Z. LIU et al.: ENHANCING THE CRYOGENIC IMPACT TOUGHNESS OF FERRITIC STEEL ...
191–199
ENHANCING THE CRYOGENIC IMPACT TOUGHNESS OF
FERRITIC STEEL BY MULTI-TYPE AND
FINE-MICROSTRUCTURE DESIGN
IZBOLJ[ANJE KRIOGENSKE UDARNE @ILAVOSTI FERITNEGA
JEKLA Z OBLIKOVANJEM ME[ANE DROBNO ZRNATE
MIKROSTRUKTURE
Zi-cheng Liu
1
, Peng Gao
*,2
, Jia-kuan Ren
3
, Cheng-gang Li
3
, Wu-bo Zhong
1
,
Zhan Hu
1
, Zhen-yu Liu
*,3
1
Zhanjiang iron and Steel Co., Ltd., Baosteel, Zhanjiang 524000, People’s Republic of China
2
Jiujiang University, School of Mechanical and Intelligent Manufacturing, Jiujiang 332005, People’s Republic of China
3
Northeastern University, State Key Laboratory of Rolling and Automation, Shenyang 110819, People’s Republic of China
Prejem rokopisa – received: 2020-12-14; sprejem za objavo – accepted for publication: 2023-02-24
doi:10.17222/mit.2020.234
Based on pure melting, multi-type and fine-microstructure design, a ferritic steel with an outstanding combination of cryogenic
impact toughness and strength was fabricated. The microstructure was characterized by means of optical microscopy, electron
back-scattered diffraction and transmission electron microscopy. The steels with different final cooling temperatures of 663 °C
and 560 °C show ferrite and perlite microstructure, and ferrite grain size can be refined from 3.15 μm to 2.67 μm by lowering
the final cooling temperature from 663 °C to 560 °C. Moreover, a higher volume fraction of acicular ferrite and polygonal ferrite
can be obtained for a lower final cooling temperature of 560 °C. Hence, the cryogenic impact toughness can be enhanced, and
the brittle failure can be suppressed, even at –110 °C, showing a higher impact toughness compared with other ferritic steels.
Keywords: Ferritic steel; thermo-mechanical control process; ferrite grain morphology, cryogenic impact toughness
Avtorji so s pomo~jo ~istega taljenja oziroma uporabe zelo ~istih osnovnih surovin in oblikovanja me{ane drobno zrnate
mikrostrukture izdelali feritno jeklo z izjemno kombinacijo kriogene udarne `ilavosti oz. udarne `ilavosti pri zelo nizkih
temperaturah in mehanske trdnosti. Mikrostrukturo izdelanega jekla so okarakterizirali s pomo~jo svetlobnega mikroskopa
(OM), spektroskopijo povratno sipanih elektronov na vrsti~nem elektronskem mikroskopu (SEM/EBSD) in presevno
elektronsko mikroskopijo (TEM). Feritno jeklo so ohlajali s kon~nih ohlajevalnih temperatur 663 °C in 560 °C in pri tem
ustvarili me{ano feritno-perlitno mikrostrukturo. Pri zni`anju kon~ne temperature ohlajanja na 560 °C se je velikost feritnih
kristalnih zrna zmanj{ala s 3,15 μm na 2,67 μm. Opazili so, da se je pove~al dele` igli~astega in poligonalnega ferita. Tako so
pomaknili pojav za~etka krhkega loma jekla celo do –110 °C in s tem mo~no izbolj{ali udarno `ilavost jekla pri zelo nizkih
temperaturah v primerjavi z ostalimi jekli te vrste.
Klju~ne besede: feritno jeklo; termomehansko kontroliran proces; morfologija feritnih kristalnih zrn, kriogenska udarna `ilavost
1 INTRODUCTION
Steels have been widely used in infrastructure, heavy
machinery and other fields. In order to reduce the weight
and improve the safety of transportation, it is necessary
to enhance the cryogenic toughness of steels. However,
strength and toughness are usually mutually exclusive in
conventional metals.
1–4
Nearly all steels show a duc-
tile-brittle transition. Hence, it is important to develop
steels with an excellent cryogenic toughness.
Grain refinement is an effective way to lower the
ductile-brittle transition temperature (DBTT) of high-
strength steels. However, when the grain size is refined
below 1 μm, the plasticity and toughness of the steel are
significantly reduced.
5–7
Delamination or splitting caused
by anisotropic microstructures (such as the orientation of
grains and the second phase along the rolling direction)
can improve the toughness of metals at low tempera-
tures. However, in the process of reducing the DBTT,
delamination toughening usually reduces the toughness
of the ductile fracture zone.
8–9
In addition, Mg-Ni-Ti
composite materials with dual, continuous, interpenetrat-
ing phase structures can be developed through three-di-
mensional printing technology. The composite material
has a unique combination of mechanical properties, such
as good energy-absorption efficiency and better strength
and toughness indicators, and its strength can be im-
proved efficiently at ambient temperature and even at
high temperature.
10
Some researchers also improved the
strength and toughness by developing high-entropy al-
loys.
11
Nickel can be added to greatly improve the
strength of the steel and keep the steel’s good tough-
ness.
12
Medium-manganese steel has been widely studied
due to its reasonable cost and excellent mechanical prop-
erties. In medium-manganese steel, by adjusting the
microstructure of the retained austenite, steels with good
strength and toughness indexes can be obtained.
13
Materiali in tehnologije / Materials and technology 57 (2023) 2, 191–199 191
UDK 669.15-194.57:620.178.2:536.483 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(2)191(2023)
*Corresponding author's e-mail:
gaopeng20060707@163.com, (Peng Gao)

To ensure the low-temperature impact toughness of
the steel plate, low-carbon, ultra-low-sulfur and inclu-
sion morphology control are adopted in the steelmaking
process. When the content of sulfur and phosphorus in
the steel is high, the impact toughness is little affected at
high temperature, but the impact toughness is sensitive to
sulfur and phosphorus at low temperature. Therefore, it
is of great significance to improve the strength and
toughness of steel plate by adopting a pure smelting
technology and controlling the content of sulfur and
phosphorus at the minimum value.
14–16
In this paper the mechanical properties and micro-
structure of ferritic steel were researched for the mecha-
nism of improving the strength and toughness of steel
plate. Pure smelting technology was used to control the
content of sulfur and phosphorus. The effects of the final
cooling temperature on the microstructures and mechani-
cal properties were researched. A steel plate with excel-
lent comprehensive mechanical properties was obtained
by multi-type and fine-microstructure design.
2 EXPERIMENTAL PART
2.1 Materials
To fabricate high-quality ferritic steel with high
strength and cryogenic toughness, a series of processes
including a hot metal pre-treatment, converter blowing
and RH vacuum refining were adopted to reduce the con-
tents of phosphorus, sulfur and nitrogen in the steel. It is
important to control a reasonable chemical composition
to improve the strength and toughness of steel.
17,18
The
chemical composition of the steel is shown in Table 1.
2.2 Thermo-mechanical processing
The rolling experiment was carried out on a 450-mm
rolling mill. The conventional two-stage rolling was
adopted. According to the empirical formula of T
NR
,
19
the
estimated non-recrystallization temperature of the steel is
924 °C. Therefore, the final rolling temperature of
recrystallization-controlled rolling is in the temperature
range 1000–1020 °C and the initial rolling temperature
of recrystallization-controlled rolling is controlled at
1050 °C. The final temperature of non-recrystallization-
controlled rolling is about 850 °C. In addition, according
to the empirical formula of A
e3
,
20
the estimated A
e3
tem-
perature of the steel is 786 °C, which is like the thermo-
dynamic calculation result of 820 °C. According to the
empirical formula of A
e1
, the estimated A
e1
temperature
of the steel is 709 °C. Considering that the A
r1
tempera-
ture is lower than the A
e1
temperature, the final set cool-
ing temperature of 550 °C and 650 °C was chosen. The
schematic diagram of thermo-mechanical control process
is shown in Figure 1, and the actual measured parame-
ters of thermo-mechanical control process are shown in
Z. LIU et al.: ENHANCING THE CRYOGENIC IMPACT TOUGHNESS OF FERRITIC STEEL ...
192 Materiali in tehnologije / Materials and technology 57 (2023) 2, 191–199
Table 1: Chemical composition of the steel (w).
CS iM nA lN bT iC uN iC rPSN
0.08 0.13 1.52 0.044 0.016 0.012 0.12 0.12 0.015 0.0063 0.0006 0.0034
Table 2: The parameters of thermo-mechanical processing.
Steel
Tin of rough
rolling, °C
Tout
of rough
rolling, °C
Tin of finish
rolling, °C
Tout
of finish
rolling, °C
Tout
of
cooling, °C
Cooling
time, s
Colling rate,
°C/s
LT 1026 995 817 827 560 12 20.58
HT 1032 992 877 852 663 12 14.08
Figure 1: Schematic diagram of thermo-mechanical processing

Table 2. Moreover, the steel with a low final actual mea-
sured cooling temperature of 560 °C is designated as
"low-temperature (LT) steel", while the one with a high
final actual measured cooling temperature of 663 °C is
designated as "high-temperature (HT) steel".
2.3 Microstructure characterization and mechanical
property tests
The samples for microstructure characterization were
cut from the hot-rolled plates, and the longitudinal sec-
tion was polished and etched in 4 % nitric alcohol solu-
tion for 15 s. The microstructure was observed on an
optical microscope (LEICA DMIRM). In addition, the
metallographic samples were used for EBSD analysis af-
ter electropolishing. The electrolyte contained 12.5 %
perchloric acid and 87.5 % absolute ethanol. The volt-
age, current and time of electropolishing were 30 V,
1.8 A and 15 s, respectively.
A CMT-5105 tensile testing machine was used to
measure the strength and ductility along the rolling di-
rection. Three specimens were tested for each batch. The
tensile samples were machined and stretched according
to GB/T 228-2002. The Charpy V-notch impact samples
of 10 mm × 10 mm × 55 mm was tested with a JBW-500
impact testing machine at different temperatures, and the
test is in accordance with GB / T 229-1994 national stan-
dard. The testing temperatures were 20 °C, –40 °C,
–80 °C and –110 °C. The samples for TEM analysis
were cut from the hot-rolled plate with a wire-cutting
machine. The thickness was reduced to less than 45 μm
with abrasive paper, and then some wafers with a diame-
ter of 3 mm were punched using a Dervee 1700-3A
puncher. Then these wafers were further thinned with a
twin-jet electrolytic polisher in 10 % perchloric acid
alcohol solution, and the operating voltage and tempera-
ture were 32 V, –20 °C, respectively. Finally, these sam-
ples were examined in a Fei TECNAI G
2
F20 transmis-
sion electron microscope (TEM) operated at 200 kV.
3 RESULTS
3.1 Mechanical properties
The yield strength, tensile strength and total elonga-
tion of the steels are shown in Figure 2. Both the yield
strength and tensile strength can be slightly enhanced by
lowering the final cooling temperature. Both steels show
similar ductility. The impact absorbed energy of the
steels is shown in Figure 3, revealing that the impact ab-
sorbed energy of the LT steel is higher than that of the
HT steel, especially at –100 °C.
3.2 Optical microstructure
The microstructures of the LT and HT steels are
shown in Figure 4. Both steels show ferrite and pearlite
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Materiali in tehnologije / Materials and technology 57 (2023) 2, 191–199 193
Figure 3: Impact absorbed energy of the LT and HT steels at different
temperature
Figure 2: Tensile properties of the LT and HT steels
Figure 4: Microstructure of: a) LT and b) HT steels

microstructures. Among them, ferrite accounted for a
large proportion (82–93 %). The grain size of the LT
steel is relatively small and the grain boundary was ser-
rated. The grain size of the HT steel was large and the
grain boundary was smooth. The comparison of the grain
sizes is shown in Table 3. The average grain size of the
LT steel was smaller than that of the HT steel, and the
number fraction of grains with a size below 5 μm is also
high in the LT steel. This is because the undercooling at
the final cooling temperature of 560 °C is larger than that
at the final cooling temperature of 663 °C, which gives
full play to the role of ultra-fast cooling in the phase
transformation region and is beneficial to the grain re-
finement. Figure 5 shows the distribution of the grains’
long axes with different final cooling temperatures. The
long axis of most grains was below 10 μm and the aspect
ratio was below 6.
3.3 EBSD analysis
The EBSD results are shown in Figure 6. The grain
size distribution of both processes is shown in Figure 7.
Z. LIU et al.: ENHANCING THE CRYOGENIC IMPACT TOUGHNESS OF FERRITIC STEEL ...
194 Materiali in tehnologije / Materials and technology 57 (2023) 2, 191–199
Table 3: Grain size of the LT and HT steels.
steel <1μm <3μm <5μm <7μm <9μm <11μm <13μm <15μm <17μm <23μm <29μm <33μm <39μm
LT 63.062 25.181 7.257 1.887 1.306 0.508 0.435 0.0726 0.145 0.0726 0.0726 0 0
HT 60.423 20.282 9.296 4.085 1.972 0.845 0.845 0.563 0.704 0.563 0.141 0.141 0.141
Figure 7: Grain size distribution of: a) LT and b) HT steels
Figure 5: Grain morphology of the LT and HT steels
Figure 6: EBSD analysis on: a) LT and b) HT steels

The average grain size in Figure 7a was 2.67 μm, and
that in Figure 7b was 3.15 μm. The grain misorientation
statistics are shown in Figure 8. The number fraction of
high-angle grain boundaries in Figure 8a was 61.8 %,
and that in Figure 8b was 59.15 %. At a low final cool-
ing temperature of 560 °C, the grain size was small, and
the proportion of high-angle grain boundaries was high,
resulting in better overall mechanical properties.
It is concluded that the final cooling temperature of
560 °C can lead to a fine microstructure, high yield
strength, tensile strength and impact energy. This is be-
cause the grain size mainly depends on the nucleation
rate and growth rate. With the increase in undercooling,
the nucleation rate and growth rate increase, but the in-
creasing rate is different. When the undercooling is
small, the increasing rate of the nucleation rate is less
than the growth rate. However, when the undercooling is
large, the increasing rate of nucleation rate is greater
than the growth rate. During the transformation of aus-
tenite to ferrite, the grain size of ferrite and pearlite can
be refined by increasing the undercooling. The higher the
cooling rate is, the greater the undercooling is. There-
fore, an increasing cooling rate is conducive to refining
the grain size and improving the strength and toughness.
3.4 TEM analysis
Figure 10 shows the TEM microstructure and precip-
itate morphology of the LT and HT steels. The micro-
structure with a final cooling temperature of 560 °C was
composed of acicular ferrite and polygonal ferrite. The
ferrite grain was fine. The diameter of smallest polygo-
nal ferrite grain was about 200 nm, and the width of
acicular ferrite was about 220 nm. Dislocations begin to
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Materiali in tehnologije / Materials and technology 57 (2023) 2, 191–199 195
Figure 8: Misorientation distribution of the a) LT and b) HT steels
Figure 9: TEM micrographs of the a) LT and b) HT steels

Z. LIU et al.: ENHANCING THE CRYOGENIC IMPACT TOUGHNESS OF FERRITIC STEEL ...
196 Materiali in tehnologije / Materials and technology 57 (2023) 2, 191–199
Figure 11: Impact fractographs of the HT steel: a), c) –40 °C, b), d) –110°C
Figure 10: Impact fractographs of the LT steel: a), c) –40 °C, b), d) –110°C

migrate within the polygonal ferrite and at the grain
boundaries. There were carbide precipitates in the polyg-
onal ferrite with the size about 50 nm. The micro-
structure with the final cooling temperature of 663 °C
was mainly composed of polygonal ferrite and pearlite.
The polygonal ferrite had larger grain size, fewer dislo-
cations and no carbide precipitation.
3.5 Impact fractographs
Figure 10 shows the impact fractographs of the LT
steel. The shear lip and fiber zone of the fracture surface
at –40 °C and –110 °C were large, and the radiation zone
was small, resulting in a higher impact energy. As shown
in Figure 10c, many ovate dimples with certain direc-
tions were seen, which were deep and flat at the bottom.
Some small dimples were also distributed in coarse dim-
ples. The directivity of the dimples indicates that the
sample is fractured by tearing, and the tearing needs to
absorb a lot of energy. As shown in Figure 10d, coarse
dimples and a certain number of small dimples were
seen. However, there were some river patterns. The frac-
ture surface was composed of fiber zone and cleavage
zone, but mainly in the fiber zone. Therefore, the sample
still maintains a high toughness.
Figure 11 shows the impact fractographs of the HT
steel. At –40 °C, the shear lip and fiber zone were larger
than the radiation zone, resulting in a higher impact en-
ergy. The fracture surface at –110 °C is a typical brittle
fracture mode with low impact energy. As shown in Fig-
ure 11c, there was mainly the fiber zone, and many ovate
dimples were seen. The dimples were deep, and thus the
toughness is better. As shown in Figure 11d, there were
fewer small dimples and many river patterns. The frac-
ture surface was mainly composed of cleavage zone.
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Materiali in tehnologije / Materials and technology 57 (2023) 2, 191–199 197
Figure 13: TEM micrographs beneath fracture surface of: a) LT and b) HT steels
Figure 12: EBSD micrographs of the a) LT and b) HT steels

Therefore, the toughness is relatively low and the impact
absorbed energy is 190 J.
4 DISCUSSION
4.1 Crack propagation
The microstructure beneath the fracture surface was
characterized by EBSD, as shown in Figure 12. The
crack prolongation paths were depicted by white lines in
Figure 12a and 12b, with the decrease of final cooling
temperature, the more tortuous crack propagation path
can be observed. The finer grain size caused by a lower
final cooling temperature of 560°C resulted in a higher
volume fraction of high-angle grain boundaries
(HAGBs). Many researchers indicated that the ferrite
with HAGBs can effectively prevent crack propaga-
tion.
19–20
Thus, the impact toughness is obviously en-
hanced.
The substructure near by the main crack after the im-
pact test at –110 °C was characterized by TEM, as
shown in Figure 13. Polygonal ferrite grain of 600 nm
and acicular ferrite of 220 nm with high density disloca-
tion caused by a lower final cooling temperature of 560
°C are observed. It is obviously that grain boundaries can
prevent the slip of dislocations. So that dislocations are
more likely to become entangled at grain boundaries due
to the higher volume fraction of HAGBs (Figure 8a),
which contributed to a higher impact toughness. How-
ever coarse grain with lower volume fraction of HAGBs
under a higher final cooling temperature of 663 °C ex-
hibited a lower impact toughness.
5 CONCLUSIONS
Steels with two different final cooling temperatures
were obtained by combinations of pure smelting and
controlled rolling and ultra-fast cooling. The micro-
structure and mechanical properties of the steels were in-
vestigated.
(1) The microstructure of both steels is composed of
ferrite and pearlite. However, a lower final cooling tem-
perature of 560 °C can refine the ferrite grains and the
average grain size is refined from 3.15 μm to 2.67 μm by
lowering the final cooling temperature from 663 °C to
560 °C. The grain-refinement effect is not pronounced.
This may be a main reason why the tensile strength is not
greatly enhanced.
(2) Note that a lower final cooling temperature of
560 °C can greatly change the ferrite grain morphology,
resulting a higher volume fraction of acicular ferrite and
polygonal ferrite. Hence, the cryogenic impact toughness
can be enhanced, and the brittle failure can be sup-
pressed even at –110 °C.
Author contribution statement
Z.-Y. L. and Z.-C. L. conceived the experiments; P.
G. and C.-G. Li performed the experiments; Z.-C. L. and
P. G. wrote the paper; W.-B. Zhong and Z. H. processed
the data; J.-K. R. revised the paper and contributed to the
discussion of the results.
Declaration of competing interest
The authors declare that they have no known compet-
ing financial interests or personal relationships that could
have appeared to influence the work reported in this pa-
per.
Acknowledgments
This work was supported by the Dedicated Fund for
Promoting High-Quality Economic Development in
Guangdong Province (Marine Economic Development
Project) (grant number GDOE[2019]A08), and also was
supported by Science and Technology Research Project
of Jiangxi Provincial Department of Education (grant
number GJJ211821).
Data availability
The raw/processed data required to reproduce these
findings cannot be shared at this time as the data also
forms part of another ongoing study.
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