Hezio Rosa Silva

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The Effect of Restoration Process on the
Mechanical Behavior of Ultrafine Grain Size Nb-Ti
Steel Processed by Warm Rolling and Sub and
Intercritical Annealing
Hezio Rosa Silva Gustavo Gonçalves Lourenço
Luciana Helena Reis Braga Dagoberto Brandão
Santos
Federal University of Minas Gerais - UFMG -
Brazil
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Concepts Review
Ferrite grain size refining increases both
mechanical strength and toughness of steels Low
carbon content enhances good welding
characteristics
Mechanical Behavior of High Strength Steels
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Historical Review
Development through the years
Dr. D. Ponge Max Planck Inst.
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Introduction
(se )HSLA 350 to 850 MPa 3(se)carbon steels
without alloying
automotive industry
pipe lines for low temperatures operations
plates for the naval industry
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Objectives
Evaluate the restoration process in refinement of
ferrite in low C-Mn steel microalloyed with Nb
and Ti. Produce ferrite grain size of about 1?m
with different microstructural constitution
Besides ferrite and MA constituent, a
combination of
Ferrite dispersed cementite particles
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Experimental Procedure
Chemical Composition () -------------------------
----------------------------------------- C
Mn Nb Ti
0.11 1.41 0.028 0.012
Following steps
Specimens reheated at 900C and then ice brine
quenched
Specimens reheated at 740C, submitted to warm
rolling at 700C, with three equal pass
reductions and then air cooled
The annealing schedule was employed on all
specimens at temperatures of 550ºC or 800ºC for
different times, and after the soaking time the
samples were air cooled.
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Experimental Procedure
Final microstructure revealed by nital 2 and
LePera etching.
Ferrite grain size and volume fractions were
determined using image analyzer software and the
ASTM standards.
Vickers microhardness measured using a 0.3 N (300
gf).
Tensile tests and sub-size Charpy impact tests at
-20ºC
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Results and Discussion
Ice brine quenching of non-deformed samples
To obtain a metastable microstructure to increase
the ferrite nucleation rate during the warm
rolling and in the subsequent annealing.
Nital Etched, MO
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Results and Discussion
Ice brine quenching of non-deformed samples
The mean prior austenite grain sized was about
10.1 ?m.
Picral Etched, OM
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Results and Discussion
Warm Rolled Microstructure
Microstructure highly deformed due to the work
hardening during warm rolling, and presents
islands of martensite over a ferritic matrix.
Nital Etched, MO
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Results and Discussion
Warm Rolled Microstructure
Rolling temperature was not enough to start
recrystallization
The austenite formed in the ferrite grain
boundaries.
2 nital etched, SEM micrographs
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Results and Discussion
Intercritical and Subcritical Annealing
550ºC, 300 s OM and SEM respectively, nital 2
etched
7200 s
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Results and Discussion
Intercritical and Subcritical Annealing
800ºC, 300 s
800ºC, 7200 s
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Results and Discussion
Intercritical and Subcritical Annealing
Two types of ferrite grains were observed
polygonal ferrite grains which were formed on
cooling from annealing temperature
ferrite sub-grains formed as a result of recovery
and recrystallization of deformed grains.
Some ferrite grains with cell dislocation
structure in which recovery took place are also
present
As soon as austenite become homogeneous, it has
sufficient time to coalesce, providing larger MA
islands.
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Results and Discussion
Intercritical and Subcritical Annealing
300 s
7200 s
550ºC
800ºC
OM, LePera etched
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Results and Discussion
Intercritical and Subcritical Annealing
Some elongated grains with high dislocation
density
Observed
Islands and blocks of MA in the final
microstructure
Minor constituents present were granular bainite
and pearlite
Carbides prevail for subcritical annealing, while
MA for intercritical annealing.
Fv(800ºC) for MA practically suffers no variation
with the annealing time.
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Results and Discussion
Mechanical Behavior
As the annealing time increases, the hardness and
tensile strength decreases
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Results and Discussion
Mechanical Behavior
whereas the average ferrite grain size increases
continuously.
Result of the competition between three processes
taking place I - recovery and recrystallization
of ferrite II - grain growth of recrystallized
ferrite grains III - austenite formation and
their grain growth during intercritical annealing
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Results and Discussion
Mechanical Behavior
The beginning of the recrystallization can be
observed by an inflection in the curve of
hardness versus annealing temperature at
approximately 550ºC.
Microhardness values for specimens annealed
during 1800 s at different temperatures.
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Results and Discussion
Mechanical Behavior
The MA constituent formation is responsible for
lower yield strength of the samples annealed at
800ºC as the same way that happen in DP steels.
The higher ductility for samples annealed at
800ºC can be explained in the same way.
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Results and Discussion
Mechanical Behavior
The MA constituent formation is responsible for
low energies of the samples annealed at 800ºC.
Annealing at 500ºC led to an increase in
strength, but low values for absorbed energies.
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Conclusion
The mean ferrite grain size obtained was between
1.3 and 3.8 µm, respectively, for the highest and
lowest annealing times, respectively. Reaching
the maximum level of refinement about 87 with an
initial austenitic grain size of 10.1 µm.
The microhardness has changed from 175 to 220
VHN, that shows the influence of austenitic grain
size prevail about MA volume fraction. There was
an improvement of 20 on mechanical properties in
comparison to initial sample (hot rolled 187
VHN).
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Conclusion
The quenching from 900ºC led to the formation of
a martensite homogeneous microstructure.
The results indicates it is possible to project
an alloy with tensile strength near by 630 MPa
for a 300 s annealing at 550ºC or a lower value
for a 7200 s annealing (570 MPa). For annealing
at 800ºC these values are lower.
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Conclusions
The low carbon steel has been subjected to warm
rolling followed by intercritical and subcritical
annealing. The results have shown the significant
refinement of ferrite grain structure and
corresponding improvement in mechanical
properties.
The optimum combination of strength and ductility
has been achieved in samples subjected to 900ºC
austenitizing, quenching and then 0.66 reduction
during warm rolling and 60 min annealing at 800ºC.
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Conclusion
The ferrite grain refinement led to an increase
by 20 in mechanical properties compared to
industrially hot rolled steel. This was
accompanied by similar improvement in ductility
and work hardening behaviour.
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Special Thanks To
Your attention Aknowledgements Conselho
Nacional de Desenvolvimento Científico e
Tecnológico CNPq Project FVA number
400609/2004-5
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