**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).
