Steels_ Metallurgy and Applications, Third Edition

16 Steels: Metallurgy and Applications

10

C:

O4

(a)

1(OOl) (i 2) (1 ii o o) l

[~o] [~o1 [1~o1 [1~o1,, I

I/ I. t! I

70%

50%

0%

(b)

-o')

90%

85%

,50% I

30%

9 A A A AA ,

HB

I I , ., i m I I

o o 30 o 600 9o o 60 o Oo o

Figure 1.12 Development of the roiling texture of an aluminium-killed 0.007 wt% C steel
with cold reduction, (a) ~ fibre (b) skeleton line of y fibre (Schlippenback and Lucke. 27)
The skeleton line is the line close to the y fibre exhibiting the maximum orientation density

planes in particular slip directions. For a body-centred cubic structure, such as
ferrite, slip can occur in the four [111] directions on 12 slip planes divided
between (011), (112) and (123). This gives 48 possible slip systems. 29 For a
face-centred cubic-structure, such as austenite, slip can occur in the three [011]
slip directions on the four (111) slip planes giving 12 possible slip systems. 29
In any sample containing grains with a range of orientations, grains with certain
orientations will deform with greater difficulty than other grains because their
possible slip systems are oriented less favourably than others to the direction of
the applied stress. The result is that grains with certain orientations develop a
higher stored energy of deformation than other orientations. Dillamore et al. 30
showed that after 70% cold reduction the subgrain size decreased from (100)
through (112) to (111) and (110) planes, as illustrated in Figure 1.13. In addition
the subgrain misorientation increased in the same order. The result was that the
total stored energy of deformation increased in the order (100), (112), (111) and
(110). Other crystallite size work 31 and X-ray line broadening studies 32 have
confirmed that the stored energy of deformation varies with orientation.
The stored energy of deformation is important because it can lead to selective
nucleation of new grains during reerystallization when the steel is heated. Decker
and Harker 33 suggested that the highest energy regions would be able to nucleate
recrystallized grains first and that they would then be able to consume regions
with low initial energy that would not nucleate so easily. The dislocation densities