1. Vol.25 No.2 LI Xincheng et al: Effect of Nanophase on the Nucleation…228
DOI 10.1007/s11595-010-2228-8
Effect of Nanophase on the Nucleation of
Intragranular Ferrite in Microalloyed Steel
LI Xincheng1
, WANG Xinyu2
,FENG Xiaotian1
(1. Advanced Forming Technology Institute, Jiangsu University, Zhenjiang 212013, China;
2. School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China)
Abstract: MnS, MnS＋V(C, N) complex precipitates in micro-alloyed ultra-fine grained steels
were precisely analyzed to investigate the grain refining mechanism. The experimental results shows
that MnS, MnS＋V(C, N) precipitates provide nucleation center for Intra-granular ferrite (IGF), so that
refined grain remarkably. Moreover, substructures such as grain boundary, sub-boundary, distortion
band, dislocation and dislocation cell in austenite increased as the deformation energy led by heavy
deformation at low temperature (deformation temperature≤800 ℃, deformation quantity≥50%). As
a result, V(C, N) nanophase precipitated at these substructures, which pinned and stabilized sub-
structures. The substructures rotated and transformed into ultra-fine ferrite. 20 nm-50 nm were the best
grain size range of V(C, N) as it provided nucleating center for intragranular ferrite. The grain size of
V(C, N) were less than 30 nm in the microalloyed steels that with volume ratio of ultra-fine ferrite more
than 80% and grain size less than 4 μm.
Key words: intra-granular ferrite (IGF); vanadium; MnS; nanophase; substructure
1 Introduction
For a long term, there were few studies on structure
refinement effect of nanophase such as V(C, N) and MnS
in steels. The extremely rigorous constraint on this study
was mainly because the carbonitride of microalloy ele-
ments in steel is very few, MnS and other nanophase
(grain size less than 100 nm) is regarded as harmful in-
clusions that has not been caused attention of its effect on
grain refine in microalloyed steel and high-accuracy
method to detect the morph and location of these pre-
cipitates is lacked. Studies on this aspect are only about
niobium and titanium micoalloyed steel[1-3]
, so far a sys-
tematic research on the nucleating effect and grain re-
fining mechanism of V(C, N) particles in V-microalloyed
steel is rarely seen. The recent work shows that V(C, N),
MnS and other nanophase have very significant structure
refinement and nucleating promotion effect on intra-
granular ferrite in V-microalloyed steel. In order to ex-
plore the nucleating effects of nanophases on structure
refinement as well as its refining mechanism, it’s neces-
sary to go deep into observation and analysis on structure
pattern and precipitation phase in metallographic, TEM
micrograph and complex photos of vanadium microal-
loyed steels (ultra-fine grain steel), then grain refinement
can be explored, so as to lay a theoretical foundation of
ultra-fine grain steel prototype preparation and industrial
production.
2 Experimental
The test steels used mid-component of Q235 as ba-
sic ingredient and a pinch of vanadium were added into it.
Specific chemical compositions are shown in Table 1
(Steel A in Table 1 was used as contrast steel). A 25 kg
vacuum inductance furnace was used to smelt the test
steels. The ingot was forged into a φ14 mm cylinder
specimen, and then it was machined to form aφ8 mm×
12.4 mm compression specimen. The phase transforma-
tion point Ar3 was measured by Foremaster Digital full
automatic phase transformation machine and metal-
lographic analysis apparatus under the condition of
holding 5 min at 1200 ℃, the heating and cooling rate
was 10 ℃/s. Table 1 shows Thermo-Calc calculations of
the equilibrium phase transition point Ae3 in steels mi-
croalloyed with vanadium.
©Wuhan University of Technology and Springer-VerlagBerlin Heidelberg2010
(Received: Nov.25, 2008; Accepted: May 18, 2009)
李李李LI Xincheng( ): Prof.; Ph D; E-mail:lixincheng@ujs.edu.cn
Funded by the National Natural Science Foundation of China (50475125)
and the Universities Natural Science Fund Key Project of Jiangsu Province
(04KJA430021)

2. Journal of Wuhan University of Technology-Mater. Sci. Ed. Apr.2010 229
Table 1 The Ae3, Ar3 and chemical composition of the test steels/%
Single-pass compression test were proceeded on a Glee-
ble-2000 thermal simulator. Specimens were austenitized at
1200 ℃for 5 min to make sufficient solutionizing of vana-
dium, followed by cooling at cooling rate of 10 ℃/s and then
deformed throughout undercooled austenite range. Deforma-
tion temperature(T) were 800 ℃(Ae3)-750 ℃(Ar3), deforma-
tion amount(ε) were30%, 50%, 60% and 70%, respectively,
and the rate of deformation were 50 s
－1
.The influences of
static recovery and recrystallization on ferrites were eliminated
by water quenching to room temperature immediately after
deformation.
Microstructure observation was performed by a
scanning and transmission electron microscope (SEM
and TEM), respectively. For SEM observations, 2%-4%
nital was used for etching. TEM thin foil specimens
were prepared by mechanical thinning followed by
twin-jet reduction method. TEM observation was per-
formed by using a JEM-2010 UHR electron microscope.
MnS and V(C, N) particles and structure components in
microstructure were analyzed by a SISC-IAS8.0 image
analysis software. There are few particles which sizes
are greater than 100 nm, so the particles less than 100
nm are regarded as nanaphase. The average sizes of
particles that make up 60% of the precipitation were
used as the grain size of nanophase, ensuring that it was
representative.
3 Results and Discussion
3.1 Nucleation promote effect of MnS and
MnS + V(C, N) on IGF
MDCSobral and GMiyamoto’s work[3,4]
shown that
use metallic oxide as nucleating center, MnS＋V(C, N)
complex precipitate can promote the nucleation of intra-
granular ferrite idiomorphs on it. Fig.1 (a) and (b) are
respectively micrograph and EDS of intragranular ferrite
formed on single MnS phase of the non-vanadium test
steel A in Table 1. Fig.1 (c) shows that intragranular
ferrites nucleate on precipitates which are presumably
incoherent MnS＋V(C, N). EDS measurement (Fig.1 (d))
reveals that this complex precipitate is V(C, N) precipi-
tated on incoherent MnS in austenite.
Random selected 62 incoherent MnS in austenite of
test steel B and C in Table 1 were investigated to explore
the nucleating effect of MnS and MnS＋V(C, N) com-
plex precipitates on intragranular ferrite. Table 2 shows
analyzing result of IGF nucleating rate on MnS and MnS＋
V(C, N) precipitates.
Results in Table 2 show that intragranular ferrite
nucleating rate of MnS＋V(C, N) complex precipitates is
higher than that of single MnS obviously. These results
show that the nucleating mechanisms of MnS and MnS＋
V(C, N) on intragranular ferrite are difference. Manga-
nese is austenite stabilizer; manganese-poor area exists
around MnS increased driving force of γ/α transfor-
mation, thus promote ferrite nucleate on MnS and then
growing into intragranular ferrite. However, compared
with epitaxial growth ferrite on MnS＋V(C, N) complex
precipitates, the promotion effect on form new phase of
constitutional supercooling caused by manganese-poor is
very small. Fig.2 shows EDS measurement of
un-nucleated MnS in austenite. It reveals that their exist
higher ferrite peak in MnS. So it can be inferred that this
kind of precipitate constitute by (MnFe)S which belongs
to atypical MnS precipitates. The constitutional super-
cooling degree around (MnFe)S can not satisfy the re-
quest of form new precipitate.
Steel C Si Mn S P V N Ae3/℃ Ar3/℃
A
B
C
0.086
0.091
0.089
0.16
0.18
0.20
0.57
0.56
0.58
0.007
0.005
0.006
0.011
0.0089
0.0094
—
0.064
0.14
0.0032
0.0031
0.0063
862
866
873
718
705
695
Table 2 IGF nucleating rate on MnS and MnS＋V(C, N) in test steel B and C
MnS/
MnS+V(C,N)
Nucleated MnS/
analyzed MnS
Nucleated MnS+V(C, N)/
analyzed MnS+V(C, N)
Nucleating rate of MnS
Nucleating rate of
MnS+V(C, N)
22/40 14/22 33/40 63.6％ 82.5％
Fig.1 TEM and EDS of MnS and MnS＋V(C, N)

3. Vol.25 No.2 LI Xincheng et al: Effect of Nanophase on the Nucleation…230
Statistical analysis on MnS precipitates show that it
also has relationship with the size of MnS whether MnS
could be a nucleating center for intragranular ferrite. The
present work observed that the MnS precipitates size
greater than 100 nm cannot be nucleating center of IGF,
those IGF nucleating centers are all belongs to nanophase
(size less than 100 nm). This is because the differences of
precipitation temperature affect grain size of MnS as well
as the probability to be nucleating center of intragranular
ferrite. The solubility product[9]
of MnS in δ and γ
shows in formula (1) and formula (2).
(1)
(2)
Precipitation temperature (1200 ℃-1390 ℃) of
MnS can be inferred from formula (1) and (2). It means
MnS precipitate in austenite after γ/α transformation.
Most of MnS precipitate on grain boundary of austenite
and growing into large grains (200 nm-300 nm) rapidly,
which not belongs to nanophase. It can not satisfy the
request nanophase grain sizes of intragranular ferrite
nucleating center.
The Refs.[9,10] revealed that the diffusion of man-
ganese was controlling factor for precipitate growth of
MnS, which is because the diffusion coefficient of
manganese in γ is much smaller than sulphur. Diffu-
sion speed of manganese get slow following the decrease
of temperature. The residual manganese and sulfur con-
tents will nucleate on oxide inclusions or high-energy
deformation band formed in austenite granular. At this
point, MnS has no growing tendency. The sizes of MnS
are very tiny, many are the circular nanometer, and it may
become IGF nucleating center.
3.2 Effects of deformed austenite and V(C,
N) on IGF
By analyzing the test steel B in Table 1, the results
shows that partial of ultra-refined ferrite formed in
V-microalloyed steel are caused by heterogeneous nu-
cleation effect of deformation enhanced precipitates in
austenite on intragranular ferrite. A noticeable phe-
nomenon shows that new secondary nucleating ferrite
which have no correlations with precipitates were ob-
served that exist around ultra-refined ferrite nucleated by
precipitates (shows in Fig.3). Actually, the amount and
volume ratio of precipitates are obviously inadequate if
only consider ferrite grains nucleated by precipitates to
make up whole microstructure of ultra-refined steels. The
experimental results inferred that whole microstructure
consists of secondary nucleating ferrite and ultra-refined
ferrites formed by ferrite (second nucleation) dynamic
recrystallization and other unknown grain refining
mechanism.
3.2.1 Effect of deformed austenite fragmentation on IGF
That can be presumed that unknown grain refining
mechanism have relationship with structure evolution of
deformation austenite. High density dislocation relaxa-
tion caused by deformation in austenite made the poly-
gonization of austenite grain, thus refined the grain size
furthermore. There were cellular structures which were
divided by the dense dislocated walls formed in the aus-
tenite. Deformed austenite grains were divided into sub-
grains which has the minimum misorientation by dislo-
cated walls. As deformation carried on, γ/α trans-
formation in austenite subgrains continued, and the aus-
tenite grains were refined. The more deformation of
specimens, the thinner of austenite grains and subgrains.
For that, the size of ferrite grains decreased obviously.
The misorientation of austenite subgrains caused by ro-
tation of austenite grain in the process of deformation
increased as the increasing of deformation amount. In
subsequent process of quenching, ferrite grains were
refined furthermore for there was orientation relation-
ship[11]
between the ferrite and the austenite matrix.
lg[Mn][S] 9090 / 2.929 ( 215 / 0.097)
[Mn] 0.07[Si]
T T= − + − − +
−
lg[Mn][S] 105900 / 4.2489 0.07[Si]T= − + −
Fig.2 EDS of un-nucleated MnS in austenite
Fig.3 Microstructure of test steel B in Table 1

4. Journal of Wuhan University of Technology-Mater. Sci. Ed. Apr.2010 231
Fig.4 shows the shape of cellular structures in aus-
tenite grains of test steel B in table 1 after 30% and 70%
compression deformation respectively at 750 ℃. Fig.4 (a)
shows that the size of cellular structure was uneven and
its shape varied not only massive but also strip, it inferred
that the deformations of austenite grains were severe
inequality. Its microstructure also exists a very serious
phenomenon of mixed crystal, grain size of ferrite were
uneven (2-9 μm), volume ratio of ultra-fine ferrite was
only 10%. Cellular structures in Fig.4 (b) are small and uni-
form, ferrite grain size was only 2.4 μm in its microstructure
anda volume ratio of ultra-fine ferrite was morethan 80%.
3.2.2 Effects of V(C, N) precipitated in austenite on IGF
Precipitation occurred in the grain boundary, the
sub-grain boundaries, dislocation lines, and the disloca-
tion cell wall of austenite in the process of deformation
enhanced transformation. This is because there exist
higher energy than average free energy on austenite ma-
trix at those defects. In addition, the micro-alloy solute
and C, N solute atoms themselves easy-to-segregate at
these defects, and thus in favor of forming nucleating
center for micro-alloy carbon and nitrogen compounds in
these places. Fig.5 shows the shape of precipitation in the
grain boundary, the sub-grain boundaries, dislocation
lines, the dislocation cell wall and α phase of test steel
B and C in Table 1 after 60% compression deformation.
Fig.4 Microstructure of B test steel in Table 1 after
30% and 70% deformation at 800 ℃
Fig.5 Microstructure of precipitation in different defects of test steel B and C after different deformation (50%-70%) at 750℃

5. Vol.25 No.2 LI Xincheng et al: Effect of Nanophase on the Nucleation…232
Fig.5 (a) shows acicular ferrite precipitated in the
grain boundary of austenite. Such ferrites had fuzzy in-
terface, non-axis and irregular-shaped grains, its grain
size were tinier than common transformation ferrite. It
split the original austenite grains, and then limited the
growth of transformation ferrite. As a result, the
pre-generated acicular ferrite had a significant promoting
effect on obtaining fine-grained ferrite in the continuous
cooling process.
Fig.5 (f) shows that two kinds of precipitation were
observed in the same austenite grain. A was intragranular
ferrite precipitate in grain boundary and grown into
transgranular (grain inner). B was precipitation in dislo-
cation cell wall which stabilized the strip dislocation cell
wall where it was precipitated. As sub-structures of aus-
tenite recovery and transformed into ultra-fine grains,
disperse precipitation of nanophase can prevent the
coarsening of ferrite grain.
V(C, N) precipitated in defects of the grain bound-
ary, the sub-grain boundaries, dislocation lines, and the
dislocation cell wall had its special pinning effects on
refine grains. Austenite sub-structure were spun by V(C,
N) precipitates and limited to grown up after it trans-
formed into ferrite. Fig.6 shows that V(C, N) precipitates
have significant spinning effects.
It can be inferred from Fig.6 that the size of trans-
formation ferrite in three test steels were very tiny (less
than or equal 4 μm). Arguably the coarsening driving
force should be very large, but the experimental results
shown that the coarsening ratio of ferrite grain in test
steel C was sluggish. Growing from 2.8 μm to 4 μm
holding for 300 seconds, its growth rate G was about
0.004 μm/s. But at the same experiment condition, the
size of ferrite grain in non-vanadium test steel A was
growing from 3.8μm to 7.4 μm, its growth rate G was
about 0.012 μm/s, about 3 times than test steel C. The
growth rate of test steel B between it of test steel A and C.
As shown in Fig.6, the higher the amount of vanadium,
the more stable of the size of transformation ferrite. The
significant differences of grain growing trend between tree
test steels were due to the differences of micro-precipitation
mechanism of vanadium content in steels.
MNiikura[12]
discussed the effect of microalloy
element vanadium in steel by the first-principles of
quantum mechanics. The segregation energy of vana-
dium on fcc ferrite grain boundary was about 2.35 eV, as
well as its impurity formation energy in crystal was about
0.96 eV. Therefore in the austenite, vanadium can present
in the crystal boundary may also exist in transgranular,
and occupy the crystal boundary. That was, regarded
from energy point, adding vanadium will increase the
number of austenitic grain boundary and at the same time
reduce the grain size. Vanadium existed in the crystals
will cause changes of stress field; it can serve as a new
nucleating center for austenite. If Vanadium existed in
the grain boundary, it had a strong interaction with other
atoms around it. This was an effective way to prevent the
migration of sub-grain boundary and grain boundary and
inhibit growing of grains. V(C, N) could be a
non-spontaneous nucleating center for intragranular fer-
rite and play a very important role on grain refinement.
In summary, the ultra-fine ferrite grains in
V-microalloyed steels were mainly formed due to com-
bined effect of fragment of deformed austenite, the pin-
ning of V(C, N) nanophase on substructure, intragranular
nucleation and secondary nucleation(show in Fig.7).
Moreover, in order to investigate the appropriate particle
size of V(C, N) that can provide nucleation center for
intra-granular ferrite, a model of α nucleated on V(C,
N) interface was established(shows in Fig.8).
The free energy △G can be obtained by formula (3):
(3)
where, ΔGV is the transformation kinetics of unit volume,
r the radius ofα.
Fig.9 shows the relations between △G and r cal-
culated by formula(3). It can be inferred from Fig.9 that
the smallest nucleation energy △G was on the interface
of V(C, N). So the nucleation position for intra-granular
ferrite should be V(C, N) nanophase.
Differentiate on r in formula (3), then can obtain the
critical nucleation energy △Gcrit and criticalradius rcrit ofα.
3 2
4 ( ) / 3 4 ( )VG r G f r fγαθ σ θΔ = π Δ ⋅ + π ⋅
( )=(1 cos )(2+cos )/4f θ θ θ−
cosX Xγ α γασ σ σ θ− = ⋅
Fig.6 Ferrite grain size of test steel A, B and C holding for
different times after 70% deformation at 750 ℃
Fig.7 Formation of ultra-fine ferrite due to combined effect of fragment of deformed austenite, pinning of nanophase on substructure,
intragranular nucleation and secondary nucleation

6. Journal of Wuhan University of Technology-Mater. Sci. Ed. Apr.2010 233
(4)
(5)
The critical diameter of V(C, N) is:
(6)
By the above analysis, V(C, N) particle must have an
appropriate size to become nucleation center for in-
tra-granular ferrite. The size of V(C, N) particle changes
along with the change of △GV. Fig.10 shows the relationship
between the size of V(C, N) precipitates, the volume ratio of
ultra-fine ferrite and the grain size of ultra-fine ferrite in test
steel B and C in Table 1. For greatest display the grain re-
finement function of microalloyed contents, the size of V(C,
N) shouldbe controlledunder 100 nm. It canbe inferredfrom
Fig.10 that 20 nm-50 nm were the best size range. The grain
size of V(C, N) were less than 30 nm in the microalloyed
steels that with volume ratio of ultra-fine ferrite grater than
80% and grain size less than 4 μm.
4 Conclusions
a) In the experiment condition of present work, MnS,
in the traditional sense was a kind of harmful inclusions
in steels, could be transformed into beneficial inclusions.
It provided nucleating center for V(C, N) and intra-
granular ferrite, so that refined grains remarkably.
b) The experimental results strongly indicate that the
grain refining mechanism of MnS and MnS＋V(C, N)
complex precipitates were its intercrystalline and secon-
dary nucleation can increase nucleating center for intra-
granular ferrite. Moreover, substructure such as grain
boundary, sub-boundary, distortion band, dislocation and
dislocation cell wall in austenite increased as the defor-
mation energy leaded in by heavy deformation at low
temperature. As a result, V(C, N) nanophase precipitated
at these substructures, which pinned and stabilized sub-
structure, thus refined ferrite grains furthermore.
c) The nucleating energy of IGF was the smallest on
V(C, N) phase interface. 20 nm-50 nm were the best
grain size range of V(C, N) as it provide nucleating center
for intragranular ferrite.
d) It was shown that there exist a linear relationship
between the size of precipitates and the grain size of fer-
rite. The grain size of V(C, N) were less than 30 nm in the
microalloyed steels that with volume ratio of ultra-fine
ferrite more than 80% and grain size less than 4 μm.
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3 2
crit 4 ( ) (1 cos )(2 cos ) / 3( )VG Gγασ θ θΔ = π − + Δ
crit 2 / Vr Gγασ= − Δ
crit crit2 4 / Vd r Gγασ= ⋅ = − Δ
Fig.8 Model ofαucleated on V(C, N) interface
Fig.9 Relations between ΔG and radius ofα
Fig.10 The relationship between the size of precipitates, the vol-
ume ratio of ultra-fine ferrite and the grain size of ultra-fine
ferrite in test steel B and C in Table 1