MECHANICAL BEHAVIOR OF AN HIGH STRENGHT STEEL (AHSS) WITH MEDIUM MN CONTENT IN TWO ROLLING CONDITIONS: HOT AND WARM

Abstract

The microstructure before intercritical annealing of an AHSS (Advanced High Strength Steel) with medium Mn content plays an important role in the final mechanical properties, since the transformations occurring during annealing modify phases, composition, and morphology. The microstructural changes that occur during intercritical annealing treatment of a medium Mn steel were examined. Two starting material comes from different conditions, hot rolling at 1200°C and warm rolling (initial rolling at 1200°C and subsequent at 680°C). The mechanical properties were related to the transformation phenomena that occur in these steels, mainly TRIP (Transformation Induced Plasticity) and TWIP (Twinning Induced Plasticity) effects. The transformations were verified by SEM (Scanning Electron Microscopy) and X-RD (X-Ray Diffraction). Tensile strength values of 1111 MPa and 17% elongation were obtained by hot rolling route. For the warm rolling route, 35% deformation and a tensile strength of 1357 MPa were obtained. The strain hardening curve was analyzed, showing the presence of the TWIP effect subsequently "saw" behavior related to the discontinuous TRIP effect. The mechanic properties values are related to the difference in morphology phases present. An acicular morphology of a/? (ferrite/austenite) provides a higher value of tensile strength, but low elongation percentage, and a mixture of lamellar and globular morphologies, provides an optimized combination of strength and ductility. Key words: AHSS, medium manganese steel, rolling, heat treatment, discontinuous TRIP effect, TWIP.