materials
ArticleHow the Thermomechanical Processing Can Modify the High
Strain Rate Mechanical Response of a Microalloyed Steel
Remigiusz Błoniarz 1, *, Janusz Majta 1, * , Bogdan Rutkowski 1 , Grzegorz Korpała 2 , Ulrich Prahl 2 ,Jacek Janiszewski 3 and Paulina Lisiecka-Graca 1
1

2

3

*

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Citation: Błoniarz, R.; Majta, J.;
Rutkowski, B.; Korpała, G.; Prahl, U.;
Janiszewski, J.; Lisiecka-Graca, P.
How the Thermomechanical
Processing Can Modify the High
Strain Rate Mechanical Response of a
Microalloyed Steel. Materials 2021, 14,

Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science and Technology,
30-059 Krakow, Poland; rutkowsk@agh.edu.pl (B.R.); graca@agh.edu.pl (P.L.-G.)
Institute of Metal Forming, TU Bergakademie Freiberg, 09599 Freiberg, Germany;
grzegorz.korpala@imf.tu-freiberg.de (G.K.); ulrich.prahl@imf.tu-freiberg.de (U.P.)
Faculty of Mechatronics, Armament and Aerospace, Military University of Technology,
00-908 Warsaw, Poland; jacek.janiszewski@wat.edu.pl
Correspondence: bloniarz@agh.edu.pl (R.B.); majta@metal.agh.edu.pl (J.M.)

Abstract: The effects of thermomechanical processing (TMP) on the mechanical response of microalloyed steels subjected to dynamic loading conditions were examined. The deformation conditions in
the thermomechanical laboratory rolling processes were selected on the basis of dilatometric tests.
It allowed (with a constant value of total deformation) us to obtain microstructures with different
compositions and morphology of the particular components. Several samples characterized by aparticularly complex and unexpected representation of the obtained microstructures were selected
for further research. Plastometric tests, i.e., compression and tensile tests, were performed underquasi-static loading with digital image correlation (DIC) analysis, and under dynamic loading on
the Split Hopkinson Pressure Bar (SHPB) apparatus with strain rates of 1400 and 2000 s−1 . Samples
deformed in such conditions were subjected to microstructural analysis and hardness measurements.
It has been observed that the use of various combinations of TMP parameters can result in the formation of specific microstructures, which in turn are the source of an attractive mechanical response
under dynamic loading conditions. This opens up new possible areas of application for such popular
structural materials which are microalloyed steels.

6062. https://doi.org/10.3390/
ma14206062

Keywords: thermomechanical processing; microalloyed steel; dynamic loading; mechanicalproperties; microstructure development

Academic Editor: Adam Grajcar
Received: 31 August 2021
Accepted: 7 October 2021
Published: 14 October 2021

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Licensee MDPI, Basel, Switzerland.
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Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

1. Introduction
The manufacturing of metal products in finished form or as semi-finished productswith excellent quality and an optimal combination of mechanical properties has long been
the main goal of technological development. Particularly high requirements exist forthese products if they are intended to operate in extremely difficult conditions, e.g., where
dynamic loads occur or may occur. Fortunately, the current state of knowledge allows
for a properly selected chemical composition to design parameters of thermomechanicalprocessing. This makes it possible to obtain the expected microstructures forming an
attractive metallic composite of soft and hard components with the desired morphology.
The current level of knowledge in the field of physical and mechanical metallurgy, andthe continuous progress in the automation of metal forming processes, enable the use of
precise settings of technological parameters and their control in industrial conditions.
A microalloyed steel, characterized by the increased strength due to the presenceof Nb, V and Ti precipitates, is a well-known and widely used structural material [1,2].
Contrary to plain carbon steels, the microstructure development in these steel grades isvery pronounced. Microalloyed steels are strengthened by precipitation, solid solution,
work hardening, grain refinement mechanisms as well as multi-phase microstructure. The

Materials 2021, 14, 6062. https://doi.org/10.3390/ma14206062

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results of research published so far on the influence of thermomechanical processing conditions on microstructural effects of microalloyed steels show that it is possible to obtain adifferent mechanical response for the same chemical composition [3,4]. The most importantones, from the point of view of creating mechanical properties, are the composition of the
microstructures and the morphology of individual components. It is commonly assumed
that the most advantageous for obtaining good plasticity (toughness) and high strength(yield stress, YS; tensile strength, TS) is to guarantee a significant microstructure refinement.
It turns out, however, that due to the appropriate phase composition, morphology ofindividual components and additional effects of controlled microstructural inhomogeneity, resulting from the specific arrangement of microstructural elements, it is possible toproduce an attractive structural materials. Such materials are dedicated to withstanding
specific loads, such as those observed under impact loading. It is already proved that
reducing the average grain size and additionally the formation of the bimodal and complex
phase (CP) microstructures lead to an optimization of the mechanical properties [3–5].
Further activity towards improving the mechanical properties, especially of microalloyedsteels, may be the use of a bimodal microstructure in order to improve the ductility andplasticity of these materials under conditions of operation of very high strain rates [6–9].
One of the factors hindering the broad application of high-strength steel products is theapparent ductility decreasing, especially under dynamic loading conditions. This problem
generally applies to body centered cubic crystal lattice (b.c.c.) metals whose strain ratesensitivity is high. Recently published works have suggested several ideas to improve theductility of modern steels by using dispersed oxide, cementite or martensite [10–12], theuse of a bimodal microstructure or distribution of grain size [13]. It is also possible to apply
deformation of the ultrafine grained (UFG) material at a very low temperature and/or
high strain rate, thereby increasing the high strain rate sensitivity or introducing heavy
twinning into the material during processing [14]. Hence, it can be assumed that not only
the microstructure refinement, but also the controlled and specific inhomogeneity of thecomplex microstructural components for a given chemical composition can be significant
from the point of view of high strain rate mechanical response of the microalloyed steels.
The main advantages of this approach are the avoidance of the need to use complex chemical compositions and the use of new technological processes, which is definitely more
expensive compared to the production of already well-mastered processes for the manufacture of microalloyed high strength low alloy (HSLA) steels. There is still no substantial
amount of research concerning the strain rate dependance of the mechanical responseof the complex phase compositions of microalloyed steels. In particular, plasticity andstrength at strain rate higher than 103 s−1 (corresponding to dynamic loading condition)
are important for applications in automotive bodies and in energy, naval or defense industries. The basic process parameters for obtaining a fine-grained and homogeneous
microstructure of the microalloyed steels with particular chemical composition are soaking
and deformation temperatures, prior austenite grain size, amount of deformation in theaustenite non-recrystallization region and the cooling rates during and after the austenitetransformation [15–18]. The recent progress and the latest achievements in thermomechanical processing of the first, second and third generations of advanced high-strength
steels (AHSSs) and NANO (New Application of Nano Obstacles for dislocation movement)
Nano Hiten steels have been very widely reviewed and thoroughly discussed in several
published works, such as that of Zhao and Jiang [19].
The aim of the study was to investigate the influence of thermomechanical treatment
on microstructural changes and the resulting mechanical response under high strain rateloading. Variations of all of the most important parameters of the thermomechanicalprocessing of Nb-treated microalloyed steel were included in the applied laboratory rolling
schemes. The evaluation of the influence of the obtained various microstructures for different histories of the deformation process was based mainly on the results of plastometrictests performed with the use of the Split Hopkinson Pressure Bar (SHPB) technique.

Materials 2021, 14, 6062

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Summarizing, the main purpose of the present study is to clarify how the specificmicrostructures produced by the thermomechanical processing of Nb-microalloyed steel
can affect strength and plasticity at very high strain rates. A wide and fully substantiatedset of experimental tests of microalloyed steels is presented in this work. It may prove to
be an indispensable tool for thermomechanical process control in order to produce new
structural materials intended for applications under dynamic loading conditions.
2. Materials and Methods
Microalloyed steels of 0.06 wt.% C, 0.058 wt.% Nb, 0.30 wt.% Si, 1.63 wt.% Mn,
0.045 wt.% V, 0.08 Mo, 0.02 wt.% Ti, 0.23 wt.% Ni and 0.006 wt.% N were examined. All
the material specimens were made from commercial sheet with a thickness of 14.6 mm.
The samples were first cold rolled on a 4-high laboratory rolling mill with a working rolldiameter of φ 100 mm, at the Department of Metal Forming, AGH UST, Cracow, Poland.
Total applied true strain was ε = 0.74 (thickness reduction z = 52% from 14.6 to 7 mm). Then,samples of φ 5 × 10 mm were cut according to longitudinal direction, and dilatometrictests were performed. Dilatometric tests as well as forming dilatometric tests were performed with the use of a quenching and forming dilatometer BÄHR Thermoanalyse GmbH
(Hüllhorst, Germany) DIL 805 A/D equipped with an inductive heating system. The
samples taken from cold-rolled flat bars were heated on a deformation dilatometer to theselected temperatures, 890, 900, 920 and 1000 ◦ C, with a heating rate of 1 ◦ C/s. The samples
were held at the preset temperature for 3 min. Next, one sample for each temperature wassubjected to compression with a strain equal to ε = 0.34, while the other sample was notdeformed. After deformation, the samples were cooled to room temperature at the rate4 ofMaterials 2021, 14, x FOR PEER REVIEW
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100 ◦ C/s. On the basis of dilatometric tests, thermomechanical rolling diagrams (presented
in Figure 1) were designed.

(a)

(c)

(b)

(d)

(e)

127

Figure 1. Schemes of applied thermomechanicaltreatment
for schedule A (hottreatment
rolling withpass) A(a)(hot
androlling
schedule
Figure 1. Schemes ofapplied thermomechanicalfor oneschedule
withB one
◦ C (c) and 1000 ◦ C(hot rolling with two passes) (b).
Theinitialmicrostructuresoftheausteniteobtainedafter
heatingto900pass) (a) and schedule B (hot rolling with two passes) (b). The initial microstructures of the(d). Examples of thermomechanicalrolling
schemes
forheating
individual
variants(e). 1000 °C (d). Examples of thermomechanicalausteniteobtained
after
to 900°C (c) androlling schemes for individual variants (e).

Microstructure studies were performed using Carl Zeiss AG (Oberkochen, Germany)
Axio Imager M1m and Reichert Optical Company (Vienna, Austria) Metavar microscopes.
Electron Back Scattering Diffraction (EBSD) analysis was performed using an FEI
Company (Hillsboro, United States) NanoSEM 450 microscope, with post-processing

128
129
130
131
132
133
134
135

Materials 2021, 14, 6062

4 of 17Microstructure studies were performed using Carl Zeiss AG (Oberkochen, Germany)
Axio Imager M1m and Reichert Optical Company (Vienna, Austria) Metavar microscopes.
Electron Back Scattering Diffraction (EBSD) analysis was performed using an FEI Company (Hillsboro, OR, USA) NanoSEM 450 microscope, with post-processing using Oxford
Instruments (Witney, United Kingdom) Chanel 5 software. Each EBSD measurement was
performed with a step size of 50 nm, and the limit for one grain qualification was 8 pixels.
Additionally, in the grain size analysis, all grains with an area smaller than 0.2 µm2 were
rejected for further processing. A Bruker Corporation (Billerica, MA, USA) AXS D8 Advance apparatus with a cobalt lamp generating X-rays of 1.7902 Å wavelength was used
for X-ray diffraction (XRD) analysis.
Two groups of samples characterized by a particularly complex representation of theobtained microstructures were selected for further research. Under quasi-static conditions,the samples prepared from flat bars were examined under quasi-static tensile loading using
Materials 2021, 14, x FOR PEER REVIEW
19
Instron (Norwood, MA, USA) 4502, whereas the cylindrical specimens were used 5inofcompression tests using for tensile configuration, and the Zwick Roell Group (Ulm, Germany)
Z250 machine was used for upsetting configuration. Tensile tests were accompanied byDantecbar
Dynamics(Skovlunde,Denmark)
Q400 DIC
strainanalysis
Hardness 157
striker
was driven
by a compressed
air systemwitha barrellengthequipment.
and inner diameter
maps
were
made
using
Zwick
Roell
Group
(Ulm,
Germany)
3212
Hardness
Tester.
of 1200 mm and 12.1 mm, respectively. The impact striker bar velocities applied during 158
High strain wererate compression
tests21wereusingrates
the classicalthe experiments
between 9 andm/s,performed
ensuring strain
betweensplit
150 Hopkinsonand 2200 159−1pressurebar(SHPB)
systembuilt
attheMilitary
UniversityofTechnology,
Warsaw,s for the specified size of specimens used. Cylindrical samples of 4 mm length and 4Poland.
mm 160The
basic wereparameters
shown in Figure 2 are as follows: input and output 161
diameterappliedofinthe
allbarthesystem
SHPB experiments.
bar 1200
mm long,
strikerbar 250mm
longand all bars
mmin diameter.
The bars 162
The pulse
shapingtechniquewasused
to minimize
the 12wave
dispersion
by damping
weremade of the heat treatedmaragingoscillations
steel gradeand
MS350(yield strength—2300
MPa; 163
the Pochhammer–Chreehigh-frequency
to facilitate
stress equilibrium.
Young
modulus—196
GPa;elastic
wave
speed—4960
m/s).
The
strikerbar
wasdrivenThe pulse shaper size was chosen for the given striker impact velocity and mechanicalby 164
a compressed
systemwith aItbarrel
lengththat
andforinner
diameter
1200
mm anda 12.1mm, 165
responseof theairtestedmaterial.
was founda given
SHPBoftest
condition,copperrespectively.
The
impactstrikerbarvelocities
appliedduringtheexperiments
werebetweenpulse shaper with a 3 mm diameter and thickness ranging from 0.1 and 0.3 mm guarantees 166
9 and 21 m/s,
strain oscillations
rates between
and 2200
s−1 forstressthe specified
sizeinof 167
dampingof the ensuringhigh-frequency
and150achieving
dynamic
equilibrium
specimens
used.
Cylindrical
4 mm
lengthmm diameter
wereappliedthespecimen.
A greasebasedsamples
on MoS2ofwas
appliedtoand
the 4contact
interfaces
betweenthein 168
all the SHPB
specimen
andexperiments.
the bars to minimize the interfacial friction.
169

170

FigureFigure 2.
2. Scheme
Scheme ofof thetheSHPB
SHPBsetup.
setup.

171

The pulse
wave shaping
signals in
the inputwas
andusedoutputbars were
capturedusing strain
gauges 172
techniqueto minimize
thewave dispersion
by damping
attached
symmetricallytotheoppositesurfaces
of
the
bars
and
in
their
middlelength.
The 173
the Pochhammer–Chree high-frequency oscillations and to facilitate stress equilibrium.
straingaugeswereconnected
toafull
Wheatstonebridgeconfiguration.
Typicalelectrical
The pulse shaper size was chosen for the given striker impact velocity and mechanical 174
straingauges
1.6 mmgaugeIt length
werethat
used
VishayaMicro
responseof theoftested
material.
was foundfor (CEA-13-062UW-350,
a given SHPB test condition,copper 175
Measurements
Inc.a [Raleigh,
UnitedandStates]).
The ranging
amplifiedsignalsof thestraingauges 176
pulse
shaper with
3 mm diameter
thicknessfrom
0.1 and
0.3 mmguarantees
wererecordeda signal conditioning
frequency
bandstressof 1MHz(SGA-0Bin 177
dampingof theusing
high-frequency
oscillationsunit
andwithachieving
dynamic
equilibrium
V5
conditioning
amplifiers,
ESA
Messtechnik,
Olching,
the Wheatstone
specimen. Abridge
greasewith
basedsignal
on MoS
wasappliedtothecontact
interfaces
betweenthe 178
2
Germany)
and thea dataacquisition
system(LeCroy Chestnut
specimen andbars
to minimize
the interfacial
friction.Ridge [Chestnut Ridge, United 179
States],high-speed
digitalTheWJ354A
wave signals
in theinputoscilloscope).
and output bars were captured using strain gauges 180
Samples
deformed
duringthetests
subjected
to 181
attached symmetrically to the oppositeplastometricsurfaces of thebarswereand inagain
their middle
length.
microstructural
analysis(optical microscopy,
OM; transmission
electron microscopy,Thestrain gauges
were connected
to a full Wheatstonebridge configuration.
Typical 182
TEM)
andstrain
hardnessmeasurements.
Lamellaeforwerethe TEM
were pre-paredelectricalgaugesof 1.6 mm gauge
lengthused observation(CEA-13-062UW-350,
Vishay 183
withCompany (Hillsboro,
UnitedHelios
NanoLab™
600i.gauges
The 184
MicroFEI
Measurements
Inc. [Raleigh,NC,States)USA]).SEM/Ga-FIB
The amplified
signalsof the strain
TEM microscopeusedaforstructure
analysis unit
was with
FEI Company(Hillsboro,
States) 185
wererecorded using
signalconditioning
frequency
band of 1United
MHz (SGA-0B
Tecnai G2 20 TWIN. Mapping of structural orientation based on diffraction patterns was 186
performed by NanoMEGAS SPRL (Brussles, Belgium) ASTAR hardware with ACOM 187
software.
188
3. Results and Discussion

189

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5 of 17

V5 Wheatstone bridge with signal conditioning amplifiers, ESA Messtechnik, Olching,
Germany) and a data acquisition system (LeCroy Chestnut Ridge [Chestnut Ridge, NY,
USA], WJ354A high-speed digital oscilloscope).
Samples deformed during the plastometric tests were again subjected to microstructural analysis (optical microscopy, OM; transmission electron microscopy, TEM) and hardness measurements. Lamellae for the TEM observation were pre-pared with FEI Company(Hillsboro, OR, USA) SEM/Ga-FIB Helios NanoLab™ 600i. The TEM microscope used forstructure analysis was FEI Company (Hillsboro, OR, USA) Tecnai G2 20 TWIN. Mappingof structural orientation based on diffraction patterns was performed by NanoMEGASSPRL (Brussles, Belgium) ASTAR hardware with ACOM software.
3. Results and Discussion
Thermomechanical processing is a metallurgical process that integrates work hardening and heat treatment into a single process [20]. Thus, by using appropriate combinationsof deformation history and cooling rates in the appropriate temperature ranges in austenite or during its phase transition, we can, for a given microalloyed steel with specified
chemical composition, produce the expected mechanical properties that are primarily afunction of the steel0 s microstructure [21,22]. Generally, all microstructures obtained in thepresent study consist of ferrite/bainite matrix with a small amount of retained austenite,martensite and perlite. Nevertheless, the basic differences between the obtained materials
result from the content of individual structural components and their specific morphology.
On the other hand, favorable combinations of mechanical properties, including, in particular, strength and ductility under dynamic loading conditions, result from the structural
inhomogeneity being a direct result of the thermomechanical processes carried out.
3.1. Dilatometric Tests
The main goal of the present study was to evaluate mechanical properties of samples
with various, specifically prepared during the thermomechanical processing microstructures, deformed with high strain-rates. In order to precisely determine the critical temperatures for the austenitic range, the experiments began with dilatometric tests. Information
was obtained on the starting temperature of the austenite–ferrite transformation for different reheating rates of the samples to the temperatures at which the rolling processes werecarried out. The results obtained from the conducted tests are presented in Table 1. The key
heating transformation temperatures were established by dilatometric testing at different
reheating rates: 1, 10, 30, 50, 70 and 100 K/s. The approximate temperature of the end of
the austenitic transformation of Ac1 was taken as 880 ◦ C. The differences in the durationof phase transitions resulting from the reheating rate turned out to be negligible from thetechnological point of view.
Table 1. Critical temperatures determined during reheating at different rates.
Transformation during Reheating up to 1200 ◦ C
Ac1 , ◦ C
Ac3 , ◦ C

1 K/s
800
892

10 K/s820
870

30 K/s820
880

50 K/s820
880

70 K/s820
883

100 K/s
825
885

The austenite grain size is essential for the production of a specific final microstructure, strength and toughness of microalloyed steels after thermomechanical processing.
Qualitatively and quantitatively, the austenite microstructure is already formed at the stageof reheating to the austenitizing temperature. Therefore, information from the dilatometrictests, which allowed for an assessment of the role of reheating rate, were investigated at
the beginning of the present study.
Subsequent samples were tested in order to assess the influence of austenite deformation and austenitization temperature on phase transformations during cooling. Fouraustenitization temperatures were selected. After annealing (reheating rate 1 K/s) andMaterials 2021, 14, 6062

6 of 17austenitization (withstanding for 180 s), one sample was cooled and the other was deformed with a strain of 0.34, and then cooled. The cooling medium was nitrogen gas.
Materials 2021, 14, x FOR PEER REVIEW
of 19 3,
Recorded dilatometric curves with assigned critical temperatures are presented in 7Figure
where the final microstructures after the tests are also presented.

231

Figure 3. Influence of initial deformation on the course of dilatometric curves (a) and
Figure 3. Influence of initial deformation on the course of dilatometric curves (a) and microstructures of the specimens (b).
microstructures of the specimens (b).

232
233

A temperature of 890 ◦ C has been chosen as the nearest temperature at which stable
A temperature of 890 °C has been chosen as the nearest temperature at which stable 234
austenite exist. However, dilatometric tests do not correspond with real rolling processes
austenite exist. However, dilatometric tests do not correspond with real rolling processes 235
because of the temperature drop during the transfer of the billet from the furnace to the rolls.
because of the temperature drop during the transfer of the billet from the furnace to the 236
This temperature drop is estimated as 10 ◦ C. For this reason, an austenitization temperaturerolls. This temperature drop is estimated as 10 °C. For this reason, an austenitization 237
of 900 ◦ C was applied in order to achieve an 890 ◦ C billet temperature during deformation.
temperature of 900 °C was applied in order to achieve an 890 °C billet temperature during 238
During the laboratory rolling process, the billet size is relatively small compared todeformation.
239
industrial processes, so the correct choice of temperatures may be disturbed due to the
During the laboratory rolling process, the billet size is relatively small compared to 240
temperaturefluctuations and non-uniform distribution that occur. Moreover, in order toindustrial processes, so the correct choice of temperatures may be disturbed due to the 241
assesstheimpactof the intensityof these changes
on the kinetics
of austenitetransformation
temperature fluctuations
and non-uniform
distributionthat occur.
Moreover,in order to 242◦ C was included
andthemorphology
ofits
products,
anaustenitization
temperatureof920
assess the impact of the intensity of these changes on the kinetics of austenite 243
intransformation
the adopted thermomechanicalschemes.
The influenceof the austenite
and the morphology processingof its products,
an austenitization
temperatureof 920 244◦
deformationsattemperatureshigherthanaustenite
recrystallization(estimatedas 980°C was included in the adopted thermomechanical processing schemes. The influenceof C,245
based
on chemicalcomposition)require higherhigher
austenitization
temperature,so 1000 ◦ C246
the austenite
deformationsat temperatures
than austenite
recrystallization
waschosen as
temperature.
different kinds
coolingaustenitization
strategies were247(estimatedas the980 last°C, test
based
on chemicalTwo
composition)requireofhigher
applied:in air so
andin water.
temperature,1000°C was chosen as the last test temperature. Two different kinds of 248
In thefirst stage
ofapplied:the conducted
research,cooling
strategies
werein air andin water.the images of microstructures and the249
resultingmechanicalof samples
deformed
underofdifferent
conditions
ther-250
In thefirst stagepropertiesof the conducted
research,
the images
microstructures
andofthemomechanicalprocessing
with thetotaldeformed
deformation
were
analyzed.
Thus, forofthe251
resulting mechanicalproperties
ofsamesamplesunderdifferent
conditions
same
thickness of rolled
flat bars,
severalstructural
materials ofdifferent
properties
were252
thermomechanicalprocessing
withthe same
total deformation
were
analyzed.
Thus, forobtained.
thermomechanicaltreatmentin theofpresent
are pre-253
the same The
thicknessof rolled flat bars,
several schemes
structuralusedmaterials
differentstudy
properties
were obtained.
thermomechanicaltreatment
schemes usedwein thepresent
sentedin Figure The
1. However,for the following
investigations,decidedto study
study,aremore254
presented inonly
Figure1. However,for Onthe thefollowing
investigations,decidedto study,
particularly,
selectedschemes.
basis ofdilatometric wetests,
samples
for ther-255
more particularly,
onlywereselectedschemes.
On the 1A,
basis
of 3A,
dilatometrictests,
momechanicalrolling
prepared.
Samples
1B,
3B, 4A, 4B,
6Asamplesand 6B for
were256
thermomechanicalrolling were prepared.
1A, 1B, for
3A,selecting
3B, 4A, 4B,
6A and for
6B wereselected(rolling schedules—see
Figure 1).Samples
The criterion
samplesfurther257
selected(rollingschedules—see
1). The criterion
for selecting samples
further 258
tests
resultedfromthe degree ofFigure
differentiationof microstructuresat room fortemperature
tests
resulted
the degree
of indifferentiationof microstructuresat room
temperature
259
and
the
processfrom
parameters
used
the completed
thermomechanicalrolling
tests.
and the process parameters used in the completed thermomechanical rolling tests.
260
3.2. Microstructural Studies
3.2. Microstructural
Microstructural Studies
investigations of the specimens produced by the TMP were performed.261
Microstructuralinvestigations
the specimens
producedcomponents
by the TMP
were
Both thephase composition
and the ofmorphology
of individualof thestud-262
performed.
Both
the
phase
composition
andthemorphologyofindividualcomponents
of of263
ied microstructures were assessed. Examples of the results of the obtained images
the studied microstructures were assessed. Examples of the results of the obtained images 264
of microstructures are shown in Figure 4. The microstructural analysis was performed 265
using samples cut, in a plane parallel to the rolling direction, from (i) the near surface (1 266

Materials 2021, 14, 6062

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microstructures are shown in Figure 4. The microstructural analysis was performed using
samples cut, in a plane parallel to the rolling direction, from (i) the near surface (1 mm slice
for prior austenite grain revealing due to low ability of that steel for martensitic transformation) and (ii) mid-thickness location of a 5 mm thick final flat bar. The microstructuralcharacterization was carried out by optical and scanning electron microscopy (SEM) withEBSD technique. High mechanical properties, including impact toughness, require not
only higher strength of the microstructure components, such as non-polygonal ferrite,
acicular ferrite, bainites or martensite achieved due to dedicated chemical compositionand high cooling rates, but also a controlled degree of their refinement and finally, in thecase of many phases, of their proper arrangement in the micro-regions. Microstructural
studies have revealed that bainite is the dominant microstructural component for the
produced materials. Nevertheless, from the point of view of mechanical properties, an
important factor characterizing the obtained microstructures is their heterogeneity andthe morphology of individual components. It can be noticed (Figure 4) that in the case ofsamples 1A and 1B, the obtained microstructure is characterized by the highest proportionof ferrite with a fairly uniform degree of refinement, especially in the case of sample 1A.
The greatest microstructural inhomogeneity can be seen in the images of samples 4A,
especially 4B (Figure 4). The latter is dominated by a characteristic image of areas marked
with granular ferrite, corresponding to the primary austenite structure, filled with a mixture of bainite, martensite and acicular ferrite. The microstructure revealed in sample 3A(Figure 4) shows a distinct bimodal character, i.e., there are uniformly distributed grains ofvery fine and much larger diameters. Such a microstructure, as already mentioned in [6–9],
is very advantageous from the point of view of the optimal combination of ductility andhigh strength.
It should be expected that the observed characteristics of the microstructures, i.e.,significant differences in morphology, phase composition and structure refinement, will
have a significant impact on the mechanical responses of the tested samples under bothquasi-static and dynamic loading conditions.
The quantitative assessment of the retained austenite was also the result of the performed analysis. The thermomechanically rolled specimens were examined using X-ray
diffraction analysis (XRD). Based on the diffractograms, the volume fraction of retained
austenite in the tested samples was calculated. Based on the evaluation of the microstructures, the volume fraction and the morphology of individual structural components andhistograms showing the degree of disorientation within the grain boundaries, the dominant
phases occurring in each of the microstructures were identified. The grain sizes presented
in Table 2 were determined on the basis of maps from the EBSD analysis, where 0.2 µm2
was assumed as the minimum grain area.
Table 2. The results of the analysis of microstructure components for the applied thermomechanical rolling schemes(Figure 1) and the quantitative assessment of the degree of refinement as well as the amount of retained austenite.
SchemeMicrostructure Components

Average Grain Diameter (EBSD), µm

Retained Austenite Fraction, %

1A
1B
2A
2B
3A
3B
4A
4B
5A
5B
6A
6B

Granular bainite
Granular bainite
Granular bainite
Granular bainite/upper bainite
Granular bainite
Upper bainite
Granular bainite
Upper bainite
Upper bainite
Upper bainite
Upper bainite/lower bainite
Upper bainite/lower bainite

2.28
2.33
2.23
2.12
2.19
2
1.76 (1.67 µm taken from thin film from TEM)
1.99
1.89
2.02
1.73
1.71

5.73
3.47
2.83
4.2
1.12
4.88

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8 of 17
9 of 19290

Figure4. Microstructuresof samples
produced byrolling.
variousMicrostructures
thermomechanicalrolling.
Figure 4. Microstructures ofsamples
produced by various
thermomechanicalobtainedby opticalobtained by optical microscopy and SEM using the EBSD technique.
microscopy and SEM usingMicrostructures
the EBSD technique.

It should be expected that the observed characteristics of the microstructures, i.e.,significant differences in morphology, phase composition and structure refinement, will

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9 of 173.3. Plastometric Tests
On the basis of the microstructural tests performed and the aforementioned criterionof the selection of the research material, samples were selected for plastometric tests. In
the first stage of the study, the quasi-static tensile tests were carried out. In addition tothe obtained stress–strain curves, images of strain distributions were also produced using
the DIC system. These tests allowed us to determine the mechanical characteristics of all
Materials 2021, 14, x FOR PEER REVIEW
11 oftests
19 andtested samples under quasi-static loads. The examples of results of the tensile
measurements with the DIC system are shown in Figure 5. The representation of the initial
microstructures in the samples before test is also shown.
319

320

Figuretensiletesttest
results(a).(a).
Measurement
results
and deformation
distributions
made madebyFigure 5.
5.Examples
Examplesofofthethequasi-static
quasi-static
tensileresults
Measurement
results
and deformation
distributions
by 321means of DIC (b). The representations of the initial microstructures (c) in the specimens are also presented.
322
means of DIC (b). The representations of the initial microstructures (c) in the specimens are also presented.

As it was mentioned earlier, the microstructures obtained after thermomechanicalrolling were formed mainly by ferrite–pearlite (specimens 1A, 1B) or bainite–martensite
and acicular ferrite reaching (specimens 4A, 4B). The results presented in Figure 4 clearlyshow that hard microstructural components (perlite, bainite, martensite) exerted asignificant influence on the mechanical response of the tested samples. These observations
are consistent with literature [23,24] where it was shown that the ferrite was the soft

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As it was mentioned earlier, the microstructures obtained after thermomechanicalrolling were formed mainly by ferrite–pearlite (specimens 1A, 1B) or bainite–martensite andacicular ferrite reaching (specimens 4A, 4B). The results presented in Figure 4 clearly showthat hard microstructural components (perlite, bainite, martensite) exerted a significant
influence on the mechanical response of the tested samples. These observations are consisMaterials 2021, 14, x FOR PEER REVIEW
12 of 19
tent with literature [23,24] where it was shown that the ferrite was the soft microstructurecomponent with high elongation, whereas bainite was the hard microstructure component
with high strength. We can expect that in the high strain rate regime, the retained austenite
strain rate incomparison
the ferrite andbainite, whichofusually
positivestrain rate
(specimens
1A,1B) can betoconsidered
as independent
strain show
rate incomparison
to the 332
333sensitivity.
ferrite and bainite, which usually show positive strain rate sensitivity.
An
importantfeatureofstructural
materials,
especiallythoseintendedfor
An important feature of structural materials, especially those intended for applications 334
applications
in various
loaddueconditions
to the mechanicalstate, susceptibility
is their susceptibility
in various loadconditions
to the due
mechanicalstate, is their
to work 335
to work hardening.
For this reason,in theofcase
the materials
producedin thisstudy
hardening.
For this reason,
also in also
the case
theofmaterials
producedin thisstudy
after 336
afterthermomechanicalrolling,tests
were
carriedoutthatallowedustocomparethethermomechanical rolling, tests were carried out that allowed us to compare the mechanical 337
mechanical response of individual samples in the function of deformation distribution 338
response
of individual samples in the function of deformation distribution and as a result
and as a result of work hardening. First, the samples with a special shape (Figure 6) were 339
of work hardening. First, the samples with a special shape (Figure 6) were subjectedsubjected to the tension tests with this same range of the total elongations. During loading, 340
to the tension tests with this same range of the total elongations. During loading, thethe measurements of strain distributions were made using the DIC system [25], to 341
measurements of strain distributions were made using the DIC system [25], to determine
determine the degree of the localizations of deformation. Then, the deformed samples 342
the degree of the localizations of deformation. Then, the deformed samples were subjected
were subjected to hardness measurements in order to assess the characterization of work 343
to hardness measurements in order to assess the characterization of work hardening
hardening of the various microstructures. The results of the performed quasi-static tensile 344of the various microstructures. The results of the performed quasi-static tensile tests
tests indicate significant differences in the ability of work hardening between the tested 345
indicatesignificant differences in the ability of work hardening between the tested samples.
samples. Comparing samples 1A and 1B with 4A and 4B, it can be seen that in the case of 346
Comparingand 1B tendencywith 4A and
4B, it can beofseen
in the caseformer,the former,samples
there is 1A
a stronger
for localization
the thatdeformation.
In ofthethecaseof 347thereisastrongertendencyforlocalizationofthedeformation.
In
the
case
of
sample6, in 348
sample 6, in general, higher hardness values and weaker tendency for strain localization
general,higherhardnessvaluesandweakertendencyforstrainlocalizationwere
observed.
were observed.
349

350

Figure 6.andMapsof strain
and hardness
in samples
produced as arolling.
result ofSpecimens
Figure 6. Maps of strain distributions
hardnessin distributions
samples produced
as a resultof thermomechanicalthermomechanicalrolling.
Specimens
withaspecial,arcuate
shape
of
the
deformation
zone werewith a special, arcuate shape of the deformation zone were elongated until the maximum tensile force was reached.
elongated until the maximum tensile force was reached.

351
352
353

It should be mentioned here that, as it is generally known, the formation of a local
It should
mentioned
here
that, asit is ofgenerally
known, the formation
a localstress stateat thebemicrolevel onvarioustypes
stress concentrators,e.g., hardofphaseand 354stress
state
at
the
micro
level
on
varioustypes
ofstressconcentrators,
e.g.,
hard
phase
and
soft ferrite interfaces, facilitates the dissipation of deformation energy on a macro scale.355
soft ferrite interfaces, facilitates the dissipation of deformation energy on a macro scale. 356
The
possibility of evaluating the characteristics of work hardening, which is presented
The possibility of evaluating the characteristics of work hardening, which is presented in 357
in Figure 5 as an effect in the form of localization of both deformation and strengthening
Figure 5 as an effect in the form of localization of both deformation and strengthening 358(hardness), is of key importance in this case. Therefore, it is reasonable to follow the(hardness), is of key importance in this case. Therefore, it is reasonable to follow the 359
relationships between the local inhomogeneous states of the microstructure, hard andrelationships between the local inhomogeneous states of the microstructure, hard and soft 360
components, and the resulting macro-scale consequences of the mechanical response. This 361
dependence is of particular importance in dynamic load conditions, where the wave 362
pattern of stress spreading is observed.
363
The next stage of the research included the measurements of impact toughness and 364

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13 of 19soft components, and the resulting macro-scale consequences of the mechanical response.
materialin the initialstate, basedon the analysisof theload
fracture
energy,the producedThis dependence
is of particularimportancein dynamic
conditions,
wherethe wavematerials
clear, dominantdivision in terms of cooling conditions. The samples
pattern of show
stress aspreading
is observed.
The
nextstagein
ofair
thehave
researchincluded
theslightly
measurementsof impact
toughnesscooledafter
rollinga fractureenergyhigher thanor equal
to that ofandthetheinterpretationoftheobtained
results.
It
canbenoticed(Figure7)
thatin
relationsample in the input state, while for samples cooled in water, this energy is much lower.tothe material in the initial state, based on the analysis of the fracture energy, the produced
Figure7. Energy
for breaking
in Charpy(impact
toughness)for V-notchedsamplesmaterials
showconsumed
a clear, dominantdivisionin testterms
of coolingconditions.
The samples
with
not-normalized
dimensions:width/thickness/length=
10/4/55mm.
The
highestvaluescooled after rolling in air have a fracture energy slightly higher than or equal to thatofof theenergypercentage
deviation
in forthe samples
negative arepresented.
sampleandin theinput state,whilecooledin water, this energy is much lower.

366
367
368
369
370
371
372

373

Figure 7. Energy consumed for breaking in Charpy test (impact toughness) for V-notched samples
analyzing
the obtained
results in more detail,it can beseen
thatin thevalues
case ofwith When
not-normalized
dimensions:width/thickness/length= 10/4/55
mm.
The
highestofthermomechanicalrolling
accordingtoschemeA,samples
1A
and
2A
almostdonotdifferenergy and percentage deviation in the negative are presented.

in the breaking energy from the input material. The increase in impact toughness wasobservedinanalyzing
the case ofthe
theobtained
sample resultsrolled according
to theitscheme
3A andshouldbeWhenin more detail,can be seen
thatitin
the case
notedthat this increaserollingis the according
greatest among
all tested
samples.
the imageofof thermomechanicalto schemeA, samples
1AObserving
and 2A almost
do notthemicrostructure
for energy
this sample
shouldbe assumed
that highintoughness
value indifferin the breaking
from3A,
theitinput
material.
The increase
impact toughness
thisis a direct
ofthe
thesample
bimodaldistribution
ofto
significantlylarge
was case
observedin theresultcase ofrolledaccordingthe schemerefined
3A andand
it should
be noted
that thisincrease
the greatest
among
all testedinsamples.
image
grains[6–9,13].
The
rollingispattern
B (twohot passes),turn, in Observing
each case theshows
anof the microstructure
forenergy
this sample
3A, it should
be assumed
high toughness
increasein the fracture
for air-cooled
samples,and the thatobtained
results arevaluevery
in this caseis a other.
direct resultof the bimodal distributionof significantly
andsimilarto eachThermomechanicallyrolled samples
according torefined
schemes
1Blargeandgrains[6–9,13]. Theby
rolling
patterntoughness,
B (two hotand
passes),
in be
turn,
in eachcaseshows an2B
are characterized
high impact
this canrelatedto themorphology
increase
the fracture
for air-cooled
samples,
obtained results
are veryof
their instructure.
The energy
observed
SEM fractures
andandthethemicrostructuresindicateasimilartoeachother.
Thermomechanicallyrolledsamplesaccordingtoschemes
1B
2B
relatively uniform distribution of both larger and finely comminuted grains, whichandon theare characterized
impacttoughness,and thisfrom
can thebe related
to theofmorphologyofone
hand ensuresby
anhighincreasein strength
resultingrefinement
the structure,
theiratstructure.
fracturesand themicrostructuresindicate
relatively
andthe same The
timeobserved
does not SEM
generate
privileged
places(as in the caseof verya fine
grain
uniform distributionof bothandlargerandfinely comminuted grains, which on the one
conglomerates)
for initiation
crack
propagation.
handTheensuresanincreaseinstrengthresultingfrom the properties
refinementofofthe
thetested
structure,andinfluence of the strain rate on the mechanicalsamples
at
the
same
time
does
not
generate
privileged
places
(as
in
the
case
of
very
fine
grain
was presented in Figure 8, in which the quasi-static and dynamic stress–strain curves
conglomerates)
for initiation
andpropagation.
obtainedfor samples
subjected
to crack
different
histories of thermomechanical treatment wereThe
influenceofthestrainrateonthemechanical properties of the tested samples wascollected.
presented in Figure 8, in which the quasi-static and dynamic stress–strain curves obtained
for samples subjected to different histories of thermomechanical treatment were collected.

374
375
376
377
378
379
380
381
382
383
384
385
386
387
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395

396
Figure 8. Quasi-static and high strain rate curves for microalloyed steel subjected to thermomechanical rolling. MicrostrucFigure
8.
Quasi-staticandhighstrainrate
curvesformicroalloyedsteel
subjected
to397
ture images for water-cooled thermomechanical variants after high strain rate compression were added.
thermomechanical rolling. Microstructure images for water-cooled thermomechanical variants
398
afterComparisonshigh strain rate ofcompression
wereadded.
the obtained results for samples subjected to different histories of399

thermomechanical treatment and subsequent compression in quasi-static conditions andComparisons of the obtained results for samples subjected to different histories of 400
SHPB are shown in Figure 8. The representation of the final microstructures in dynamicallythermomechanical treatment and subsequent compression in quasi-static conditions and 401
deformed samples is also shown. The microstructure analysis performed with the use of
SHPB are shown in Figure 8. The representation of the final microstructures in 402
optical microscopy showed a uniform image of the microstructure on the sample crossdynamically deformed samples is also shown. The microstructure analysis performed 403
sections after dynamic deformation. This is consistent with the expectation that in thewith the use of optical microscopy showed a uniform image of the microstructure on the 404
SHPB tests, the distribution of the effective strain on the sample cross-section should be as
sample cross-sections after dynamic deformation. This is consistent with the expectation 405
uniform as possible [26–28]. Using the comparisons of the obtained stress–strain curves forthat in the SHPB tests, the distribution of the effective strain on the sample cross-section 406
different
strain rates, the increments of flow stress of the tested samples were determinedshould be as uniform as possible [26–28]. Using the comparisons of the obtained stress– 407
asstrainan effectof fortheirstrain ratesensitivity
3). Thisseems
to beofthe
the finest408
curvesdifferent
strainrates, the(Table
incrementsof flow
stresstheresulttestedof
samplesrefinementofthemicrostructure
obtainedwith
scheme4A.
were determined as an effect of their strain rate sensitivity (Table 3). This seems to be the 409
result of the finest refinement of the microstructure obtained with scheme 4A.

410

Table 3. Increases in the flow stresses due to the strain rate sensitivities at strain 0.1 for stress–strain
411
curves presented in Figure 8.
SampleFlow Stress under
Quasi-Static Loading

Flow Stress under
Dynamic Loading

Increase in the
Flow Stress

Strain Rate
Sensitivity, m

6A
6B
4A
4B
3A
3B
1A
1B
Initial material

1190
1150
928
1035
703
786
698
759
770

1332
1300
1229
1177
841
955
849
935
961

142
150
301
142
138
169
151
176
191

0.00930.01000.02290.01050.01510.01640.01630.01760.0186The selected stress–strain curves clearly show the possibility of obtaining significant changes in the mechanical properties of the tested microalloyed steels subjected tothermomechanical processing. It results directly from the fact that the HSLA steels arecharacterized by the highest number of strengthening mechanisms, i.e., deformation, solid
solution, precipitation, complex phase and grain refinement strengthening. In turn, thesestrengthening mechanisms are activated in different ways in different microstructural

Materials 2021, 14, 6062

13 of 17

compositions produced by a thermomechanical processing. Multiple microalloying additions also have a clear synergistic effect on hardenability. A favorable combination of highstrength and ductility at high strain rates was obtained due to the developed multi-phasemicrostructure and its dedicated inhomogeneity in the micro and meso scales.
The results of plastometric tests presented in Figure 8 allow for the formulation ofsome general conclusions. The differences in the behavior of materials heated to 900 ◦ C
and 1000 ◦ C (Figure 1) can be explained by the presence of more precipitates in the steel
heated to a lower temperature, because the vanadium micro-additions did not dissolve
sufficiently when reheated to the rolling temperature at 900 ◦ C. In steel reheated to 1000 ◦ C,
the vanadium present in the solution did not have enough time to form carbides and theaustenite with dissolved vanadium underwent a bainite transformation.
Sample 4A is characterized by the highest work hardening among water-cooled
specimens. It contains the smallest share in the structure of upper bainite, and due to thelowest reheating temperature, it contains the highest number of carbides in the structure. At
the given reheating temperatures, samples rolled in two passes (Figure 1) are characterizedby higher strength in the quasi-static compressions than those rolled in one pass whencooled in the air. When cooled in water, single pass rolled specimens represent higher
strength. The exception is sample 4A, for which the yield stress is lower than for sample 4B.
In the case of air-cooled samples, for high strain rate regime, the differences between
samples rolled according to schemes A and B are the same as for quasi-static compression.
This is not the case with water-cooled samples. The increase in plastic flow stress ofsamples reheated before rolling to 1000 ◦ C is the same, regardless of whether the rolling
was carried out in one or two passes. In samples reheated before rolling to 900 ◦ C, theincrease in the yield stress of a sample rolled in one pass is much greater than in a sample
rolled in two passes. The compression yield stress of the sample 4A at a strain rate of2100 s−1 is higher than that of the sample 4B compressed at a strain rate of 2200 s−1 .
Significant differences in the strain rate sensitivity were observed in case of samples
1A, 1B and 4A, 4B. The compression curves shown in Figure 8 also show that the sample 4A,
characterized by a fine-grained structure of granular bainite strengthened with particles offoreign phases, shows the highest sensitivity to the strain rate while maintaining a highstrain hardening ability. Hence, it can be concluded that for given strain rate and strainlevel, the microalloyed steel with heterogeneous, multiphase microstructure exhibits higher
strain rate sensitivity than fully ferritic/pearlitic microstructure.
The analysis of the obtained results of compression tests clearly shows that materialof sample 1A represents the highest work hardening rate, which may be due to the low
yield stress associated with the presence of a relatively large amount of retained austenite.
Additionally, the presence of a substantially high volume of retained austenite makes the
sample material highly susceptible to work hardening.
Summarizing the results of compression tests presented in Figure 8, it should bestated that samples 1A, 1B and 4A, 4B exhibit an exceptionally attractive combinationof mechanical properties under dynamic loading conditions. Sample 4A shows a higher
sensitivity to strain rate even though the structure morphology is relatively similar to 1A,
although sample 4A has a finer microstructure and sample 1A contains a significant amountof retained austenite. It follows that under dynamic load conditions, the grain refinement
has a stronger positive effect on toughness than increasing the amount of retained austenite.
At the end of the study, a more detailed analysis of the relationship between the historyof thermomechanical rolling, the resulting microstructure and the mechanical properties,
especially at high strain rates, was carried out. TEM microscopy and diffraction analysis
were used in these studies. Sample images of the microstructures are shown in Figure 9.

Materials 2021, 14, 6062

refinement has a stronger positive effect on toughness than increasing the amount ofretained austenite.
At the end of the study, a more detailed analysis of the relationship between thehistory of thermomechanical rolling, the resulting microstructure and the mechanicalproperties, especially at high strain rates, was carried out. TEM microscopy
14 ofand17
diffraction analysis were used in these studies. Sample images of the microstructures areshown in Figure 9.

466
467
468
469
470
471
472

473

FigureScheme4A4A
after
thermomechanicalrolling:
the TEM(a) diffraction
Figure 9.
9. Scheme
after
thermomechanicalrolling:
the TEM
imageimage(a) diffraction
patternpattern(b) and(b)
andof structural
after compression
under (c)quasi-static(c) anddynamic
maps
of maps
structuralorientationorientation
after compression
under quasi-staticand dynamic
(SHPB
–2000
(SHPB
s−1 ) (d) conditions.
s−1) (d)–2000
conditions.

474
475
476

Due
differentiationof microstructuraleffects,
selectedcasescases
were were
anaDuetotothe
thevery
verylarge
large
differentiationof microstructuraleffects,
selectedlyzed
indetail.
The
microstructure
analysiswascarriedoutonsamplesmadeofrolledsteel
analyzed in detail. The microstructure analysis was carried out on samples made of rolledaccording to variant 4A (cold deformation, heating to 900 ◦ C, rolling in one pass, cooling inwater). The samples were analyzed at the condition after rolling and after deformation withthe value of ε = 0.25 with two strain rates: 0.01 s−1 and 2100 s−1 . TEM analysis showedthat the original structure of sample 4A was composed of finely divided components withno visible signs of deformation. The diffraction pattern shows over 15 rings, formed outof numerous spots, the evidence of the large number of small grains. Such a complex
diffraction pattern corresponds to a more complex microstructural features, than ferrite
structure only. These rings are probably derived from numerous carbides. The dots on thediffraction pattern are clear and not stretched, which shows that the accumulated strainenergy is not high and the material is relatively free from microstructural strain effects. The
structure also includes bainite grains and numerous precipitates of various sizes, from a
few to several hundred nm. The sample after deformation with a strain rate of 0.01 s−1shows a strong development of the dislocation substructure and a large change in the crystallographic orientation within one grain. The presence of the precipitates facilitates theaccumulation of microstructural effects of deformation. The dislocation substructure tends
to be orderly. It is visible on the diffraction map, where the outlines of the subgrains can be
seen. Dislocations are arranged in tangles, which transform into subgrain boundaries withfurther deformation.
The deformation effects can be observed much more significantly in the sample
deformed with the strain rate of 2000 s−1 . Optical microscope images of microstructures
show that the microstructure is clearly finer than in the sample deformed at a lower
strain rate. The change in the crystallographic orientation within the grains is smoothand does not form a distinct substructure. The photos of the dislocation substructure
show the significant accumulation of dislocations blocked on numerous highly dispersive

477
478

Materials 2021, 14, 6062

15 of 17precipitates. This effect is characteristic for steels with Nb, Ti and V microalloying additions,and also for the microalloyed steel tested in this study.
The presented research has shown that it is possible to form the microstructure in thefinished product in such a way as to achieve the expected mechanical responses at highstrain rates without the need to change the chemical composition of the microalloyed steel.
For this reason, further study on the problem discussed in this paper should be directed
towards the creation of appropriate models that can be used directly for designing an online
control of more and more complex thermomechanical processing of the microalloyed steels.
The results of the present study are important not only for the automotive, defense andmining industries, but also for the further development of processes under dynamic
forming conditions.
4. Conclusions
In the present study, a series of thermomechanical rolling tests and plastometrictests at low and high strain rates were presented. The main conclusions may be drawn
based on the investigations of 0.06 wt.% C, 0.058 wt.% Nb, 0.30 wt.% Si, 1.63 wt.% Mn,
0.045 wt.% V, 0.08 Mo, 0.02 wt.% V, Ti, 0.23 wt.% Ni and 0.006 wt.% N microalloyed steel.
These compositions were selected to provide representation of the commonly used grade
X70. The basic feature of the tested steel is the presence of two main strengthening mechanisms, i.e., precipitation strengthening due to Nb, V and Ti, as well as grain refinement.
Multiple microalloying additions also have a clear synergistic effect on hardenability. A
favorable combination of high strength and ductility at high strain rates was obtained
thanks to the developed multi-phase microstructure and its dedicated inhomogeneity inthe micro and meso scales. The paper presents several possible strategies for modifying
the dynamic response of the examined microalloyed steels. Depending on the boundary
conditions of the thermomechanical process, it is possible to change the history of theprocessing, i.e., reheating temperature, deformation schedule, cooling rate, or to change thefinishing temperature from conventional austenite rolling to austenite–ferrite intercriticalrolling, or a combination of both. The presented research results and their discussion clearly
indicate the need for further research for selected histories of thermomechanical processing
in order to more precisely determine the changes in the strain rate sensitivity of the studied
microstructures. The obtained results, both from the study of mechanical responses andmicrostructural analyzes, allow for the formulation of the following conclusions.
1.

2.

3.

4.

5.

The microstructural and plastometric tests performed showed that under dynamic
loading conditions, the refinement of the grains has a stronger positive effect on
toughness than the increase in the amount of retained austenite.
It has been shown that the bimodal and gradient microstructures with large fractionof bainite/martensite clusters surrounded by ultra-fine-grained ferrite are capableof producing steels with an attractive combination of strength and ductility at highstrain rates.
Observing the image of the microstructure for the sample 3A, it should be assumed
that good toughness in this case is a direct result of the bimodal distribution ofsignificantly refined and large grains.
The analysis of the obtained results of plastometric tests clearly shows that sample
1A represents the highest work hardening rate, which may be due to the low yieldstress associated with the presence of a relatively large amount of retained austenite.
Additionally, the presence of a substantially high volume of retained austenite makes
the sample highly susceptible to work hardening.
For given strain rate and strain, the microalloyed steel with heterogeneous, multiphase microstructure exhibits higher strain rate sensitivity than fully ferritic/pearlitic
microstructure. This is a suggestion for the further development of new highstrength ultra-fine-grain steels that can exhibit attractive mechanical behavior underdynamic loading.

Materials 2021, 14, 6062

16 of 17

Author Contributions: R.B. and J.M. developed the idea for the study. Methodology, J.M.; formal
analysis, U.P.; investigation, R.B., G.K., B.R. and J.J.; writing—original draft preparation, J.M. andR.B.; writing—review and editing, J.M., B.R. and J.J.; visualization, R.B.; supervision, J.M.; project
administration, J.M. and R.B.; validation and DIC analysis, P.L.-G. All authors have read and agreed
to the published version of the manuscript.
Funding: The research project was financed by the Ministry of Science and Higher Education (AGH
research subsidy no. 16.16.110.663).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: The authors would like to thank M. Kwiecień for help in carrying out the rolling
processes and A. Bunsch for the XRD study.
Conflicts of Interest: The authors declare no conflict of interest.

References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.

Villalobos, J.C.; Del-Pozo, A.; Campillo, B.; Mayén, J.; Serna, S. Microalloyed Steels through History until 2018: Review ofChemical Composition, Processing and Hydrogen Service. Metals 2018, 8, 351. [CrossRef]
DeArdo, A.J. Niobium in modern steels. Int. Mater. Rev. 2003, 48, 371–402. [CrossRef]
Muszka, K.; Hodgson, P.; Majta, J. A physical based modeling approach for the dynamic behavior of ultrafine grained structures.
J. Mater. Process. Technol. 2006, 177, 456–460. [CrossRef]
Ardehali Barani, A.; Li, F.; Romano, P.; Ponge, D.; Raabe, D. Design of hogh-strength steels by microalloying and thermomechanicaltreatment. Mater. Sci. Eng. A 2007, 463, 138–146. [CrossRef]
Song, R.; Ponge, D.; Raabe, D.; Speer, J.; Matlock, D. Overview of processing, microstructure and mechanical properties ofultrafine grained bcc steels. Mater. Sci. Eng. A 2006, 441, 1–17. [CrossRef]
Song, R.; Ponge, D.; Raabe, D. Mechanical properties of an ultrafine grained C–Mn steel processed by warm deformation andannealing. Acta Mater. 2005, 53, 4881–4892. [CrossRef]
Majta, J.; Zurek, A.; Pietrzyk, M. The Effect of Deformation in the Two-Phase Region of C-Mn and Microalloyed Steels on the
Mechanical Behavior of the Resulting Structure. J. Phys. IV Fr. 1997, 7, 397–402. [CrossRef]
Storojeva, L.; Ponge, D.; Kaspar, R.; Raabe, D. Development of microstructure and texture of medium carbon steel during heavy
warm deformation. Acta Mater. 2004, 52, 2209–2220. [CrossRef]
Saito, Y. Mathematical model of hot deformation resistance in austenite-ferrite two phase region. Trans. Iron Steel Inst. Jpn. 1987,
27, 419–424. [CrossRef]
Belyakov, A.; Sakai, Y.; Hara, T.; Kimura, Y.; Tsuzaki, K. Effect of Nano-Sized Oxides on Annealing Behaviour of Ultrafine Grained
Steels. Mater. Trans. 2004, 45, 2252–2258. [CrossRef]
Tsuji, N.; Ueji, R.; Minamino, Y.; Saito, Y. A new and simple process to obtain nano-structured bulk low-carbon steel with superiormechanical property. Scr. Mater. 2002, 46, 305–310. [CrossRef]
Huyashi, T.; Nagai, K. Improvement of strength-ductility balance for low carbon ultrafine-grained steels through strain hardening
design. Trans. Jpn. Soc. Mech. Eng. 2002, 68, 1553–1558. [CrossRef]
Benito, J.A.; Tejedor, R.; Rodríguez-Baracaldo, R.; Cabrera, J.M.; Prado, J.M. Ductility of Bulk Nanocrystalline and Ultrafine Grain
Iron and Steel. Mater. Sci. Forum 2009, 633–634, 197–203. [CrossRef]
Narayana Murty, S.V.S.; Torizuka, S. Mechanical properties of ultrafine grained steels processed by large strain-high Z
deformation—A review. Mater. Sci. Forum 2010, 633–634, 211–221. [CrossRef]
Majta, J.; Lenard, J.G.; Pietrzyk, M. Modelling the Evolution of the Microstructure of a Nb Steel. ISIJ Int. 1996, 36, 1094–1102.
[CrossRef]
DeArdo, A.J.; Hua, M.J.; Cho, K.G.; Garcia, C.I. On strength of microalloyed steels: An interpretive review. Mater. Sci. Technol.
2009, 25, 1074–1082. [CrossRef]
Senuma, T. Advances in Physical Metallurgy and Processing of Steels. Physical Metallurgy of Modern High Strength Steel Sheets.
ISIJ Int. 2001, 41, 520–532. [CrossRef]
Varlinden, B.; Driver, J.; Samajdar, I.; Doherty, R.D. Thermo-Mechanical Processing of Metallic Materials, 1st ed.; Pergamon Materials
Series; Elsevier: Oxford, UK, 2007; Volume 11.
Zhao, J.; Jiang, Z. Thermomechanical processing of advanced high strength steels. Prog. Mater. Sci. 2018, 94, 174–242. [CrossRef]
DeGarmo, E.P.; Black, J.T.; Kohser, R.A. Materials and Process in Manufacturing, 9th ed.; John Wiley & Sons: Hoboken, NJ,
USA, 2003.
Prasad, S.; Sarma, D. Influence of thermomechanical treatment on microstructure and mechanical properties of a microalloyed(Nb + V) weather-resistant steel. Mater. Sci. Eng. A 2005, 399, 161–172. [CrossRef]

Materials 2021, 14, 6062

22.
23.
24.
25.
26.
27.
28.

17 of 17

Chen, J.; Tang, S.; Liu, Z.-Y.; Wang, G.-D. Microstructural characteristics with various cooling paths and the mechanism ofembrittlement and toughening in low-carbon high performance bridge steel. Mater. Sci. Eng. A 2013, 559, 241–249. [CrossRef]
Slycken, V.J.; Verleysen, P.; Degrieck, J.; Bouquerel, J.; De Cooman, B.C. Dynamic response of aluminum containing TRIP steel
and its constituent phases. Mater. Sci. Eng. A 2007, 460–461, 516–524. [CrossRef]
Khan, A.S.; Baig, M.; Choi, S.-H.; Yang, H.-S.; Sun, X. Quasi-static and dynamic responses of advanced high strength steels:
Experiments and modeling. Int. J. Plast. 2012, 30–31, 1–17. [CrossRef]
Graca, P.; Muszka, K.; Majta, J.; Perzyński, K. Digital Image Correlation (DIC) System as a Verification Tool for Constitutive
Models of Deformation with Complex Strain Path Changes. Comput. Methods Mater. Sci. 2016, 16, 47–53.
Gama, B.A.; Lopatnikov, S.L.; Gillespie, J.W. Hopkinson bar experimental technique: A critical review. Appl. Mech. Rev. 2004, 57,
223–250. [CrossRef]
Pankow, M.; Attard, C.; Waas, A.M. Specimen size and shape effect in split Hopkinson pressure bar testing. J. Strain Anal. Eng.
Des. 2009, 44, 689–698. [CrossRef]
Pei, P.; Pei, Z.; Tang, Z. Numerical and Theoretical Analysis of the Inertia Effects and Interfacial Friction in SHPB Test Systems.
Materials 2020, 13, 4809. [CrossRef]