A. Grajcar - Corrosion Resistance of High-Mn austenitic steels for the automotive industry.pdf

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Corrosion Resistance of High-Mn
Austenitic Steels for the Automotive Industry
Adam Grajcar
Silesian University of Technology
Poland
1. Introduction
Significant progress in a field of development of new groups of steel sheets for the
automotive industry has been made in the period of the last twenty years. From the aspect
of materials, this development has been accelerated by strong competition with non-ferrous
aluminium and magnesium alloys as well as with composite polymers, which meaning has
been successively increasing. From the aspect of ecology, an essential factor is to limit the
amount of exhaust gas emitted into the environment. It is strictly connected to fuel
consumption, mainly dependent on a car weight. Application of sheets with lower thickness
preserving proper stiffness requires the application of sheets with higher mechanical
properties, keeping adequate formability. Figure 1 presents conventional high-strength
steels (HSS) and the new generations of advanced high-strength steels (AHSS) used in the
automotive industry. Steels of IF (Interstitial Free) and BH (Bake Hardening) type with
moderate strength and high susceptibility to deep drawing were elaborated for elements of
body panelling. However, the increasing application belongs to new multiphase steels
consisting of ferritic matrix containing martensitic islands (DP – Dual Phase) or bainitic-
austenitic regions (TRIP – Transformation Induced Plasticity). These steels together with CP
(Complex Phase) and MART steels with the highest strength level are the first generation of
advanced high-strength steels (AHSS) used for different reinforcing elements (International
Iron & Steel Institute, 2006).
Nowadays, apart from limiting fuel consumption, special pressure is placed on increasing
the safety of car users. The role of structural elements such as frontal frame members,
bumpers and other parts is to take over the energy of an impact. Therefore, steels that are
used for these parts should be characterized by high product of UTS and UEl, proving the
ability of energy absorption. It is difficult to achieve for conventional HSS and the first
generation AHSS because the ductility decreases with increasing strength (Fig. 1).
The requirements of the automotive industry can be met by the second generation of
advanced high-strength steels combining exceptional strength and ductile properties as well
as cold formability (Fig. 1). These TWIP (Twinning Induced Plasticity) and L-IP (Light –
Induced Plasticity) steels belong to a group of high-manganese austenitic alloys but are
much cheaper comparing to Cr-Ni stainless steels (AUST SS). Their mean advantage over
first generation steels with a matrix based on A2 lattice structure is the great susceptibility of
austenite on plastic deformation, during which dislocation glide, mechanical twinning
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354
Corrosion Resistance
and/or strain-induced martensitic transformation can occur. The group of high-manganese
steels includes alloys with 15-30% Mn content. Two mean chemical composition strategies
had been worked out so far. The first includes alloys with different Mn concentration and
0.5 to 0.8% C (Ghayad et al., 2006; Jimenez & Frommeyer, 2010). The function of carbon is
stabilization of
phase
and hardening of solid solution. In the second group, the
concentration of carbon is decreased below 0.1%, whereby there is an addition up to 4% Al
and/or 4% Si (Frommeyer et al., 2003; Graessel et al., 2000). The solid solution strengthening
caused by Al and Si compensates smaller C content. Sometimes, the steels contain
chromium (Hamada, 2007; Mujica Roncery et al., 2010) or microadditions of Nb, Ti and B
(Bleck & Phiu-on, 2005; Grajcar et al., 2009; Huang et al., 2006).
Fig. 1. Conventional high-strength steels (HSS) and the new generations of advanced high-
strength steels (AHSS) used in the automotive industry (International Iron & Steel Institute,
2006).
Mechanical properties of high-manganese steels are dependent on structural processes
occurring during cold deformation, which are highly dependent on SFE (stacking fault
energy) of austenite (De Cooman et al., 2011; Dumay et al., 2007; Vercammen et al., 2002). In
turn, the SFE is dependent on the temperature and chemical composition. Figure 2 shows
that the stacking fault energy increases with increasing temperature and Al, Cu content
whereas Cr and Si decrease it (Dumay et al., 2007; Hamada, 2007). If the SFE is from 12 to 20
mJm
-2
, a partial transformation of austenite into martensite occurs as a main deformation
mechanism, taking advantage of TRIP effect.
Values of SFE from 20 to 60 mJm
-2
determine intensive mechanical twinning related to TWIP
effect. At SFE values higher than about 60 mJm
-2
, the partition of dislocations into Shockley
partial dislocations is difficult, and therefore the glide of perfect dislocations is the dominant
deformation mechanism (Hamada, 2007). In TRIPLEX steels with a structure of austenite,
ferrite and
-carbides
((Fe,Mn)
3
AlC) and for SFE > 100 mJm
-2
, the SIP (Shear Band Induced
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Corrosion Resistance of High-Mn Austenitic Steels for the Automotive Industry
355
Plasticity) effect is considered as the major deformation mechanism (Frommeyer & Bruex,
2006). High impact on the dominating deformation mechanism have also the temperature,
strain rate and grain size (Dini et al., 2010; Frommeyer et al., 2003; Graessel et al., 2000). The
key to obtain the mechanical properties regime in Fig. 1 is the high work hardening rate
characterizing the plastic deformation of high-Mn alloys. The high level of ductility is a
result of delaying necking during straining. In case of the local presence of necking, strain-
induced martensitic transformation occurs in such places (in TRIP steels) or deformation
twins are preferably generated in locally deformed areas (in TWIP steels). It leads to
intensive local strain hardening of the steel and further plastic strain proceeds in less strain-
hardened adjacent zones. The situation is repeated in many regions of the sample what
finally leads to delaying necking in a macro scale and high uniform and total elongation.
The shear band formation accompanied by dislocation glide occurs in deformed areas of
TRIPLEX steels and the SIP effect is sustained by the uniform arrangement of nano size
-
carbides coherent to the austenitic matrix (Frommeyer & Bruex, 2006).
Fig. 2. Schematic drawing of the effects of temperature and chemical composition on the
stacking fault energy (SFE) of austenite and the correlation of SFE with a main deformation
mechanism in high-Mn alloys.
2. Corrosion behaviour
2.1 General and pitting corrosion
The mean area of studies on high-manganese steels concern their high-temperature
deformation resistance (Bleck et al., 2007; Cabanas et al., 2006; Dobrzański et al., 2008;
Grajcar & Borek, 2008; Grajcar et al., 2009) and the cold-working behaviour (Dini et al., 2010;
Frommeyer & Bruex, 2006; Frommeyer et al., 2003; Graessel et al., 2000; Huang et al., 2006).
Much less attention has been paid on their corrosion resistance (Ghayad et al., 2006; Grajcar
et al., 2010a, 2010b; Hamada, 2007; Kannan et al., 2008; Mujica et al., 2010; Mujica Roncery et
al., 2010; Opiela et al., 2009). The research on Fe-C-Mn-Al alloys (Altstetter et al., 1986) for
cryogenic applications that were supposed to substitute expensive Cr-Ni steels was carried
out in the eighties of the last century. The role of manganese boils to Ni replacement and
obtaining austenitic microstructure, whereas aluminium has a similar impact as chromium.
Improvement of corrosion resistance by Al consists in formation of thin, stable layer of
oxides. As the result of conducted research it was found that Fe-C-Mn-Al alloys show
inferior corrosion resistance than Cr-Ni steels and they can be used as a substitute only in
some applications (Altstetter et al., 1986). The addition of 25% Mn to mild steels was found
to be very detrimental to the corrosion resistance in aqueous solutions (Zhang & Zhu, 1999).
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356
Corrosion Resistance
The Fe-25Mn alloy was difficult to passivate, even in such neutral aqueous electrolytes as
1M Na
2
SO
4
solution. With increasing Al content up to 5% of the Fe-25Mn-Al steel, the
anodic polarization curves exhibit a stable passivation region in Na
2
SO
4
solution, but it
shows no passivation in 3.5wt% NaCl solution.
Recently, corrosion resistance of Fe-0.05C-29Mn-3.1Al-1.4Si steel in acidic (0.1M H
2
SO
4
) and
chloride-containing (3.5wt% NaCl) environments was investigated (Kannan et al., 2008).
Moreover, the corrosion behaviour of the tested high-Mn steel with that of IF-type was
compared. Performing immersion and polarization tests it was found that Fe-Mn-Al-Si steel
has lower corrosion resistance than IF steel, both in acidic and in chloride media. The
corrosion resistance of the high-manganese steel in chloride solutions is higher compared to
that observed in acidic medium. The behaviour of Fe-0.2C-25Mn-(1-8)Al steels with
increased concentration of Al up to 8% wt. in 3.5wt% NaCl was also investigated (Hamada,
2007). Hamada reported that the corrosion resistance of tested steels in chloride
environments is pretty low. The predominating corrosion type is the general corrosion, but
locally corrosion pits were observed. In steels including up to 6% Al with homogeneous
austenite structure, places where the pits occur are casually, whereas in case of two-phase
structure, including ferrite and austenite (Fe-0.2C-25Mn-8Al), they preferentially occur in
phase.
The corrosion resistance of examined steels can be increased trough anodic
passivation in nitric acid, which provides modification of chemical composition and
constitution of the surface layer (Hamada et al., 2005). This was done by reducing the
surface concentration of Mn and enriching the surface layer in elements that improve the
corrosion resistance (e.g. Al, Cr).
A better effect was reached by chemical composition modification. It was found that
addition of Al and Cr to Fe-0.26C-30Mn-4Al-4Cr and Fe-0.25C-30Mn-8Al-6Cr alloys
increases considerably the general corrosion resistance, especially after anodic passivation
ageing of surface layers in an oxidizing electrolytic solution (Hamada, 2007). Cr-bearing
steels passivated by nucleation and growth of the passive oxide films on the steel surface,
where the enrichment of Al and Cr and depletion of Fe and Mn have occurred. The positive
role of Cr in obtaining passivation layers in 0.5M H
2
SO
4
acidic solution was recently
confirmed in Fe-25Mn-12Cr-0.3C-0.4N alloy (Mujica et al., 2010; Mujica Roncery et al., 2010).
The steel containing increased Cr, C and N content shows passivity at the current density
being five orders of magnitude lower compared to the Fe-22Mn-0.6C steel.
2.2 Effect of deformation
Results of corrosion tests described above concern steels in the annealed or supersaturated
state. The influence of cold plastic deformation on corrosion behaviour in 3.5wt% NaCl was
studied in Fe-0.5C-29Mn-3.5Al-0.5Si steel (Ghayad et al., 2006). It was found on the basis of
potentiodynamic tests, that the steel shows no tendency to passivation, independently on
the steel structure after heat treatment (supersaturated, aged or strain-aged). Higher
corrosion rate of deformed specimens than that of specimens in supersaturated state, was a
result of faster steel dissolution caused by annealing twins, which show a different potential
than the matrix. The highest corrosion rate was observed in strain-aged samples, as a result
of ferrite formation, which creates a corrosive galvanic cell with the austenitic matrix. The
enhanced corrosion attack at the boundaries of deformation twins was also observed in Fe-
22Mn-0.5C steels (Mazancova et al., 2010).
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Corrosion Resistance of High-Mn Austenitic Steels for the Automotive Industry
357
2.3 Hydrogen embrittlement and delayed fracture
Generally, increasing the strength of steels, their hydrogen embrittlement susceptibility
increases. This is one of the main problem to use AHSS. If hydrogen content reaches the
critical value, it can induce a reduction of strength and ductile properties. A critical
concentration of hydrogen is various for different steels (Lovicu et al., 2010; Sojka et al.,
2010). Hydrogen embrittlement is usually investigated by performing slow strain rate
tensile tests on hydrogenerated samples. Austenitic alloys are considered to be immune to
this type of corrosion damage. However, the stress- or strain-induced martensitic
transformation of austenite taking place in TRIP-aided austenitic alloys can be a reason of
their embrittlement. This can happen due to the high difference in solubility and diffusion
rate of hydrogen in the BCC and FCC lattice. Austenite is characterized by high solubility
and low diffusivity of H in the A1 lattice and thus acts as a sink for hydrogen lowering its
mobility and increasing the hydrogen concentration. Due to the slow diffusion rate of
hydrogen in austenite, it is hardly to enrich it homogeneously to a hydrogen content
causing embrittlement. However, it was shown (Lovicu et al., 2010) that the hydrogen
concentration in surface regions of the high-Mn steel is much higher than in the centre
zone. It can lead to the intragranular fracture in these regions because of strain-induced or
hydrogen-induced martensitic transformation and finally to reduction of strength and
ductility.
When the formed automotive element is exposed to the air the delayed fracture can occur.
The technological formability is usually investigated in cup forming tests (Otto et al., 2010;
Shin et al., 2010). It was observed (Shin et al., 2010) that the 0.6C-22Mn steel cup specimen
underwent the delayed fracture when exposed to the air for seven days, even though the
specimen was not cracked during forming. This is because the strain-induced martensitic
transformation occurred during the cupping test in places of stress concentration. When the
addition of 1.2% Al was added the steel cup forms with the high share of mechanical
twinning instead the
’
transformation. It leads to lower stress concentration and finally
to improvement in cup formability (Shin et al., 2010).
3. Experimental procedure
3.1 Material
The chapter addresses the corrosion behaviour of two high-Mn steels of different initial
structures in chloride and acidic media. Their chemical composition is given in Table 1.
Steel grade
C
Mn
26Mn-3Si-3Al-Nb 0.065 26.0
25Mn-3Si-1.5Al-Nb-Ti 0.054 24.4
Si
3.08
3.49
Al
Nb
Ti
S
P
N
O Structure
2.87 0.034 0.009 0.013 0.004 0.0028 0.0006

1.64 0.029 0.075 0.016 0.004 0.0039 0.0006
+
Table 1. Chemical composition of the investigated steels, wt. %
The vacuum melted steels have similar C, Mn and Si concentration. Significant impact on
the SFE of austenite has the difference in Al and Ti content. The lower SFE of 25Mn-3Si-
1.5Al-Nb-Ti steel compared to 26Mn-3Si-3Al-Nb steel is a result of the lower Al content.
Moreover, several times higher Ti content in a first steel provides a decrease of
phase
stability, as a result of fixing the total nitrogen and some carbon (Grajcar et al., 2009).
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