Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 77

would be insufficient to develop the necessary structures. At the top annealing
temperature, the two-phase structure of ferrite and austenite is developed with
a decreasing proportion of ferrite as the temperature is increased. 13~ The carbon
clearly partitions between the two phases to give a much higher concentration of
carbon in the austenite, but there may also be a smaller partitioning of manganese
and silicon with the manganese content higher and the silicon content a little
lower in the austenite than in the ferrite. During cooling, the volume fraction
of the austenite may decrease and its carbon content may increase, particularly
for slow cooling rates, as illustrated in Figure 1.85. In some cases, the carbon
diffusing into the austenite may not reach equilibrium and may, therefore, develop
a carbon-rich layer, just within the surface of the austenite.
At the austempering temperature, some of the austenite may transform gradu-
ally to bainite to leave a mixture of bainite and residual retained austenite. The
latter could be in several positions, for example, between the laths of the bainite
structure 137 or along ferrite grain boundaries. 1as
When the hold time and austempering temperature are relatively low, the
carbon in the austenite remains at its initial relatively low value formed on
cooling. On further cooling to room temperature, therefore, much of this austenite
transforms to martensite because the Ms temperature can be above room temper-
ature. As the transformation to bainite proceeds, carbon rejected from the bainite
builds up in the austenite with the result that its Ms temperature eventually
decreases to below room temperature. The austenite is then stable at room temper-
ature and no martensite is formed on cooling. The volume fraction of austenite
retained at room temperature, therefore, increases. On further holding, the volume
fraction of retained austenite at room temperature decreases because more of it
will have transformed to bainite before final cooling. Eventually, the tendency to
form carbide in the austenite will increase and carbide may form, but it is well
known that this tendency is inhibited by the presence of silicon. The result is that
a silicon addition leads to the retention of a higher volume fraction of austenite
with a higher carbon content.

0.6

A 0.5

c: 0.4

c$

8 ~ 0.3

~.c_

0.1

3~

IO~

30~

0.0~

500 600 700 800 900

Temperature (~

Figure 1.85 Evaluation of carbon enrichment in untransformed y during cooling from
825 ~ C. After Minote et al. 13~