Abstract:
Although the austenitic stainless steels 304 and 321 are often treated nominally as equivalents
in their hot rolling characteristics, the question remains whether any subtle differences
between the two allow further optimisation of their respective hot rolling schedules. The hot
workability of these two types of austenitic stainless steels was compared through single-hit
Gleeble simulated thermomechanical processing between 800℃ and 1200℃ while the strain
rate was varied between 0.001s
and 5s
. It was found that the constants for the
hyperbolic sinh equation for hot working of AISI 321 steel are Q = 465 kJ/mol, =
9.76 × 10
MPa
, = 0.009 MPa
and ! = 6.1 while for 304 steel the constants are
Q = 446 kJ/mol, = 2.14 × 10
MPa
, = 0.008 MPa
and ! = 6.1. It is shown
that the occurrence of dynamic recrystallisation starts when the Zener-Hollomon parameter
Z ≈ 6.4 × 10
s
for both steels but that the differences in the values of Q and A3 (the
structure factor) between the two steels does lead to consistently lower steady state stresses
for the steel 321 than is found in the steel 304 at the same Z values. This may, therefore, offer
some scope for further optimisation of the hot rolling schedules and in particular in the mill
loads of these two respective steels.
A modelled constitutive equation derived from hot working tests to predict hot rolling mill
loads is proposed and validated against industrial hot rolling data for AISI 321 stainless steel.
Good correlation is found between the predicted Mean Flow Stress, the Zener-Hollomon Z
parameter and actual industrial mill load values from mill logs if allowances are made for
differences in Von Mises plane strain conversion, friction and front or back end tension. The
multipass hot working behaviour of this steel was simulated through Gleeble
thermomechanical compression testing with the deformation temperature varying between
1200℃ down to 800℃ and the strain rate between 0.001s
-1
and 5s
-1
. At strain rates greater
than 0.05s
-1
, dynamic recovery as a softening mechanism was dominant, increasing the
dynamic recrystallisation to dynamic recovery transition temperature DRTT to higher
temperatures. This implies that through extrapolation to typical industrial strain rates of about
60s
-1
,most likely no dynamic recrystallisation in plant hot rolling occurs in this steel but only
dynamic recovery. Grain refinement by DRX is, therefore, unlikely in this steel under plant
hot rolling conditions. Finally, mill load modelling using the hot working constitutive
constants of the near-equivalent 304 instead of those specifically determined for 321, introduces measurable differences in the predicted mill loads. The use of alloy-specific hot
working constants even for near-equivalent steels is, therefore, recommended.
Key words: dynamic recrystallisation (DRX), dynamic recovery (DRV), AISI 321,
constitutive equation, mean flow stress (MFS), dynamic recrystallisation to dynamic recovery
transition temperature (DRTT), hot working, modelling.
