Diapositiva 1

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Influence of Processing Parameters on
Recrystallised Microstructure of Extra-Low Carbon
Steels C. Capdevila, J.P. Ferrer, F.G.
Caballero and C. García de Andrés Solid-Solid
Phase Transformations Group (MATERALIA)
Department of Physical Metallurgy Centro
Nacional de Investigaciones Metalúrgicas
(CENIM) Consejo Superior de Investigaciones
Científicas (CSIC) Avda. Gregorio del Amo,
8 E-28040 Madrid, Spain www.cenim.csic.es
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Focus the problem
The recrystallisation in the as-coiled during
production of thin hot strips of extra low carbon
steel (ELC) for direct application, is obtained
if the finishing rolling temperature (FRT) is low
enough (FRT ranging from 750 to 820 ºC) to strain
the ferrite and the coiling temperature high
enough (above 600 ºC) to recrystallise. Bearing
in mind that that the inner and outer wraps, as
well as the edges, cool faster than the centre,
the coiling temperature must generally be kept
between 700C and 650C in an industrial line. As
a consequence, it is very difficult to obtain a
fully recrystallised material after coiling in
the thinnest sheets due to thermal looses. In a
joint project with ARCELOR and CRM it has been
proposed a new strategy which consists on
transferring a hot coil (2 4 mm) from the hot
strip mill (HSM) directly to an additional
rolling stand. After recoiling, a heavy warm
deformation (HWD) is performed below the ? - ?
transformation temperature to reduce the
thickness down to 1.2 mm or lower, followed by
recoiling. The conditions of this last rolling
process and the coiling temperature are of vital
importance to obtain a fully recrystallised
material. In this sense, the goal of the present
paper is to study the influence of those
parameters on microstructural evolution of ELC
steels.
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Simulation of the process
Soaking 1250C - 60min
hot rolling FRT 900C, 850C 20-12-6.5-4
ROT cooling 4-5C/s
final coiling
coil transfer
HWR
CT THWR 50C
entry thickness 20mm
Study of rex-kinetics at CENIM
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Simulation of the process
0.03C-0.17Mn-0.009P-0.005Si-0.006S-0.041Al-0.003N
(in wt.)
Specimen processing parameters
Sample CT (ºC) T of HWD (ºC) HWR ()
ELC-C 640 590 46
ELC-F 680 631 66
ELC-H 690 634 48
ELC-I 660 608 55
ELC-L 580 536 54
After HWD material is annealed at temperatures
ranging from 525 to 700 ºC for 1.5 h to simulate
coiling
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Evolution of the microstructure
Thermo-Electric Power (TEP) measurements have
been used to track microstructure evolution
DS DV / DT
?S depends on the microstructure -Decrease of
elements in solid solution ?S ? -Increase of
dislocations ?S?
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Evolution of the microstructure
Influence of annealing temperature after HWD
(simulation of coiling)
final coiling
Annealing temperatures ranging from 525 to 700
ºC. Holding time of 1.5 h. Heating rate up to
desire temperature of 50 ºC/s.
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Evolution of the microstructure
A detailed analysis of TEP results presented in
this figure revealed several common patterns that
should be highlighted
1- Higher TEP values are recorded in samples with
high CT temperature (ELC-F, ELC-H, and ELC-I) as
compare with those with low CT temperature (ELC-C
and ELC-L).
1- Higher TEP values are recorded in samples with
high CT temperature (ELC-F, ELC-H, and ELC-I) as
compare with those with low CT temperature (ELC-C
and ELC-L).
2- The same TEP value is reached at annealing
temperature of 700 ºC for all the studied
samples.
2- The same TEP value is reached at annealing
temperature of 700 ºC for all the studied
samples.
3- There is a drop in TEP first, followed by a
sharp increase.
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Evolution of the microstructure
Role of cementite
Simulation of TEP evolution assuming cementite
dissolution as the only phenomenon occurring
ELC-F at 575ºC
ELC-F at 625ºC
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Evolution of the microstructure
Role of AlN
Cooling from hot rolling is fast enough to avoid
AlN precipitation ELC-C and ELC-L samples where
transfer was undergone at ?650 and ?600 ºC,
respectively, present the lowest TEP values. This
is consistent with the assumption that most of Al
and N are still in solid solution in this
material. By contrast, steels ELC-F, ELC-H and
ELC-I where transfer was undergone at
temperatures around 700 ºC present a higher TEP
values which indicate that some AlN precipitation
events have occurred during the transfer and HWD
The rise in TEP at temperatures above 625 650
ºC during the reheating cycles could be also
explained in base of AlN precipitation. Moreover,
the similar results achieved in all the materials
studied after annealing at 700 ºC suggest that
most of the AlN precipitation events have taken
place
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Evolution of the microstructure
Role of AlN
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Influence of AlN precipitation on recrystallised
grain morphology
Evolution of hardness and ReX volume fraction
with annealing temperature
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Influence of AlN precipitation on recrystallised
grain morphology
For the same deformation level The higher the CT
is, the bigger the delay in the beginning of ReX
is (ELC-H vs ELC-C, and ELC-I vs ELC-L) Those
samples with lowest CT temperature (ELC-C and
ELC-L), and the highest HWR (ELC-F) present the
earliest recrystallization start temperature.
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Influence of AlN precipitation on recrystallised
grain morphology
Evolution of grain size and grain density
Two different tendencies can be
distinguished 1- ELC-C, ELC-F and ELC-L
samples (low CT with exception of ELC-F) present
smaller grain size and higher values of n as
annealing temperature is increased up to 650ºC.
2- ELC-H and ELC-I (high CT) present increasing
grain size and hence decreasing n values as
annealing temperature is increased.
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Influence of AlN precipitation on recrystallised
grain morphology
ELC- C HWD46 CT590 ºC
525 ºC
550 ºC
575 ºC
600 ºC
625 ºC
650 ºC
ELC- H HWR48 CT634 ºC
575 ºC
600 ºC
625 ºC
650 ºC
700 ºC
CT ? ? ReX starts at lower temperatures CT ? ?
Coarser ReX grain size. Nucleation is difficulted
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Conclusions
The results obtained after different warm rolling
conditions and reheating temperatures have been
analyzed to conclude that slight modification on
HWD conditions could induce big differences on
the subsequent recrystallisation processes which
take place on reheating. The absence of AlN
precipitation, and a higher stored energy,
produce finer and more equiaxed recrystallised
grains in materials with high HWR and/or low CT
temperatures. High transfer temperature
promotes the AlN precipitation during the
transfer. This precipitation, together with the
low stored energy for recrystallisation because
of low HWR, leads to coarser and irregular in
shape recrystallised grains after subsequent
reheating. The lower the transfer temperature,
the small amount of AlN precipitation during the
transfer is. The low store energy in samples
with high CT temperatures and low HWR induce the
overlapping of recrystallisation and AlN
precipitation, promoting the block of
recrystallization.