**2. Corrosion behaviour**

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

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,

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

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

(Bleck & Phiu-on, 2005; Grajcar et al., 2009; Huang et al., 2006).

2006).

mechanism, taking advantage of TRIP effect.
