**1. Introduction**

352 Corrosion Resistance

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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 bainiticaustenitic 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

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, straininduced 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 strainhardened 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).

mechanism in high-Mn alloys.

**2.1 General and pitting corrosion** 

**2. Corrosion behaviour** 

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

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).

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 highstrength 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)3AlC) and for SFE > 100 mJm-2, the SIP (Shear Band Induced 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, straininduced 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 strainhardened 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.
