STRENGTH AND DUCTILITY OF SINGLE PHASE STEELS
FROM USUAL TO NANO-SCALE MICROSTRUCTURES
O. Bouaziz
ArcelorMittal Research, Voie Romaine-BP30320,
57283 Maizières-lès-Metz Cedex, France.
olivier.bouaziz@arcelormittal.com
Keywords : steels, ductility, nano-structured, single phase
1. Introduction
In order to achieve strengths higher than 1500MPa in single phase steels, the developed metallurgical
solutions are the martensitic grades [1,2] or more recently austenitic steels Fe-Mn-C with TWining
Induced Plasticity [3-5]. An alternative suitable strategy is based on a very strong grain size refinement
in the sub-micron domain where even strengths in excess of 2000MPa are possible in the single phase
iron based system [6,7].
In the field of the development of nano-materials two main challenges have to be tackled by research
in order to support this possible new family of structural materials :
- the control of the ductility,
- the realistic processing routes.
The present paper focuses only on the first one. The topic is not simple, as there is some confusion in
literature dedicated to nano-grained metals regarding the definition of what ductility we are talking
about. Indeed there are two main ductility indicators used to compare the developed nano-structured
alloys and this comparison controls a lot the conclusions and the orientations related to the best
metallurgical solutions. The two indicators are the uniform elongation and the total elongation at
fracture of the tensile specimen. Unfortunately, the second one is well known not to be an intrinsic
property of the material. It depends on the size and shape of the specimen and is affected also by the
extensometer used. As a matter of fact it is probably dangerous and meaningless to use elongation at
fracture for two reasons :
- the comparison between data collected in the literature may not be relevant due the different
sample geometries used by the various research teams,
- scientific conclusions concerning the understanding of the relationships between the
microstrutural scale and the ductility are not generally possible.
Thus this article is only based on the analysis of the effect of the microstructural length scale on :
- the uniform elongation,
- the fracture strain given by the area reduction.
Indeed the first one is relevant to analysis of the effect of the microstructure refinement on the strainhardening. The second one is useful to assess the damage resistance which is a valuable property for
ultra-high strength structural alloys.
2. Uniform elongation and fracture strain in single phase steel
2.1. The case of ferrite
Fifty years ago pioneering works investigated the relationship between the mechanical properties of
single phase steels and the ferritic grain size [8,9]. The first one focusing on the evolution of the
uniform elongation has been done by Morrison through the analysis of the properties of mild steel with
a carbon content between 0,05 and 0,13wt% with grain size in the range from 150 to 1.6µm [10].
Assuming the Hollomon power law for the stress-strain curve, it was shown that the strain-hardening
parameter n (assumed to be equal to the uniform elongation) is expressed as a function of the grain size
as [10] :
u 
5
10  d 
1
(1)
2
where d is the grain size in mm.
Using recent data related to the measurement of the uniform elongation for pure iron [6,7] and for IF
(Interstitial Free) steels [11-13] it is now possible to extend the analysis to a much more wider range of
grain size, especially to the sub-micron domain. As shown in Fig1 the decrease in the uniform
elongation with the grain size refinement is obviously confirmed. It is also noticed that the fit proposed
by Morrison (Eq.1) overestimates the uniform strain especially for grain size lower than one
micrometer. Thus a more accurate fit can be proposed as :

 d 


 u  0.27  1  exp   
6

(2)
where d is in µm.
To complete the picture, Fig2 summarizes the variation of the yield stress and the ultimate tensile
strength with the grain size for the same single phase ferritic steels.
0,3
0,25
Data
New fit
Morrison's fit
Strain
0,2
0,15
0,1
0,05
0
0,01
0,1
1
10
100
Grain size (µm)
Figure 1: Uniform elongation as a function of the grain size for pure iron and IF steels (data from [6, 7,
11, 12, 13]).
3000
YS
UTS
Stress (MPa)
2500
2000
1500
1000
500
0
0,01
0,1
1
10
100
Grain size (µm)
Figure 2: Variation of the yield stress (YS) and the ultimate tensile strength (UTS) with the grain size
for pure iron and IF steels (data from [6, 7, 11, 12, 13]).
As mentioned in the introduction, this review also aims at highlighting the behaviour of the fracture
strain as a function of the grain size, which is also a very important parameter to represent ductility
more completely. Surprisingly, there is a general lack of data in the literature concerning the influence
of grain size on area reduction in monophase ferritic steels, particularly in the submicron domain.
Indeed, complete investigations have been only done on ferrite-pearlite steels showing that the area
reduction increases as grain size decreases in the range between 2 and 100µm [10,14]. The reported
sensitivity of the area reduction to the variation of the grain size is given by [14] :
(3)
RA  0.785  0.0033d
where d is expressed in µm.
This sensitivity is quite small compared to that of the uniform elongation. The weak sensitivity seems
to be confirmed by a detailed investigation of the influence of the grain size in a stable austenitic steel
(i.e. exhibiting twinning and no induced martensite) and for grain size from 30µm to 0,5µm [10] as
summarized in Fig3. Finally it is probably reasonable to conclude that the area reduction is quite
constant with the grain size. This parameter should be more seriously measured and reported because
it is a relevant information with regard to the ductility of single phase steels as a function of the grain
size refinement.
1
650
550
450
Uniform elongation
Reduction area
Stress at 1%
0,5
350
Stress (MPa)
Strain
0,75
0,25
250
0
0,1
1
10
150
100
Grain size (µm)
Figure 3: Evolution of the uniform elongation, the area reduction and the flow stress at 1% strain as a
function of the grain size for a stable austenitic steel.
2.2. The case of martensite
In the domain of single phase steels with possible strengths higher that 1500MPa another example of
well known nanostructure is related to martensite. In order to compare the ductility of monophase
ferritic steels with sub-micron grain size, the uniform elongation and the area reduction are drawn as a
function of the carbon content (internal data for carbon content lower than 0.3wt% and data from [2]
for higher content). Strengths higher than 1500MPa can be achieved with a better uniform elongation
than for the ultra-fine grain ferrite (carbon content between 0.2 and 0.3wt%) but possibly with an
inferior area reduction at fracture.
0,6
2400
2200
0,5
2000
1800
0,3
Uniform elongation
Reduction area
UTS
0,2
1600
UTS (MPa)
Strain
0,4
1400
0,1
1200
0
1000
0,1
0,15
0,2
0,25
0,3
0,35
Carbon content (wt%)
0,4
0,45
0,5
Figure 4: Variation of the uniform elongation, the area reduction at fracture and the ultimate tensile
strength with the carbon content for as quenched martensite (internal data for carbon content lower
than 0.3wt% and data from [2] for higher content).
3. How to go further
3.1. Martensite tempered at low temperature
Interesting combinations between strength and uniform elongation have been highlighted in the
previous section concerning as-quenched martensite. But area reduction at fracture needs to be
improved for carbon contents higher than 0.3wt%. Fig5 illustrates the capability of a tempering
treatment at low temperature to improve the area reduction without decreasing the uniform elongation
and with only a slight decrease in strength.
0,6
2300
Reduction area
Uniform elongation
UTS
2250
0,4
2200
0,3
2150
0,2
2100
0,1
2050
0
1
10
100
1000
10000
UTS (MPa)
Strain
0,5
2000
100000
Time (s)
Figure 5: Evolution of the uniform elongation, the area reduction at fracture and the ultimate tensile
strength with the tempering time at 150°C for 0.4%C martensite (data from [2].
.
3.2. The twins in austenite
It has recently been shown by Lu and co-workers [15-17] that an excellent combination of yield stress,
ultimate tensile strength and work-hardening can be achieved in nanostructured metals by introducing
a high density of coherent growth twins in electrodeposited copper films. By exploiting the observed
thermal stability of the high density of twins induced by plastic straining in an Fe-Mn-C austenitic steel
illustrated in Fig6, such an excellent combination of properties can also be achieved by a suitable
rolling and recovery treatment, which removes dislocations but not the twinning nano-structure [18], as
shown in Fig7 and Fig8. The balance between the complexity of the metallurgical route and the
obtained mechanical properties is very promising for bulk nanostructured single phase steels.
Figure 6: TEM bright field micrograph of Fe-22Mn-0.6C alloy cold rolled to 50% reduction and
partially recrystallised by annealing at 625°C for 120s. The specimen was tilted so that the zone axis of
the unrecrystallised grain was close to [011].
1800
1600
True stress (MPa)
1400
1200
1000
stress TD
stress RD
800
600
400
200
0
0
0,02
0,04
0,06
0,08
0,1
True strain
Figure 7: Tensile behaviour of an Fe-22Mn-0.6C TWIP steel after 50% cold reduction and a recovery
treatment at 500°C for 1h (RD : rolling direction, TD : transverse direction).
1750
1500
Stress (MPa)
70
YS-as rolled
UTS-as rolled
YS-recovered
UTS-recovered
Uel-as rolled
Uel-recovered
60
50
1250
40
1000
30
750
20
500
Uniform elongation (%)
2000
10
250
0
0
0
0,1
0,2
0,3
0,4
0,5
Equivalent strain
0,6
0,7
0,8
Figure 8: Evolution of the yield stress (YS), the ultimate tensile strength (UTS) and the uniform
elongation for an Fe-22Mn-0.6C TWIP steel as a function of the equivalent prestrain by rolling with
and without a recovery treatment at 500°C for 1h.
4. Conclusions
In the framework of single phase steels with strengths higher than 1500MPa, a review of the behaviour
of uniform elongation as a function of the grain size in ferrite has been presented. It has been shown
that if the ductility of the submicron grain sized monophase ferritic steels is only discussed based on
this indicator, the ductility is poor. However, the point has been made that the area reduction at fracture
is also an important aspect of the ductility. A general lack of data on the area reduction as a function of
the grain size in the monophase ferritic steels, especially in submicron range, has been noticed. This
parameter should be more seriously measured and reported because it provides relevant information
for discussing the ductility. In particular, the available data seem to show that this quantity tends to be
less sensitive to the grain size than uniform elongation. Indeed, the former one is relevant to analysis of
the effect of the microstructure refinement on the strain-hardening. The latter one is useful in assessing
the damage resistance which is a valuable property for ultra-high strength structural alloys.
Ductility of single phase martensitic steels has also been discussed. Strength higher than 1500MPa can
be achieved with a better uniform elongation than for the ultra-fine grain ferrite but with probably with
less favourable area reduction at fracture.
Finally, promising ways of achieving a desirable balance between strength, uniform elongation and
area reduction have been highlighted. Martensite tempered at low temperature or TWIP steels with
prestrain and recovering appear to be very promising as candidates for developing bulk nanostructured
single phase steels with relevant mechanical properties and realistic processing.
References
[1] G.Krauss: Met. Trans. A Vol. 32 (2001), p. 861
[2] M. Saeglitz, G.Krauss: Met. Trans. A Vol. 28 (1997), p. 377
[3] C. Scott, N. Guelton, S. Allain, M. Faral, in: Proceeding of the MS&T’05 Conference, Pittsburgh,
USA, (2005)
[4] O. Grassel, L. Kruger, G. Frommeyer, L. W. Meyer: Int. J. Plast. Vol.16 (2000), p.1391
[5] O. Bouaziz, S. Allain, C. Scott: Scripta Mat. Vol. 58 (2008), p. 484.
[6] S. Takaki, K. Kawasaki, Y. Kimura: J. of Mat. Proc. Tech. Vol. 117 (2001), p. 359
[7] D. Jia, K.T. Ramesh, E. Ma: Acta Mater. 51 (2003), p. 3495
[8] E.O. Hall: Proc. Phys. Soc. Vol. B64 (1951), p. 747
[9] N.J. Petch: J. Iron Steel Inst.Vol. 174 (1953), p. 25
[10] W.B. Morrison, R.L. Miller, in: Proc. of 16th Sagamore Army Materials Research Conference,
New York, (1969), p.183
[11] N. Tsuji, Y. Ito, Y. Saito, Y. Minamino, Scr. Mat. Vol.47 (2002), p. 893
[12] O. Bouaziz, T. Iung, M. Kandel, C. Lecomte : J. Phys IV Vol.11 (2001), p.223, 2001.
[13] O. Bouaziz, A. Aouafi, S. Allain : Mat. Sc. Forum Vol.584-586 (2008), p. 605
[14] F.B. Pickering, T. Gladman: Iron&Steel Institute, special report 81(1963), p. 10
[15] L. Lu , Y.F. Shen, X.H. Chen, L.H. Qian, K. Lu: Science Vol.304 (2004), p. 422
[16] E. Ma , Y.M. Wang, Q.H. Lu, M.L. Sui, L. Lu, K. Lu: Appl. Phys. Letter Vol.85 (2004), p. 4932
[17] M. Dao, L. Lu, Y. Shen, S. Suresh: Acta Mater. Vol.54 (2006), p. 5421
[18] O. Bouaziz, C. Scott, G. Petitgand: Scripta Mat. Vol.60 (2009), p.714