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Static strain ageing behaviour of dual phase steels
Article  in  Materials Science and Engineering A · July 2008
DOI: 10.1016/j.msea.2007.08.056
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Materials Science and Engineering A 486 (2008) 63–71
Static strain ageing behaviour of dual phase steels
Süleyman Gündüz ∗
Zonguldak Karaelmas University, Karabük Technical Education Faculty, Department of Materials, 78200 Karabük, Turkey
Received 27 February 2007; received in revised form 20 August 2007; accepted 22 August 2007
bstract
In this work an investigation was conducted into the cold deformation ageing susceptibility of a carbon steel and a microalloyed steel, both
ith dual phase micro-structure. Ageing experiments after different prestrains were carried out at temperatures ranging from 25 to 250 ◦C. It
as found that yield strength (YS) and tensile strength (UTS) of the steels with different dual phase micro-structures exhibit maximum values
t ageing temperature of 100 ◦C after different prestrains. It is assumed that the first rise is based on the formation of solute atom atmospheres
round dislocations and the further strengthening in the second step is caused by the low-temperature carbide precipitation in ferrite. When the
geing temperature increased to 150, 200 or 250 ◦C, YS decreased due to tempering effect in martensite. It was also found that the ageing of the
icroalloyed steel occurred more slowly than that of the carbon steel. The slow occurrence of ageing was clearly observed at temperatures of 100,
50, 200 and 250 ◦C and was attributed to the chemical composition of the steels.
 2007 Elsevier B.V. All rights reserved.
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eywords: Microalloyed steels; Static strain ageing; Prestrain
. Introduction
Dual phase steels belong to high-strength low-alloy steels
HSLA) characterised by micro-structures mainly consisting of
dispersion of hard martensite grains in a soft ferrite matrix
1–5]. These steels have a combination of strength, ductility,
nd formability that makes them attractive for reducing mass
nd costs and improve crash energy management in the auto-
obile industry [2]. Many large-scale research programs on
hese steels are conducted in industrial laboratories and univer-
ities. Today’s automobile manufacturers are increasingly using
ual phase steels. Vehicle impact components can have relatively
omplex shapes, yet still have thin gages. Dual phase steel allows
he versatility of mass reduction combined with improved driver
nd passenger safety—an advantage that alternative materials
annot match [1–5].
By intercritical annealing heat treatments, it is possible to
roduce a dual phase steel micro-structure (ferrite and marten-
ite) which is obtained by heating a low-alloy hypoeutectoid
teel between the Ac1 and Ac3 temperatures forming ferrite and
ustenite with subsequent rapid cooling in order to transform the
∗ Tel.: +90 370 4338200/1503; fax: +90 370 4338204.
E-mail address: sgunduz1@gmail.com.
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921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
oi:10.1016/j.msea.2007.08.056
ustenite to martensite. Control of the particle size, dispersion
nd amount of martensite in the ferritic matrix is also possi-
le. Many studies [1–7] have shown that martensite content is
ominant in controlling tensile properties and that increasing the
mount of martensite decreases ductility. It has been suggested
hat optimum properties are obtained for dual phase steels con-
aining 20% martensite. Other factors that have been reported
o influence the ductility of dual phase steels include the com-
osition of the martensite, alloy content of the ferrite, retained
ustenite and amount of new ferrite (also called epitaxial ferrite)
8–13].
Plain carbon steels and high-strength low-alloy steels are sub-
ect to strain ageing, which reduces their ductility and increases
heir strength. Dual phase steels have been developed to meet
emands for high-strength steels with good formability, partic-
larly in the automotive industry. Strain ageing has also been
bserved in dual phase steels [14,15]. However, the composi-
ional and micro-structural factors, which influence or control
he strain ageing behaviour of steels in the dual phase conditions,
ave not yet been established in detail. Strain ageing behaviour
s important not only to enhance the fundamental understand-
ng of dual phase steel structure, but also to provide practical
uidelines for the use of dual phase steels [16].
In the present work, the ageing behaviour of a carbon steel and
microalloyed steel with different martensite volume fractions
mailto:sgunduz1@gmail.com
dx.doi.org/10.1016/j.msea.2007.08.056
64 S. Gündüz / Materials Science and Engineering A 486 (2008) 63–71
Table 1
Chemical compositions of the steels investigated (wt.%)
C Si S P Mn V Ti Al Nb
M 0.020
C 0.025
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duced a dual phase micro-structure with a ferritic matrix
surrounding martensite islands in both type of steels as seen
in Fig. 3a and b. The measurementof phase volume fraction
(Table 2) showed that the dual phase carbon steel had a higher
icroalloyed steel 0.11 0.10 0.010
arbon steel 0.22 0.40 0.020
as studied. The variation of mechanical properties, especially
he increase in YS was measured by tensile tests. The effects of
he martensite volume fractions and amount of prestraining on
train ageing behaviour of steels were also examined.
. Experimental procedure
The steels used in this investigation are commercially
roduced a carbon steel (without alloying elements) and
microalloyed steel with a chemical composition shown
n Table 1. All the specimens were first subjected to an
nnealing treatment at 900 ◦C for 30 min followed by air-
ooling to homogenize the micro-structure. Samples used
or the annealing treatment are of dimensions approximately
70 mm × 30 mm × 4.5 mm. A Carbolit furnace capable of
perating up to 1200 ◦C was used. The temperature in the heat
reatment furnace was measured using a K-type thermocouple
nd temperature variation during heat treatment did not exceed
3 ◦C.
Dual phase micro-structure was achieved by annealing at the
ntercritical temperature of 750 ◦C for 15 min and cooling in cold
ater. Through these heat treatments, the temperature of each
pecimen was monitored by a K-type thermocouple mounted to
he head of the specimens in a 15 mm deep hole. Manufacture
f tensile test specimens was carried out after heat treatment to
liminate the effect of oxidation and decarburization caused by
eat treatment. The dimension of tensile test pieces with gauge
ength parallel to the rolling direction is shown in Fig. 1.
The specimens were prestrained in tension by 2, 4 or 6%.
fter this, they were unloaded and aged at 25, 100, 150, 200
nd 250 ◦C for 30 min. After ageing of the specimens, they were
ubjected to a tensile test at ambient temperature at a crosshead
peed of 2 mm min−1. At least three specimens were tensile
ested for each ageing temperature and average values were cal-
ulated. The increase in flow stress as a result of restraining was
aken as the strain ageing, �Y2, which is illustrated in Fig. 2.
or samples, prestrained in tension, �Y2 was determined with
ingle specimen by the difference between lower yield stress
fter ageing and the flow stress at the end of the prestraining.
Fig. 1. Dimension of tensile test specimen (in mm).
F
a
1.20 0.10 0.05 0.020 0.070
1.40 – – 0.015 –
In the present work optical microscopy and scanning electron
icroscopy studies (SEM) were carried out to characterize steel
icro-structures and fracture surfaces, respectively. Samples for
icro-structural studies were prepared and etched with 2% Nital
ollowed by immersion in an aqueous solution (10% sodium-
etabisulfite). This etching technique revealed martensite as
rown. The optical examination of the samples was carried out
sing an Epiphot 200 Nikon type microscope capable of magni-
cations between 50× and 1000×. Determination of grain sizes
nd volume fractions of ferrite and martensite were also carried
ut by using mean linear intercept and point counting methods
n etched metallographic specimens at appropriate magnifica-
ions. Fractures of the broken tensile specimens aged at 25, 100
r 250 ◦C were examined in the scanning electron microscope.
. Results and discussion
.1. Optical metallography
The original micro-structure of the carbon steel and microal-
oyed steel consisted of equiaxed grains of varying size and
errite with pearlite. However, the heat treatment applied pro-
ig. 2. Stress–strain curve for low carbon steel strained to point A, unloaded,
nd then restrained immediately (curve a) and after ageing (curve b).
S. Gündüz / Materials Science and Engineering A 486 (2008) 63–71 65
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ig. 3. Micro-structure of dual phase carbon steel (a) and dual phase microal-
oyed steel (b) annealed at 750 ◦C for 15 min and quenched in water.
artensite volume fraction than the dual phase microalloyed
teel due to higher carbon concentration. Speich and Miller
17] indicated that the amount of martensite in dual phase steel
ncreased with the increase in intercritical temperature or carbon
ontent.
Table 2 also shows the mean linear intercept ferrite and
artensite grain sizes in dual phase carbon steel and microal-
oyed steel. It is noted that the mean linear intercept ferrite and
artensite grain sizes in dual phase carbon steel were approx-
mately 8.6 and 5.4 �m, respectively. However, dual phase
icroalloyed steel had finer grain sizes, which are 6.3 and
.9 �m for ferrite and martensite. This is due to the presence
f microalloyed elements such as vanadium, titanium and alu-
inium in microalloyed steel. It is well established in microal-
oyed steels carbonitride precipitation plays an important role
n controlling the micro-structure and mechanical properties. In
act, it has been reported that the ferrite grain size is influenced by
he formation of fine precipitate such as VCN, TiCN or AlN dur-
ng and after transformation. The presence of these second-phase
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ean linear intercept grain sizes and volume fractions of ferrite and martensite phase
teels Ferrite (%) Martensite (%)
icroalloyed steel 80 20
arbon steel 60 40
ig. 4. Variation of stress–strain curves of the dual phase carbon steel at different
geing temperatures for the prestrains of 2% (a), 4% (b) and 6% (c).
articles dramatically changes the grain coarsening characteris-
ics due to their pinning effect on the austenite grain boundaries.
he fine austenite grains lead to fine ferrite and martensite grains
uring cooling to room temperature [14,18].
.2. Mechanical properties
Figs. 4 and 5 show the stress–strain diagrams of the dual phase
arbon steel and the microalloyed steel prestrained in tension by
, 4 or 6%, aged at different temperatures, and restrained. As
hown, the dual phase carbon steel and the microalloyed steel,
rior to any ageing, exhibits continuous yielding which has been
ommonly attributed to mobile dislocations introduced during
ooling from the intercritical annealing temperature. Many dis-
ocation sources come into action at low strain and plastic flow
egins simultaneously through the specimen, thereby suppress-
ng discontinuous yielding [19].
s in the carbon steel and the microalloyed steels
Ferrite grain sizes (�m) Martensite grain sizes (�m)
6.3 2.9
8.6 5.4
66 S. Gündüz / Materials Science and En
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ig. 5. Variation of stress–strain curves of the dual phase microalloyed steels at
ifferent ageing temperatures for the prestrains of 2% (a), 4% (b) and 6% (c).
However, when the both type of steels prestrained in tension
y 2, 4 or 6% were aged at 25, 100, 150, 200 and 250 ◦C, they
howed discontinuous yielding behaviour and developed yield
lateaus. This is due to the diffusion of interstitial solute atoms
o free dislocations generated during cooling from the intercrit-
cal annealing temperature or plastic deformation; in this way
he dislocations are anchored and yield point returns. It is also
vident that segregation needs to stop when all the dislocation
ites allowing strong interaction with solute atoms are occupied.
his absence of a saturation effect must be due to some form
f precipitation of the solute atoms collected by the dislocations
20]. Waterschoot et al. [21] indicated that in dual phase steel,
everal stages must be considered in both the ferrite (static strain
geing processes) and the martensite (tempering processes): (1)
he Cottrell atmosphere formation stage, (2) the carbon cluster-
ng stage in the ferrite, (3) the precipitation stage in ferrite and
4) the effects due to the tempering of the martensite, such as the
olume contraction of martensite during tempering, the changes
n the martensite strength and the additional carbon clustering
r precipitation near the ferrite/martensite interfaces.
Dual phasesteel may contain more available interstitial solute
han is needed to complete Cottrell locking after prestrain. This
n general, atmosphere formation and precipitation hardening
an both contribute to the increase in yield strength observed
�
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gineering A 486 (2008) 63–71
uring strain ageing. Davies [22] observed an increase in yield
trength and yield point elongation in both air-cooled and water-
uenched dual phase steels after ageing for 20–30 min at about
70 ◦C; this time and temperature are typical of present paint
ven conditions and this ageing is often referred to as bake hard-
ning. He suggested that this is a result of the excess carbon in
olution diffusing so as to pin the free dislocations in the ferrite
nd/or form fine carbides. It seems probable that precipitates
re able to form on dislocations during strain ageing because,
ven on slow cooling, ferrite is supersaturated with carbon and
itrogen. Dislocations are known to be very effective nucleation
ites, so that the solute is able to precipitate in the presence of
islocations but not in their absence [23].
Whilst low-ageing steels have been produced for many
ecades by using aluminium, titanium or boron, to form a stable
itride, the extension of this idea to form a stable carbide is rel-
tively recent. The amount of carbide/nitride former that needs
o be added has to exceed the stoichiometric ratio required to
orm the carbide and nitride. When the carbon and nitrogen con-
ent of the steel is limited, then smaller microalloying additions
re required [24]. It is clear from the foregoing discussion that
he general schema of the effect of carbide and nitride formers
s fairly clear. Their effectiveness in preventing strain ageing
ncreases with increasing affinity for carbon and nitrogen [23].
In spite of the alloying additions, free interstitial solute atoms
ay still be present in microalloyed steels. It is difficult to
etermine quantitatively the free interstitial content of commer-
ial steels by established techniques because of the interaction
etween alloying additions and the interstitials, but the presence
f free interstitials may be established indirectly. For instance, in
he as rolled condition most microalloyed steels exhibit a yield
oint elongation, which increases when the steel is aged. This
uggests that on ageing, solute atoms such as carbon and nitro-
en which were frozen in a random distribution throughout the
atrix during rapid cooling migrate to free dislocations, thereby
ausing an increase in the yield point elongation [25].
As also seen from Figs. 4 and 5 that dual phase microalloyed
teel containing 20% martensite volume fractions showed more
ronounced discontinuous yielding than the dual phase carbon
teel containing 40% martensite volume fractions for the tem-
eratures ranging from 25 to 250 ◦C. This is due to the presence
f different percentage of martensite in both steels. Speich [26]
howed that at lower temperatures, the segregation of carbon to
islocations and the elimination of residual stresses results in an
ncrease in the yield strength and return of discontinuous yield-
ng, but only if the volume fraction of the martensite phase is
elow 30%. At higher volume fraction of martensite, the initial
ielding behaviour appears to be less affected by ageing because
f higher dislocation density and higher residual stresses.
Tables 3 and 4 illustrate the strain ageing behaviour of the
ual phase carbon steel and the microalloyed steel by showing
nitial yield strength, strength after straining, final yield strength
fter ageing, �Y1 (increase in stress produced by prestrain),
Y2 (increase in stress produced by ageing), �Y3 (increase
n stress due to straining and ageing, �Y1 + �Y2), UTS and
longations. As can be seen in both the dual phase carbon steel
nd the microalloyed steel show a decrease in �Y2 as the amount
S. Gündüz / Materials Science and Engineering A 486 (2008) 63–71 67
Table 3
The tensile properties of the dual phase carbon steel with different prestrain rates and ageing temperatures
Prestr. (%) Age. temp. (◦C) Int. YP (MPa) Strength after
str. (MPa)
Final LYP
(MPa)
�Y1 (MPa) �Y2 (MPa) �Y3 (MPa) UTS (MPa) Total
elong. (%)
2
25 548 721 752 173 31 204 860 8
100 513 737 792 224 55 279 882 7
150 520 712 729 192 17 209 819 7
200 542 715 703 152 −12 161 811 8
250 546 731 700 185 −31 81 726 8
4
25 560 831 838 271 7 278 873 5
100 541 834 883 224 49 342 891 4
150 544 840 839 299 −1 292 886 6
200 550 836 820 292 −16 263 875 7
250 541 832 783 291 −49 242 843 7
25 534 875 877 341 2 343 883 5
100 567 870 916 303 46 349 923 6
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6 150 561 871 860
200 564 873 850
250 566 863 806
f tensile prestrain is raised form 2 to 4 or 6% for the ageing
emperatures in the range studied.
For example, dual phase carbon steel samples prestrained in
ension by 2% and then aged at 100 ◦C for 30 min show the high-
st increase in �Y2 (55 MPa); it then decreases to 49 and 46 MPa
hen the amount of prestrain was raised to 4 or 6%, respectively.
ince increasing prestrain effectively raises the dislocation den-
ity and reduces the work hardening rate, more dislocations
ust be pinned at high prestrains to achieve the same level of
Y2 which can be developed at low prestrains with fewer dis-
ocations. The mechanism responsible for decreasing �Y2 with
ncreasing prestrain has not been widely discussed in the litera-
ure. Lee and Zuidema [27] suggest that the effect is due to stress
elaxation in the strain fields of dislocations during the ageing.
n the other hand, according to some other investigators [28,29],
t is due to the fact that if prestrain increases, the dislocation den-
ity becomes higher and, consequently, the amount of carbon per
islocation decreases, leading to the decrease in �Y2.
o
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2
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able 4
he tensile properties of the dual phase microalloyed steel with different prestrain rat
restr. (%) Age. temp. (◦C) Int. YP (MPa) Strength after
str. (MPa)
Final LYP
(MPa)
2
25 426 544 561
100 429 543 619
150 429 543 563
200 428 545 557
250 428 542 546
4
25 428 615 625
100 433 617 674
150 431 611 625
200 431 613 623
250 439 619 621
6
25 435 624 638
100 432 617 664
150 433 610 622
200 426 613 616
250 431 618 612
303 −11 300 905 6
295 −23 291 903 7
287 −57 240 871 7
In contrast to the negative effect of prestrain on the change in
Y2 produced by subsequent ageing, it was found that increas-
ng prestrain markedly increased the UTS of both dual phase
arbon steel and microalloyed steel (see Tables 3 and 4). For
xample dual phase carbon steel samples exhibited the highest
ncrease in UTS (882 MPa) after prestraining by 2% followed
y ageing at 100 ◦C; it then showed further increase to 892 and
23 MPa as the prestrain rate was raised to 4 or 6%, respec-
ively for the ageing temperature of 100 ◦C (see Table 3). This
s consistent with the results obtained by Wilson and Russel
30] who showed that the amount of tensile prestrain has only a
mall effect on the change in yield stress after ageing, but with
igher prestrain, a greater lower yield elongation and a greater
ncrease in UTS are obtained. They also showed that the amount
f prestrain has a strong effect on the rate of work hardening,
or example; doubling the amount of prestrain in the range of
–7% extension approximately doubles the final values of �U
change in UTS due to straining and ageing). This contrast in
es and ageing temperatures
�Y1 (MPa) �Y2 (MPa) �Y3 (MPa) UTS (MPa) Total
elong. (%)
118 17 135 637 10
114 76 190 672 11
114 20 134 639 13
119 12 129 577 12
117 4 118 572 13
187 10 197 673 11
184 57 241 689 11
180 14 194 662 11
180 10 192 642 10
176 2 182 640 12
189 14 203 660 11
185 47 232 692 7
177 12 189 657 9
187 3 190 635 10
187 −6 181 626 11
6 nd En
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8 S. Gündüz/ Materials Science a
he effect of prestrain on �Y2 and �U was noted previously
y Hundy [31]. The results obtained from the present investiga-
ion therefore support a conclusion that the rate of decrease in
Y2 is rather insensitive to dislocation density and is principally
ependent on the solute segregation per dislocation.
It is also seen from Tables 3 and 4 that both steels show a
ignificant increase in YS, UTS and �Y2, however the elon-
ation decreases as the ageing temperature is raised from 25 to
00 ◦C for the prestrains in the range studied. Changes in yield
henomena are generally associated with the classical Stage I
f strain ageing related to atmosphere formation at dislocation
32]. The yielding and Lüders phenomena, which are charac-
eristic of plastic flow in the ferrite, show the clearest evidence
f ageing. Stage 2 ageing behaviour, classically observed as an
ncrease in tensile strength and a decrease in total elongation due
o precipitation at dislocation [30], is clearly observed in these
teels. This may indicate the precipitation of carbonitride on dis-
ocations during strain ageing. Stage 2 of ageing corresponds to
regime where the stress required to generate new dislocations
s less than the unpinning stress, so that the increase in the strain
ardening rate in this stage was due to the activation of new
islocation sources, and the consequent increase in dislocation
ensity [33].
The results obtained from the present study are also consis-
ent with the results obtained by Davies [34] who showed that
hese changes in mechanical properties of vanadium containing
ual phase steel are due to ageing effect at ambient tempera-
ure. This is associated with a reduction in the number of mobile
islocations, due to formation of Cottrell atmospheres around
he dislocations. Cottrell atmospheres occur as a result of the
ccumulation of nitrogen and carbon atoms around disloca-
ions sometimes referred to as clouds, influences the plastic flow
ehaviour of the steel in as much as plastic flow is the result of
he movement of dislocations. Because the nitrogen and carbon
toms segregate to dislocations represent a more stable condi-
ion (lower misfit energy), they tend to ‘immobilize’ or ‘lock’ or
pin’ the dislocations, thus making plastic strain more difficult
i.e., they raise the stress required to cause dislocations to move)
35].
However, YS and UTS fell with increasing testing temper-
ture to 150, 200 or 250 ◦C, while percentage elongation to
racture increased. These are signs of overageing probably due
o tempering effect of martensite and coarsening of the precip-
tates on the dislocations [23,36]. Abdollah [14] also observed
hat increase in time and temperature cause a reduction in yield
trength in the plain carbon dual phase steel. This phenomenon
s associated with the tempering effect that starts in martensite
hase.
The changes in �Y2 were greater in the dual phase microal-
oyed steel than in the carbon steel for the prestrains and the
geing temperatures in the range studied. Also, �Y2 values
f the dual phase carbon steel decreased to minus values after
geing at 200 and 250 ◦C for different prestrain rates (see
ables 3 and 4). These results indicate that overageing occurred
ore slowly in the dual phase microalloyed steel than the dual
hase carbon steel. This was associated with the chemical com-
osition of dual phase microalloyed steel which, in addition
f
p
d
gineering A 486 (2008) 63–71
o carbon atoms, contains nitrogen and carbide forming ele-
ents such as titanium, vanadium and aluminium which may
orm as precipitates such as TiCN, VCN and AlN that causes
econdary hardening. The precipitation of these particles, espe-
ially VCN, on dislocations has two important effects on the
echanical properties of dual phase steels. Firstly the dislo-
ations are pinned by precipitates and this delays recovery of
he tangled dislocation structure introduced during the marten-
ite transformation. In effect this retards the normal softening
rocess, which occurs on ageing. Secondly the pining of the dis-
ocations increases the strength of material. Tekin [37] showed
hat when the overageing occurs, the precipitates on dislocations
oarsen and become more widely spaced. Eventually the dislo-
ations must be able to free themselves from these precipitates
o that recovery can begin. When this happens the precipitates
an still contribute to the hardening by a conventional precipita-
ion hardening mechanism, but the absence of dislocation pining
nd the ensuing recovery process will both lead to softening. The
ombined loss of strength by freeing of the dislocations from the
recipitates and by the recovery of the dense dislocation tangles
hould lead to a marked softening. This is consistent with the
ecrease in strength as the ageing temperatures are increased
see Tables 3 and 4).
Rashid [38] has studied the change in strength and ductil-
ty of a number of different dual phase steels after prestraining
p to 25% and ageing for 1 h at 205 ◦C. In all cases the yield
trength increased and the ductility decreased. The change in ten-
ile strength was more complex, increasing for all decrease of
restrain in microalloyed steels but decreasing for low degrees
f prestrain in steels without microalloying additions. Rashid
xplains these differences by a reversal from a net soften-
ng when the tempering of martensite phase is important to a
et strengthening when ferrite strengthening resulting from the
nteraction between dislocations and NbCN or VCN precipitates
s important. Davies [19] also proposed that the strain ageing of
icroalloyed steel is influenced by the interaction of carbon
toms with clusters of vanadium atoms in the ferrite matrix.
The microalloyed dual phase steel showed greater ductility
han the dual phase carbon steel. This was evidenced by supe-
ior elongation and reduction in area as well as fracture surface
nalysis. Figs. 6 and 7 show fracture surfaces of the dual phase
icroalloyed steel and the carbon steel prestrained in tension
y 4 or 6% and then aged at 25, 100 and 250 ◦C. As seen in
igs. 6 and 7, both dual phase microalloyed steel and carbon
teel showed certain amount of cleavage pattern, typical of brit-
le fracture after ageing at 100 ◦C; this was manifest as low
ercent elongation prior to fracture. The reduction in area also
ecreased at the ageing temperature of 100 ◦C, which corre-
ponds to embrittlement temperature due to static strain ageing
esult of the interaction between dislocations and interstitial
toms and/or precipitate particles. However, ductile dimpling
as found in both the steels after ageing at 250 ◦C which led
o increase in percentage elongation due to overageing resulted
rom tempering effect of martensite and/or coarsening of the
recipitates on dislocations.
Dual phase carbon steel and microalloyed steel also showed
ifferent fracture behaviour after ageing at 25 ◦C. For example,
S. Gündüz / Materials Science and Engineering A 486 (2008) 63–71 69
F strain
3
d
s
w
f
T
a
e
m
f
[
v
b
ig. 6. Fracture surface micrographs for the dual phase microalloyed steel, pre
0 min followed by restraining.
ual phase carbon steel with 40% martensite volume fractions
howed brittle fracture, whilst dual phase microalloyed steel
ith 20% martensite volume fractions exhibited a certain sur-
ace roughness, typical of ductile fracture (Figs. 6 and 7a).
his sharp difference in fracture pattern over such a low-
geing temperature for a short period of time cannot be
xplained by the effect of strain ageing and support argu-
ents that introducing different percentage martensite into
i
s
a
ed in tension by 4% and then aged at 25 ◦C (a), 100 ◦C (b) and 250 ◦C (c) for
errite matrix affects the fracture behaviour. Kocatepe et al.
39] indicated in their work that with increasing martensite
olume fractions, the fracture pattern changed from ductile to
rittle.
The results presented confirm that static strain ageing occurs
n both dual phase carbon steel and dual phase microalloyed
teel in the differentamount of prestrains and ageing conditions
s evidenced by an increase in �Y2 (increase in stress produced
70 S. Gündüz / Materials Science and Engineering A 486 (2008) 63–71
F in ten
f
b
m
s
p
n
r
t
l
h
a
p
a
i
4
s
ig. 7. Fracture surface micrographs for dual phase carbon steel, prestrained
ollowed by restraining.
y ageing). The changes in �Y2 were greater in dual phase
icroalloyed steel than in carbon steel. These results could be
ignificant for the use of dual phase steel after straining and
aint baking. For example, during the cold forming of compo-
ent, dual phase steel undergoes different strain level and this
esults in an increase in yield strength during the bake hardening
reatment. Free interstitial such as carbon will move to the dis-
ocations generated in the cold forming process during the bake
ardening treatment. In effect bake hardening is a type of strain
geing process which is ordinarily detrimental due to accom-
anying loss in ductility but which can be used to considerable
w
s
2
T
sion by 6% and then aged at 25 ◦C (a), 100 ◦C (b) and 250 ◦C (c) for 30 min
dvantage after forming operation to provide a strengthening
ncrement.
. Conclusion
Static strain ageing behaviour of dual phase microalloyed
teel with 20% martensite volume fractions and carbon steel
ith 40% martensite volume fractions were investigated. The
pecimens were prestrained in tension by 2, 4 and 6%, aged at
5, 100, 150, 200 and 250 ◦C for 30 min followed by restraining.
he main conclusions from this study are as follows:
and
1
2
3
4
5
6
7
A
c
S
t
S
R
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[36] S.H. Mousavi Anijdan, H. Vahdani, Mater. Lett. 59 (2005) 1828–
S. Gündüz / Materials Science
. Dual phase carbon steel had a larger martensite volume frac-
tion than the dual phase microalloyed steel due to its higher
carbon content.
. Both steels displayed significant changes in appearance as
the ageing temperature was increased for the prestrain in the
range studied. This indicated that static strain ageing takes
place in both dual phase carbon steel and microalloyed steel.
. In contrast to the negative effect of prestrains on the change
in �Y2 produced by subsequent ageing, it was found that
increasing prestrain markedly increased the change of UTS
of both dual phase carbon steel and microalloyed steel. This
indicated that �Y2 value is insensitive to dislocation density
and is principally dependent on the solute segregation per
dislocation.
. Both dual phase carbon steel and microalloyed steel showed
significant increases in YS, UTS and �Y2, however the
percentage elongation to fracture decreased as the ageing
temperature was raised from 25 to 100 ◦C for the prestrain
in the range studied. This is due to atmosphere formation at
dislocation and precipitation of carbonitride on dislocations
during strain ageing.
. Further increase in ageing temperature to 150, 200 and
250 ◦C caused a reduction in YS, but an increase in per-
centage elongation. These are signs of overageing probably
due to tempering effect of martensite and coarsening of the
precipitates on the dislocations.
. The ageing in the dual phase microalloyed steel occurred
more slowly than the dual phase carbon steel. This was
associated with the chemical composition of dual phase
microalloyed steel which, in addition to carbon atoms,
contained nitrogen and carbide forming elements such as
titanium, vanadium and aluminium.
. The fracture surface analyses indicated that both dual phase
microalloyed steel and carbon steel exhibited certain amount
of cleavage pattern, typical of brittle fracture after ageing
at 100 ◦C. However, ductile dimpling was found in both the
steels after ageing at 250 ◦C due to overageing resulted from
tempering effect of martensite and/or coarsening of the pre-
cipitates on dislocations.
cknowledgements
The author whishs to acknowledge with gratitude the finan-
ial support of Zonguldak Karaelmas University, Institute of
cience and the Project and Science Research Commission for
heir support. The author would like to thank also Dr. Hüseyin
¸ . SOYKAN in Turkey for his kind material supports.
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[
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[
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https://www.researchgate.net/publication/248473521
	Static strain ageing behaviour of dual phase steels
	Introduction
	Experimental procedure
	Results and discussion
	Optical metallography
	Mechanical properties
	Conclusion
	Acknowledgements
	References