Steels_ Metallurgy and Applications, Third Edition

Low.carbon structural steels 151

By introducing a delay between roughing and finishing, rolling can be made to

take place at a temperature below 950"C, where the strain-induced precipitation

of Nb(CN) or TiC is sufficiently rapid to prevent recrystaUization before the next

pass. According to Cohen and Hansen, 11 austenite recrystaUization and carboni-

tride precipitation are interlinked during this process, substructural features in

the deformed austenite providing nucleation sites for carbonitride precipitation

which in turn pins the substructure and inhibits recrystallization. This results in

an elongated pancake morphology in the austenite structure and the austenite is

said to be conditioned. The deformation substructure that is introduced within

the austenite grains has a particularly beneficial effect in developing a finer grain

size. This arises from the fact that the substructure provides intragranular sites

for ferrite nucleation in addition to those at the austenite grain boundaries.

The controlled-rolling operation can be intensified by depressing the defor-

mation process below Ar3 and into the two-phase y + c~ region. In addition to

further grain refinement, rolling in this region also produces a significant change

in the microstructure. Thus a mixed structure is produced, consisting of polyg-

onal ferrite grains which have transformed from deformed austenite and deformed

ferrite grains which were produced during the rolling operation.

Transformation to ferrite

Although the mean ferrite grain size is related to the thickness of the pancaked
austenite grains, other factors also play an important part in the control of the
finished microstructure and properties. As discussed previously, alloying elements
depress the austenite to ferrite transformation temperature and thereby decrease
the ferrite grain size. A further important effect is the rate of cooling from the
austenite (or y + or) range and a combination of controlled rolling and accelerated
cooling is now being used to produce further improvement in properties.
The benefits of accelerated cooling can be used in two ways:

1. To increase the strength compared with air-cooled, controlled-rolled material.

5OO

~.~ 450

oa

E

~400 alized

~" I ~/" / ~ ~ Controlled

350~ ~rolled (CR)

t CR + Accelerated

300 cooled

i I ...... I ..... I

0.3 0.35 0.4 0.45 0.5

Carbon equivalent

Figure 2.9 Exploitation of controlled rolling and accelerated cooling for improved prop-
erties in plate steels