Steels_ Metallurgy and Applications, Third Edition

60 Steels: Metallurgy and Applications

~- =,.. 40

2

g 2O

9 ~ 10

c 8

"~ 6

u. 2

~,. Recrystallized

Deformed

austenite

0 40 80 120 160 200

Effective austenite grain boundary

area per unit volume mm-1

Figure 1.64 Variation of ferrite grain size with austenite grain boundary area per unit
volume for deformed and recrystallized austenite. (After Kasper etal. 1~

af

N r

= 7

o~ (p 5

"~ =d 3

-1

Ferrite grain size

ASTM no.

8 \9 \10

0 10 20 30 40 50 60 70 80 90

Rolling reduction below recrystalllzaUon temperature (%).

Figure 1.65 Effect of recrystaUized austenite grain size and total reduction below recrys-
taUization temperature on ferrite grain size (After Sekine et al. 1o3)

formation of strain-induced fine precipitates may still lead to a finer recrystallized
austenite grain size than would otherwise have been the case. This again leads to
a finer ferrite grain size on subsequent cooling and transformation. Figure 1.64
shows that the ferrite grain size produced on transformation depends on the
austenite grain boundary surface area per unit volume before transformation
and that a much finer ferrite grain size, for a given grain boundary surface
area, is produced from an unrecrystallized austenite than from a recrystallized
austenite. It is clear that austenite grain boundaries provide nucleation sites for
the formation of ferrite grains. The grain boundary surface area per unit volume
itself depends on the austenite grain size, but for the unrecrystallized structure,
it also depends on the strain given after the last previous recrystallization since
this largely determines the grain shape which in turn influences grain boundary
area per unit volume. Figure 1.65 shows how the ferrite grain size varies with