1. Vol.23 No.6 LI Xincheng et al: Precipitation and Hetero-nucleation Effect of…844
DOI 10.1007/s11595-007-6844-x
Precipitation and Hetero-nucleation Effect of V(C, N)
in V-Microalloyed Steel
LI Xincheng1
, ZHAO Liangyi1
, WANG Xinyu2
, ZHAO Yutao2
(1.Advanced Forming Technology Institute, Jiangsu University, Zhenjiang 212013,China;
2.School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013,China)
Abstract: The precipitation behavior of V(C, N) in steels microalloyed with vanadium was
researched using a thermal simulator during single-pass deformation at 800-750 ℃. The V(C, N) pre-
cipitates and its nucleation effect on ferrite were investigated by TEM and EDS. The experimental
results show that there are two remarkable heterogeneous nucleation effects of V(C, N) particles pre-
cipitated before γ→αphase change: primary reason is that high coherency between V(C, N) and
ferrite promotes V(C, N) to become a nucleating center of intragranular ferrite; secondary reason is that
the coarsening of V(C, N) causes locally solute-poor region in austenite, thus expedites the nucleation
of intragranular ferrites further. Furthermore, the relationship between the size and shape of V(C, N)
was studied, and identification method was provided for distinguishing interphase precipitation and
general precipitation to avoid erroneous judgment and misguide.
Key words: V(C, N); heterogeneous nucleation; deformation enhanced ferrite transformation
1 Introduction
For a long term, there were few studies on defor-
mation precipitation and heterogeneous nucleation effect
of carbonitrides of microalloy elements in steels[1, 2]
. The
extremely constraints on this study are mainly because
the carbonitride of microalloy elements in steel is negli-
gible, and lack of high-accuracy method to detect the
morph and location of these precipitates. Studies on this
aspect are only about niobium and titanium micoalloyed
steel, so far it is rarely seen a systematic research on the
nucleating effect and grain refining mechanism of V(C, N)
particles in V-microalloyed steel. The present works
indicate that V(C, N) particles have very obvious het-
erogeneous nucleation effect in V-microalloyed steel. In
order to explore the contribution of nucleating effects of
V(C, N) particles to structure refinement as well as its
refining mechanism, it’s necessary to go deep into ob-
servation and analysis on structure pattern and precipita-
tion phases of microalloyed steels.
2 Experimental
2.1 Raw materials
Carbon structural steel Q235 (steel A in table 1) was
used as a contrast steel and the mid-component of Q235
was used as basic ingredient. Microelement vanadium
was added into the steels. The contents of vanadium and
nitride were mainly adjusted. The effect of V(C, N) par-
ticles on intragranulary nucleated ferrites was investi-
gated. Specific chemical compositions are shown in Table 1.
The phase transformation point Ar3 was measured by a
foremaster digital full automatic phase transformation
machine and a metallographic analysis apparatus under
the conditions of holding 5 min at 1200 ℃ with 30%
deformation amount, the heating and cooling rate was
10 ℃/s. Table 1 shows Thermo-Calc calculations of the
equilibrium phase transition point Ae3 in microalloyed
steels with vanadium.
A 25 kg vacuum inductance furnace was used to
smelt the test steels. The ingot was forged into cylinder
specimens. Forging temperature was 1200-950 ℃, and
normalization temperature was 950 ℃. The specimens
were isothermally treated at 950 ℃ for 10 min and then
cooled to room temperature by air. Then it was machined
to form a φ8×12.4 mm compression specimen.
(Received: Nov. 12, 2007; Accepted: May 6 2008)
李李李LI Xincheng( ): Prof.;Ph D;E-mail:lixincheng@ujs.edu.cn
Funded by the National Natural Science Foundation of China (50775102)
and the Universities Natural Science Fund Key Project of Jiangsu Province
(04KJA430021)

2. Journal of Wuhan University of Technology-Mater. Sci. Ed. Dec. 2008 845
2.2 Procedures
Single-pass compression test were proceeded on a
gleeble-2000 thermal simulator. Transformation proc-
esses were as follows. Specimens were austenitized at
1200 ℃ for 5 min to make sufficient solutionizing of
vanadium, followed by cooling at rate of 10 ℃/s and then
deformed throughout undercooled austenite range. De-
formation temperature(T) were 800 ℃(Ae3)-750 ℃(Ar3),
deformation amount(ε) were 30%, 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.
Structure detection and analysis schedules are as
follows. Specimens were parted at 1/3 diameter along the
compression axial direction and cor ％roded by 2 - ％4
nital for investigating the metallographic structure after
thermal simulation deformation. Foils for TEM were
prepared by the twin-jet reduction method at 30 V and
cooled by liquid nitrogen. Twin-jet fluid was composed
of ％5 perchloric acid ％and 95 carbinol. Complex foils
for TEM were prepared by extraction complex technique.
The ingredients of phases were analyzed by a JEM-2010
UHR. V(C, N) particles and structure components in
microstructure were analyzed by a SISC-IAS8.0 image
analysis software. Specimens for chemical phase analysis
were prepared by electroextraction method. Extract was
composed of 10% KCl and 0.5% citric acid monohydrate.
The temperature of electroextraction was 0-5 ℃ and
current density was 20 mA/cm2
. There were few particles
whose sizes are greater than 100 nm, so the particles less
than 100 nm were regarded as nanaphase. The average
sizes of particles 60% made up of the precipitation were
used as the grain size of nanophase, ensuring that it was
representative.
3 Results and Discussion
3.1 Heterogeneous nucleation effect of
V(C, N) precipitates on IGF
The specimens were analyzed by TEM and the re-
sults discovered that V(C, N) particles contributed a two
ways of heterogeneous nucleation effect before γ→α
transformation. First, high coherency between V(C, N)
and ferrite promoted V(C, N) to become a nucleating
center of intragranular ferrite; second, the coarsening of
V(C, N) caused locally solute-poor region in austenite,
thus expedited the nucleation of intragranular ferrite
further(Fig.1).
Fig.1 (a) shows a polygonal V(C, N) particle pre-
cipitated in austenite grain. An island ferrite formed up-
side it and a complete epitaxial growth ferrite formed at
the bottom plane of the V(C, N) particle. The explanations
is that higher interfacial energy on the pien of particle
make particle grow outside at this position and form a
austenite stable element poor region outside the upper
contour, then form into island ferrite for constitutional
supercooling (this region coincides with the upper con-
tour of the particle). At the bottom of the particle, the
growth of ferrite epitaxially depended on the interface. It
is a representative growth pattern of heteroepitaxy Stran-
ski-Krastanov(S-K)[3]
. For the misfit degree between the
(001) plane of V(C, N) and intragranular ferrite is only
2.0%, high coherency of the lattice decreases specific
Table 1 The Ae3, Ar3 and chemical composition of the test steels(mass fraction, %)
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
Fig.1 Microstructure of quenched C test steel in table 1 with 60% deformation at 800℃

3. Vol.23 No.6 LI Xincheng et al: Precipitation and Hetero-nucleation Effect of…846
surface energy as well as the nucleation free energy, so
V(C, N) become a nucleation center for intragranular
ferrite[4]
. Precipitate in Fig.1(a) has dual grain refining
action, “fragment crystal” action introduced by the
modification of crystallography orientation in austenite
local region upside the particle and hetero-nucleation
effect at the bottom plane of the particle.
Fig.1(b) shows that there exists island structure
which has difference with the ferrite matrix around single
circle V(C, N) particle precipitated in austenite grain, but
the EDS diagram shows that there is Fe peak only (Fig.2).
It is inferred that a special region(island ferrite) is formed
by solute-poor. Crystallography orientation in austenite
local region was changed by island ferrite, and then
caused fragmenting of local austenite. The austenite grain
was fragmented into plenty of subgrains (A in Fig.1(b))
and cellular structure (B in Fig.1(b)), which has the
smallest misorientation. As the proceeding of deforma-
tion, the island ferrites formed continuously in frag-
mented austenite, and thus fragment austenite further.
After that ultra-fine equiaxed grains were obtained.
Fig.2 EDS pattern of island structure
3.2 Formation of “wrapped ferrite”
An experimental phenomenon was observed in
metallographic structure, which has not been reported
before. Some circle, triangular or irregular black patch
structure in ferrites formed in intracrystalline distortion
zone or some granular ferrite were observed. According
to the amplification factor it can be inferred that those
black patch structure have grain sizes less than or equal to
2 μm, which is not precipitates (Fig.3(a)). These parti-
cles were wrapped in ferrite and had clear boundaries
(Fig.3(b)).
Foils that had such feature were analyzed further-
more using TEM and EDS (Fig.4). The results shows that
these small particles wrapped in ferrite grains are also
ferrites using V(C, N) as nucleation center which has
obvious ferrite boundaries. Thus it can be judged that
these black patches are a kind of ferrites which have more
tiny structure than those wrapped outside. These black
patches were called as “wrapped ferrite”.
Fig.3 Microstructure of B test steel in Table 1
(a) Microstructure of granular ferrite
(b) Local zoom in
Fig.4 TEM and EDS images of granular ferrite
3.3 Relation between precipitation shape
and grain size of V(C, N)
Fig.5 shows the shape of V(C, N) particles in C test
steel after 70% compression deformation at 800 ℃. They
were obtained by quenching and holding for different
times. It is indicated that there are few particles with
small sizes, assumed circular or sub-circular before pre-
cipitation (Fig.5(a) and Fig.5(b)). The amount of V(C, N)
particles increases with the increasing of the holding time,
and they distributes uniformly (Fig.5(c)). When the sizes
coarsened to 30 nm, the circles fade-away and changed
into square and sub-square. And then it changed com-
pletely into square and sub-square when the grain al-
ligator was be yond 50 nm (Fig.5(d)). T he explanation

4. Journal of Wuhan University of Technology-Mater. Sci. Ed. Dec. 2008 847
is that there exists a parallel orientation relation between
austenite and the precipitation of microalloy carbonitride
on it[5]
: (100)V(C, N)//(100)γ，(010) V(C, N)//(010).
Dominated by this relation, the effect of interfacial en-
ergy between austenite and V(C, N) particles were very
obvious and misfit degrees were homogenous to each
direction. At present the precipitation of microalloy
carbonitride should be sphere cement out in austenitic
matrix. But interfacial energy will not be leading role
anymore after the growing up of precipitates. Ledges on
interface will be best adherence reaction place for new
diffusion atoms. V(C, N) particles will change into cubic
step wise and show square and sub-square shape under
transmission electron microscope.
For proving whether the precipitates in Fig.5 were
the production of deformation enhanced transformation
during deformation, another three repeated tests were
performed on C test steel in Table 1 with 70% compres-
sion deformation at 800 ℃. The results showed that ag-
ing time of deformation enhanced ferrite transformation
was very short (about 0.025-0.026 s). Moreover, once the
phase transformation occurred, it finished immediately. It
means that precipitates in Fig.5 are certainly the produc-
tion of deformation enhanced transformation during de-
formation.
Fig.6 shows the relation between sizes of V(C, N)
particles and holding time when the C test steel In table 1
was deformed with 70% deformation at 800 ℃. The
particles would grow up when the crystal nucleus was
formed. For microalloy carbonitride nucleated along
dislocation line, diffusion rate of solute atoms along the
dislocation pipeline are faster than atoms on the other
direction. But the segregated solute atoms on dislocation
line have been consumed at the beginning of nucleation.
Oversaturated solute atoms in austenitic matrix during
precipitated growing processes will firstly move to the
dislocation line by lattice diffusion, and then diffuse and
grow up on dislocation line. Lattice diffusion of solute
atom is absolutely a control action in this procedure. This
matches the theory of diffusion control growth.
Fig.6 Relation between size of precipitated phase and holding time
3.4 Effect of deformation amount on pre-
cipitation of V(C, N)
Effect of deformation amount on precipitation of
V(C, N) was examined on specimens austenitized at 1200 ℃
for 5 min followed by air cooling to 750 ℃ with defor-
mation amount(ε) of 0%, 30%, 50% and 70%, respec-
tively. Deformed specimens isothermally treated at 750 ℃
for several seconds. Precipitation mechanism of V(C, N)
at different deformation amounts(ε) was investigated by
TEM and chemical phase analysis method.
The results shows that the precipitation of V(C, N) in
undeformed (ε＝0%) austenite is very sluggish. There
was no precipitation of V(C, N) when holding time is less
than 780 s, and ferrite start nucleating on austenite grain
boundary at this time. When holding time was increased
from 780 s to 1560 s, there occurred many V(C, N) par-
ticles successively with sizes of 30-100 nm. This ex-
perimental results indicate that the precipitation of V(C, N)
in undeformed (ε＝0%) austenite is very sluggish. It
needs about 13-26 min to finish. Table 2 shows the
measuring result of V(C, N) particles in specimens with
deformation amount (ε) of 30%, 50% and 70% respec-
tively. It is clearly seen from Table 2 that the precipita-
tion of V(C, N) is promoted significantly by deformation
amount. At 750 ℃ with 70% deformation, precipitation
amount of V is 0.063%, about 44.4% of total V-content in
steel.
Fig.5 Microstructure of C test steel in table 1 holding for different times at 800℃ with deformation of 70%

5. Vol.23 No.6 LI Xincheng et al: Precipitation and Hetero-nucleation Effect of…848
3.5 Identification of interphase precipi-
tation and general precipitation
Some tiny spotted second-phase precipitated in the
specimens (volume ratio of ultrafine grain ferrite were
70% and the grain size were 2.8 μm) arranged in align-
ment were observed. These carbonitrides attached on the
interface ofγ→αphase transformation. New precipi-
tated phase was formed row by row by repeated migrat-
ing ofγ→αinterfaces to γ grains, and this is inter-
phase precipitation. Interphase precipitation consists of
irregularly spaced and curved sheets of V(C, N) particles
(Fig.7(a)). This is possibly the pinning of solute particle[6]
in front of migratory phase boundary lead to solute drag,
thus the precipitates had enough time to nucleate. Pre-
cipitates pinned phase boundary efficiently, made the
migration ofγ→αphase boundary blocked, thus lead to
the repeated formation of ordered arrangement and
curved sheets of interphase precipitation. The present
research shows that the similar interphase precipitation
can be observed in the specimens except the contrast steel
(A test steel in Table 1). Another strange phenomenon is
that general precipitation can be observed in different
viewing field in the same specimens (Fig.7(b)). Grain
size of the two kind particles were equal, coinciding with
the conclusion of the Ref.[7]. This leads to derivation of a
more considerable question that interphase precipitation
grains precipitated at higher temperature followsγ→α
phase transformation but general precipitation is the
oversaturated precipitation of ferrite. Grain size of the
two kind precipitates should have obvious difference.
A ledge growth model for ferrite and carbonitride in
interphase precipitation procedure was established to
explain this strange phenomenon (Fig.8). Carbonitride
grains nucleated onγ→αinterface and grown in ferrite.
Ferrite grew up as a ledge pattern in horizontal and ver-
tical orientation. The carbonitride nucleus may be formed
onγ→αinterface after the ferrite grown a wide region
in vertical orientation, and then several narrow ferrite
banding were formed continuously (Fig.9). Microalloyed
carbonitride of interphase precipitation distributed ir-
regularly on planes that once were theγ→αinterface.
Stereoscopic model for transmutation product in inter-
phase precipitation can be shown in Fig.9. Observed from
direction A, precipitated phase grains distributed parallel
onγ→αinterface, but seen from direction B, it distrib-
uted irregularly. For the difference of viewing directions,
interphase precipitation was misidentificated as general
precipitation. As a result, it can be inferred that if the
grain size are equal, they are all interphase precipitation.
So grains in Fig.7(b) are interphase precipitation even
though they look like general precipitation. It should be
emphasized that the description of interphase and general
precipitation in former documents[2,7]
should be carefully
examine and distinguish.
Table 2 Quantitative measuring results of carbonitride
in samples after different deformation
Deforma-
tion tem-
pera-
ture/℃
Defor-
mation
amount
/%
Solution
amount
of V / %
Precipi-
tation
amount
of V/%
Solution
amount/Pre
cipitation
amount
×100%
750
30
50
70
0.117
0.095
0.079
0.025
0.047
0.063
17.6
33.1
44.4
Fig.8 Step growth model for interphase precipitation
Fig.7 Microstructure of C test steel in table 1
with 50% deformation at 760 ℃

6. Journal of Wuhan University of Technology-Mater. Sci. Ed. Dec. 2008 849
Fig.9 Stereoscopic model for interphase precipitation in microalloy
4 Conclusions
a) There were two remarkable heterogeneous nu-
cleation effects of V(C, N) particles precipitated before
γ→α phase change: first, high coherency between V(C,
N) and ferrite promoted V(C, N) to become a nucleating
center of intragranular ferrite; second, the coarsening of
V(C, N) caused locally solute-poor region in austenite,
thus expedited the nucleation of intragranular ferrite
further.
b) The size of precipitated particles had osculating
relations with its form. Grain sizes of sub-circle were
20-30 nm, and square’s and sub-square’s were usually
greater than 50 nm.
c) A step growth model was established for inter-
phase precipitation and general precipitation, and identi-
fication method was suggested for avoid erroneous
judgments and misguide.
References
[1] Yasuya. Microstructural Evolution with Precipitation of
Carbides in Steels[J]. ISIJ International, 2001, 41(6):
554-565
[2] M Sobral, P R Mei, H J Kestenbach. Effect of Carbonitride
particles formed in Austenite on the Strength of Microal-
loyed Steels[J]. Materials Science and Engineering, 2004,
367(2):317-321
[3] X C Li, M Zhang, X J Xu. The Effect of Nanophase on the
Nucleation of Intragranular Ferrite in Microalloyed Steel[J].
Chinese Journal of Mechanical Engineering, 2007(in Chi-
nese)
[4] X C Li, Y F Jiang, S G Cai. The Effect of V, Nb and De-
formation Condition on Deformation Enhanced Transfor-
mation in Microalloying Steel[J]. Transactions of the Chi-
nese Society for Agricultural Machinery, 2007(in Chinese)
[5] D Q Bai, F Hamad, J Asante. Precipitation Strengthening in a
low Carbon Microalloyed Steel[J]. Materials Science Forum,
2005, 4:481-488
[6] Y Li, D N Crowther, T N Baker. The Effects of Vanadium,
Niobium, Titanium and Zirconium on the Microstructure and
Mechanical Properties of Thin Slab Cast Steels[J]. ISIJ In-
ternational, 2004, 44(6):1093-1102
[7] S S Campos, E V Morales, H J Kestenbach. Detection of
Interphase Precipitation in Microalloyed Steels by Micro-
hardness Measurements[J]. Materials Characterization,
2004, (52):379-384