figure 1.66 Effect of niobium additions on yield strength of hot strip and cold-reduced
strip after batch-annealing and temper rolling (After Bordignon et al. t~