1
8nd International Conference on Physical and Numerical Simulation of Materials Processing, ICPNS’16
Seattle Marriott Waterfront, Seattle, Washington, USA, October 14-17, 2016
The Effect of Mn and Cr on Microstructures and Mechanical
Properties in Nanobainitic Steels
Hui Guo1, Aimin Zhao12*, Chao Zhi2, Jianguo He3, Ran Ding 2
Innovation Center of Steel Technology, University of Science and Technology Beijing,
Beijing,10083, China
2Engineering Research Institute, University of Science and Technology Beijing, Beijing, 10083, China
1Collaborative
ABSTRACT
The effects of Mn and Cr on nanobainitic transformation kinetics, microstructures and mechanical properties in
high-carbon Si-Al-rich alloy steels were determined by dilatometry, scanning electron microscopy (SEM), X-ray
diffraction (XRD) and tensile tests. The results showed that Mn and Cr would extend bainitic incubation period
and completion time. And with the increase of Mn and Cr, the bainitic ferrite plate thickness decreased and the
volume fraction of retained austenite increased. TRIP effect was observed during tensile testing which improved
the overall mechanical properties. The addition of Mn and Cr can improve the ductility of nanobainitic steels
which transformed at a low temperature. Nanobainitic steel which was austempering at 230 °C exhibited
excellent mechanical properties with ultimate tensile strength of 2146 MPa, total elongation of 12.95%. Please
put your abstract here.
Keywords: Nanobainitic, Kinetics, Retained austenite, TRIP effect
1.INTRODUCTION
In the recent decade, nanostructured, high-carbon
silicon-rich alloy bainitic steels involving fine-scaled
carbide-free bainitic ferrite plates and uniform
dispersion of carbon enriched austenite have been
developed by Caballero and Bhadeshia (Caballero,
Bhadeshia, Mawella, Jones& Brown, 2002). This
bainitic steels exhibit are remarkable ultimate tensile
strength in excess of 2.3 GPa and a toughness of 30
MPa•m1/2by isothermal treatment at low temperature
for a long time (e.g., 125 oC for 29 d while 190 oC for
14 d) (Caballero & Bhadeshia, 2004). The prominent
mechanical properties have aroused widespread
concern within the academics.
Although the nanobainitic steels have excellent
mechanical properties, the slow rate of bainitic
transformation limits the development in a practical
point of view. In order to shorten transformation time,
the selection of rational alloy elements has been
conducted to accelerate bainitic transformation. The
addition of Al and/or Co can increase the chemical
driving for the transformation of austenite into ferrite
and the transformation can be easily accelerated
(Garcia-Mateo, Caballero & Bhadeshia, 2003;
Yoozbashi, Yazdani& Wang, 2011). The
investigation of Huang et al.(Huang, Sherif& RiveraDíaz-del-Castillo, 2013)showed that reducing the
concentration of Mn has a much greater effect than
increasing Co in terms of accelerating the bainitic
reaction which can bring significant cost reduction.
C. GOULAS et al. (Goulas, Mecozzi & Sietsma,
2016)studied the effect of chemical inhomogeneity
on the isothermal bainite formation, which has
shown that the growth of bainite in the high Mn and
Cr concentration regions were retarded.
In this research, a new composition has been
designed using the base composition, and the effect
of adjustment of Mn and Cr content on the bainitic
transformation rate, microstructures and mechanical
properties have been investigated.
2.Material and methods
A total of three steels have been investigated in
detail for this study, and their chemical compositions
are listed in Table 1. The steels were produced by a
50 kg vacuum medium frequency induction melting
furnace. The ingots were forged and then
homogenized at 1200 °C for 24 h. Thermal
simulation tests were carried out by dilatometry
using a DIL 805A dilatometer to determine the
bainite transformation time after austempering at 230
°C. Cylindrical samples with a length of 100 mm and
a diameter of 12 mm were cut from the forging
stocks and austenitized at 950 °C for 30 min, then
transferred to a salt bath for austempering at 230 °C,
and finally air cooled to room temperature.
2
C
Si
Mn
Cr
Al
Fe
NB1
0.83
2.44
0.43
---
0.73
Bal.
NB2
0.83
2.54
1.01
---
0.89
Bal.
NB3
0.81
2.48
0.98
1.04
0.91
Bal.
Tensile tests were performed at room temperature in
specimens of 12 mm diameter and 25 mm gauge
length with a cross-head speed of 0.1 mm•min-1. The
engineering stress-strain curves were converted into
the true stress–strain curve, and strain hardening
was characterized by the instantaneous workhardening exponent n that was calculated from the
true stress–strain curve as the Equation (1):
d ln σ
n
(1)
d ln ε
Where σ represents true stress and ε represents true
strain.
The microstructure morphologies were observed
using a ZEISS ULTRA 55-type field emission
scanning electron microscope (FE-SEM).
Metallographic samples were ground, mechanically
polished and etched with 2% nital. The evolution of
microstructure change in tested steels before and
after tested was achieve by X-ray diffraction (XRD)
analysis using Cu-Kα radiation with a voltage of 40
kV and a current of 150 mA. The hardness of the
samples were determined by Rockwell harness
tester.
3.Results and Discussion
3.1 Kinetics of bainite transformation
Fig.1 presents the time temperature transformation
(TTT) curves of three experimental steels calculated
by the MUG83 thermodynamic model, where Ms, P
and B stand for the martensite start temperature,
pearlite and bainite, respectively. As the content of
Mn is increased from 0.43% (NB1steel) to 1.01%
(NB2 steel), the whole C-shaped curve is reduced
that results in the retardation of the pearlite reaction.
In addition, the martensite reaction is depressed to a
lower temperature. After the concentration of Cr is
further increased to 1.0% on the base of NB2 steel,
the curve shift steeply to bottom-right of the image
and the Bs and Ms decreased significantly, the
hardenability of tested steel is remarkably improved.
From the current results, it appears that the addition
of Mn and Cr clearly has a strong effect in enhancing
the hardenability of steel and reducing the bainite
800
NB1
700
NB2 NB3
P
600
。
No.
transformation temperature. Recent researches have
demonstrated that the reduction of isothermal
transformation temperature is a very effective mean
to achieve the nano-size plates of bainitic ferrite
(Meng, Feng, Zhou, Zhao, Zhang& Qian, 2015),
accordingly the rational adjustment of alloy element
is considered as a key requirement to carry out the
bainite transformation at low temperature.
Temperature/ C
Table 1 Chemical compositions of experimental steels (wt.%)
500
400
B
300
Ms（ N1） = 221℃
200
Ms（ N2） = 214℃
Ms（ N3） = 186℃
100
-1
10
10
0
10
1
10
2
10
3
10
4
10
5
Time/s
Figure.1Time temperature transformation diagram of
experimental steels
Fig. 2 presents the overall kinetics of experimental
steels for isothermal treatment at 230 oC, Fig. 2(b)
displays the corresponding reaction rate of Fig. 2(a).
The time at which generates 1-3% and 95-98%
volume fraction of bainite is defined as the start and
completion time of isothermal bainite transformation,
respectively. It can be seen that higher Mn (NB2
steel) and Cr content (NB3 steel) are accompanied
by longer transformation incubation time and lower
transformation rate, and bainite reactions are both
retarded. When Mn content increases to 1.01%, the
incubation period of NB2 steel is more than three
times as much as NB1 steel, subsequently with
further increasing Cr content, the need time of
bainite start transformation of NB3 steel is almost the
same as the terminal time of NB1 steel. Likewise,
the transformation rate rapidly decreases with the
increasing of Mn and Cr content, and the maximum
transformation rate of N1 steel after austempering at
230 oC is 0.01 μm•s-1 much higher than NB3 steel.
This phenomenon is attributed to the raising of
supercooled austenite strength, caused by the
increasing of phase transformation resistance and
the decreasing of transformation rate due to the solid
solution strengthening of Mn and Cr (Suh, Park&
Kim, 2008; Miyamoto, Usuki & Furuhara, 2010). On
the another hand, the diffusion of carbon atom in
austenite is markedly retarded by the increasing of
Mn and Cr content which results in a gradual
extension of incubation period.
3
NB3 steel possesses the 30.62%. Mn and Cr, as the
substitutional solid strengthening element are
beneficial for the enhancing of austenitic stability and
mechanical strength, and this effect can lead to an
increase trend in the final retained austenite.
(a)
50
Dilatation/ μm
40
14400s
NB1
NB2
NB3
24450s
46660s
30
20
block RA
10
1180s
3820s
BF
13600s
block RA
film RA
BF
0
100
1000
Time/s
film RA
10000
(b)
Transformation rate/ μ·ms
-1
0.012
NB1
NB2
NB3
0.010μ·ms
-1
BF
0.009
film RA
block RA
0.005μ·ms
-1
0.006
0.002μ·ms
-1
0.003
Figure.3 SEM micrographs of experimental steels: (a) NB1, (b)
NB2, (c) NB3, in which BF and RA stand for bainitic ferrite and
retained austenite, respectively.
0.000
100
1000
Time/s
10000
Figure.2Transformation kinetic of experimental steels for
isothermal treatment at230 oC,(a) dilatation-time curves and (b)
transformation rate
3.2Microstructure observation
The scanning electron microscopy micrographs of
obtained microstructures of experimental steels after
austempering at 230 °C for 24 h are shown in Fig. 3,
which comprise bainitic ferrite (concave
microstructures) and retained austenite (convex
microstructures), this latter exists in two different
morphologies, as film-like and as blocky. As it can be
seen, corresponding to the NB1 and NB2 steel, the
NB3 steel exhibits a more blocky microstructure.
From the Table 2, it is clearly that the thickness of
bainitic ferrite plate is refined and the volume fraction
of retained austenite is increasing with the increasing
of Mn and Cr content.
3.3Mechanical properties evolution
The mechanical properties of experimental steels
are shown in Table 2 and Fig. 4(a). With increase in
Mn content, the ultimate tensile strength and total
elongation of NB2 steel are both slightly decrease,
by further increasing the Cr content, the total
elongation of NB3 is almost two times as much as
NB1 and NB2 steel reaching 12.95%, but the
ultimate tensile strength is 2146 MPa slightly lower
than NB2 and NB1 steels. This phenomenon is
attributed to the increasing of retained austenite with
the increasing of Mn and Cr content, especially the
Table 2 Mechanical properties of experimental steels
t
Before
tensile
After
tensile
nm
VRA/%
VRA/%
709.6
56.64
17.47
8.87
6.19
698.4
50.54
23.85
17.89
12.95
686.7
46.61
30.62
15.36
UTS
TE
MPa
%
NB1
2330
6.56
NB2
2231
NB3
2146
No.
Hard
ness
HV
where UTS, TE and t respectively stand for the ultimate tensile
strength, total elongation and thickness of bainitic ferrite, VRA
stands for the volume fraction of retained austenite.
Fig. 4(b)shows the work-hardening index curves of
experimental steels, during the plastic deformation,
the n-value of NB1 and NB2 steel rapidly decrease
before the occurrence of necking, and the change of
volume fraction of retained austenite before and after
tensile tests is less than 10% (Table 2). Meanwhile,
the NB1 and NB2 steel are consisted of higher
amount of film-like stable retained austenite and
lower amount of instability blocky retained austenite,
Fig. 3(a) and (b). This phenomenon indicates that
only a small quantity of blocky retained austenite has
been transformed at the initial stage of tensile tests,
and the potential of the TRIP effect remains unused,
which results in little or null benefit on the increasing
of plasticity. Nerveless the value-n of NB3 steel
remains constant about 0.07 till the necking
occurrence, which shows that the retained austenite
gradually transforms into martensite during the workhardening after plastic deformation that can be
favorable to reduce stress centralization and delay
4
Acknowledgment
(a) 2500
REFERENCES
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This research was supported by the National Natural
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0.1
0.0
0.00
0.03
0.06
0.09
0.12
True strain
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4.Conclusion
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Spring Steels Accounting for Chemical
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as follows:
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Qian, L. (2015). Effects of Austempering
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of Low-C High-Al/Si Carbide-Free Bainitic
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(1) The alloy element of Mn and Cr can decrease the
Bs and Ms, shift the C-curve to bottom-right based
on the analysis of thermodynamic calculation. With
the increasing of Mn and Cr content, the incubation
period and terminal time increase and the bainite
reaction is inhibited.
Suh, D. W., Park, S. J., & Kim, S. J. (2008).
Influence of Cr and Ni on Microstructural Evolution
during Heat Treatment of Low-Carbon
Transformation Induced Plasticity
Steels. Metallurgical and Materials Transactions
A, 39(9), 2015-2019.
(2) The thickness of bainitic ferrite is about 50nm
after austempering at 230 °C for 24h, and the
addition of Mn and Cr can refine the thickness of
bainitic ferrite plates.
Miyamoto, G., Usuki, H., Li, Z. D., & Furuhara, T.
(2010). Effects of Mn, Si and Cr addition on reverse
transformation at 1073K from spheroidized
cementite structure in Fe–0.6 mass% C alloy. Acta
Materialia, 58(13), 4492-4502.
(3) The volume fraction of blocky retained austenite
increases with the increasing of Mn and Cr, and
during the plastic deformation the TRIP effect is
taken place which is beneficial for mechanical
properties.