Characterization of microstructure and mechanical properties of thermomechanically processed high strength steel

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME
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MICROSTRUCTURAL CHARACTERIZATION AND ELASTOPLASTIC
BEHAVIOUR OF HIGH STRENGTH LOW ALLOY STEEL
Shatrughan Soren1
, R.N. Gupta2
, N. Prasad3
and M. K. Banerjee4
1
Assistant Professor, Dept. of Fuel & Mineral Engineering, ISM Dhanbad, India
2
Associate Professor & Head, Dept. of Metallurgical Engineering, BIT Sindri, India
3
Ex-Professor, Dept. of Metallurgical Engineering, BIT Sindri, India
4
Steel Chair Professor, Dept. of Metallurgical and Materials Engineering, MNIT Jaipur, India
ABSTRACT
The ferrite grain refinement is a powerful mechanism to improve the strength and toughness
in steels. High strength low alloy steel is controlled rolled at a temperature just above its A3
temperature and then water cooled. In the present investigation an attempt has been made to
produced ultrafine ferrite grained (1–3 µm ) steels through relatively simple Thermomechanical
Controlled Processing (TMCP). The microstructure of the steel was characterized by Electron Back
Scattered Diffraction (EBSD) technique and nanoindentation method was used to characterize the
elastoplastic behaviour of the steel. It is found that about 20 percent prior austenite undergoes
dynamic strain induced transformation with grain size 3µ or less. The ferrite formed after direct
cooling having varying elastoplastic characteristics and that the observed variation owes its origin to
difference in carbon content of ferritic formed at different temperatures.
Keywords: Thermomechanical controlled processing, ultra fined ferrite, Electron back scattered
diffraction, Nanoindentation.
1. INTRODUCTION
Due to excellent formability advanced high strength steels are widely employed in
automotive industries [1, 2]. Quite often high strength low alloy steels are subjected to conventional
thermo-mechanical treatment. Controlled thermo-mechanical processing has often more use direct
cooling after controlled rolling. Under such situation of continuous cooling multiphase
microstructure are reported to result [3, 4]. However the micro constituents formed are dependent
upon cooling rate. In recent times dynamic strain induced transformation (DSIT) of austenite to
ferrite is reported by a number of workers; in this case deformation and transformation takes place
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simultaneously [5-7]. In order to have a deeper understanding about microstructural change during
DSIT, electron back scatter diffraction has been used by previous workers [8]. It appears that the
ferrite formed in case of continuous cooling after hot working is of transient character in respect of
mechanism of formation. It is anticipated that the ferrite formed through different mechanism will
have different elastoplastic behaviour. Nanoindentation technique has been used by some researchers
to study the elastoplastic behaviour of microconstituent in multiphase microstructure [9, 10].
However there is no report of relating the elastoplastic behaviour of mechanistically transient ferrite
crystals with chemical composition and deformation parameters. The present investigation aims at
characterizing elastoplastic behaviour of ferrite crystals formed through different mechanism during
continuous cooling of high strength low alloy steels following control rolling just above upper
critical temperature.
2. EXPERIMENTAL
The chemical composition of the steel used for the present investigation is furnished below in
Table 1
Table 1 Chemical composition of the experimental steel (weight %)
C Mn Si Cr Mo Ni Cu Al Ti Nb Fe
0.13 2.24 1.25 0.34 0.027 0.048 0.111 0.017 0.007 0.061 bal
As supplied steel plate of thickness 6mm was cut into small pieces (20mmx20mm); the
sample pieces were soaked at different temperatures viz. 800o
C, 845o
C, and 900o
C in the electrical
resistance furnace for fixed holding time of 20 minutes. After soaking, the samples were rolled upto
50 % thickness reduction and directly quenched in water. The mechanical behaviour of the rolled
specimen were studied by nanoindentation test. The microstructural characterization were carried out
using optical, scanning electron microscope. Electron back scattered diffraction technique was used
to understand the character of transformation of austenite. DSC studies was made to get the idea of
As and Af temperatures.
3. RESULTS AND DISCUSSIONS
When the steel is rolled at 800o
C, within the two phase field, both austenite and proeutectoid
ferrite are deformed. The austenite so deformed is converted to bainite during subsequent cooling
(Fig. 1a). The proeutectoid ferrite is also deformed and undergoes partial recovery. Some austenite
undergoes dynamic strain induced transformation (DSIT) and forms very small grained ferrite. The
SEM picture in Fig. 1(b) substantiates the above observation by way of showing acicular bainitic
ferrite, ultrafine DSIT ferrite and recovered ferrite. When rolling of the steel is carried out at a
temperature, 845o
C, which is near to A3 temperature of the steel, DSIT ferrite formation is enhanced
(Fig. 2a); the deformed austenite remaining in the microstructure finally transform to bainite. As
corroborated by the scanning electron microscopic observation some proeutectoid ferrite of small
size is also found in the microstructure (Fig. 2b). When the rolling is raised to 900o
C, the austenite is
recrystallized; this crystallized austenite undergoes bainite transformation following Kardjumov-
Sach relationship. Some austenite also undergoes dynamic strain induced transformation to ferrite.
This bainite structure is clearly seen in Fig. 3(a & b).

9
Fig. 1 Microstructure of steel deformed 50% after soaking at 800o
C (a) Optical (b) SEM
In view of the fact that microstructures indentified by microorientation can provide extra
information than the mere microstructural study, EBSD analysis was performed on samples
deformed at 900o
C by 50% reduction. The orientation data (Table. 2) and corresponding histogram
(Fig. 4) shows that 20% of the boundaries ferrite crystals are low angle boundaries having
misorientation 3.5o
or less. However 73% of the boundaries are high angle boundaries having
misorientation greater than 15o
. The corresponding microstructure shows the predominance of
acicular ferrite/bainite as microconstituent. This means that rolling just above the A3 line with a
moderate deformation of 50% thickness reduction has undergone recrystallization of austenite. The
recrystallized austenite during fast cooling has given rise to the formation of acicular bainite αo
B. It is
pertinent to state that the existence of appreciable amount of boundaries of low misorientation, is
indicative of occurrence
20 µ
a b
a
20 µ
b
20 µ
a b

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue
of dynamic strain induced transformation of austenite, where deformation and transformation take
place simultaneously; however the ferrite so formed could not be recrystallized even statically owing
to pining effect of microalloy carbides
microstructure does not show any evidence of dynamic recrystallization of ferrite and most of ferrite
with or without low angle boundaries appears acicular in shape.
Table.2
Fig. 4
The final microstructure of the said steel is therefore acicular ferrite with large angle
boundaries; these ferrite has originated from recrystallized austenite; the ferrite of small
misorientation can result only if transformation of austenite to ferrite and deformation of ferrite to
acicular morphology, take place simultaneously.
found that only 8.5 percent grains have sizes one micron and less whereas 23 percent of grains are
found to be of size less than 3 microns (Table.
low misorientation, one finds a good correspondence between percentage of grains of size less than 3
micron and the percentage of ferrite of low misorientation of grains. This tends to lead one to
conclude that dynamic strain induced transformation (DSIT) ca
conventional thermomechanical treatment.
As evident from the microstructure, DSIT ferrite is deformed and has not undergone
recrystallization (Fig. 5). Therefore it is expected that they should exhibit a deformation t
However recrystallized austenite of 77 percent has undergone bainitic transformation and it is known
that this bainite transformation leads to stronger texture in comparison to polygonal ferrite formed
urnal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976
6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME
10
to pining effect of microalloy carbides formed, strain induced, at relatively low temperature. This
with or without low angle boundaries appears acicular in shape.
Table.2 Showing Misorientation Angle
Fig. 4 Misorientation histogram
ferrite has originated from recrystallized austenite; the ferrite of small
acicular morphology, take place simultaneously. From the analysis of grain size distribution, it is
found to be of size less than 3 microns (Table. 3). Comparison of this observation with the ferrite of
w misorientation, one finds a good correspondence between percentage of grains of size less than 3
conclude that dynamic strain induced transformation (DSIT) can lead to grain refinement better than
conventional thermomechanical treatment.
recrystallization (Fig. 5). Therefore it is expected that they should exhibit a deformation t
urnal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
October (2013), © IAEME
formed, strain induced, at relatively low temperature. This
ferrite has originated from recrystallized austenite; the ferrite of small
From the analysis of grain size distribution, it is
). Comparison of this observation with the ferrite of
w misorientation, one finds a good correspondence between percentage of grains of size less than 3
n lead to grain refinement better than
recrystallization (Fig. 5). Therefore it is expected that they should exhibit a deformation texture.

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue
intragranularly under slower cooling rate duri
expected to be weak, due to the fact that a less volume fraction of this phase has been formed. When
recrystallized austenite produces polygonal ferrite in the microstructure a weak and random texture
would result. However in this particular case a further cooling has produced acicular morphology of
ferrite. This transformation takes place with definite habit and orientation between parent and
product phases. Also its percentage is quite high (~77%); ther
characteristic grain orientation may be reflected in the form of microscopic texture.
Admittedly the microstructure consists of ferrite of similar morphology but with different
genesis. Major fraction is bainitic ferri
The other type is dynamic strain induced transformation of austenite to ferrite with limited dynamic
recrystallization in ferritic phase. However the pole figure (Fig. 6) generated in EBSD anal
not show isolines but shows dots, thereby being less conclusive in information. Again the pole figure
does not show symmetry along RD or TD; therefore the deformation process could not be
orthorhombic. Again only a limited number of grain are inv
information being less statistical it is difficult to derive the representative texture if there be any. This
is for why no specific comments on the evolution of distinctly different textures for ferrites of
different origin can be made.
Table 3. Grain size distribution and
Fig.5 SEM microstructure showing deformed DSIT
urnal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976
6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME
11
intragranularly under slower cooling rate during TMCP (ref bavis). In case of DSIT, the texture is
uld result. However in this particular case a further cooling has produced acicular morphology of
product phases. Also its percentage is quite high (~77%); therefore there is reason to believe that
characteristic grain orientation may be reflected in the form of microscopic texture.
genesis. Major fraction is bainitic ferrite formed by shear transformation of recrystallized austenite.
recrystallization in ferritic phase. However the pole figure (Fig. 6) generated in EBSD anal
orthorhombic. Again only a limited number of grain are involved in mapping; thus textural
Grain size distribution and corresponding area fraction
SEM microstructure showing deformed DSIT
urnal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
October (2013), © IAEME
ng TMCP (ref bavis). In case of DSIT, the texture is
uld result. However in this particular case a further cooling has produced acicular morphology of
efore there is reason to believe that
te formed by shear transformation of recrystallized austenite.
recrystallization in ferritic phase. However the pole figure (Fig. 6) generated in EBSD analysis does
olved in mapping; thus textural

12
Fig. 6 Pole figure generated by Fig. 7 Map of phases generated by
EBSD technique EBSD technique
The colour coded EBSD map of phases have recoded the presence of about 2% retained
austenite (Fig. 7). This follows from transformation mechanism of γ→α which in this case has been
martensitic for about 77% recrystallized austenite; this produces bainite ferrite of high angle
misorientation. Thus our conjecture that bainitic transformation occurs in this steel to a large extent
is supported by the observation that some retained austenite is present in the material.
In the present experiments, the steel was rolled at 900o
C after soaking for 20 minutes and a
total of 50% deformation was given. From the DSC heating curve it is also clear that the above
temperature of deformation is just above the A3 temperature of the experimental steel. From the
microstructure of the steel it is further observed that the structure is constituted by bainitic ferrite,
DSIT and some proeutectoid ferrite (Fig.8). The microstructure of the sample shows acicular ferrite,
some ferrite of irregular morphology occasional presence of proeutectoid ferrite of massive
appearance is occasionally observed. The deformation temperature being above Ar3, the austenite is
stable and transformation during straining of stable austenite is characteristically different from that
of the metastable austenite. The deformation load within austenite leads to creation of intragranular
nucleation sites for ferrite. The ferrite nucleated intra-granularly undergoes continuous deformation
and recrystallization and become finer. Such equiaxed ferrite formed after recrystallization appears
extremely fine in the microstructure. During the course of rolling, deformed austenite cooled fast
subsequently transforms at a lower temperature and produces bainite or granular bainite depending
upon transformation temperature. Elastoplastic behaviour of ferrite present in its microstructure is
attempted to be studied by nanoindentation methods. The result of nanoindentation in respect of
hardness of the measured phase, its yield strength and elastic modulus are furnished in the Table.4. It
is known that nanoindentation involves stresses along all directions and compressive, tensile and
shear stresses are produced in nanoindentation against where pure tensile or compressive stresses is
involved [9]. It is reported that construction of indentation stress-strain curve from the observed
load-depth of indentation curve enables one to relate them [8]. With the concept used elsewhere [11]
the present data have been generated to describe the elastoplastic behaviour of microconstituents in
this rolled steel.

13
Fig. 8 Microstructure of steel showing different phases
Table. 4 Mechanical properties of steel characterized by Nanoindentation methods
Fig: 9. Nanoindentation Hardness Vs Yield Fig. 10 Load versus penetration dept showing
Strength curve elastic recovery for indentation
From the result of nanoindentation in Table 3 it is observed that hardness, yield strength and
also the elastic constants are different for different ferritic phase. Elastic constant varies with yield
strength of ferrite; it is known that yield strength is highly structure sensitive properties and owes it
origin to the interaction of interstitial atom with Cottrell-Lomer barriers. Table 3 however does not
SL.
No. Hardness
(VHN)
Yield
Strength
(GPa)
Elastic
Constant
(GPa)
1 535 5.78 246
2 622 6.70 263
3 778 8.40 316
4 649 7.00 279
5 691 7.40 275
6 703 7.50 271
7 766 8.20 315
8 973 10.50 360
9 439 4.70 439
400 500 600 700 800 900 1000
4
5
6
7
8
9
10
11
YieldStrength,GPa
Hardness, VHN
0 100 200 300 400
0
4
8
12
16
20
Indent 1
Indent 2
Indent 3
Indent 4
Indent 5
Indent 6
Indent 7
Indent 8
Indent 9
Indent10
Load,mN
Penetration Dept, nm

14
show any specifically defined pattern of variation; nevertheless it is found that increasing yield
strength has increased elastic constant of the materials. It is known that elastic constant is a material
property and it is structure insensitive. Therefore the observed variation of this structure insensitive
property along with a structure sensitive property must stem from the difference in chemistries of
ferrite appearing in the microstructure. Fig 9 shows that hardness of the ferrite crystals vary linearly
with yield strength of the phase. Such one to one correspondence is indicative of predominance of
shear deformation in determining the hardness of the phase. Yield strength is the manifestation of
onset of plastic deformation through shear. Indentation test similarly measures the resistance to
permanent deformation; so it is natural that they will have co-relation. From all these relations it
appears that dynamic transformation of austenite under rolling strain has envisaged mechanistically
transient phase transformation. In fact such transient transformation of austenite has been reported
earlier. The microstructural evidence is also suggestive of the same (Fig 8). Due to difference in
mechanism of transformation, there is kinetic constraints in solute partitioning. The ferrite formed
later at lower temperatures are rich in solute content due to transformation being diffusionless, not
permitting solute partitioning. However carbon rich bainitic ferrite is unlikely to exhibit such a large
difference in Young Modulus (228 GPa in test 1 to 360 GPa in test 9); rather it may be conjectured
that other substitutional elements are also responsible for creating difference in elastic constants in
the steel.
Fig. 11 (a) Nanoindentation Load-penetration curves and (b) Corresponding stress- strain curves for
spot ten
Ferrite of different carbon content forming at different transformation temperature will
envisage different degrees of breaking away of Cottrell atmosphere and hence will exhibit different
yield strength; higher is the ferritic carbon, more will be the stress required to break the barrier and to
initiate plastic deformation and hence yield strength will be higher. Elastically strained lattice of
high solute ferrite will require high stress to produce equivalent elastic strain; hence modulus of
elasticity will increase. From the degree of elastic recovery (Fig. 10), it seems that about four distinct
compositionally different ferrite has formed. The ferrite at spot ten has a massive look and appears to
be proeutectoid ferrite formed at early stage of transformation (Fig. 8). The ferrite at spot nine
showing maximum hardness, yield strength and Young modulus is clearly the bainitic ferrite rich in
solute content and being highly dislocated (Fig. 8). The microstructure is seen to corroborate the
nanoindentation results. Dynamically transformed austenite produces ferrite of different solute
contents and dislocation densities and accordingly they show different elastoplastic behaviour. In
view of low carbon of this steel, the elastic recovery is usually very low. There are few ferritic areas
for which elastic recovery is very small (Fig 10). Such a high level plasticity entices to conjectured
0 1 00 20 0 300 400 500
0
4
8
12
16
20
Load,mN
P enetration depth, nm
0.00 0.05 0.10 0.15 0.20
0
1x10
8
2x10
8
3x10
8
4x10
8
Stress,Pa
Strain

15
that this ferrite is formed through low range diffusional means and is of extremely low carbon,
leaving behind a carbon richer austenite. This is of globular morphology and shows low hardness.
The load penetration curves for this phase exhibits discontinuities (Fig. 11a). As observed elsewhere
such discontinuities are normally associated with the formation of cracks. For such large plastic
deformation, initiate that a very high shear stress had been generated. This high shear stress is
seemingly responsible for the formation of cracks in this material. In the corresponding indentation
stress strain curve (Fig. 11b), a sudden decrease in indentation stress is observed at high strain. This
leads to lower hardness. A similar observation is made for spot 1 where the elastic recovery is 17%
(Fig 12a & 12b). The general value of elastic recovery in the ferrite found through dynamic
transformation of austenite is found to lie within 20-23% . The corresponding
Fig. 12 (a) Nanoindentation Load-penetration curves and (b) Corresponding stress- strain curves for
spot one
stress-strain curve also reveals plastic deformation with little elastic strain. Hence plastic
deformation has also led to discontinuities in concerned load-penetration curves which are of crack
formation indicative. high shear stress had been generated. This high shear stress is seemingly
responsible for the formation of cracks in this material. In the corresponding indentation stress strain
curve (Fig. 11b), a sudden decrease in indentation stress is observed at high strain. This leads to
lower hardness. A similar observation is made for spot 1 where the elastic recovery is 17% (Fig 12a
& 12b). The general value of elastic recovery in the ferrite found through dynamic transformation of
austenite is found to lie within 20-23%. The corresponding stress-strain curve also reveals plastic
deformation with little elastic strain. Hence plastic deformation has also led to discontinuities in
concerned load-penetration curves which are of crack formation indicative.
4. CONCLUSIONS
The authors wish to conclude that rolling of the experimental HSLA steel just above upper
critical temperature insures dynamic strain induced transformation of austenite to ferrite. About 75%
of recrystallized austenite transforms to bainite. It is further concluded that the grain size of the DSIT
ferrite is quite small, ~ 3µm or less. The authors also conclude that ferrite of different compositions
are present in the microstructure and these ferrite crystals exhibit different degrees of elastic recovery
during nanoindentation; again due to variation in composition these ferrites have different elastic
modulii. The highly plastic ferrite present in the microstructure undergoes cracking due to shear
involved in indentation test.
0 100 200 300 400
0
4
8
1 2
1 6
2 0
Load,mN
P en etratio n d epth (n m )
0.00 0.05 0.10 0.15 0.20
0
1x10
8
2x10
8
3x10
8
4x10
8
5x10
8
Stres,Pa
strain

16
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Asfahani and G. Tither (eds.) International Symposium on Low Carbon Steel for the 90s,
1993, p. 389
[2] M. R. Barnett and J.J. Jonas, ISIJ Int. 39(1999) 856
[3] M.K. Banerjee, P.S. Banerjee and S. Datta, ISIJ Int. Vol. 41 (2001) No. 3, pp. 257-261
[4] M. K. Banerjee, D. Ghosh and S. Datta: Scand J. Metall., 29 (2000).
[5] A. J. DeArodo: ISIJ Int., 35 (1995), 946.
[6] G. Krauss and S. W. Thompson: ISIJ Int., 35 (1995), 937.
[7] K. Shibata and K. Asakura: ISIJ Int., 35 (1995), 982.
[8] A. Oudin, P.D. Hodgson and M.R. Barnett, Mater. Sci. Eng. A 486(2008) 72-79
[9] E. Martinnez, J. Romero, A. Lousa and J. Esteve, Appl. Phys. A 77, 419-426 (2003)
[10] P. Hones, M. Diserens, R. Sanjines and F. Levy, J. Vac. Sci. Technol. B 18, 2851 (2000)
[11] W.C. Oliver and G.M. Pharr.J. Mater. Res. 7, 1564 (1992)
[12] Bevis Hutchinson, Lena Ryde, Eva Lindh, et al J. Mat. Sc. Engg. (A) 257, 9-17(1998)

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