microstructural characterization and elastoplastic behaviour of high strengt

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

7

MICROSTRUCTURAL CHARACTERIZATION AND ELASTOPLASTIC

BEHAVIOUR OF HIGH STRENGTH LOW ALLOY STEEL

Shatrughan Soren1, R.N. Gupta

2, N. Prasad

3 and M. K. Banerjee

4

1Assistant Professor, Dept. of Fuel & Mineral Engineering, ISM Dhanbad, India

2Associate Professor & Head, Dept. of Metallurgical Engineering, BIT Sindri, India

3Ex-Professor, Dept. of Metallurgical Engineering, BIT Sindri, India

4Steel 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. 800oC, 845

oC, and 900

oC 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 800

oC, 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, 845oC, 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 900oC, 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 800oC (a) Optical (b) SEM

Fig. 2 Microstructure of steel deformed 50% after soaking at 845oC (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 900oC 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 αoB. It is

pertinent to state that the existence of appreciable amount of boundaries of low misorientation, is

indicative of occurrence

Fig. 3 Microstructure of steel deformed 50% after soaking at 900

oC (a) Optical (b) SEM

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

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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 900oC 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

Yie

ld S

tren

gth

,GP

a

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

Lo

ad

, m

N

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 0 0 2 0 0 3 0 0 4 0 0 5 0 00

4

8

1 2

1 6

2 0

Lo

ad

, m

N

P enetratio n depth , nm

0.00 0.05 0.10 0.15 0.20

0

1x108

2x108

3x108

4x108

S

tres

s,

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 1 00 2 0 0 30 0 4 0 00

4

8

1 2

1 6

2 0

Lo

ad

, m

N

P e n e tra tio n d e p th (n m )

0.00 0.05 0.10 0.15 0.20

0

1x108

2x108

3x108

4x108

5x108

Str

es,

Pa

strain



16

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