Steels_ Metallurgy and Applications, Third Edition

150 Steels: Metallurgy and Applications

Rolling

Tamura et al. 9 recognize three distinct stages during controlled rolling:

1. Deformation in the recrystallization temperature range just below the reheating
   temperature.


2. Deformation in the temperature range between the recrystallization tempera-
   ture and Ar3.


3. Deformation in the two-phase y + ct temperature range between Ar3 and Arl.

At temperatures just below the reheating temperature, the rate of recrystallization
is rapid, increasing with both temperature and degree of deformation. However,
refinement of the austenite structure is produced by successive recrystallization
between passes, provided the strain per pass exceeds a minimum critical level.
Recrystallization is retarded to some extent by the presence of solute atoms,
such as aluminium, niobium, vanadium and titanium, and the process is known
as solute drag. However, the major effect of niobium and other micro-alloying
elements in retarding recrystaUization and grain growth arises from the strain-
induced precipitation of fine carbonitrides during the rolling process.
As the rolling temperature decreases, recrystallization becomes more difficult
and reaches a stage where it effectively ceases. Cuddy l~ has defined the
recrystallization stop temperature as the temperature at which recrystallization
is incomplete after 15 seconds, after a particular rolling sequence. Using this
criterion, the effect of the common micro-alloying elements on recrystallization
behaviour is shown in Figure 2.8 and this illustrates the very powerful effect of
niobium. The retardation effects of the various elements are dependent on their
relative solubilities in austenite, the least soluble (niobium) having the largest
driving force for precipitation at a given temperature and creating a proportionally
greater effect in raising the recrystallization temperature than the more soluble
elements such as aluminium and vanadium.

1050 9

-~ :3 lOOO

ca- E 950

e~_

o 900

e-.

o ~ 850

~ N

~ 800

o

n-- 750 9

0

Nb

0.050 O. 1 O0 O. 150

Initial solute content (atom per cent)

0.200 0.250

Figure 2.8 Effect on recrystallization stop temperature of increase in micro-alloy content
in 0.07% C 1.4% Mn 0.25% Si steel (After Cuddy l~