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. Al-

steels 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]:

X a = 1- exp[−0 .693 ( tt0 .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:

t 0. 5=Aε p ε̇q Ds expQ x

RT (2)

where is the strain applied, ε̇ 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

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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

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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 V-

microalloyed 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:

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X p=1−exp [ ln0 . 95( tt0.05 )

n ](3)

If Xp = 0.95 in Eq. (3), the following expression may be deduced:

t 0. 95=(ln 0 .05ln 0 .95 )

1/n

⋅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:

t 0. 95=14 .15 (t 0. 05 )1 . 0001(5)

V and V-Ti steels:

t 0. 95=7 . 26 (t 0 .05 )0. 9999(6)

Al steels:

t0.95=2 .1 ( t0 .05)1. 02(7)

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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. 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]

8

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.

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.

9

These results have important implications on the evolution of microstructure during

and at the end of thermomechanical processing of microalloyed steels and help to

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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82, 1213.

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[13] J.S. Perttula, L.P. Karjalainen, Mater. Sci. Technol. 1998, 14, 626.

[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

[20] H.S. Zurob, C.R. Hutchinson, Y. Brechet, G.R. Purdy, Mater. Sci. Eng. A 2004,

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.

[24] Z. Wang, Q. Yong, X. Sun, Z. Yang, Z. LI, C. Zhang, Y. Weng, ISIJ Int. 2012,

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[25] M.G. Akben, I. Weiss, J.J. Jonas, Acta Metall. 1981, 29, 111.

[26] M.G. Akben, B. Bacroix, J.J. Jonas, Acta Metall. 1983, 31, 161.

[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) Nb-

microalloyed 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% V-

0.02% 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 0.063 0.019 0.0174

TV2 0.34 0.22 1.08 0.009 0.055 0.024 0.0182

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 0.024 0.006

U7

0.095 0.321 1.52

5

0.029 0.003 0.004 0.0042

Y1

0.099 0.297 1.46

3

0.037 0.002 0.01

Y7

0.102 0.284 1.47

9

0.02 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 R.T(ºC)

D(μm)

ε̇(s-1)

VC0.75 VN NbC0.87 NbN NbC0.7N0.2 TiC TiN AlN Ar3

(°C)V1

1230 172 0.20/0.35 3.63 730 9681100 786

V21230 167 0.20/0.35 3.63 758 1012

1120 782

V3 1100/1230 125/165 0.20/0.35 3.63 785 1070

1117 784

V4 1100/1200 95/180 0.35 3.63 791 1023

1095 768

V51200 165 0.20/0.35 3.63 833 1051

1130 716

V61200 170 0.20/0.35 3.63 773 957

1069 715

V71200 162 0.35 0.91/3.63 847 1082

1183 718

V81200 157 0.20/0.35 3.63 879 1126

1176 721

TV1 1200 31 0.20/0.35 3.63 849 1050 1207 1505 693TV2

1200 53 0.35 1.09/3.63 809 1041 1174 15301133 718

N1 1230 122 0.20/0.35 3.63 1122 1151 1165 919 786N2 1230 116 0.20/0.35 3.63 1228 1232 1249 925 786N3 1250 210 0.20/0.35 3.63 1126 1054 1146 976 768N4 1250 190 0.20/0.35 3.63 1241 1129 1233 994 769N5

1275 430 0.35 1.09/3.63 1237 1107 12261053 674

N7 1295 415 0.20/0.35 3.63 1302 1141 1272 967 751N8 1250 140 0.20/0.35 3.63 987 965 1037 957 770N9

1250 190 0.20/0.35 3.63 1090 1022 11161084 704

NT1 1250 55 0.20/0.35 3.63 1144 1069 1161 1114 1438 979 768U7 1200 127 0.20/0.35 3.63 719 878 912 953 109

6Y1 1200/130

0165/550 0.20/0.35 3.63 746 124

4Y7 1200/130

0151/550 0.20/0.35 3.63 122

1

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List of Figure Captions.

Fig. 1. Recrystallized fraction (Xa) versus time (t). (a) V-microalloyed steel; (b) Nb-

microalloyed 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