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Materials Science and Engineering A 378 (2004) 304307

Analysis of the martensitic transformation at various scales in TRIP steel

M.R. Berrahmoune a,, S. Berveiller a, K. Inal a, A. Moulin b,1, E. Patoor a

a LPMM, UMR CNRS 7554-ENSAM: 4, rue Augustin Fresnel Technopole, 57078 Metz Cedex 3, Franceb LEDEPP, 17 Avenue des Tilleuls B.P. 70011, 57191 Florange Cedex, France

Received 17 June 2003; received in revised form 29 September 2003

Abstract

The mechanical behavior of transformation induced plasticity (TRIP) steels depends on the martensitic transformation of the retainedaustenite. The studied material is a three-phase TRIP steel with ferrite base containing less than 1% of bainite and 8% of retained austenite.

Tensile tests were performed at various temperatures ranging from 60 to +120 C. X-ray diffraction was used to determine the kinetics

of transformation at various temperatures. First results show that at low temperatures, the austenite transforms into martensite very rapidly,

whereas the kinetics of transformation are much slower at high temperatures. The stress state in the austenite at room temperature was also

obtained by X-ray diffraction. After pre-strain, it is noticed that retained austenite is in tension probably due to the carbon content which is

more important in austenite than in ferrite.

2004 Elsevier B.V. All rights reserved.

Keywords: TRIP effect; Martensite transformation; X-ray diffraction; Three-phase material; Mechanical behavior

1. Introduction

The use of high strength steels with excellent formability

becomes a primary objective for economical and safety rea-

sons. In this context, the transformation induced plasticity

(TRIP) steels with their high strength and excellent forma-

bility compromise can answer to the demand for weight

decrease in the automotive industry. Their excellent me-

chanical properties result from the martensitic transforma-

tion of metastable retained austenite, induced by thermo-

mechanical loading. The TRIP steels possess a multi-phase

microstructure, consisting typically of ferrite, bainite and re-

tained austenite. The microstructure is obtained after an in-

tercritical annealing and a subsequent isothermal annealing

in the bainitic transformation region, called austempering.

The carbon content in austenite is increased both during the

intercritical annealing and the austempering. The carbon en-

richment during austempering is the result of the suppression

of the formation of carbides during the bainitic transforma-

tion, due to the presence of the alloying elements such as sili-

con and aluminum. The enrichment of carbon in the austenite

Corresponding author. Tel.: +33-387375430; fax: +33-387375470.

E-mail address: reda.berrahmoune@metz.ensam.fr

(M.R. Berrahmoune).1 Tel.: +33-382515241; fax: +33-382514260.

increases its thermal stability and consequently, the austen-

ite can be retained upon cooling to room temperature [15].A quantitative determination of the volume fraction of

the existing phases, especially the retained austenite, is es-

sential for the evaluation of the TRIP steels properties. Ex-

perimental methods that have been reported in the literature

include X-ray diffraction (XRD) [16], optical microscopy

combined with image analyses [7], scanning electron mi-

croscopy [8], dilatometry [9], and magnetic measurements

[10]. Among them, the XRD method is the most frequently

used as it is a suitable technique. In fact, it allows to study

the crystallographic texture of the material, and also to de-

termine the stress state in the material that evolves with the

phase transformation. Indeed, internal stresses are generated

during phase transformation. These internal stresses come

from different sources:

The internal stresses resulting from the incompatible

transformation strain accompanying martensitic phase

transition (shear strain of 0.2 and change in volume of

0.04 for steels) [4].

The internal stresses associated with plastic flow of prod-

uct and parent phase due to dislocation motion [11].

The present work aims to determine the mechanical be-

havior of the (commercial) TRIP 600 steel by characterizing

the martensitic transformation using XRD.

0921-5093/$ see front matter 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.msea.2003.10.372

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M.R. Berrahmoune et al. / Materials Science and Engineering A 378 (2004) 304307 305

Table 1

Chemical composition of the alloy in wt.% (balance iron)

Steel C Mn Si P Al

TRIP600 0.08 1.70 1.55 0.015 0.029

2. Experimental procedure

2.1. Material and mechanical testing

Experiments were performed on thin sheets of rolled TRIP

600 steel (1.5 mm thickness) provided by Arcelor. The chem-

ical composition is given in Table 1.

After polishing, specimens were etched with sodium

di-sulfite and picral solution. The microstructure was inves-

tigated using light optical microscopy (LOM). Tensile tests

were performed in the rolling direction using flat samples

with 25mm gauge length and 4mm width. Strain rate of

2.6 103 s1 was used. High temperature tests (80 and

120 C) were done using resistance heated whereas in low

temperature tests (0, 30 and 60 C) liquid nitrogen was

used.

2.2. Determination of retained austenite content

The retained austenite content was measured at the surface

of samples by X-ray phase analysis, using Co K radiation.

The measuring accuracy of the retained austenite by this

method is about 1%. A step scan within the 2 interval

between 45 and 150 was done (Fig. 1). The reflection

intensities were deduced from the pole figures of {2 0 0}

and {2 1 1} planes of ferrite/martensite phase, and {2 2 0}and {3 1 1} planes of austenite phase. To compare retainedaustenite at the surface and in the volume of samples, the

magnetic method was used. For magnetic measurements,

the full austenite decomposition in the reference sample was

obtained by annealing some samples at 600 C for 4 h.

2.3. Determination of residual stresses

For stress analysis, we used the classical sin2 method.

is the angle between the surface normal and the direc-

0

50

100

150

200

250

300

350

45 65 85 105 125 1452

Intensity

(cps)

{1 1 0}

{3 1 1} {2 2 0}g{2 2 0}

{2 0 0}

{2 1 1}

Fig. 1. 2-scan showing peaks of each phase after a pre strain of 0.5%

at 20 C.

tion of the strain being measured. In the sin2 method, lat-

tice strains are measured at each -tilt of the specimen. For

isotropic materials, macroscopic diffracting plane strain val-

ues are linearly dependent on sin2 . The slope of the

curve leads to the value of the stress . X-ray measure-

ments were carried out on {3 1 1} crystallographic planes

of austenite using Co K radiation. A step width of 0.1

and measuring time of 60 s at 2 angles between 108 and

113, for each of the 13 angles are chosen. The center of

gravity method was used for the determination of the peak

positions. Then, corresponding strains were computed and

the stresses were calculated by linear analysis regression.

3. Results and discussion

3.1. Microstructure

Fig. 2 shows the initial microstructure of the three-phasesteel which consists in ferrite base with grains of various

sizes from 15 to 20m, containing bainite islands (dark)

located on the grain boundaries and retained austenite is-

lands (white) being inside ferrite grains and also on the

grain boundaries. An optical phase proportioning gives 8%

of retained austenite and less than 1% of bainite content.

The low content of retained austenite makes observation of

the martensite formed after pre-strain more difficult. The

Ms temperature was measured using differential scanning

calorimetry and is about 65 C.

3.2. Mechanical properties

The stressstrain curves of the TRIP 600 steel at various

temperatures ranging from 60 to +120 C are presented

in Fig. 3. At low temperatures (T = 0 C), the curves

present a plateau in the beginning of plastic range. This

phenomenon can be explained by the fact that at low tem-

peratures, retained austenite is less stable than at high tem-

peratures and transforms to martensite easily. It corresponds

to GreenwoodJohnson effect that results in plastic accom-

modation carried out at constant stress [12]. At higher tem-

peratures (T 20 C), the plateau disappears. The Fig. 4

Fig. 2. Optical micrograph of TRIP 600 steel. F: ferrite; B: bainite; RA:

retained austenite.

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306 M.R. Berrahmoune et al. / Materials Science and Engineering A 378 (2004) 304307

0

100

200

300

400

500

600

700

800

0 20 40 60 80

Strain ( %)

Stress(MPa)

5

1

23

4

1 --- T = - 60C

2 --- T = - 30C

3 --- T = + 20 C

4 --- T = + 80 C

5 --- T = + 120C

Fig. 3. Tensile stressstrain curves of the TRIP 600 at various temperatures.

370

380

390

400

410

420

430

440

450

460

-100 -50 0 50 100 150

temperature (C)

Yieldstrentgh

(MPa)

Fig. 4. The evolution of yield strength as function of temperature.

presents the evolution of yield strength as a function of the

temperature. It can be seen that the yield strength decreaseswith temperature. However, a peak is observed around

T = 20 C. This could correspond to Ms temperature that

is the transition temperature between the stress-assisted

transformation and strain induced transformation domains.

Indeed, from the Fig. 5, it can be seen that the mecha-

nism of the transformation depends on the temperature.

Spontaneous transformation at pre-existing nucleation sites

occurs on cooling to below the Ms temperature (point A).

Stress-assisted nucleation on the same sites will occur at

SM

SM dM

C

A

Yield strengthof austenite

Temperature

Stress assisted

nucleation

Strain induced

nucleation

Stres

s

Fig. 5. Schematic representation of stress-assisted and strain-induced

regimes of mechanically induced transformation.

0

5

10

15

20

25

30

-100 -50 0 50 100 150

Temperature (C)

uniform

elongationA%

Fig. 6. The evolution of uniform elongation as function of temperature.

temperatures above Ms at increasingly higher stresses for

increasing temperatures until Ms (point C). Above Ms

new nucleation sites are introduced by the plastic strain.

So the yield strength of austenite decreases with increasing

temperatures. Near Ms both modes will operate [13].

The Fig. 6 represents the evolution of the uniform elon-

gation as a function of the temperature. The elongation in-

creases with temperature until T = 70 C, temperature at

which the TRIP effect is maximum. The increase of elon-

gation corresponds to the additional strain which due to

GreenwoodJohnson and the Magee effect [12,13].

3.3. Volume fraction of retained austenite

From the Fig. 7, it can be seen that the rate of the trans-

formation is zero at the beginning and gradually increases

to a maximum after a considerable amount of strain. Afterthis the rate gradually decreases until no further transforma-

tion occurs upon further straining and a limiting value for

the martensite content is reached. In the other hand, at low

temperatures (60 up to 0 C), the austenite transforms into

martensite very rapidly: for = 5% at T = 30 C, the

fraction of retained austenite has decreased from 6 to 2%,

and it is about 1% at failure. On the contrary, the kinetics

of transformation at high temperatures are much slower at

0

1

2

3

4

5

6

78

9

1 0

0 5 1 0 1 5 2 0 2 5

Strain(%)

retainedaustenite(%

)

6

5

32

1

4

1- T = -60C2- T = -30C

3- T = 0C4- T = 20C5- T = 80C

6- T = 120C

Fig. 7. The volume fraction of retained austenite as function of strain at

various temperatures.

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M.R. Berrahmoune et al. / Materials Science and Engineering A 378 (2004) 304307 307

Table 2

Stress values in austenite at different pre-strain determinated by X-ray

diffraction

Pre-strain (%) Phase Plane Stress (MPa) Remark

0 Austenite {3 1 1} 75 5 In compression0.5 Austenite {3 1 1} +130 15 In tension1 Austenite {3 1 1} +275 5 In tension

the beginning. Moreover, the fraction of retained austenite

at failure is about 34%. The high rate of transformation at

low temperatures is due to the fact that these temperatures

are close to Ms and the martensitic transformation is stress

assisted. At this stage, the martensitic transformation occurs

before the plastic deformation of austenite. At high temper-

atures, the low kinetics of transformation is due to the high

stability of retained austenite.

In addition, by XRD analyses (curves 13, 5, and 6) the

initial volume fraction of retained austenite is about 6%whereas it is about 8% by magnetic (curve 4) and optical

measurements. This can be explained by the fact that during

polishing, the retained austenite at the surface of the sam-

ples transforms to martensite, and X-ray diffraction mea-

surements are carried out at the surface. To avoid that, it

would be necessary to etch samples with electrochemical

etching on the depth of 100m.

3.4. Residual stresses distribution

The strain measurements were done only for the austenitic

phase only. Measurement on martensite could not be done

because it is difficult to distinguish the peak of martensitephase from the peak of the ferrite. Stress values with their

uncertainty at different pre-strain in austenite are given in

Table 2.

It is noticed that the austenite phase is in tension and the

stress values increase with increasing pre-strain except at the

initial state where the stress value is negative due to surface

preparation. However, the fact that austenite phase is in ten-

sion (I > 0) shows that this phase is not the softest one.

Indeed, the positive residual stresses value in austenite after

unloading corresponds to the fact that during the loading,

the local behavior of the austenite will be above the macro-

scopic behavior and then the austenite phase is harder thanthe ferrite phase. This result is in agreement with nanoinden-

tation tests carried out by Furnmont et al. [14] on different

TRIP steels which show that the hardness values increases

in the following order: ferrite, bainite, austenite, and marten-

site. The high value of the austenite hardness is due to the

carbon content which is greater in austenite than in ferrite

phase.

4. Conclusion

Mechanical properties of the TRIP 600 steel were deter-mined using tensile tests at various temperatures ranging

from 60 to 120 C. The TRIP effect is maximum for T =

70 C. The temperature Ms was deduced from the yield

strength vs. temperature curves, it is about 20 C. The kinet-

ics of transformation are high at low temperatures whereas

the transformation is much slower at high temperatures.

Stress distribution, determined by X-ray diffraction, shows

that after unloading, the austenite phase is in tension. How-

ever, measurement on martensite could not be done because

of the difficulty to distinguish martensite from ferrite peaks.

The next step in this work is to distinguish these peaks us-

ing deconvolution method. On the other hand, in situ tests

will be performed for the determination of stress distributionduring the loading.

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