Effect of Mn Segregation on TRIP Mechanism in Medium-Mn Duplex Steel

Abstract

Recently, interest in global warming, emission gas and fine dust has been heightened, and steel and automobile companies have been actively researching on the weight reduction of vehicles. Many efforts have been directed towards applying lightweight materials such as Al or Mg alloys, despite their high costs. However, because of poor formability and limited tensile strength, the application is restricted for these alloys. Many consumers' demands and expectations are headed towards increasing vehicle weight, aiming to improve vehicle safety, reliability and passenger comfort. In order to satisfy both of these two conditions, lightweight and safety, many research is actively operated to apply Advanced High Strength Steel (AHSS) to the body-in-white (BIW). It is anticipated that application of AHSS will catch two goals of thickness reduction and safety assurance at the same time. Various steels have been studied as potential candidates for AHSS, but studies on medium-Mn steels are actively underway. In medium-Mn steels, intercritical annealing is performed after hot rolling or cold rolling. Retained austenite is intentionally produced by the austenite reverted transformation (ART) and austenite grain growth from the partitioning of austenite stabilizing element, C and Mn. This retained austenite is stabilized through C/Mn partitioning from surrounding martensite or ferrite. Therefore, when external stress or strain is applied, the energy required for martensite transformation is met and the transformation-induced plasticity (TRIP) behavior is shown. Due to TRIP behavior during deformation, medium-Mn steels show better strain hardening than conventional (ferrite + martenstie) dual-phase steels. After intercritical annealing, martensite is in a tempered state and high yield strength can be obtained. However, retained austenite must have appropriate stability so that it exhibit continuous TRIP behavior until fracture and contribute to improvement of tensile properties. However, when rolling the medium-Mn steel with the Mn range between 3-10 wt.%, the Mn segregation band is inevitably generated along the rolling direction. These Mn segregation bands are formed by solidification process: solidification occurred from the dendrite core and pushed the solute atom outward, and showed a high solute concentration in the interdredritic space. In general, it is reported that the Mn segregation has an adverse effect on the formability of the material. In-depth studies on microstructural evolution and finding proper fraction and stability of ret20130882ained austenite in medium-Mn (austenite + martensite) duplex steels have been insufficient. In the present study, the medium-Mn (austenite + martensite) duplex hot-rolled steel showing the operation of TRIP behavior was developed by varying tempering temperature of an Fe-0.1C-10Mn-1Si-0.3Mo-0.5V steel sheets, and tensile properties were evaluated. Detailed microstructural evolution by both thermally and mechanically were investigated in relation with Mn segregation band by electron back-scatter diffraction (EBSD) and transmission electron microscopy (TEM) analyses. The volume fractions of each phases such as austenite, ?’-martensite, and ?-martensite were identified by X-ray diffraction (XRD). Since the electron probe micro-analysis (EPMA) has a limitation on observing Carbon atom, detailed chemical compositions in segregated regions were also examined by an atom probe tomography (APT). Then the correlation between microstructural evolution process and tensile ductility was verified.