Influence of the microalloying elements on the temporary
inhibition of static recrystallization by strain-induced precipitates
Manuel GÓMEZ1)*, Alberto QUISPE2), Sebastián F. MEDINA1)
1
National Centre for Metallurgical Research, CENIM-CSIC, Av. Gregorio del Amo 8;
28040 Madrid, Spain.
2
National University “Jorge Basadre”, Av. Miraflores s/n, University City, Tacna, Peru.
*Corresponding author: E-mail: mgomez@cenim.csic.es
1
The kinetics of static recrystallization of austenite and its transitory inhibition by
strain-induced precipitates have been characterized in several microalloyed steels with
different compositions. This inhibition can be seen by the formation of “plateaus” in the
curves of static recrystallization obtained from isothermal double-deformation tests. The
influence of the type of microalloying element (Nb, V, Al) and the mean size of the
precipitates on the duration time of the plateau of recrystallization inhibition has been
studied and empirical relationships between these variables have been obtained. Alsteels present a much coarser particle size and a considerably shorter plateau compared
to Nb and V-microalloyed steels.
KEY WORDS: Microalloyed steel; Austenite; Static recrystallization; Precipitation;
Kinetics; Plateau of recrystallization inhibition
2
1. Introduction
The amount and type of microalloying elements play an important role on the shape
and the nature of nanoprecipitates formed in microalloyed steels. The elements most
typically considered as microalloying elements are Ti, Nb and V, although Al is
frequently considered as a microalloying element as well. At equal level of alloying, the
precipitates of the microalloying elements are soluble in austenite as follows[1]:
Ti<Al<Nb<V, and their carbides are usually more soluble than their nitrides.
The static recrystallization is different before and after strain-induced precipitation.
At higher temperatures, when elements are in solution, the recrystallization kinetics of
austenite can be described by an Avrami equation[2]:

 t 

X a = 1 - exp - 0.693 

t
0.5 


n



(1)
where Xa is the recrystallized volume fraction and t0.5 is the time corresponding to 50%
recrystallization, which depends on all the major variables that intervene in hot
deformation and whose most general expression is:
t0.5  A p q D s exp
Qx
RT
(2)
where  is the strain applied,  the strain rate, D the austenite grain size, Qx the
activation energy for recrystallization, T the absolute temperature, R=8.3145 Jmol-1K-1,
and p, q and s are parameters. While p and q are negative values, s is positive. Under
certain conditions of deformation and below a critical temperature, strain-induced
precipitation starts and the recrystallization kinetics cannot be described only by Eq. 1.
The fine precipitates formed by the microalloying elements and interstitials (C, N) exert
a pinning force on austenite grain boundaries in motion. As a result, recrystallization is
temporarily inhibited and a horizontal “plateau” appears in the curves.[3-5] After longer
3
times, precipitates coarsen, so pinning forces are again lower than recrystallization
driving forces, the plateau ends and the value of Xa grows again.[6-9]
Few works have previously focused on the quantitative influence of the type of
microalloying on the duration time of the plateaus. This work presents empirical
equations that illustrate the strong variation in the extent of temporary blockage of
recrystallization for Nb, V and Al microalloyed steels.
2. Experimental
Data related to kinetics of static recrystallization and strain-induced precipitation
from more than twenty steels with different compositions were collected and analyzed.
The steels contained a range of combinations of C, N and single or complex additions of
precipitate-forming elements such as V, Nb, Al and Ti (Table 1). Most of the steels
were manufactured by Electroslag Remelting (ESR) in a laboratory unit capable of
producing 30 kg ingots. Recrystallization and precipitation were studied by means of
isothermal hot torsion tests using specimens with a gauge length of 50 mm and a
diameter of 6 mm. Before deformation, the specimens were austenitized. The reheating
temperature varied according to composition and was set to be higher than the solubility
temperature in order to completely dissolve precipitates. Ti-added steels represented an
exception, as the low solubility of TiN precipitates in austenite makes it difficult or even
impossible to reach complete dissolution.[1,10] After austenitization, the specimens were
rapidly cooled to the deformation temperature in order to prevent precipitation prior to
deformation. The deformation temperatures were between 1150ºC and 800ºC and the
recrystallized fraction was determined for several post-deformation holding times. The
double deformation technique [11,12] was used to calculate Xa, in particular applying the
4
method known as “back extrapolation”.[13,14] The accuracy of this method has been
verified by comparing with metallographic observations.[4] Strain rate varied between
0.91 and 3.63 s-1. The applied strains of 0.20 and 0.35 were insufficient to promote
dynamic recrystallization. It is known that critical strain for dynamic recrystallization is
slightly lower than peak strain (p). Empirical expressions published elsewhere [15,16]
allowed to calculate p as a function of initial grain size (D0), Zener Hollomon
parameter (Z), activation energy (Q) and composition. It was confirmed that the values
of strain applied  remained below the critical strain, as the calculated values of p were
much higher than  (most of them between 0.5-1.5). This was also confirmed by
observing the values of critical strain found by other authors for similar steels and strain
rates [17]. Phase transformation temperature Ar3 was determined by dilatometry tests.
Table 2 summarizes the testing conditions, solubility temperatures[1] and Ar3
temperatures for the steels studied. Carbon extraction replica technique was used to
study the precipitation state by means of transmission electron microscopy (TEM).
3. Results and Discussion
The torsion test gives the values of torque applied versus the number of turns made
on the specimen, which are transformed respectively into equivalent stress and strain
using Von Mises criterion.[18]
Fig. 1a shows an example of the evolution of Xa versus the time after deformation for
a V-microalloyed steel. The progress of recrystallization following Avrami´s law before
and after the formation of the plateau of inhibition of recrystallization caused by the
pinning effect of precipitates (as described above) can be seen in the figure. Fig. 1b
shows another example of recrystallization kinetics for a Nb-microalloyed steel. In this
5
particular case, two successive plateaus were distinguished. It has been previously
described that the formation of two types of precipitates with similar stoichiometries
and precipitation temperatures can facilitate the formation of two plateaus in Nb and Vmicroalloyed steels.[7,19,20] Curves like those shown in Fig. 1 where the plateau is well
defined can be used to deduce the temperatures and times corresponding to different
recrystallized fractions. The points that define the start and the end of the plateau are
taken to plot the induced precipitation start (Ps) and finish (Pf) curves, respectively. In
this way recrystallization–precipitation–time–temperature (RPTT) diagrams can be
drawn.[6] The value of Xa does not vary between the Ps and Pf curves and is represented
by a horizontal line. Once the Pf curve is reached, the lines of each Xa drop again. Fig. 2
shows examples of RPTT diagrams of a V and an Al-microalloyed steel.
In general, the length of the plateaus in curves as those shown in Fig. 1 is longer for
the case of Nb-microalloyed steels, compared to V-steels. Al-microalloyed steels
usually present the shortest plateaus of recrystallization inhibition[6] or even no
plateaus.[21] This can be also observed comparing the time interval (distance) between
Ps and Pf curves in RPTT diagrams like those shown in Fig. 2. In steels where Ti is the
sole microalloying addition, the occurrence of plateaus in recrystallization curves is
possible[22] but less frequent. This is due to the high solubility temperature of TiN that
restricts the strain induced precipitation. If strain induced precipitation does not take
place or if it is insignificant, static recrystallisation follows the sigmoidal law versus
time continuously.[23]
Similar to recrystallization, the strain induced precipitation occurring mostly during
the time interval of the plateau can be supposed to obey Avrami's law[24], and the
precipitated fraction (Xp) can be expressed as:
6
n

 t  
 
X p  1  exp ln 0.95

 t0.05  

(3)
If Xp = 0.95 in Eq. (3), the following expression may be deduced:
1/ n
 ln 0.05 
t0.95  

 ln 0.95 
 t0.05
(4)
The times to reach 5% and 95% of precipitated volume fraction (t0.05 and t0.95,
respectively) can be assumed to coincide approximately with tN and t'N, the times
corresponding to the nose of the Ps and Pf curves, respectively.[4] Fig. 3 shows the
values of t0.95 as a function of t0.05 for all the steels studied. It can be seen that the steels
can be classified in three distinct groups according to the microalloying element. The
regression of the values of t0.05 and t0.95 for the different categories of microalloyed
steels gives the following equations:
Nb and Nb-Ti microalloyed steels:
t0.95  14 .15t0.05 1.0001
(5)
V and V-Ti steels:
t0.95  7.26 t0.05 0.9999
(6)
Al steels:
t0.95  2.1t0.05 1.02
(7)
Eqs. (5) to (7) confirm that strain-induced precipitation follows Avrami's law, as in
all cases the exponent found for the parameter t0.05 is close to 1, which agrees with Eq.
7
4. Previous figures and Eqs. (5)-(7) also show that the length or duration of the plateau
of recrystallization inhibition is not constant, but it is a function of the type of
microalloying element and follows the order: Nb>V>Al. The much shorter length of the
plateaus for Al-microalloyed steels can be well explained by the considerably coarser
sizes of AlN precipitates compared to other strain-induced precipitates, as seen in Fig. 4.
It is known that the pinning forces exerted by precipitates decrease for lower
precipitated volume fractions and coarser sizes.[6,22] The coarser size of AlN particles
results from the larger diffusion coefficient of Al compared to Nb and V, as well as the
higher precipitation temperature of AlN.[23] For similar reasons, it has been found that
Nb(C,N) particles nucleate earlier and grow faster than VN particles.[19] However, the
plateaus in Nb-microalloyed steels are longer than for the case of V-steels. This means
that, apart from the mean particle size (considering similar levels of precipitated
volume), other factors might have an effect on the duration of the inhibition of
recrystallization by precipitates. It has been previously suggested that the highest
effectiveness of Nb to inhibit static recrystallization comes from the smaller effect of V
in solution (solute drag) compared to Nb[14,25], but results presented in this work
correspond to times and temperatures were strain-induced precipitation is taking place.
Another possible explanation could be sought on the differences in the interaction
between precipitates and grain boundaries in Nb and V-steels. On this regard, other
alloying elements such as Mn and Mo can modify the activity coefficients of
microalloying and interstitial elements and the solubility of precipitates.[26]
Consequently, this can exert a complex influence on the retardation of the start and the
end of precipitation and on the extent of the temporary inhibition of recrystallization.
8
Finally, it must be taken into account that Nb precipitates have higher solubility
temperatures than V precipitates (as seen in Table 2) and the same can be said for the
nose temperatures. At higher precipitation temperatures diffusion is faster and
supersaturation is lower, so it can be expected that the balance between growth and
nucleation rates will be different in Nb precipitates and V precipitates. V particles
(formed at lower temperatures) will nucleate faster and will grow less than Nb particles,
which explains their lower average size. However, it has been previously found [27] that
Nb microalloyed steels can present a bimodal distribution of precipitate sizes at the end
of the plateaus, with an important amount of very fine precipitates. This can be
explained by the more sluggish nucleation at higher temperatures that allows to have a
significant population of “new” and relatively fine precipitates capable of inhibiting
recrystallization after long post-deformation times, despite the coarser average size of
Nb particles compared to V precipitates.
4. Conclusions
The extent of the temporary inhibition of recrystallization by strain-induced
precipitates is a function of microalloying element. Empirical equations obtained from
the study of more than twenty steels confirm that strain-induced precipitation kinetics
obeys Avrami's law and show that the duration of the plateau in the curves of
recrystallization kinetics follows the order: Nb>V>Al. When single additions of Ti are
employed, the formation of plateaus associated to strain-induced precipitation is
difficult because of the low solubility of TiN.
These results have important implications on the evolution of microstructure during
and at the end of thermomechanical processing of microalloyed steels and help to
9
explain why Nb is the most adequate element to obtain unrecrystallized austenite at the
end of hot rolling.
Acknowledgements
The authors gratefully acknowledge the financial support of Spanish Ministry of
Economy and Competitiveness thorough the project ref. MAT2011-29039-C02-02.
10
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S. Vervynckt, K. Verbeken, P. Thibaux, Y. Houbaert, Mater. Sci. Eng. A 2011,
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[14] H.L. Andrade, M.G. Akben, J.J. Jonas, Metall. Trans. A 1983, 14, 1967.
[15] S.F. Medina, C.A. Hernández, Acta Mater. 1996, 44, 137.
[16] S.F. Medina, C.A. Hernández, Acta Mater. 1996, 44, 149.
[17] C. Ghosh, V.V. Basabe, J.J. Jonas, Steel Research Int. 2013, 84, 490.
[18] A. Faessel, Rev. Métall. Cah. Inf. Tech. 1976, 33, 875.
[19] S.F. Medina, M. Gomez, P.P. Gomez, J. Mater. Sci. 2010, 45, 5553.
11
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382, 64.
[21] P. P. Suikkanen, V. T. E. Lang, M. C. Somani, D. A. Porter, L. P. Karjalainen,
ISIJ Int. 2012, 52, 471.
[22] M.I. Vega, S.F. Medina, A. Quispe, M. Gómez, P.P. Gómez, ISIJ Int. 2005, 45,
1878.
[23] S.F. Medina, M. Gomez, P. Valles, Steel Res. Int. 2010, 81, 1010.
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[25] M.G. Akben, I. Weiss, J.J. Jonas, Acta Metall. 1981, 29, 111.
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[27] S. F. Medina, A. Quispe, P. Valles, J. L. Baños, ISIJ Int. 1999, 39, 913.
12
(a)
(b)
Fig. 1. Recrystallized fraction (Xa) versus time (t). (a) V-microalloyed steel; (b) Nbmicroalloyed steel.
13
(a)
(b)
Fig. 2. RPTT diagrams. (a) V-microalloyed steel; (b) Al-microalloyed steel.
14
Fig. 3. Time to reach 95% of precipitated volume fraction (t0.95) as a function of the
time for 5% of precipitation (t0.05).
15
(a)
(b)
Fig. 4. TEM images of carbon replicas. (a) Nb carbonitrides in a 0.040% Nb steel;
(b) Al nitrides (coarse rectangles) and VN precipitates (fine particles) in a 0.05% V0.02% Al steel.
16
Table 1. Chemical compositions of steels studied [mass %].
Steel
C
Si
Mn
Al
V
Nb
Ti
N
V1
0.11
0.24
1.1
0.012
0.043
0.0105
V2
0.12
0.24
1.1
0.012
0.060
0.0123
V3
0.11
0.24
1.0
0.01
0.093
0.0144
V4
0.21
0.2
1.1
0.009
0.062
0.0134
V5
0.33
0.22
1.24
0.011
0.076
0.0146
V6
0.35
0.21
1.23
0.008
0.033
0.0121
V7
0.42
0.24
1.32
0.012
0.075
0.02
V8
0.37
0.24
1.42
0.012
0.120
0.019
TV1
0.55
0.29
1.06
TV2
0.34
0.22
1.08
0.009
N1
0.11
0.24
1.23
0.002
0.041
0.0112
N2
0.11
0.24
1.32
0.002
0.093
0.0119
N3
0.21
0.18
1.08
0.007
0.024
0.0058
N4
0.21
0.19
1.14
0.008
0.058
0.0061
N5
0.51
0.25
1.2
0.008
0.026
0.0105
N7
0.29
0.22
1.3
0.006
0.066
0.0062
N8
0.20
0.2
1.0
0.006
0.007
0.0056
N9
0.46
0.24
1.25
0.011
0.009
0.01
NT1
0.21
0.22
1.18
0.007
0.028
U7
0.095
0.321
1.525 0.029
0.003
Y1
0.099
0.297
1.463 0.037
0.002
Y7
0.102
0.284
1.479 0.02
0.063
0.019
0.0174
0.055
0.024
0.0182
0.004
0.024
0.006
0.0042
0.01
0.0158
17
Table 2. Testing conditions (reheating temperature, initial austenite grain size, strain,
strain rate), solubility temperatures[1] and phase transformation temperature Ar3 for the
steels studied.
Solubility temperatures (ºC)
Steel
V1
V2
V3
V4
V5
V6
V7
V8
TV1
TV2
N1
N2
N3
N4
N5
N7
N8
N9
NT1
U7
Y1
Y7
R.T
(ºC)
1230
1230
1100/1230
1100/1200
1200
1200
1200
1200
1200
1200
1230
1230
1250
1250
1275
1295
1250
1250
1250
1200
1200/1300
1200/1300
D
(μm)
172
167
125/165
95/180
165
170
162
157
31
53
122
116
210
190
430
415
140
190
55
127
165/550
151/550

0.20/0.35
0.20/0.35
0.20/0.35
0.35
0.20/0.35
0.20/0.35
0.35
0.20/0.35
0.20/0.35
0.35
0.20/0.35
0.20/0.35
0.20/0.35
0.20/0.35
0.35
0.20/0.35
0.20/0.35
0.20/0.35
0.20/0.35
0.20/0.35
0.20/0.35
0.20/0.35
𝜀̇
VC0.75
(s-1)
3.63
730
3.63
758
3.63
785
3.63
791
3.63
833
3.63
773
0.91/3.63 847
3.63
879
3.63
849
1.09/3.63 809
3.63
3.63
3.63
3.63
1.09/3.63
3.63
3.63
3.63
3.63
3.63
3.63
3.63
VN NbC0.87 NbN NbC0.7N0.2 TiC
968
1012
1070
1023
1051
957
1082
1126
1050
1041
719
746
18
TiN
AlN
1100
1120
1117
1095
1130
1069
1183
1176
1122
1228
1126
1241
1237
1302
987
1090
1144
878
1151
1232
1054
1129
1107
1141
965
1022
1069
912
1165
1249
1146
1233
1226
1272
1037
1116
1161
953
1207 1505
1174 1530 1133
919
925
976
994
1053
967
957
1084
1114 1438 979
1096
1244
1221
Ar3
(°C)
786
782
784
768
716
715
718
721
693
718
786
786
768
769
674
751
770
704
768
List of Figure Captions.
Fig. 1. Recrystallized fraction (Xa) versus time (t). (a) V-microalloyed steel; (b) Nbmicroalloyed steel.
Fig. 2. RPTT diagrams. (a) V-microalloyed steel; (b) Al-microalloyed steel.
Fig. 3. Time to reach 95% of precipitated volume fraction (t0.95) as a function of the
time for 5% of precipitation (t0.05).
Fig. 4. TEM images of carbon replicas. (a) Nb carbonitrides in a 0.040% Nb steel; (b)
Al nitrides (coarse rectangles) and VN precipitates (fine particles) in a 0.05% V-0.02%
Al steel.
List of Table Captions.
Table 1. Chemical compositions of steels studied [mass %].
Table 2. Testing conditions (reheating temperature, initial austenite grain size, strain,
strain rate), solubility temperatures[1] and phase transformation temperature Ar3 for the
steels studied.
19