E. KALINOWSKA-OZGOWICZ et al.: RECYSTALLIZATION OF HSLA STEEL AUSTENITE AS ...
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RECYSTALLIZATION OF HSLA STEEL AUSTENITE AS
REVEALED WITH A HOT-COMPRESSION TEST
REKRISTALIZACIJA AUSTENITNEGA HSLA JEKLA, DOLO^ENA
Z VRO^IM TLA^NIM PREIZKUSOM
El¿bieta Kalinowska-Ozgowicz1, Roman Kuziak2, Wojciech Ozgowicz3
1Lublin University of Technology, Fundamentals of Technology Faculty, 38 Nadbystrzycka Str., 20-618 Lublin, Poland
2Institute for Ferrous Metallurgy, 12 K. Miarki Str., 44-100 Gliwice, Poland
3Silesian University of Technology, Mechanical Engineering Faculty, Institute of Engineering Materials and Biomaterials,
18A Konarskiego Str., 44-100 Gliwice, Poland
wojciech.ozgowicz@tlen.pl
Prejem rokopisa – received: 2017-05-30; sprejem za objavo – accepted for publication: 2017-10-02
doi:10.17222/mit.2017.065
Hot deformation of the HSLA-type steel was investigated applying the method of the relaxation of stresses by means of a
Gleeble 3800 thermomechanical simulator. The influence of the conditions of plastic deformation on the shape of stress-strain
flow curves and the kinetics of thermally activated static processes were determined. It was found that the microstructure of the
investigated steel and the state of the precipitations of Nb and Ti carbonitrides after hot deformation depend mainly on the
degree of deformation and the time of the relaxation of stresses.
Keywords: HSLA steel, hot compression test, stress relaxation method, softening kinetics, microstructure
Avtorji v ~lanku opisujejo raziskavo vro~e deformacije malo legiranega jekla z veliko trdnostjo process (HSLA, angl.: High
Strength Low Alloyed) z uporabo metode sprostitve napetosti na termomehanskem simulatorju Gleeble 3800. Dolo~ili so vpliv
plasti~ne deformacije na obliko krivulj te~enja napetost-deformacija in kinetiko termi~no aktiviranih procesov. Ugotovili so, da
sta mikrostruktura preiskovanega jekla ter izlo~anje Nb in Ti karbonitridov po vro~i deformaciji, v glavnem odvisna od stopnje
deformacije in ~asa potrebnega za sprostitev napetosti.
Klju~ne besede: malolegirano jeklo z veliko trdnostjo (HSLA), vro~i tla~ni preizkus, metoda sprostitve napetosti, kinetika
meh~anja, mikrostruktura
1 INTRODUCTION
High-strength low-alloy (HSLA) steels are already
well established in the fields of numerous technical
applications due to the possibility of reducing the masses
of structures and an improvement in the strength of steel
products. The attainment of favorable mechanical and
exploitable properties of this group of material depends
particularly on the proper hot deformation, recrys-
tallization and final cooling of the products.1–4 This
process results in the creation of adequate mechanisms
of strain hardening and recrystallization of steel ensuring
the required properties of the products at the stage of
their exploitation.5–7 The mechanisms of hot deformation
and recrystallization are mostly analyzed based on the
shape of the stress-strain ( – ) flow curves and the
kinetic curve of the recrystallized fraction in the function
of the time of isothermal holding or cooling after the
deformation. These curves are usually determined with
various kinds of mechanical tests supported by a
metallographic investigation.8–12
The aim of the researches of the microalloyed
C-Mn-Nb steel was to determine the influence of the
parameters of the hot-compression tests of axially
symmetric samples on the process of strain hardening
and dynamic recrystallization and on the static kinetics
of thermally activated processes occurring isothermally
after the deformation. The method of the relaxation of
stresses was used for this purpose, applying a computer-
controlled Gleeble 3800 thermomechanical simulator.
2 EXPERIMENTAL PART
2.1 Material and methods
The tests were performed on an industrial melt of
microalloyed C-Mn-Nb steel (grade S355NL, according
to standard PN-EN10025-2:2007) of the HSLA type, the
chemical composition of which is seen in Table 1.
The steel was delivered as a profile of the economical
channel 240E in its raw state after hot rolling. Test
samples were cut from the profile in the direction of
Materiali in tehnologije / Materials and technology 52 (2018) 2, 157–161 157
UDK 66.065.53:669.15-194.56:669.15:620.173.25 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(2)157(2018)
Table 1: Chemical composition of the tested steel, in mass fractions (w/%)
Steel C Si Mn P S Cu Al N Nb V Ti Fe
C-Mn-Nb 0.16 0.29 1.48 0.030 0.017 0.049 0.01 0.0098 0.037 0.002 0.004 bal.
rolling. Plastometric tests applying the method of axially
symmetric hot compression were carried out on the
simulator Gleeble 3800, using cylindrical samples with a
diameter of 7 mm and a length of 8.4 mm. The samples
were resistance heated in an Ar atmosphere. In order to
reduce the friction on the surface, the samples were
lubricated with graphite and interlayered with a foil of
Ta. During the tests, the force was measured as a func-
tion of the anvil displacement, the temperature on the
side surface of the samples, and the shape of the samples
after upsetting. The drop in the compressive force was
recorded as a function of time. The kinetics of the static
softening after hot deformation was tested applying the
method of stress relaxation in the case of logarithmic
strain ( = ln(h0/h) where h0 and h are the initial and final
sample heights, respectively) within a range of 0.2–0.6 at
a temperature of 800–1200 °C and with the applied
strain rate () amounting to about 1 s–1. The samples
were austenitized for 360 s at 1200 °C. After the com-
pression test, the samples were immediately cooled
down in water so that their microstructure could be
analyzed. Etching with a saturated picric-acid solution
(Mi7Fe) was used, at a temperature of 70 °C, to reveal
prior austenite grains. Metallographic tests were carried
out on an OLYMPUS GX 71 (Japan) optical microscope
as well as on an electron microscope of the S/TEM/Titan
(FEI) type, within a range of 245–300 kV and with a
resolution of the image of less than 0.10 μm. Foils were
mechanically thinned and ionically polished in a PIPS
(GATAN) device.
3 RESULTS AND DISCUSSION
3.1 Analysis of the flow curves and the kinetics of
recrystallization
The – flow curves of the investigated steel,
obtained with the hot-compression tests, permitted us to
determine the influence of the conditions of plastic
deformation upon the strain hardening and the recrys-
tallization of deformed austenite. The steel deformed to
 = 1.2 displays flow curves varying in their shape,
depending generally on the temperature and the strain
rate (Figure 1). The flow curves recorded at the defor-
mation temperature of 1200 °C show a distinct maxi-
mum of flow stresses for the minimum strain rate and a
less obvious maximum of these stresses for the maxi-
mum strain rate (Figure 1a). However, the curves
obtained in the  test range clearly show that dynamic
recrystallization processes occur under these compres-
sion conditions, whereas at a lower strain temperature
(900 °C), flow curves in the same range of  indicate
unambiguously that only dynamic recovery occurs under
these compression conditions (Figure 1b).
The curves of the relaxation of stresses of the inves-
tigated steel in the range of the compression temperature
(800–1200 °C) generally display three distinct states of
softening (Figure 2). The share of the first and the third
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158 Materiali in tehnologije / Materials and technology 52 (2018) 2, 157–161
Figure 2: Curves of the stress relaxation of C-Mn-Nb steel, recorded
in the hot-compression test and the method of determining the
coefficients of linear equations of the stages of softening (I–III):
a) Tdef = 1200 °C,  = 0.14,  = 1s–1; b) Tdef = 800 °C,  = 0.2,  = 1s–1
Figure 1: Effect of deformation temperature and strain rate of the
compression test on the flow curves of the investigated steel
austenitized at 1200 °C
state in the drop of stresses is linear and occurs in the
case of a steady and relatively low change rate of the
stresses. This can be described with the logarithmic
dependence in Equation (1):
 = o –  log  (1)
where :  – actual stress,  – time of relaxation, o,  –
constants.
In the second state of softening, however, a violent
drop in the level of stresses was observed. The respond-
ing values of the coefficients in Equation (1) determined
for the analyzed stress relaxation curves were tabulated
in Figure 2. It was found that in the case of the inves-
tigated conditions of the compression test, the coefficient
of the inclination of the straight line () in the third state
of stress relaxation is distinctly lower than for the first
state and that the o values increase with the reducing
temperature of deformation. It may be suggested that this
depends essentially on the coefficients of the inclination
of stress relaxation versus the level of stresses. One
should think that the slope of the straight lines of
relaxation is a function of the material state, resulting
from the course of the static or metadynamic recovery
process, or a process similar to the creep of the material
rather than – as it was suggested previously11 – a result
of the machining effect of the Gleeble 3800 device.
The intensive drop in the level of stresses between
the first and the third state of the stress relaxation is
caused mainly by the process of static recrystallization or
metadynamic recrystallization. The respective curves of
the kinetics of these processes were determined analy-
tically based on dependence (Equation (2)), concerning
the investigated HSLA steel, and they are plotted on
Figure 3:
 = (1 – x) (o1 – I log ) + x(oIII – III log ) (2)
where: x – the share of the recrystallized fraction after
the time  starting at the recrystallization, I, III – indices
corresponding to the first and third state of the stress
relaxation.
It was found that the sigmoidal course of the kinetic
curves corresponds to the model exponential curve given
by L. P. Karjalainen.12 It was also found that the
isothermal holding of the investigated steel after its
deformation in the range of  (0.14–0.27) at 1200 °C
leads to the removal of the results of strain hardening
caused by the static recovery and recrystallization
(Figure 3a). At a lover temperature of deformation
(about 900 °C), besides these processes, a probable stage
of metadynamic recrystallization was also revealed
(Figure 3b).
3.2 Metallographic analysis
A metallographic analysis was performed with the
determination of both the deformation-time-tempera-
ture-transformation (DTTT) diagram and the characteris-
tic temperatures of phase transformations AC1 = 710 °C
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Materiali in tehnologije / Materials and technology 52 (2018) 2, 157–161 159
Figure 4: Influence of the degree and temperature of deformation of
C-Mn-Nb steel on the structure of the initial grains of austenite:
a)  = 0.268, Tdef = 1200 °C, b)  = 0.6, Tdef = 900 °C (TA = 1200 °C,
 = 1 s–1)
Figure 3: Influence of the degree of deformation on the process of
isothermal softening of C-Mn-Nb steel after its austenitization at 1200 °C
and compression tests at: a) 1200 °C, b) 900 °C, at the strain rate
amounting to 1 s–1
and AC3 = 809 °C. The knowledge provided with this
diagram allowed a detailed analysis of the phase trans-
formations of the examined steel under hot compression,
followed by cooling at a rate of approximately 70 °C/s to
about 30 °C/min. The martensitic transformation tempe-
rature was about 380 °C, while the bainitic transforma-
tion temperature was about 530 °C.
The results of the metallographic analysis of the
investigated HSLA steel, hot deformed in the compres-
sion test, were gathered in microphotographs (Figures 4
and 5). It was found that the revealed shape and size of
the austenite grains after the transformation 	 – ’
depend mainly on the state of the deformed austenite
structure and the course of thermally activated static
processes. A significant influence on these processes is
exerted mainly by the temperature and degree of de-
formation (Figure 4). The investigated steel compressed
with a deformation  of about 0.27 at the temperature of
1200 °C, indicates dynamically recrystallized fine grains
	 in the matrix of primary austenite grains sized about
70 μm (Figure 4a). In the case of a lower temperature of
deformation (800–900 °C) and when  is about 0.2–0.6,
the steel contains primary grains 	 in its structure, dist-
inctly indicating merely the effects of strain hardening or
dynamic recovery (Figure 4b).
TEM observations of the structures of thin foils also
revealed a significant influence of the hot-compression-
test conditions and isothermal-holding time for the
samples before the 	-’ transformation on the martensite
morphology and the state of precipitations in the tested
steel. This was identified after the steel had been cooled
down from the temperature of deformation in the
Gleeble device.
The resulting effects of the precipitation of interme-
tallic interstitial carbide and nitride phases – or probably
carbonitride Nb and Ti phases – inherited in martensite,
were generally identified with the technique of electron
diffraction and the method of dark field.
The deformation of the investigated steel in the
temperature range of 800–1200 °C and its isothermal
holding after the deformation results in a varying degree
of the formation of fine static precipitations of NbC and
TiN sized about 60 nm in the matrix of the steel, which
nucleate in the 	 phase and appear mainly at the boun-
daries and the lath of the š phase of a highly dense
dislocation (Figures 5 and 6) as well as in the zones of
twinned martensite. The images of diffractions also
revealed reflexes Fe3C, suggesting the possibility of a
formation of bands of lower bainite in the course of
cooling the HSLA steel (Figure 6). Similar effects of the
phase identification for HSLA steels were also dealt with
in the papers published by other authors.1,6,10
4 CONCLUSIONS
The results of the investigation concerning the
C-Mn-Nb steel of the HSLA type lead to the following
conclusions:
• Plastometric tests of the C-Mn-Nb HSLA steel
performed on the Gleeble 3800 device, according to
the method of hot compression and the relaxation of
stresses in the temperature range of 800–1200 °C,
facilitate a successful analysis of dynamic recrys-
tallization and the kinetics of recovery, static and
metadynamic recrystallization.
• The microstructure of the investigated steel and the
state of the precipitations of Nb and Ti carbonitrides,
following the hot deformation, at a strain rate of about
1s–1 within the temperature range of 800–1200 °C,
are mainly determined with the degree of defor-
mation and the time of the relaxation of stresses.
• Hot deformation in the investigated range of tem-
perature and strain rate ensures a refinement of the
initial 	 grains of the investigated steel to about
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Figure 6: Microstructure of the lath martensite with TiN and Fe3C
precipitations: a) bright field, b) dark field of (111)TiN plane,
c) diffraction, d) diffraction solution, C-Mn-Nb steel, TA = 1200 °C,
Tdef = 1100 °C,  = 1 s–1,  = 0.2,  = 30 s/water
Figure 5: Microstructure of the lath martensite with residual austenite
and precipitations of NbC: a) bright field, b) diffraction pattern,
c) solution of diffraction pattern, C-Mn-Nb steel, TA = 1200 °C,
Tdef = 1100 °C,  = 0.2,  = 1 s–1,  = 30 s/water
40 μm, proportionally to the decrease in the tempe-
rature and increase in the degree of deformation.
• The decrease in the temperature deformation of the
tested steel to about 800 °C and increase in the
degree of deformation to about 0.6 impede the
dynamic recrystallization and static removal of the
consequences of strain hardening, not only as a the
result of the recovery and static recrystallization, but
also due to metadynamic recrystallization.
5 REFERENCES
1 HSLA Steels 2011 – Proc. of the 6th Inter. Conf. on HSLA Steels,
Beijing, 2011
2 Y. Weng, H. Dong, Y. Gan, Advances Steels, The recent scenario in
steel science and technology, Part IV, Advanced HSLA Steels,
Metall. Press and Springer-Verlag, Berlin, Heidelberg, 2011
3 W. Ozgowicz, M. Opiela, A. Grajcar, E. Kalinowska-Ozgowicz, W.
Krukiewicz, Metallurgical products of microalloy constructional
steel, Journal of Achiev. in Mater. and Manuf. Eng., 44 (2011), 7–34
4 J. Patel, B. Wilshire, The challenge to produce consistent mechanical
properties in Nb-HSLA strip steels, Journal of Mater. Proc. Techn.,
120 (2002), 316–321
5 C. M. Sellars, From trial and error to computer modelling of
thermomechanical processing, Ironmaking and Steelmaking, 38
(2011) 4, 250–257
6 G. Purdy, Modeling recrystallization of microalloyed austenite:
effect of coupling recovery, precipitation and recrystallization, Acta
Mater., 50 (2002), 3075–3092
7 S. Chao, K. Kang, J. J. Jonas, The Dynamic, Static and Metadynamic
Recrystallization of a Nb-Microalloyed Steel, ISIJ Inter., 41 (2001),
63–69
8 B. Dutta, E .J. Palmiere, Effect of Prestrain and Deformation Tem-
perature on the Recrystallization Behavior of Steels Microalloyed
with Niobium, Metall. and Mater. Trans. A, 34 (2003) 6, 1237–1247
9 E. Kalinowska-Ozgowicz, Structural and mechanical factors of the
strengthening and recrystallization of hot plastic deformation of
steels with microadditives, Open Access Library, 20 (2013) 2, 1–246
10 W. J. Liu, J. J. Jonas, Nucleation kinetics of carbonitrides in
microalloyed austenite, Metall. Trans. A, 20 (1989) 4, 689–697
11 L. P. Karjalainen, Stress relaxation method for investigation of
softening kinetics in hot deformed steels, Mater. Sci. and Techn., 11
(1995), 557–565
12 L. P. Karjalainen, J. Perttula, Characteristics of static and meta-
dynamic recrystallization and strain accumulation in hot deformed
austenite as revealed by the stress relaxation method, ISIJ Inter., 36
(1996) 6, 729–736
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