Sulphide Precipitation in Ti IF Steels (PR 311 013)

Prévia do material em texto

A Detailed Study of Sulphide Precipitation
in Ti-IF Steels
G. DupuisL, R.A. Hubert2 and R. Taillardl
1Laboratory of Physical Metallurgy (C6), URA234,
University of Line I, 59655 Villeneuve d’Ascq, France
2OCAS, Research Centre of the SIDMAR group,
Flat Product Division of the ARBED group
J. Kennedylaan 3, B-9060 Zelzate, Belgium
D~+33320336226 -2+329345 1336
ABSTRACT
The precipitation behaviour of Ti-IF steels has
been studied by combining STEM-EDX measurements
with model calculations including both thermodynamic
and kinetic aspects. The measured dissolution
temperatures of Ti4C2S2 and TiXS were found to be in
good agreement with predictions based on the last
published volubility products. The Ti/S stoichiometry
ratios are 2 and 0.85, respectively. Cooling experiments
starting from a TilC2S2-free matrix ( 1300°C) show that
the precipitation of this compound begins 125°C below
its dissolution temperature. The Ti4C2S2 precipitation is
delayed by the difficulty of nucleation and by the fact
that the matrix supersaturation remains low even at -
large undercoolings because of the further growth of
TiS. Once started, the reaction proceeds however very
rapidly as shown by kinetic experiments.
1. INTRODUCTION
The microstructure and therefore the mechanical
behaviour, of interstitial-free (IF) steel are governed by
the precipitation processes which occur during the
various stages of the steel fabrication. The very low
amounts of carbon and nitrogen (often less than 30
ppm) contained in the steel are removed from solid
solution by combining with very strong carbide and
nitride formers, such as titanium, niobium or vanadium.
The definition of a relevant chemical composition for
an Ti-IF steel is however complicated because of the
existence of other kinds of precipitates such as
sulphides (TiS), carbosulphides (TilCzSJ and
phosphides (FeTiP) which compete with each other].
In usual Ti-IF steels, titanium carbosulphides are
considered to be very important because they offer the
40
possibility to completely bind carbon at relatively high
temperatures when sufficient sulphur is present. If the
chemistry and the thermal cycle are well chosen, the
finishing hot rolling can be performed with a carbon
free matrix which is highly beneficial for the texture in
case of deep drawing applicationsl’2. The carbosulphide
route is however not so easy to handle as this compound
is generally thought to be unstable above 1150°C which
is lower than usual slab reheating temperatures
(>1200”C).
At equilibrium, the competition between
sulphides and carbosulphides is described by their
respective volubility products which have been
measured by various authors 2-6. According to Mizui7,
the most reliable set of volubility products has been
given by Yang et al.3.
The TiS e Ti4C2S2 transformation has been
extensively studied by DeArdo et al.g”10who describes
the appearance of sandwich-like multi-phase
precipitates resulting from a direct solid state reaction.
In this case, TiS would reversibly transform in situ into
TidC2S2 by incorporation of titanium and carbon atoms.
To our knowledge, this observation has however not
been confirmed by other researchers, but the common
mechanism of concomitant dissolution of one
compound together with the precipitation of the other
one has also not been explicitly demonstrated.
Furthermore, the kinetic aspects of the
transformation have generally been neglected in spite of
their fundamental significance. Most papers published
up to now report simulations of typical industrial cycles
which inevitably mix various effects. In this case, it is
difficult to ascertain that the thermodynamical
equilibrium is always established making it impossible
to predict the precipitation state. Moreover, the concept
of equilibrium itself could be questioned because a
sample containing precipitates and which is maintained
in isothermal condition for a long time continues to
transform due to Ostwald ripening. Hence, the ideal
procedure would be to determine first of {Ill the
conditions leading to the quasi-equilibrium at the
temperature of interest before looking at the kinetic and
strain effects.
In this paper we present the first results of an
extensive study aiming at (1) verifying the volubility
products of TiS and Ti4C2S2 in Ti-stabilized low carbon
steels, (2) determining the conditions required for
establishing the equilibrium, (3) estimating the
influence of the thermal history, (4) characterizing the
kinetic and strain effects and (5) studying the
TiS+Ti4C2S2 reaction.
Note: In the following sections the words “precipitate”
or “particle” always designate sulphur-bearing
precipitates i.e. TiS or Ti4C2S2.
TH MWSP CONF. PROC., 1SS 1998 – 117
2. EXPERIMENTAL
operating at 200 kV. This microscope is equipped with
a thin organic window X-ray energy dispersive (EDX)
spectrometer coupled to an image analysis software.
The determination of the chemical composition of the
sulphur-bearing precipitates was carried out by means
of an automatic STEM/EDX procedure: the particles
were first image analysed and the area, shape factor and
equivalent diameter were measured. Each particle was
subsequently chemically mapped pixel per pixel. The
X-ray peak intensities were corrected using a Cliff-
Lorirne~ procedure based on home-made standards13.
The steel used was produced by laboratory
vacuum melting and casting (100 kg ingot). The chemi-
cal composition which is typical for some industrial
enameling steelsl 1 is given in table I. This steel
chemistry was chosen according to simulations per-
formed using the MODIPS software developed by one
of US]2. The software calculates the thermodynamical
equilibrium based on volubility products of all possibly
present precipitates. Whatever the volubility products
chosen for TiS and Ti4C2S2, the model predicts that
Ti4C2S2 should be the only sulphide present below
1000”C (see discussion for more details).
Table I : Heat composition in mass ppm
CIMnl P I SIN IAll Til Crl Nil Cu
961903111612041 15 I578I1OO8I2O4!216I 71<
After casting, the ingot was allowed to cool in
an unforced way to room temperature. Samples for
analysis were sawn from the ingot as 1.5 rum thin plates
before being sealed in silica tubes under residual argon
pressure.
The sulphide transformation study can be
tackled either during reheating starting from the
ambient which results in former Ti4C2S2 dissolution or
during cooling starting from high temperature which
gives rise to precipitation reactions. Fig. 1 sketches the
thermal history of the samples devoted to the study of
this precipitation. In this case, the specimens were
isothermally heat treated at 1300”C for one hour. The
samples were subsequently given a sequence of
isothermal treatments comprising 1 to 7 stages. The
steps were applied at ever decreasing temperatures, i.e.
1250, 1200, 1150, 1125, 1100, 1050 and 1000”C, and
for durations increasing with their order of occurrence.
This procedure was assumed to lead very close to the
equilibrium at each temperature. After the thermal
treatment the silica capsules were taken out of the
furnace and immediately broken in brine so as to fix the
state of precipitation at the end of each holding
sequence.
The investigation of precipitates dissolution
during heating was monitored up to 1450”C. Some
problems dealing with both the occasional oxidation of
the samples and the efficiency of brine quenching were
however encountered with temperatures higher than
1350”C.
The precipitation state was characterized by
transmission electron microscopy (TEM). Experiments
were mainly carried out in the nanoprobe electron
diffraction - and in the scanning/transmission electron
(STEM) modes. Most experiments were performed with
carbon extraction replicas using a JEOL 2010 system
118 – 40TH MWSP CONF. PROC., 1SS 1998
1400 ~
III
23456789101’1
Time (hour)
Fig. 1. Thermal cycles applied for the precipitation
experiments.
In the present paper, EDX results are often
displayed as points, each representing the corrected X-
ray peak intensity of Ti against the S intensity in a
sulphur-bearing particle. 80 to 150 particles were
considered for each thermal treatment. As the intensities
increase with the particle size the points corresponding
to particles having a given stoichiometry must align on
a straight line. Points significantly deviating from the
stoichiometry lines should be considered either as
co-precipitates clearly comprising two or more particles
having nucleated on each other or multi-phase
precipitates apparently being a single particle as
described by Hua et al.lo. In some cases co-precipitates
have been deliberately excluded from the analyses.
3. RESULTS 3.2. Precipitate Dissolution.
,
TH
After having characterized the as-cast state of
precipitation, our goal was to study samples having
undergone the thermal cycles depicted in Fig. 1 so as to
determine the precipitate evolution during cooling and
namely the TilC2S2 precipitation starting temperature.
However, it was interesting to compare this procedure
with the usual dissolution method consisting in several
independent reheating cycles at various temperatures.
3.1. As-Cast Precipitation State.
The EDX results of the as-cast sample are
reported in Fig. 2. As mentioned in the previous
section, most of the points associated with the
precipitates are located between two straight lines, The
slopes of those lines which correspond to constant Ti
over S atomic ratios are close to 2.0 and 0.85,
respectively and can be associated to titanium
carbosulphides (TilC2S2) and titanium sulphides (TixS,
hereafter sometimes named TiS). The slope values (2.0
and 0.85) are average values based on all data presented
in this paper.
As cast
m /TK.=2
.6 0
;
.5 40000-
E
3
,-
C
Ttis=o.85
g 20000-
0
0 20000 40000 60000
Sulphur intensity (A. U.)
Fig. 2. EDX results of individual particles for the as-
cast sample.
Contrary to the expectation, many titanium
sulphides were still observed which proves an
incomplete TiS+-TilC2Sz transformation. The mean
diameters of titanium carbosulphides and sulphides are
750 and 200 nm, respectively. A few phosphides FeTiP
with a size lower than 40 nm were also found as co-
precipitates in combination with other sulphur
containing precipitates 14.
All experiments presented hereafter were done
starting with fresh samples prepared from the as-cast
material.
40
The first investigations were conducted using the
usual procedure i.e. a rapid heating followed by a
soaking at the temperature of interest and a quenching
in brine.
Two thermal cycles were carried out varying the
soaking conditions. In the first case, a 3 hour soaking at
1250°C was applied. The EDX results depicted in Fig.
3, indicate that the sample still contains a large fraction
of remaining TidCzS2 with a mean size of 330 nm. The
mean TiS si~e is”3~-nm.
40000
1250”C- 3h Ti/S=2.O
,2
g 20000-
~
~ 15000-
3
“~ 10000-
i=
5000-
0
0 5000 10000 15000 20000 2!
Sulphur Intensity (A. U.)
Fig. 3. EDX results of individual particles after a 3
hour soaking at 1250”C
40000- —
35000
: n
+ TiXS
-30000 0 Ti4C2S,
?
<
@ co I mixed
- 25000
.=
~ 20000-
~
=
~ 15000-
3
“g 10000-
F
5000-
0 5000 10000 15000 2tiO0 25
Sulphur Intensity (A.U.)
00
0
Fig. 4. EDX results of individual particles after a 1
hour soaking at 1300°C.
In the second dissolution experiment a one hour
soaking at 1300°C was applied. In this case no
carbosulphides were found as indicated by Fig. 4.
Hence, it can be concluded that the dissolution
temperature of TiAC2Sz is equal to 1275A25°C for the
steel under investigation. The sample reheated at
1300”C only contains TiS particles with a mean size of
450 nm.
MWSP CONF. PROC., 1SS 1998 – 119
98
The TiS dissolution temperature could not be
reached as some 150nm titanium sulphides were still
clearly noticed after a 1450”C soaking for % hour.
3.3. Carbosulphide Precipitation
For this experiment, all samples were first
reheated in order to fully dissolve remaining titanium
carbosulphides. As shown in the previous section a 1
hour soaking at 1300°C was sufficient to fulfil this
condition. The temperature was subsequently decreased
step by step, as described in Fig. 1.
The results from the first stage (at 1250”C) are
presented in Fig. 5. In this condition, the steel contains
a lot of TiS but no Ti4C2S2, which is quite surprising,
compared with the results of the dissolution at the same
temperature as depicted in Fig. 3. Furthermore, all
stages above 1150°C show similar results, i.e. no
carbosulphides.
40000-
35000
: m
+ Tins
-30000 0 Tics422
; ~5000
@ co I mixed
>
.=O
c 2oooo -$J
=
~ 15000-
3
.-
: 10000-
,=
k
5000-
0
0 5000 10000 15000 20000 25000
Sulphur Intensity (A. U.)
Fig. 5. EDX results of 1250”C precipitation stage
45000-
40000-
-
? 35000-
~
.= 30’JOO-
~ 25000-
~
: 20000-
.: 15000-
: 10000-
11250C TiJS=2.O
n
+ TiXS
O Ti,C2Sz
@ co/ mixed
TiJS=O.85
i 5000 10000 15000 20000 25000 30000
Sulphur Intensity (AU.)
Fig. 6. EDX results of 1125°C precipitation stage
As a matter of fact, the first TiiC2S2 particles are
observed at 1125°C (Fig. 6) and occasionally at
120 – 40TH MWSP CONF. PROC., 1SS 19
1150°C. The maximum size of those newly precipitated
particles was 130 nm after 1 hour soaking at 1125°C.
At lower temperatures, the Ti4C2S2 relative
number increases dramatically to 50% at 1100”C (Fig.
7) and up to 80% after the soaking at 1000”C, As we
wanted to detect the eventual presence of mixed
precipitates at the transformation we have excluded the
co-precipitates from the 11OO°C analyse. No such
mixed particles were found (except in one uncertain
case), in the present investigation (see Fig. 7).
30000 Iloo”c TiJS=2.O 1
_ 25000
; l.____l
+ Tixs
o Ti4c2s,3 0
4 @ mixed
- 20000
,@
:
.2
~ 15000- ‘
=
.$ 10000-
$
F 5000-
0
0 5000 10000 15000 20000
Sulphur Intensity (A. U.)
Fig. 7. EDX results of 1100°C precipitation stag,e
The Ti4C2Sz mean particle size rapidly increases
at the beginning of the precipitation (around 1125°C)
and then stabilizes to a value of 500nm for temperatures
ranging from 1IOO”C to 1000”C. On the other hand, the
TiXS fraction decreases but the mean TiXS size also
stabilizes around 400 nm. Hence, the sulphide to
carbosulphide transformation is not complete at the end
of therm;l cycle depicted in Fig. 1.
0.30
E-
=
1Oorrc
0.25- n T@ @“=400nm
m Ti4C2S2 ~.500 nm
0.20-
%
5 0.l5-
Sg
~ o.1o-
0.05-
0.00 .
100 200 300 400 500 600 700 600 900 1000
Diameter (rim)
Fig. 8, Size distributions of the sulphide and
carbosulphide particles at 1000”C
Finally, some precipitate X-ray mappings were
performed. Titanium and sulphur mappings were first
prec
an
carbosulphides. Then, the furnace was cooled and
maintained at 1100”C, i.e. just below the TiJCzSz
precipitation start temperature (determined to be around
1125°C in the previous section). The furnace needed 10
min. to cool from 1300 to 1100”C. The samples were
then held at 1100°C during O, 3 or 9 min. before being
quenched.
Due to furnace cooling, few carbosulphides with
a size close to 120 nm are already present at time zero.
After 3 min. isothermal holding, numerous TiiCzSz have
been formed with a mean size of 175 nm (Figs. 10-1 1).
This size increases to 270 nm after 9 min. of treatment.
These observations clearly demonstrate the high
Size class (rim)
Fig. 11. Effect of a 1100”C holding time (3 and 9 min.)
on carbosulphide size in a sample previously
heat treated for 1 hour at 1300”C
40TH MWSP CONF.PROC., 1SS 1998 – 121
A
100 nm
Fig. 9. X-ray mapping of the Ti/S ratio for some typical
TidC2S2 co-precipitate at 1125°C -C: various Ti$
acquired and Ti/S ratios were then calculated for each
pixel taking the Cliff-Lorimer correction into account.
Large pixel counts were necessary in order to keep the
standard deviation low enough and hence provided
useful ratios without too high local variations.
200 counts per pixel was found to be a good
compromise between the statistics, the time needed for
the acquisition and the drift of the sample. However,
too few counts were obtained at the precipitate contours
resulting in artefacts visible on all mappings.
Typical results are presented in Fig. 9. At
1300”C, only TiXS particles are found (Fig. 9A) while
Ti~CzS2-TiXS co-precipitates appear at 1125°C and
below (Fig. 9B and 9C). The precipitate shapes vary
between spheres and rounded rods for both Ti4C2S2 and
TiXS so that they cannot be identified from their shape.
The Ti/S ratio is constant within each precipitate and
the concentration step at the boundary of co-precipitates
is quite steep.
3.4. Carbosulphide Formation Kinetics
Three samples were reheated at 1300”C for 1
hour in order to fully dissolve the pre-existing titanium
ipitates. A: TiXS at 1300”C- B: large TiXS with small
d TiiCzSZ at 1100”C
reaction rate of Ti4C2S2 precipitation at 1100”C.
Fig. 10 also establishes that many co-precipitates are
involved in the transformation.
30000
i
1300°C-lh
/
TiJS=2.O
llOWC-3min e I
‘_‘oooi//-’- Ii // I
o-k=r--
0 5000 10000 15000 20000
Sulphur Intensity (AU.)
Fig. 10. EDX results after 3 min soaking at 1100”C on
a sample previously heat treated for 1 hour at
1300”C
n m-l
“--- I
4. DISCUSSION
4.1. Stoichiometries
The stoichiometry of titanium carbosul hide is
P
always reported as Ti4C2S2 in the literature 5’6’1’17. The
present study confirms that the Ti/S ratio of this
compound is equal to 2. XRD and micro-diffraction
experiments also confirm the published hexagonal
structure. The Ti/C ratio could not be checked by EDS.
The stoichiometry of titanium sulphide is not so
well defined. Some publications just refer to TiS4’5 but
many ~l$firs report about TiXS with x in the range [0.8-
1.3]’”. Mentions of Ti2S~]8, TiJ3513’lb, Ti&Sg10 are
also found. Those phases and many others are identified
in the Ti-S binary phase diagram published by
Murraylg. In our study, the Ti/S ratio is close to 0.85,
the standard deviation being less than 3% for most
thermal treatments above 11OO°C. Larger positive
deviations are nevertheless observed for some cycles
and especially for those relative to our kinetic studies
($3.4.). The maximum Ti/S ratio which has been
observed on single particles is around 1.2. Larger ratios
are always associated with co-precipitates. Finally,
some TiXS particles could be indexed by rnicro-
diffraction as Ti4S5 which has a Ti/S ratio of 0.8 but the
XRD spectra performed on potentiostatically extracted
precipitates did not confirm this observation. New
investigations are currently being done to find an
explanation for this discrepancy.
4.2. Solubilities
The state of precipitation has been calculated
using the MODIPS software. Fig. 12 shows e.g. the
results obtained with the volubility products published
by Yang3. In this case, the dissolution temperatures of
TiS (point a) and TiiC@2 (point b) are 1448 and
12 11°C, respectively. Because of the higher stability of
the carbosulphide at low temperatures, TiS is not
present anymore below 1117°C (point c). The
temperatures obtained with other published volubility
products are given in Table II. The prediction of the
dissolution temperature of TiAC2S2depends on both the
volubility products of TiS and Ti4C2S2 used in MODIPS
calculations.
In the current work, the dissolution temperature
of the titanium sulphides was determined to be slightly
above 1450”C because few 150 nm titanium sulphides
are still present after 15 minutes soaking at 1450°C.
This temperature is a little higher than the predictions
deduced from the volubility products by Yang3 and
Copreauxz which are both slightly below 1450”C (see
temperature a in table II).
122 – 40TH MWSP CONF. PROC., 1SS 1998
1000-
“-”-”-””---- ‘ - . -.-.-.-,- ... ElTiN800- ----- Tis........~ c sF 422
~ 600-
(nco
.—
.,. ,---
u ----
g 400- \ ,, . .:f . . .
In
,,
:, ..\
.
2 ‘.,
E ,’ \ ‘.
200- ,’ i \/i ‘.,
,,’ \ ‘.
# ‘.
‘\
o-
~/
-b ~L a
900 1000 1100 120-0 1300 14’00- 1500
Temperature (“C)
Fig. 12. MODIPS equilibrium calculation based on
Yang’s volubility products3 (points a, b, c are
defined in the text)
Table II. Volubilityproducts and dissolution tem-
peratures of TN and T4C2S2(points a, b
and c are defined in the text)
Copreaux Yang Yoshinaga Current
et al.2 et al. 3 et al.4 work
log - 16550/T -139751T -32521T
[Ti][S] +6.92 +5.43 -2.01
log -61400m -68180rr -208321T
[Ti]4[C]2[S]2 +25.28 +31.6 -3.12
a (“c) 1448 1448 >1500 >1450
I I I I
b (“C) 1322 1211 >15m diSS:=1275
prec:=l150
I c(°C) I 1169 I 1116 I 1380 I <1000 I
In the dissolution experiment, Ti4CzS2 was still
found after 3 hours at 1250”C but had disappeared after
the 1300”C soaking. Applying Yang’s experimental
procedure, we estimate the dissolution temperature to
be 1275”C. This value is comprised between those
calculated with the equations of Yang and Copreaux
which are also based on dissolution experiments (see
temperature b in table 11). The volubility products of
Yoshinaga4 give unacceptable results for both TiS and
Ti4C2S2 as already confirmed elsewhere.
The simulations also indicate that TiS should
completely disappear below the c temperature.
However, the experiment shows that the transformation
from TiS to Ti4C2Sz is not complete even for the last
step of the thermal treatment at 1000”C. The as-cast
sample also contains a large fraction of unexpected
sulphides. This can be explained either by a false low
temperature prediction of the equilibrium state (which
it levels off rapidly during isothermal holding. These
0T
should normally be based on activities and not on
concentrations) or by the fact that the equilibrium was
not reached due to kinetic effects. As a matter of fact,
the equilibrium could be difilcult to reach with the steel
under investigation because it contains large fractions
of precipitates. A very long (1 day) soaking at 10OO°C
will be performed in the next future in order to check
this hypothesis.
4.3. TiS+TiqC2S2 Reaction
The precipitation experiment based on the
thermal cycle depicted in Fig. 1. gives results which are
completely different from the dissolution one: the first
Ti4CzSz were observed around 1150°C with the current
cooling sequence i.e. 125°C below the experimental
dissolution temperature ($3.2.). This observation is
however in agreement with a previous one made by
Mizui7. This author studied the precipitation state in a
steel containing 450 ppm Ti, 24 ppm C and 65 ppm S.
A sample reheated at 105O”C for one hour still
contained more than 75~o Ti4C2S2, but a second sample
first reheated at 1250”C for one hour and subsequently
held at 105O”C for another hour presented a totally
different state of precipitation as it contained less than
5% TiAC2S2. Hence, it appears that the dissolution-
precipitation cycle of TitC2S2 presents some kind of
hyste~esis loop-as depicted in Fig. 13.
co
.-
Z
(u
.&
u);
O*
i=
Temperature (“C)
Fig. 13. Scheme of the dissolution-precipitation
hysteresis loop
This phenomenon could be easily explained if
the kinetics played a significant role as it is the case of
the WY transformation in steel where the AG
temperature is always lower than the AC3. However,
our numerical simulations of the dissolution and
precipitation kinetics show that this effect is visible
only for heating and cooling rates above10Cks and that
4
calculations are based on the diffusion coefficient of
titanium in austenite published by Moll and Ogilviem in
1956 which is the only one available to our knowledge.
The kinetic model used here is described in other
publications12’21.
At this point of the discussion, it is necessary to
define more precisely the concept of equilibrium which
implies that the iron matrix does not present any
supersaturation and that the relative fractions of Ti4C2S2
and TiS are given by the true (unknown) volubility
products.
If we assume, with all authors having published
similar data, that the equilibrium is achieved in the
dissolution experiments, we must admit that it is not the
case for the precipitation cycle. When the temperature
is decreased below the T&C2S2 dissolution temperature
( 1275°C), this compound should begin to precipitate.
However precipitation consists of nucleation and
growth and the latter can start only when the nucleus
has reached a critical size. The start of precipitation
usually requires a large enough matrix supersaturation
which is associated with an undercooking. In this case, it
is nevertheless likely that when the temperature
decreases, the TiS further precipitates which reduces
the supersaturation and thus delays the Ti4C2S2
nucleation. It is believed that this mechanism proceeds
until it becomes energetically unfavorable i.e. when
the energy cost of the Ti4C2S2 formation is lower than
the energy gain associated with the TiS growth.
The critical size seems to be quite large as the
smallest detected TitC2Sz are always larger than 20 nm
Moreover, the formation of a Ti4C2S2 nucleus requires
the simultaneous presence of three different species, i.e.
titanium, sulphur and carbon, which are randomly
walking but which must meet at a given location in high
enough quantities to form a nucleus. This condition is
easier fulfilled in the vicinity of the TiS which can also
help the nucleation by reducing the surface energy. This
could explain the presence of numerous co-precipitates
at the beginning of the kinetic experiment at 11OO°C
($3.4.). However, a lot of newly formed iscdated
TiAC2S2are also detected (Fig. 10) and they are growing
with time as shown in Fig. 11 which indicates that they
are probably formed directly from the matrix which is
enriched by the fast dissolution of TiS.
Because of the observed high rate of the
TiS--+TiqC2S2 transformation, we ~-a~not totally exclude
a “DeArdo-like” transformation which could take
place at the very beginning of the reaction when the
system is very unstable. This reaction should be very
rapid and complete as we do not find multi-phase
particles. Further experiments are necessary in order to
explore the very beginning of the reaction.
H MWSP CONF. PROC., 1SS 1998 – 123
titanium sulphide and carbosulphide in ultra-low
5. CONCLUSION
The precipitation state of a Ti-IF steel was
studied in detail as a function of the temperature.
The measured dissolution temperatures of
TidCzSz and TiXS are in good agreement with
predictions based on the last published volubility
products. The stoichiometry ratios are 2 and 0.85,
respectively.
Cooling experiments starting from a TiqC2Sz free
matrix ( 1300”C) show that the precipitation of this
compound begins 125 ‘C below its dissolution
temperature although the isothen-nal holding before
quenching was at least one hour. The TiiCzSz
precipitation requires the dissolution of TiS but this
reaction is delayed by the difficulty of Ti4C2S2
nucleation and by the fact that the matrix
supersaturation remains low even at quite large
undercoolings because of the further growth of TiS.
Once started, the reaction proceeds however very
rapidly as shown by our kinetic experiments.
ACKNOWLEDGEMENTS
G. Dupuis is grateful to OCAS N.V. for the
funding of his PhD thesis. The authors wish to thank
Dr. C. Xhoffer for valuable discussions and for
technical assistance operating the TEM. The laboratory
for Iron and Steehnaking of the University of Ghent,
Belgium is also gratefully acknowledged for providing
the steel.
Keywords: IF, TiS, Ti4C2S2, dissolution, precipitation,
volubility, kinetics
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Author e-mails: Gregory .Dupuis@univ-lillel .fr
roger.hubert @ocas .be
Roland.Taillard @univ-lillel .fr
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