Microstructures and properties of TRansformation Induced

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A. Dimatteo, G. Lovicu, M. Desanctis, R. Valentini, A. Solina
The weight reduction of automobile, maintaining sufficient safety by using the high strength steel sheet,
is of great concern to the carmakers who have to fulfil environmental standards. In particular for the
structural and chassis parts, there has been the demand for replacing the conventional 390-440 MPa
grades steel sheets by the higher strength grades with sufficient formability. TRIP steel, where a small
portion of austenite remains in the ferritic-bainitic basic matrix, is one of the candidate materials for 590780 Mpa grades high strength steel sheet with superior formability. The excellent mechanical properties
exhibited by the TRansformation Induced Plasticity steels are mainly due to the martensitic
transformation of the metastable retained austenite induced by strain. Three TRIP steels grade have been
tested: 590, 690 and 780. Mechanical properties are determined by uniaxial tensile tests and are
correlated to the microstructures of the steels. The tensile tests up to fracture are been carried to analyse
the relation between the different strain hardening behaviours and the transformation of retained
austenite into martensite during deformation. The specimens for the metallographic investigations of
microstructure are conventionally prepared, conventional etchings such as Le Pera and nital plus sodium
metabisulphite, have been used. It has been possible by these etchings to identify the different phases
present in TRIP steel and to distinguee the different morphologies of retained austenite.
The relation between strain hardening behaviour and fraction of retained austenite is discussed.
Memorie
Microstructures and properties
of TRansformation Induced Plasticity steels
Keywords: steel, phase transformation, heat treatment, material characterization
INTRODUCTION
Antonella Dimatteo, Gianfranco Lovicu, Massimo Desanctis,
Renzo Valentini, Adriano Solina
Dipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali,
Università degli studi di Pisa, Italy
Paper presented at the 2nd International Conference
HEAT TREATMENT AND SURFACE ENGINEERING IN AUTOMOTIVE APPLICATIONS,
Riva del Garda, 20-22 June 2005
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Development of TRIP steels during the past decade leads to
a significant reduction of alloying elements addition which
makes the TRIP steels more economically available. Such a
development mainly arises from improvements of the heattreatment schedules, which consist of an intercritical annealing and a subsequent isothermal annealing in the bainitic
transformation region. The purpose for the heat is to stabilise the austenite by increasing its carbon concentration. The
carbon enrichment is due to phase separation during intercritical annealing and to the suppression of carbide formation during the bainitic transformation. The microstructure
after heat-treatment consists of ferrite, bainite, retained austenite and sometimes, martensite.
The main alloying element is carbon, which determines the
amount of intercritical austenite and leads to the retention of
the austenite in the TRIP steels. Manganese is used for hardenability and strength. It slows down the pearlite formation
and increases the strength of the material by solid solution
hardening. Silicon plays an important role on the stabilisation of the retained austenite. Silicon is an element, which
prevents and retards the carbide precipitation during bainite
formation. The important drawback of Si is that CMnSiTRIP steels are difficult to process in continuous galvanizing-line. Because of this, aluminium has been used to replace the silicon in industrial TRIP steel,(1,2). Infact both Al
and P also suppress the carbide formation. The material properties can be improved by adding Nb or Ti as alloying ele-
ments. Both elements have a similar effect. The grain size reduction effect, the suppression of undesired pearlite formation and delay of the isothermal production of bainite in the
region of the real temperature, are the main effects, through
which higher Residual-austenite contents can be realised.
The excellent mechanical properties exhibited by Transformation Induced Plasticity steels are mainly due to the martensitic transformation of the metastable retained austenite
induced by strain. In general, the retained austenite can effect the mechanical properties in several ways:
- residual blocky austenite could transform to martensite
during water quenching and improve the strength
- retained austenite interlath layers could increase the
strength by enhancing the ability of lath boundaries to obstacle dislocation movement
- retained austenite could increase the elongation due to the
TRIP effect. So, the higher volume fraction of the stable
retained austenite should correlate with improved mechanical properties.
The morphology of retained austenite has been classified into five different categories:
- blocky shape enclosed by polygonal ferrite,
- blocky or layer shape enclosed by acicular ferrite,
- blocky or layer shape enclosed by bainite packets,
- interlayer film enclosed by martensite laths,
- austenite/martensite constituent.
The stability, (4), of retained austenite depends on the carbon content in the retained austenite lattice (chemical stability), the size and the distribution of residual austenite as
well as the morphology of the surrounding phase (mechanical stability). It has been reported, (3), that the carbon distribution within residual austenite is not homogeneous, which
leads to the transformation of the low carbon retained austenite to martensite at the early stage of the deformation. The
retained austenite present between the polygonal ferrite
grains has a lower carbon content than that between the bainitic ferrite grains or laths due to the absence of carbon enrichment during the bainite reaction. These retained austenite
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Memorie
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islands tend to transform to martensite under a small strain.
The retained austenite located at a polygonal ferrite/bainite
interface might have a varying carbon distribution within
the crystal due to its location between polygonal ferrite on
one side and bainite on the other. This could lead to that part
of the retained austenite crystal in close proximity to the
polygonal ferrite, with a lower carbon content, transforming
to martensite at a lower strains than the areas in close proximity to bainite, with high carbon content.
It has also been observed that the optimum elongation behaviour is obtained when the retained austenite is present in
the microstructure in the form of the thin films between the
bainitic ferrite laths rather than as a blocky type between the
bainitic ferrite grains.
Recent research (8) has shown that retained austenite grains
larger than 1_m are unstable, and do not contribute significantly to the ductility of the material. On the other hand, retained austenite islands, which are smaller than submicron
have a low tendency to transform to martensite, even if
necking occurs, and thus do not contribute to the ductility.
The carbon content determines the chemical driving force
for the transformation of the retained austenite to martensite,
the stress-free transformation strain and the flow behaviour
of the retained austenite. It has recently been published that
the retained austenite only with optimum carbon content
(>0.5-0.6% and <1.8%) can provide the TRIP effect and increase the elongation.
The presence of other phases in the vicinity of retained austenite can also affect the strain induced transformation.
Martensite, for example, can propagate stress directly to the
retained austenite during deformation, which may then easily transforms to martensite at an early stage of straining
that diminishes the TRIP effect.
The drastic effects of microstructure on physical and mechanical properties of steels make metallographic examination
a necessity for properties understanding and enhancement.
To reveal specific microstructural characteristics, numerous
etching method have been developed, among which chemical etching is certainly the easiest and most widely used.
This technique utilizes a controlled corrosion process driven
by the electrochemical potential differences between surface
with chemical or physical heterogeneities,(6).
TRIP-aided steel are composed of four phases forming a
very fine microstructure which makes their observation difficult. For light microscopy observation, some specific etchants have been tested.
The purpose of this paper is to describe in detail the use of
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the etching techniques that have been used by several investigators to reveal the microstructure of TRIP steels. The
mechanical properties and the strain hardening behaviour of
the TRIP steel examined has been correlated to the stability
of the retained austenite.
MATERIALS AND ANNEALING CONDITIONS
Chemical composition of steels used in this study is reported
in table 1. The heat-treatment conditions applied to the
steels are shown in figure 1.
TESTING AND ANALYSING
Yield strength is defined as a lower yield point or as the
strength at 0.2% offset strain in case of the absence of a
yield point. The work-hardening behaviour is described
using the change in the instantaneous work-hardening exponent ninst defined as the following equation evaluated from
the true stress-strain curve.
ninst = dlnσt / dlnεt
The specimens for the metallographic investigations of the
microstructure are conventionally prepared and etched with
Nital and Klemm’s agent, with Nital and sodium metabisulfite’s agent, with Le Pera etching and with first with 4% Picral and then with 10% aqueous sodium metabisulfite solution. They are also etched with Behara’s etchant.
RESULTS
The following figures presents the typical multhiphase microstructure of TRIP steels consisting of bainite and retained
austenite dispersed in a ferritic matrix.
After application of Nital and Klemm’s agent, ferrite grains
show different colors, mainly blue or bright-brown, retained
austenite and martensite are white, bainite and tempered
martensite are brown and pearlite is black. Etching with Le
Fig. 2 – TRIP590 steel etched with nital and sodium metabisulfite.
Fig. 1 – Parameters of continuous annealing cycle.
Fig. 2 – Acciaio TRIP 590 attaccato metallograficamente con
Nital e metabisulfito di sodio.
Fig. 1 – Parametri del ciclo di ricottura continua.
TRIP590
TRIP690
TRIP780
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C
Mn
Si
P
S
Cr
Ni
Cu
Al
Ti
Nb
0.15
0.14
0.19
1.57
1.57
1.58
1.02
1.45
1.6
0.018
0.014
0.013
0.028
0.027
0.025
0.08
0.04
0.07
0.02
0.02
0.02
0.02
0.02
0.02
0.043
0.04
0.036
0.019
0.027
0.042
0.038
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Table 1 – Steels’ chemical
composition.
Tabella 1 – Composizione
chimica degli acciai.
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Memorie
Fig. 3 – TRIP780 steel etched with Nital and Klemm’ s agent.
Fig. 5 – TRIP690 steel etched with Behara.
Fig. 3 – Acciaio TRIP 780 attaccato metallograficamente con
Nital e Klemm.
Fig. 5 – Acciaio TRIP 690 attaccato metallograficamente con
Behara.
Fig. 4 – TRIP 590 steel etched with 4% Picral and then with 10%
aqueous sodium metabisulfite solution.
Fig. 6 – TRIP690 steel etched with Lepera.
Fig. 4 – Acciaio TRIP 590 attaccato metallograficamente con
Picral in alcool al 4% e poi con una soluzione acquosa di
metabisulfito di sodio al 10%
vent grain boundary motion even more effectively, result in
an increase in strength, but can also serve as nucleation sites
for phase transformations.
The positive effect of niobium on the volume fraction is an
higher austenite content as a result of the combination of the
different mechanisms, including grain refinement, carbon
enrichment, and martensite nucleation inhibition.
As niobium encourages the realisation of high austenite contents, the addition of niobium leads to at least equal elongation values whereas noticeably higher strength values are
obtained as consequence of grain refinement and precipitation hardening.
The mechanical properties of the steels are summarised in
table 2.
Steel
TRIP590
TRIP690
TRIP780
UTS,
MPa
Elongation,
%
YS,
MPa
thickness,
mm
617
699
792
17.6
14.7
18.0
394
400
471
2
2
2
Table 2 – Mechanical properties.
Tabella 2 – Proprietà meccaniche.
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Pera’ reagent, containing equal volumes of 4% picral and
2% aqueous Na2S2O5, the dispersed martensite islands and
the retained austenite in these microstructures are revealed
in brilliant white while the selectively attacked ferrite matrix
is tinted blue, the pearlite and bainite are brown. After application of Nital and sodium metabisulfite’ agent ferrite is tinted grey, martensite, bainite and pearlite are black, retained
austenite is white.
Etching with first with 4% Picral and then with 10%
aqueous sodium metabisulfite solution the steel microstructure reveals clearly the retained austenite as white, martensite as straw-coloured constituents, bainite can be identified
with needle-shaped austenite. The Behara’ s etchants colours ferrite, martensite, bainite and ferrite. The ferrite appears blue, the austenite yellow.
Due to its relatively big difference in atom radius niobium
enhances strength by interfering with dislocation slipping,
reduces diffusivity and slows down or even suppresses grain
boundary motion, when being located within the iron matrix. But as niobium has a strong tendency to combine with
carbon and nitrogen even more important effects arise from
the presence of small carbides and carbonitrides that form at
high and intermediate temperatures and only dissolve at
temperature above 1000 °C, (5). These small particles pre-
Fig. 6 – Acciaio TRIP 690 attaccato metallograficamente con Le
Pera.
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Fig. 7 – Variation of strain hardening coefficient with true strain
of TRIP 690 steel.
Fig. 8 – Variation of strain hardening coefficient with true strain
of TRIP 590 steel.
Fig. 7 – Variazione del coefficiente di incrudimento in funzione
della deformazione reale per l’ acciaio TRIP 690.
Fig. 8 – Variazione del coefficiente di incrudimento in funzione
della deformazione reale per l’ acciaio TRIP 590.
The high level of ductility of TRIP steels is achieved by the
transformation of metastable retained austenite under straining at room temperature. This transformation results in a
localised increase of the strain hardening coefficient during
straining, which delays the onset of necking and ultimately
leads to a higher uniform and total elongation, (7).
TRIP690 steel shows a higher n-value due to the transformation of a larger amount of retained austenite to martensite
during straining. This is consistent with the low elongation
obtained in the TRIP690 steel after testing.
REFERENCES
1. J. MAHIEU, D. VAN DOOREN and B.C. DE COOMAN, Proc. Of the Int. Conf. On TRIP-aided High
Strength Ferrous Alloys, Ghent (2002), ed. By B.C. De
Cooman, p.159.
2. WOLFGANG BLECK, Proc. Of the Int. Conf. On
TRIP-aided High Strength Ferrous Alloys, Ghent
(2002), ed. By B.C. De Cooman, p.13.
3. ILANA B. TIMOKHINA, PETER D. HODGSON and
ELENA V. PERELOMA, Metall. Trans. 35A, (2004);
p.2331.
4. P.J. JAQUES, J. LADRIERE, F. DELANNAY, Metall.
Trans. 32A, (2001); p.2759.
5. SEUNG CHUL BAIK, SEONGJU KIM, YOUNG
SOOL JIN, OHJOON KWON, ISIJ Int., 41, (2001),
p.290.
6. E. GIRAULT, P. JACQUES, P.H. HARLET, K. MOLS,
J. VAN HUMBEECK, E. AERNOUDT, and F. DELLNNAY, Materials Characterization 40, (1998), p.111.
7. O. MATSUMURA, O. SAKUMA and H. TAKESHI,
Scr. Metall. 21, (1987), p. 1301.
CONCLUSIONS
This work has demonstrated that the austenite and martensite are distinguishable etching with nital plus sodium metabisulfite and etching with Behara. With both etching the retained austenite appears white while the martensite is darkened. The retained austenite can be distinguish from non carbide bainite by shape: the retained austenite is rounded particles the other is needles.
This work has demonstrated that the retained austenite in
non-Niobium steel is more stable, the non-Niobium steel has
shown the optimum combination of mechanical properties.
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MICROSTRUTTURA E PROPRIETA’ DEGLI ACCIAI TRIP
Parole chiave:
acciaio, trasformazione di fase, trattamenti termici,
caratterizzazione materiali
Per le industrie automobilistiche che devono rispettare gli
standard ambientali è di grande importanza la riduzione in
peso delle automobili associata ad un sufficiente livello di
sicurezza tramite l’ uso di acciai alto resistenziali. In particolare per le parti strutturali e del telaio è in studio la sostituzione dei convenzionali acciai di grado 390-440 Mpa con
acciai a più elevato livello di resistenza dotati di sufficiente
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formabilità.
Le eccellenti proprietà meccaniche esibite dagli acciai TRIP
sono dovute principalmente alla trasformazione martensitica della austenite residua metastabile indotta dalla deformazione. Questa trasformazione porta ad un localizzato aumento del coefficiente di incrudimento, che ritarda l’ insorgere della strizione e porta a valori più alti dell’ allungamento percentuale uniforme e totale, (7).
Sono stati testati acciai TRIP di tre diversi gradi: 590, 690 e
780.
I provini per l’ investigazione metallografica sono stati preparati in modo convenzionale, e sono stati usati attacchi come il Le Pera e il nital insieme al metabisulfito di sodio tra-
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metabisulfito di sodio al 10% l’ austenite residua si rivela
chiaramente come le particelle bianche, la martensite come
i costituenti colorati di giallo, la bainite priva di carburi
può essere identificata con le particelle a forma di ago; figura 4. L’ attacco con il Behara colora la ferrite, martensite, la bainite e la ferrite. La ferrite appare blu, l’ austenite
sono le piccole particelle tonde bianche, la bainite senza
carburi sono le particelle bianche a forma di ago; la bainite
è marrone scuro la martensite marrone chiaro; figura 5.
Le proprietà meccaniche sono state determinate tramite test
di trazione uniassiale e sono stati correlati alla microstruttura degli acciai.
Le prove di trazione sono state effettuate per analizzare la
relazione esistente tra il coefficiente di incrudimento e la
trasformazione dell’ austenite residua in martensite durante
la deformazione. L’ acciaio TRIP 690 ha mostrato il più alto
valore di n istantaneo, figura 7, dovuto alla trasformazione
di una maggiore quantità di austenite residua in martensite
durante la deformazione. Ciò è conforme con il basso valore
dell’ allungamento percentuale a rottura dell’ acciaio TRIP
690.
Memorie
dizionalmente impiegati per gli acciai multifase. E’ stato
possibile tramite questi attacchi identificare le differenti fasi
presenti e distinguere la martensite dall’ austenite residua.
Dopo l’ applicazione del nital seguito dal Klemm i grani ferritici dovrebbero apparire principalmente blu o marrone
chiaro, la martensite e l’ austenite residua bianca, la bainite
marrone e la perlite nera. In realtà dalla figura 3 si vede che
dopo questo attacco è tutto scurito, non si riescono a distinguere le varie microstrutture. Attaccando con il Le Pera,
che contiene uguali volumi di una soluzione al 4% di acido
picrico in alcool e di una soluzione all’ 1% in acqua, le isole di martensite dispersa e l’ austenite residua appaiono
bianco brillante mentre la matrice ferritica è tinta di blu, la
perlite e la bainite sono marroni; figura 6. La martensite e l’
austenite residua sono quindi indistinguibili. Dopo l’ applicazione del nital seguito dal metabisulfito di sodio la ferrite
è tinta di grigio, la martensite, la bainite e la perlite sono
nere, l’ austenite residua è bianca; figura 2. Le particelle
bianche a forma di ago possono essere identificate con la
bainite priva di carburi. Utilizzando prima una soluzione al
4% di acido picrico in alcool e poi una soluzione acquosa di
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