Phase Transformation under Continuous Cooling Conditions in
Medium Carbon Microalloyed Steels
Manuel Gomez*, Lucía Rancel, Esther Escudero, Sebastian F. Medina
National Centre for Metallurgical Research, CENIM-CSIC, Av. Gregorio del Amo 8;
28040 Madrid, Spain.
*Corresponding author: mgomez@cenim.csic.es,
Tel.: +34 91 5538900
Fax: +34 91 5347425
1
Phase Transformation under Continuous Cooling Conditions in
Medium Carbon Microalloyed Steels
Several 35CrMo4 and 38MnV7 steels with different additions of Ti and V were manufactured by
Electro-Slag Remelting. The influence of the alloying and microalloying elements on phase
transformation at different cooling rates was studied and the continuous cooling transformation
(CCT) diagrams were plotted. In order to optimize the heat treatment and to improve the
mechanical properties, the range of cooling rates leading to a fully bainitic microstructure (without
ferrite, pearlite and especially without martensite) was determined. Bainite and martensite
transformation start temperatures (Bs, Ms) were also established and compared with the values
predicted by empirical equations. The important role of precipitates (especially VCN particles) on
final microstructure and mechanical properties was assessed.
Keywords: microalloyed steel, phase transformation, precipitation, dilatometry, CCT diagram.
2
1. INTRODUCTION
Medium carbon steels with 0.30-0.40 %C have been used during last decades as high strength
steels, especially for automotive parts manufacturing. These steels were used firstly in the quenched
and tempered (Q+T) condition, with a good compromise between strength and toughness. Later,
medium carbon steels microalloyed with V and/or Ti were developed [1]. These steels suffered a
normalizing treatment after hot stamping, in order to obtain a fine microstructure of ferrite +
pearlite with similar mechanical properties compared to Q+T steels but at a lower cost. In the last
years, microalloyed steels presenting a bainitic microstructure are being studied and introduced in
the industrial production. This microstructure offers a better behavior in terms of fracture
mechanics, because crack generated in service encounters more obstacles in its propagation through
bainitic laths and packets compared to ferrite grains [2].
Whereas ferrite-pearlite microstructure is very well known, a much deeper knowledge about bainite
microstructure is needed. Bainitic steels with a wide range of carbon contents (from less than 0.1%
to near 1%) are being investigated nowadays [3-10]. Important developments have been achieved,
especially on low carbon bainitic steels. However, a deeper research is especially necessary on
medium and high carbon steels, as the good values of strength in these steels are usually
accompanied by relatively low values of toughness. On this regard, an improved knowledge about
the thermal treatments (both under isothermal and continuous cooling conditions) to obtain
microstructures with lower, upper or granular bainite will be crucial to enhance toughness values
and fracture mechanics behavior.
Austenite-Bainite transformation is complex. First of all, it is difficult to obtain fully bainitic
microstructures, as usually other phases will appear after cooling. In order to study all these phase
transformations, one of the more useful techniques that can be applied is dilatometry. Dilatometry is
3
an experimental technique that lets to situate and follow the solid state phase transformations
occurring in different materials, particularly steels. Phase transitions bring about volume changes,
and these changes can be recorded studying the length changes of samples with normalized
dimensions during their heating or cooling. The variations in the rate and direction of length change
versus temperature (dilation/contraction) allow to determine the temperatures where phase
transformations of steel take place. In steel, dilations and contractions occur as a result of the
different crystalline structures of Fe and the phases that can arise during heating or cooling. In a
medium-carbon microalloyed steel, the main phases that can form are [11-15]: Ferrite ( and ),
Austenite (), Cementite (Fe3C), Pearlite (ferrite and cementite), Bainite (ferrite and cementite) and
Martensite, as well as carbides of the microalloying elements. The lattice parameters of the different
phases in steel can vary with the content in carbon and other alloying elements. Expressions where
the influence of temperature on lattice parameter is taken into account can be found elsewhere [16].
Optical microscopy (OM) lets to reveal bainitic packets where plates grow with the same
orientation, but the size of individual ferrite plates is usually too small to be observed by this
technique. The packet defined by OM can be considered as the “morphological packet”, but it
should be taken into account that the microstructural unit controlling crack propagation in a
cleavage fracture is the “crystallographic packet”. This packet can be measured by Electron Backscattered diffraction (EBSD). Applying a misorientation angle criterion of 15 º, the bainite packet
size is near one third of the value determined by OM [17].
Martensite is achieved at the highest cooling rates. Carbon atoms are arranged causing a distortion
in the crystal structure, and the lattice changes from body-centered cubic structure (bcc) to bodycentered tetragonal (bct). Martensite needles also develop in packets with the same direction, but in
this case there are needles that present a high misorientation angle
[18]
. As will be seen later, the
martensitic transformation start temperature (Ms) is a function of austenite carbon content. In all
4
steels with sufficiently high carbon content, the martensitic transformation cannot go to the end so a
certain fraction of austenite remains in the structure after cooling as “retained austenite” (R) [19].
Each phase in steel has its own mechanical properties, from soft and ductile ferrite to hard and more
brittle martensite. From an industrial point of view, the achievement of a particular microstructure
in steel under continuous cooling conditions is generally preferred to the same or similar result
obtained under isothermal conditions. In many steels, it is technically important to know the
microstructures obtained when cooling rates after thermal treatment close to air-cooling are applied.
On the other hand, vanadium is the most important alloying element in medium carbon
microalloyed steels. V carbonitride (VCN) particles promote a precipitation strengthening effect in
ferrite-pearlite [20] and bainitic microstructures [21]. V additions can be also beneficial for toughness
as a result of the preferential intragranular nucleation of acicular ferrite on VCN or VN particles
[22,23]
. These precipitates can also nucleate on existing TiN particles and oxides to form complex
inclusions that serve as nucleation sites for the acicular ferrite, which helps to refine bainitic
microstructure. [24]
For all the aforementioned reasons, continuous cooling transformation (CCT) diagrams have been
determined in this work for five medium carbon steels with different compositions (essentially
different Cr, V, Ti and Mn contents). These diagrams provide a useful tool to the design of
optimized thermal treatments and to generate the desired microstructures. To determine the CCT
diagrams, dilatometry tests at several cooling rates were carried out on the steels studied as
described later.
2. EXPERIMENTAL PROCEDURE
5
One reference steel and four medium carbon V/Ti microalloyed steels with different Cr, Mo, Mn
and N contents were studied. These steels (whose main application is the automotive parts
production) were manufactured by electroslag remelting (ESR) technique. Their compositions are
shown in Table 1. These steels may be used in quenched and tempered condition, but they might
also be processed under continuous cooling, since Cr, Mo and Mn additions let to obtain bainitic
microstructures under a wide range of cooling rates. Steels CR1, CR2 and CR3 correspond to a
35CrMo4 steel with relatively low Mn content. Cr and Mo additions contribute to obtain bainitic
microstructure at moderate cooling rates. Steel CR1 is the reference steel and Steels CR2 and CR3
have been respectively microalloyed with V and Ti. On the other hand, steels MN4 and MN6 are
35MnV7 steels with two levels of Mn
In order to analyze the decomposition of austenite and to determine the critical phase transformation
temperatures under continuous cooling in these steels, several dilatometry tests were carried out
using an Adamel DT-100 high resolution dilatometer. Samples for dilatometry tests where
machined from all the steels studied in the as-forged condition. These samples were cylinders with a
diameter of 2 mm and a length of 12 mm. A constant heating rate of 40 ºC/min (0.67 K/s) and
several cooling rates between 0.03 K/s and 500 K/s were applied in order to plot CCT diagrams. A
heating temperature of 1000 ºC was chosen because it lies between a typical temperature for
thermal treatments and the forging temperatures for automotive components such as crankshafts or
connecting rods. Experimental conditions of the dilatometry tests (heating, holding, cooling and
atmosphere used) are shown in Figure 1.
Different transformations that occur within similar temperatures can often overlap in the
dilatometric
curve.
Microstructural
characterization
and
Vickers
micro-hardness
(HV)
measurements on dilatometry samples cooled at different rates helped to better interpret the slope
changes in the dilatometry curves and to plot more accurate CCT diagrams. Samples were prepared
6
by metallographic techniques and microstructures were observed by optical microscopy and
scanning electron microscopy (FEG-SEM). Several samples were quenched from reheating
temperature in order to determine the initial austenite grain size. The primitive austenite grain
boundaries were revealed by etching these samples with an aqueous solution of saturated picric acid
mixed with a wetting agent. The final microstructures obtained after different cooling rates were
observed after etching with 2% nital. Vickers microhardness values were measured on several
samples according to UNE-EN ISO 6507-1 standard using a load of 5 kgf.
3. RESULTS AND DISCUSSION
Figure 2 shows two examples of austenite microstructure obtained after reheating, i.e. before
applying different cooling rates in the dilatometry test. The grain size control exerted by V
precipitates not dissolved at this moderate reheating temperature (1000 ºC) can be distinguished
comparing the microstructures of steel MN4 and reference steel CR1. Figure 3 shows an example of
a dilatometric curve (compression/dilation as a function of temperature) obtained during heating
and cooling of a sample of steel CR1. Phase transformation temperatures during heating (Ac1 and
Ac3) will be always higher than equilibrium temperatures, as a result of the relatively fast heating
rate (0.67 K/s). Similarly, the transformation temperatures during cooling will be lower than
equilibrium temperatures.
The phase transformation temperatures during cooling, necessary to plot the CCT diagrams, can be
better determined in curves such as those shown in Figure 4. Transformation temperatures are
determined from the slope changes in the curve that denote the length changes of the dilatometry
sample. The first and second derivatives were calculated to enhance the precision of this study and
were included in the plots. Depending on the composition and the cooling rate, the phase
7
transformation temperatures that can be determined in these cooling curves as shown in the figures
are:
-The onset of austeniteferrite transformation temperature (Ar3);
-The beginning and the end of eutectoid transformation or pearlite formation (Ar1s and Ar1f,
respectively);
-The beginning and the end of bainite formation (Bs and Bf, respectively);
-The martensite start temperature (Ms).
Phase transformations and the associated slope changes take place at different temperatures
depending on cooling rate. At very slow cooling rates, the phases formed are close to those
corresponding to equilibrium, i.e. ferrite and pearlite (Fig. 4a). As long as cooling rate is faster,
bainite and finally martensite appear (Figs. 4b and 4c, respectively)
[25,26]
. At intermediate cooling
rates, mixed or complex microstructures that can be beneficial to mechanical properties would be
obtained. On this regard, it has been found that an increase in the volume fraction of lower bainite
in the mixed microstructure can be beneficial for mechanical properties [27].
Transformation temperatures at different cooling rates were recorded to plot CCT diagrams (Figure
5). The values of Vickers hardness and the microstructural characterization of several samples
helped to determine the CCT diagrams. HV values (shown in CCT diagrams of Fig. 5) change in
correspondence with microstructure and increase with cooling rate. As it is known, martensite
presents the highest hardness values and ferrite the lowest, while pearlite and bainite present
intermediate values. It can be seen that hardness values measured in samples with fully bainitic
microstructures are above 300 HV in all steels (i.e. approximately above a value of 30 Hardness
Rockwell C), which means a high strength bainitic microstructure.
8
Figure 6 shows examples of microstructural characterization by optical microscopy of all steels
studied at different cooling rates. The observed microstructures correspond well with the results
from dilatometry and measured hardness values. Ferrite and pearlite form at higher temperatures
and slower cooling rates, bainite appears at intermediate and relatively fast cooling rates, depending
on the composition, and martensite forms at lower temperatures after fast cooling or quenching.
In CCT diagrams, the range of cooling rates where a fully bainitic microstructure is obtained can be
determined. Slowest and fastest cooling rates will be the tangent line to the curves corresponding to
start of ferrite and bainite formation, respectively. These values are shown in Table 2. It can be seen
that the addition of Ti in steel CR5 compared to reference 35CrMo4 steel (CR1) does not increase
the range of cooling rates that let to obtain a fully bainitic microstructure during continuous cooling.
However, when V addition is relatively high (CR2 steel), the highest cooling rate with 100% bainite
is twice faster. A similar effect is observed in steels MN4 and MN6, which also contain more than
0.1% V. It is very likely that VN or VCN particles can act as preferential intragranular nucleation
sites for the bainite packet in high V steels, as happens with the intragranular nucleation of ferrite in
V-microalloyed steels [28,29].
On the other hand, Mn notably favors bainitic transformation: when Mn% increases from 1.5 %
(steel MN4) to more than 2% (MN6) the range of cooling rates leading to a fully bainitic
microstructure is considerably wider. In fact, ferritic transformation is practically avoided in steel
MN6, as it does not appear at cooling rates as low as 0.1 K/s. It has been previously found that the
addition or enrichment of Mn can suppress the formation of grain boundary ferrite, pearlite and
acicular ferrite and promote the formation of bainite in medium carbon V-microalloyed steels [30].
Empirical expressions to predict the values of bainite (Bs) and martensite (Ms) start temperatures as
a function of chemical composition have been suggested by several authors. Experimental values
9
obtained in this work were compared to two of the most widespread expressions for Bs [31,32] and Ms
[33,34]
:
Bs (ºC) = 830-270C-90Mn-37Ni-70Cr-83Mo
(1)
Ms (ºC) =500-350C-40Mn-35V-20Cr-17Ni-10Cu-10Mo-5W+15Co+30Al
(2)
The element symbols correspond to concentrations in weight %. The calculated values are shown in
Figure 5 and it can be concluded that both expressions predict with remarkable accuracy the start
temperatures for bainitic and martensitic transformations.
Nanometric precipitates were observed by FEG-SEM (Figure 7). It has been seen before that VCN
particles can serve as bainite nucleation sites in high V-steels, which facilitates bainite formation at
faster cooling rates. On the other hand, these particles can intrinsically increase hardness values, as
it is known that the presence of fine VCN particles contributes to raise the yield strength of medium
carbon bainitic steels [35,36] due to the increase in the stress required to cause dislocation by-passing
of the particles. An example of the strengthening effect of V precipitates can be seen comparing the
HV values of steels CR1 and CR2 cooled at 1 K/s. Both samples have a fully bainitic
microstructures, but VCN particles in steel CR2 help to increase the HV value. On the other hand,
steel CR5 cooled at the same rate presents a much higher HV value. It is known that TiN
precipitates can also provide a strengthening effect (both intrinsically and thanks to a decrease in
the bainite packet and ferrite lath sizes [35]). However, in this case it must be also taken into account
that a certain fraction of martensite (much harder than bainite) is detected. It has been described
elsewhere that coarse TiN particles can be cleavage fracture initiation sites in certain cases [37].
10
4. CONCLUSIONS
 Dilatometry tests, microstructural characterization and microhardness measurements have
allowed to determine the continuous cooling transformation (CCT) diagrams in five medium
carbon 35CrMo4 or 38MnV7 microalloyed steels with different additions of Ti and V.
 Cooling rates that let to obtain any kind of microstructure for a certain steel, from ferrite +
pearlite to martensite were determined. Particularly, it was possible to find the range of cooling
rates where a fully bainitic microstructure is obtained for each steel.
 The highest cooling rate for bainite formation is twice faster for a high V microalloying addition
(8 K/s, steel CR2) compared to reference 35CrMo4 steel (3.5 K/s, steel CR1). It is concluded
that VCN particles can act as preferential nucleation sites for bainite.
 Precipitates of Ti and V contribute to increase hardness values in at least 25 HV for a particular
cooling rate, i.e. they cause a strengthening effect.
 Mn notably favors bainitic transformation: when Mn% increases from 1.5 % (MN4) to more than
2% (MN6) the range of cooling rates leading to a fully bainitic microstructure is considerable
wider. In fact, ferritic transformation is practically avoided for a 2% Mn addition, as it does not
happen at cooling rates as low as 0.1 K/s.
 Empirical models and equations used to predict bainitic and martensitic transformation
temperatures[31-34] are accurate for these steels.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the financial support of Spanish Ministry of Economy and
Competitiveness through the project ref. MAT2011-29039-C02-02.
11
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13
TABLE CAPTIONS
Table 1. Chemical composition of the steels studied (weight %).
Table 2. Highest and lowest cooling rates to obtain a fully bainitic microstructure (> 95% bainite)
in steels studied.
14
Table 1. Chemical composition of the steels studied (weight %).
Steel
%C
%Si
%Mn
%Cr
%Mo
CR1
0.38
0.24
0.82
0.83
0.17
CR2
0.38
0.28
0.9
1.01
0.2
CR5
0.36
0.38
0.94
1.16
0.23
MN4
0.38
0.25
1.53
0.19
0.041
0.11
0.0217
MN6
0.38
0.25
2.23
0.16
0.033
0.12
0.0118
15
%V
%Ti
%N
0.0090
0.12
0.0214
0.038
0.0093
Table 2. Highest and lowest cooling rates to obtain a fully bainitic microstructure (> 95% bainite)
in steels studied.
Steel
Highest (K/s)
Lowest (K/s)
CR1
3.5
0.8
CR2
8
0.8
CR5
4
0.7
MN4
18
1.8
MN6
8
0.08
16
FIGURE CAPTIONS
Figure 1. Experimental conditions for the dilatometry tests.
Figure 2. Microstructure of two dilatometry samples quenched from reheating temperature (1000
ºC) after 2 min holding time, etched to reveal prior austenite grain boundaries. a) Steel CR1; b)
Steel MN4.
Figure 3. Dilatometric curve (heating and cooling) of steel CR1.
Figure 4. Cooling curves of the dilatometry test and determination of phase transformation
temperatures for three steels at given cooling rates. The first (d’) and second (d’’) derivatives of
dilation (dL/L0) are included to enhance the precision of temperature determinations.
Figure 5. CCT diagrams of the steels studied. The predicted value of Bs and Ms temperatures and
the measured values of Vickers microhardness are indicated.
Figure 6. Examples of microstructures obtained in dilatometry samples observed by optical
microscopy. a) Steel CR1, 0.06 K/s, F+P, HV = 232; b) Steel CR2, 5 K/s, B+M, HV555; c) Steel
CR5, 10 K/s, M, HV 609; d) Steel MN4, 1 K/s, F+B, HV 327; e) Steel MN6, 1 K/s, B+M, HV 354.
(F = ferrite, P = pearlite, B = bainite, M = martensite).
Figure 7. FEG-SEM micrograph showing VN-type precipitates in the bainitic microstructure of
steel CR2.
17
1200
2
Temperature (ºC)
1000
800
Dilatometry
1) Heating rate = 0.67 K/s
2) Holding at 1000 ºC x 1 min
3) Cooling rate = 0.06 - 500 K/s
1
-5
Heating + holding: 10 MPa vacuum
Cooling: He atmosphere
600
400
3
200
0
Time (s)
Figure 1. Experimental conditions for the dilatometry tests.
18
20 m
a)
20 m
b)
Figure 2. Microstructure of two dilatometry samples quenched from reheating temperature (1000
ºC) after 2 min holding time, etched to reveal prior austenite grain boundaries. a) Steel CR1; b)
Steel MN4.
19
14
Steel CR1
Heating rate = 40 ºC/min = 0.67 K/s
Cooling rate = 1 K/s
-3
Dilation (dL/L0·10 )
12
Ac1 = 769 ºC
10
8
Ac3 = 861 ºC
6
4
2
Bf = 368 ºC
Bs = 570 ºC
0
0
200
400
600
800
1000
Temperature (ºC)
Figure 3. Dilatometric curve (heating and cooling) of steel CR1.
20
-3
Dilation (dL/L0·10 ) , d' (Dilation) , d''(Dilation)
14
Steel CR2
Heating rate = 0.67 K/s
Cooling rate = 0.1 K/s
12
10
8
6
Ar3 = 741.5 ºC
4
d'
2
d''
0
(Ar1)s = 687.5 ºC
-2
(Ar1)f = 602 ºC
-4
-6
0
200
a)
600
800
1000
-3
Dilation (dL/L0·10 ) , d' (Dilation) , d''(Dilation)
Temperature (ºC)
14
Steel MN4
Heating rate = 0.67 K/s
Cooling rate = 0.5 K/s
12
10
8
6
Ar3 = 700 ºC
4
d'
2
d''
0
-2
(Ar1)s = 611 ºC
Bs =528 ºC
Bf =373 ºC
-4
(Ar1)f =540 ºC
-6
0
200
b)
400
600
800
1000
Temperature (ºC)
-3
Dilation (dL/L0·10 ) , d' (Dilation) , d''(Dilation)
c)
400
14
Steel MN6
Heating rate = 0.67 K/s
Cooling rate = 20 K/s
12
10
8
6
4
d'
2
d''
0
-2
Ms = 321 ºC
-4
-6
-8
-10
0
200
400
600
800
1000
Temperature (ºC)
Figure 4. Cooling curves of the dilatometry test and determination of phase transformation
temperatures for three steels at given cooling rates. The first (d’) and second (d’’) derivatives of
dilation (dL/L0) are included to enhance the precision of temperature determinations.
21
1100
CR1
D =32 m
1000
900
Ae3=754 ºC
Temperature (ºC)
800
20 K/s
1 K/s
700
F
F+P
Bs
600
500
B
0.06 K/s
400
Ms
300
M
200
100
0
1
HV
640 624 564
10
100
a)
324 308 276 259 232
1000
10000
Time (s)
1100
CR2
D =19 m
1000
900
Temperature (ºC)
Ae3 =756.5 ºC
10 K/s
800
1 K/s
700
600
F+P
F
Bs
500
B
400
0.06 K/s
Ms
300
M
200
654
100
0
1
HV
645 555 541 349 332 384 235
10
100
b)
1000
10000
Time (s)
1100
CR5
D =17 m
1000
900
Ae3=744 ºC
10 K/s
Temperature (ºC)
800
1 K/s
700
F
600
F+P
Bs
500
B
0.06 K/s
400
Ms
300
M
200
640
100
0
1
HV
609 596 516 414 380
10
c)
100
1000
249 210
10000
Time (s)
1100
MN4
D =15 m
1000
Ae3=754 ºC
900
20 K/s
Temperature (ºC)
800
2 K/s
700
F
Bs
600
F+P
500
B
400
0.1 K/s
Ms
300
M
200
658
100
0
d)
1
HV
596 516 465 368 327 323
10
100
Time (s)
22
1000
261
10000
1100
MN6
D =24 m
1000
900
Ae3=726 ºC
Temperature (ºC)
800
20 K/s
1 K/s
700
F
600
F+P
Bs
500
B
400
300
M
200
639
100
0
0.03 K/s
Ms
1
HV
645 652 595 497 354 339
10
100
e)
1000
309 275
280
10000
Time (s)
Figure 5. CCT diagrams of the steels studied. The predicted value of Bs and Ms temperatures and
the measured values of Vickers microhardness are indicated.
23
a)
b)
c)
d)
24
e)
Figure 6. Examples of microstructures obtained in dilatometry samples observed by optical
microscopy. a) Steel CR1, 0.06 K/s, F+P, HV = 232; b) Steel CR2, 5 K/s, B+M, HV555; c) Steel
CR5, 10 K/s, M, HV 609; d) Steel MN4, 1 K/s, F+B, HV 327; e) Steel MN6, 1 K/s, B+M, HV 354.
(F = ferrite, P = pearlite, B = bainite, M = martensite).
25
Figure 7. FEG-SEM micrograph showing VN-type precipitates in the bainitic microstructure of
steel CR2.
26