No abstract

The grain size dependence of the tensile properties of a TWIP steel has been determined for a wide range of grain sizes obtained by grain growth after complete recrystallization of cold rolled material. The near-linear stress-strain behaviour typical of either TWIP steels or other materials that deform by twinning has been observed, the work hardening rate being larger for the smaller grain sizes. The Hall-Petch slope increases as a function of strain, from 350 MPa μm1/2 for the yield stress to 630 MPa μm1/2 for the maximum uniform strain in the tensile tests, ε  0.40. Profuse twinning is observed in deformed specimens by means of FIB-ISE.

Hot rolled, laboratory-cast, TWIP steel samples (5.4 mm thick) of 22% Mn - 0.6% C (in mass-%) were cold rolled to different reductions (from 40 % to 70 %) and subsequently isothermally annealed for various times at temperatures ranging from 450º C to 1100º C. The evolution of recrystallization and grain growth was followed by control of the softening kinetics complemented by metallographic, OIM and microtexture observations. A map of the recovery, recrystallization and grain growth in the temperature-time space was obtained. In all instances, the grain size at the end of recrystallization was very fine, D ≤ 2 µm and larger grain sizes were the result of grain growth. A range of grain sizes 2 µm ≤ D ≤ 50 µm was covered by the grain growth experiments. A phenomenological grain growth equation that is useful for the annealing control of this steel was derived from the measurements.

Fatigue behavior of four high-Mn (18 to 22 wt pct Mn) steels, after cold rolling and soft annealing, were investigated in reversed plane bending in a high-cycle regime. The surfaces of fatigued specimens were examined using an optical microscope, a scanning electron microscope (SEM), and an atom force microscope (AFM). It was discovered that the chemical composition of the steel had only a minor influence on fatigue behavior, and the fatigue limit (FL), i.e., the stress amplitude leading to more than 2AE10 6 cycles to failure, of all steels is about 400 MPa. This is about 42 to 48 pct of their tensile strength whose value is comparable to that of austenitic stainless steel and 780 MPa transformation induced plasticity (TRIP) steel. Mechanical twinning does not occur in the course of cycling; however, intense slip bands are formed. Fatigue cracks tend to nucleate at quite an early stage of fatigue life and most commonly on grain and annealing twin boundaries due to intersections of slip bands and boundaries.Savings in fuel consumption as well as the safety of passengers demand that automotive manufacturers use new materials to enable weight reductions of vehicles and improve crash resistance. In this context, high-Mn steels showing twinning induced plasticity effect (TWIP) are a promising type of material providing high tensile strength in combination with extremely good ductility. The superductility (total elongations between 60 and 95 pct) with high tensile strength (600 to 1100 MPa) of TWIP steels are related to the tendency of mechanical twinning in straining due to a low stacking fault energy (SFE) on the order of 25 mJ/m 2 . [1,2] Several investigations [3,4] have been carried out to determine the mechanical properties of TWIP steels (Fe-Mn-Al and Fe-Mn-Al-Si) in tensile loading at low and ambient temperatures. Fabrication properties such as formability and weldability have also been studied and reported. [5]